90 files changed, 19334 insertions, 0 deletions
diff --git a/dom/localstorage/ActorsChild.cpp b/dom/localstorage/ActorsChild.cpp
new file mode 100644
index 0000000000..ea7c589a66
--- /dev/null
+++ b/dom/localstorage/ActorsChild.cpp
@@ -0,0 +1,325 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ActorsChild.h"
+
+#include "ErrorList.h"
+#include "LSDatabase.h"
+#include "LSObserver.h"
+#include "LSSnapshot.h"
+#include "LocalStorageCommon.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Result.h"
+#include "mozilla/dom/LSValue.h"
+#include "mozilla/dom/Storage.h"
+#include "mozilla/dom/quota/QuotaCommon.h"
+#include "mozilla/ipc/BackgroundUtils.h"
+#include "nsCOMPtr.h"
+
+namespace mozilla::dom {
+
+/*******************************************************************************
+ * LSDatabaseChild
+ ******************************************************************************/
+
+LSDatabaseChild::LSDatabaseChild(LSDatabase* aDatabase) : mDatabase(aDatabase) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aDatabase);
+
+  MOZ_COUNT_CTOR(LSDatabaseChild);
+}
+
+LSDatabaseChild::~LSDatabaseChild() {
+  AssertIsOnOwningThread();
+
+  MOZ_COUNT_DTOR(LSDatabaseChild);
+}
+
+void LSDatabaseChild::SendDeleteMeInternal() {
+  AssertIsOnOwningThread();
+
+  if (mDatabase) {
+    mDatabase->ClearActor();
+    mDatabase = nullptr;
+
+    MOZ_ALWAYS_TRUE(PBackgroundLSDatabaseChild::SendDeleteMe());
+  }
+}
+
+void LSDatabaseChild::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnOwningThread();
+
+  if (mDatabase) {
+    mDatabase->ClearActor();
+#ifdef DEBUG
+    mDatabase = nullptr;
+#endif
+  }
+}
+
+mozilla::ipc::IPCResult LSDatabaseChild::RecvRequestAllowToClose() {
+  AssertIsOnOwningThread();
+
+  if (mDatabase) {
+    mDatabase->RequestAllowToClose();
+
+    // TODO: A new datastore will be prepared at first LocalStorage API
+    //       synchronous call. It would be better to start preparing a new
+    //       datastore right here, but asynchronously.
+    //       However, we probably shouldn't do that if we are shutting down.
+  }
+
+  return IPC_OK();
+}
+
+PBackgroundLSSnapshotChild* LSDatabaseChild::AllocPBackgroundLSSnapshotChild(
+    const nsAString& aDocumentURI, const nsAString& aKey,
+    const bool& aIncreasePeakUsage, const int64_t& aMinSize,
+    LSSnapshotInitInfo* aInitInfo) {
+  MOZ_CRASH("PBackgroundLSSnapshotChild actor should be manually constructed!");
+}
+
+bool LSDatabaseChild::DeallocPBackgroundLSSnapshotChild(
+    PBackgroundLSSnapshotChild* aActor) {
+  MOZ_ASSERT(aActor);
+
+  delete aActor;
+  return true;
+}
+
+/*******************************************************************************
+ * LSObserverChild
+ ******************************************************************************/
+
+LSObserverChild::LSObserverChild(LSObserver* aObserver) : mObserver(aObserver) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aObserver);
+
+  MOZ_COUNT_CTOR(LSObserverChild);
+}
+
+LSObserverChild::~LSObserverChild() {
+  AssertIsOnOwningThread();
+
+  MOZ_COUNT_DTOR(LSObserverChild);
+}
+
+void LSObserverChild::SendDeleteMeInternal() {
+  AssertIsOnOwningThread();
+
+  if (mObserver) {
+    mObserver->ClearActor();
+    mObserver = nullptr;
+
+    MOZ_ALWAYS_TRUE(PBackgroundLSObserverChild::SendDeleteMe());
+  }
+}
+
+void LSObserverChild::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnOwningThread();
+
+  if (mObserver) {
+    mObserver->ClearActor();
+#ifdef DEBUG
+    mObserver = nullptr;
+#endif
+  }
+}
+
+mozilla::ipc::IPCResult LSObserverChild::RecvObserve(
+    const PrincipalInfo& aPrincipalInfo, const uint32_t& aPrivateBrowsingId,
+    const nsAString& aDocumentURI, const nsAString& aKey,
+    const LSValue& aOldValue, const LSValue& aNewValue) {
+  AssertIsOnOwningThread();
+
+  if (!mObserver) {
+    return IPC_OK();
+  }
+
+  QM_TRY_INSPECT(const auto& principal,
+                 PrincipalInfoToPrincipal(aPrincipalInfo),
+                 IPC_FAIL(this, "PrincipalInfoToPrincipal failed!"));
+
+  Storage::NotifyChange(/* aStorage */ nullptr, principal, aKey,
+                        aOldValue.AsString(), aNewValue.AsString(),
+                        /* aStorageType */ kLocalStorageType, aDocumentURI,
+                        /* aIsPrivate */ !!aPrivateBrowsingId,
+                        /* aImmediateDispatch */ true);
+
+  return IPC_OK();
+}
+
+/*******************************************************************************
+ * LocalStorageRequestChild
+ ******************************************************************************/
+
+LSRequestChild::LSRequestChild() : mFinishing(false) {
+  AssertIsOnOwningThread();
+
+  MOZ_COUNT_CTOR(LSRequestChild);
+}
+
+LSRequestChild::~LSRequestChild() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!mCallback);
+
+  MOZ_COUNT_DTOR(LSRequestChild);
+}
+
+void LSRequestChild::SetCallback(LSRequestChildCallback* aCallback) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aCallback);
+  MOZ_ASSERT(!mCallback);
+  MOZ_ASSERT(Manager());
+
+  mCallback = aCallback;
+}
+
+bool LSRequestChild::Finishing() const {
+  AssertIsOnOwningThread();
+
+  return mFinishing;
+}
+
+void LSRequestChild::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnOwningThread();
+
+  if (mCallback) {
+    MOZ_ASSERT(aWhy != Deletion);
+
+    mCallback->OnResponse(NS_ERROR_FAILURE);
+
+    mCallback = nullptr;
+  }
+}
+
+mozilla::ipc::IPCResult LSRequestChild::Recv__delete__(
+    const LSRequestResponse& aResponse) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mCallback);
+
+  mCallback->OnResponse(aResponse);
+
+  mCallback = nullptr;
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult LSRequestChild::RecvReady() {
+  AssertIsOnOwningThread();
+
+  mFinishing = true;
+
+  // We only expect this to return false if the channel has been closed, but
+  // PBackground's channel never gets shutdown.
+  MOZ_ALWAYS_TRUE(SendFinish());
+
+  return IPC_OK();
+}
+
+/*******************************************************************************
+ * LSSimpleRequestChild
+ ******************************************************************************/
+
+LSSimpleRequestChild::LSSimpleRequestChild() {
+  AssertIsOnOwningThread();
+
+  MOZ_COUNT_CTOR(LSSimpleRequestChild);
+}
+
+LSSimpleRequestChild::~LSSimpleRequestChild() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!mCallback);
+
+  MOZ_COUNT_DTOR(LSSimpleRequestChild);
+}
+
+void LSSimpleRequestChild::SetCallback(
+    LSSimpleRequestChildCallback* aCallback) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aCallback);
+  MOZ_ASSERT(!mCallback);
+  MOZ_ASSERT(Manager());
+
+  mCallback = aCallback;
+}
+
+void LSSimpleRequestChild::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnOwningThread();
+
+  if (mCallback) {
+    MOZ_ASSERT(aWhy != Deletion);
+
+    mCallback->OnResponse(NS_ERROR_FAILURE);
+
+    mCallback = nullptr;
+  }
+}
+
+mozilla::ipc::IPCResult LSSimpleRequestChild::Recv__delete__(
+    const LSSimpleRequestResponse& aResponse) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mCallback);
+
+  mCallback->OnResponse(aResponse);
+
+  mCallback = nullptr;
+
+  return IPC_OK();
+}
+
+/*******************************************************************************
+ * LSSnapshotChild
+ ******************************************************************************/
+
+LSSnapshotChild::LSSnapshotChild(LSSnapshot* aSnapshot) : mSnapshot(aSnapshot) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aSnapshot);
+
+  MOZ_COUNT_CTOR(LSSnapshotChild);
+}
+
+LSSnapshotChild::~LSSnapshotChild() {
+  AssertIsOnOwningThread();
+
+  MOZ_COUNT_DTOR(LSSnapshotChild);
+}
+
+void LSSnapshotChild::SendDeleteMeInternal() {
+  AssertIsOnOwningThread();
+
+  if (mSnapshot) {
+    mSnapshot->ClearActor();
+    mSnapshot = nullptr;
+
+    MOZ_ALWAYS_TRUE(PBackgroundLSSnapshotChild::SendDeleteMe());
+  }
+}
+
+void LSSnapshotChild::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnOwningThread();
+
+  if (mSnapshot) {
+    mSnapshot->ClearActor();
+#ifdef DEBUG
+    mSnapshot = nullptr;
+#endif
+  }
+}
+
+mozilla::ipc::IPCResult LSSnapshotChild::RecvMarkDirty() {
+  AssertIsOnOwningThread();
+
+  if (!mSnapshot) {
+    return IPC_OK();
+  }
+
+  mSnapshot->MarkDirty();
+
+  return IPC_OK();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/ActorsChild.h b/dom/localstorage/ActorsChild.h
new file mode 100644
index 0000000000..d9b614e401
--- /dev/null
+++ b/dom/localstorage/ActorsChild.h
@@ -0,0 +1,283 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_ActorsChild_h
+#define mozilla_dom_localstorage_ActorsChild_h
+
+#include <cstdint>
+#include "mozilla/RefPtr.h"
+#include "mozilla/dom/PBackgroundLSDatabaseChild.h"
+#include "mozilla/dom/PBackgroundLSObserverChild.h"
+#include "mozilla/dom/PBackgroundLSRequest.h"
+#include "mozilla/dom/PBackgroundLSRequestChild.h"
+#include "mozilla/dom/PBackgroundLSSimpleRequest.h"
+#include "mozilla/dom/PBackgroundLSSimpleRequestChild.h"
+#include "mozilla/dom/PBackgroundLSSnapshotChild.h"
+#include "mozilla/ipc/ProtocolUtils.h"
+#include "nsISupports.h"
+#include "nsStringFwd.h"
+#include "nscore.h"
+
+namespace mozilla {
+
+namespace ipc {
+
+class BackgroundChildImpl;
+
+}  // namespace ipc
+
+namespace dom {
+
+class LocalStorageManager2;
+class LSDatabase;
+class LSObject;
+class LSObserver;
+class LSRequestChildCallback;
+class LSSimpleRequestChildCallback;
+class LSSnapshot;
+
+/**
+ * Minimal glue actor with standard IPC-managed new/delete existence that exists
+ * primarily to track the continued existence of the LSDatabase in the child.
+ * Most of the interesting bits happen via PBackgroundLSSnapshot.
+ *
+ * Mutual raw pointers are maintained between LSDatabase and this class that are
+ * cleared at either (expected) when the child starts the deletion process
+ * (SendDeleteMeInternal) or unexpected actor death (ActorDestroy).
+ *
+ * See `PBackgroundLSDatabase.ipdl` for more information.
+ *
+ *
+ * ## Low-Level Lifecycle ##
+ * - Created by LSObject::EnsureDatabase if it had to create a database.
+ * - Deletion begun by LSDatabase's destructor invoking SendDeleteMeInternal
+ *   which will result in the parent sending __delete__ which destroys the
+ *   actor.
+ */
+class LSDatabaseChild final : public PBackgroundLSDatabaseChild {
+  friend class mozilla::ipc::BackgroundChildImpl;
+  friend class LSDatabase;
+  friend class LSObject;
+
+  LSDatabase* mDatabase;
+
+  NS_DECL_OWNINGTHREAD
+
+ public:
+  void AssertIsOnOwningThread() const {
+    NS_ASSERT_OWNINGTHREAD(LSDatabaseChild);
+  }
+
+ private:
+  // Only created by LSObject.
+  explicit LSDatabaseChild(LSDatabase* aDatabase);
+
+  // Only destroyed by mozilla::ipc::BackgroundChildImpl.
+  ~LSDatabaseChild();
+
+  void SendDeleteMeInternal();
+
+  // IPDL methods are only called by IPDL.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  mozilla::ipc::IPCResult RecvRequestAllowToClose() override;
+
+  PBackgroundLSSnapshotChild* AllocPBackgroundLSSnapshotChild(
+      const nsAString& aDocumentURI, const nsAString& aKey,
+      const bool& aIncreasePeakUsage, const int64_t& aMinSize,
+      LSSnapshotInitInfo* aInitInfo) override;
+
+  bool DeallocPBackgroundLSSnapshotChild(
+      PBackgroundLSSnapshotChild* aActor) override;
+};
+
+/**
+ * Minimal IPC-managed (new/delete) actor that exists to receive and relay
+ * "storage" events from changes to LocalStorage that take place in other
+ * processes as their Snapshots are checkpointed to the canonical Datastore in
+ * the parent process.
+ *
+ * See `PBackgroundLSObserver.ipdl` for more info.
+ */
+class LSObserverChild final : public PBackgroundLSObserverChild {
+  friend class mozilla::ipc::BackgroundChildImpl;
+  friend class LSObserver;
+  friend class LSObject;
+
+  LSObserver* mObserver;
+
+  NS_DECL_OWNINGTHREAD
+
+ public:
+  void AssertIsOnOwningThread() const {
+    NS_ASSERT_OWNINGTHREAD(LSObserverChild);
+  }
+
+ private:
+  // Only created by LSObject.
+  explicit LSObserverChild(LSObserver* aObserver);
+
+  // Only destroyed by mozilla::ipc::BackgroundChildImpl.
+  ~LSObserverChild();
+
+  void SendDeleteMeInternal();
+
+  // IPDL methods are only called by IPDL.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  mozilla::ipc::IPCResult RecvObserve(const PrincipalInfo& aPrinciplaInfo,
+                                      const uint32_t& aPrivateBrowsingId,
+                                      const nsAString& aDocumentURI,
+                                      const nsAString& aKey,
+                                      const LSValue& aOldValue,
+                                      const LSValue& aNewValue) override;
+};
+
+/**
+ * Minimal glue IPC-managed (new/delete) actor that is used by LSObject and its
+ * RequestHelper to perform synchronous requests on top of an asynchronous
+ * protocol.
+ *
+ * Takes an `LSReuestChildCallback` to be invoked when a response is received
+ * via __delete__.
+ *
+ * See `PBackgroundLSRequest.ipdl`, `LSObject`, and `RequestHelper` for more
+ * info.
+ */
+class LSRequestChild final : public PBackgroundLSRequestChild {
+  friend class LSObject;
+  friend class LocalStorageManager2;
+
+  RefPtr<LSRequestChildCallback> mCallback;
+
+  bool mFinishing;
+
+  NS_DECL_OWNINGTHREAD
+
+ public:
+  void AssertIsOnOwningThread() const {
+    NS_ASSERT_OWNINGTHREAD(LSReqeustChild);
+  }
+
+  bool Finishing() const;
+
+ private:
+  // Only created by LSObject.
+  LSRequestChild();
+
+  // Only destroyed by mozilla::ipc::BackgroundChildImpl.
+  ~LSRequestChild();
+
+  void SetCallback(LSRequestChildCallback* aCallback);
+
+  // IPDL methods are only called by IPDL.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  mozilla::ipc::IPCResult Recv__delete__(
+      const LSRequestResponse& aResponse) override;
+
+  mozilla::ipc::IPCResult RecvReady() override;
+};
+
+class NS_NO_VTABLE LSRequestChildCallback {
+ public:
+  NS_INLINE_DECL_PURE_VIRTUAL_REFCOUNTING
+
+  virtual void OnResponse(const LSRequestResponse& aResponse) = 0;
+
+ protected:
+  virtual ~LSRequestChildCallback() = default;
+};
+
+/**
+ * Minimal glue IPC-managed (new/delete) actor used by `LocalStorageManager2` to
+ * issue asynchronous requests in an asynchronous fashion.
+ *
+ * Takes an `LSSimpleRequestChildCallback` to be invoked when a response is
+ * received via __delete__.
+ *
+ * See `PBackgroundLSSimpleRequest.ipdl` for more info.
+ */
+class LSSimpleRequestChild final : public PBackgroundLSSimpleRequestChild {
+  friend class LocalStorageManager2;
+
+  RefPtr<LSSimpleRequestChildCallback> mCallback;
+
+  NS_DECL_OWNINGTHREAD
+
+ public:
+  void AssertIsOnOwningThread() const {
+    NS_ASSERT_OWNINGTHREAD(LSSimpleReqeustChild);
+  }
+
+ private:
+  // Only created by LocalStorageManager2.
+  LSSimpleRequestChild();
+
+  void SetCallback(LSSimpleRequestChildCallback* aCallback);
+
+  // Only destroyed by mozilla::ipc::BackgroundChildImpl.
+  ~LSSimpleRequestChild();
+
+  // IPDL methods are only called by IPDL.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  mozilla::ipc::IPCResult Recv__delete__(
+      const LSSimpleRequestResponse& aResponse) override;
+};
+
+class NS_NO_VTABLE LSSimpleRequestChildCallback {
+ public:
+  NS_INLINE_DECL_PURE_VIRTUAL_REFCOUNTING
+
+  virtual void OnResponse(const LSSimpleRequestResponse& aResponse) = 0;
+
+ protected:
+  virtual ~LSSimpleRequestChildCallback() = default;
+};
+
+/**
+ * Minimal IPC-managed (new/delete) actor that lasts as long as its owning
+ * LSSnapshot.
+ *
+ * Mutual raw pointers are maintained between LSSnapshot and this class that are
+ * cleared at either (expected) when the child starts the deletion process
+ * (SendDeleteMeInternal) or unexpected actor death (ActorDestroy).
+ *
+ * See `PBackgroundLSSnapshot.ipdl` and `LSSnapshot` for more info.
+ */
+class LSSnapshotChild final : public PBackgroundLSSnapshotChild {
+  friend class LSDatabase;
+  friend class LSSnapshot;
+
+  LSSnapshot* mSnapshot;
+
+  NS_DECL_OWNINGTHREAD
+
+ public:
+  void AssertIsOnOwningThread() const {
+    NS_ASSERT_OWNINGTHREAD(LSSnapshotChild);
+  }
+
+ private:
+  // Only created by LSDatabase.
+  explicit LSSnapshotChild(LSSnapshot* aSnapshot);
+
+  // Only destroyed by LSDatabaseChild.
+  ~LSSnapshotChild();
+
+  void SendDeleteMeInternal();
+
+  // IPDL methods are only called by IPDL.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  mozilla::ipc::IPCResult RecvMarkDirty() override;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_localstorage_ActorsChild_h
diff --git a/dom/localstorage/ActorsParent.cpp b/dom/localstorage/ActorsParent.cpp
new file mode 100644
index 0000000000..7951e447e2
--- /dev/null
+++ b/dom/localstorage/ActorsParent.cpp
@@ -0,0 +1,9022 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ActorsParent.h"
+
+// Local includes
+#include "LSInitializationTypes.h"
+#include "LSObject.h"
+#include "ReportInternalError.h"
+
+// Global includes
+#include <cinttypes>
+#include <cstdlib>
+#include <cstring>
+#include <new>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "ErrorList.h"
+#include "MainThreadUtils.h"
+#include "mozIStorageAsyncConnection.h"
+#include "mozIStorageConnection.h"
+#include "mozIStorageFunction.h"
+#include "mozIStorageService.h"
+#include "mozIStorageStatement.h"
+#include "mozIStorageValueArray.h"
+#include "mozStorageCID.h"
+#include "mozStorageHelper.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Logging.h"
+#include "mozilla/MacroForEach.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/OriginAttributes.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/Result.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/Services.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/StoragePrincipalHelper.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Unused.h"
+#include "mozilla/Utf8.h"
+#include "mozilla/Variant.h"
+#include "mozilla/dom/ClientManagerService.h"
+#include "mozilla/dom/FlippedOnce.h"
+#include "mozilla/dom/LSSnapshot.h"
+#include "mozilla/dom/LSValue.h"
+#include "mozilla/dom/LSWriteOptimizer.h"
+#include "mozilla/dom/LSWriteOptimizerImpl.h"
+#include "mozilla/dom/LocalStorageCommon.h"
+#include "mozilla/dom/Nullable.h"
+#include "mozilla/dom/PBackgroundLSDatabase.h"
+#include "mozilla/dom/PBackgroundLSDatabaseParent.h"
+#include "mozilla/dom/PBackgroundLSObserverParent.h"
+#include "mozilla/dom/PBackgroundLSRequestParent.h"
+#include "mozilla/dom/PBackgroundLSSharedTypes.h"
+#include "mozilla/dom/PBackgroundLSSimpleRequestParent.h"
+#include "mozilla/dom/PBackgroundLSSnapshotParent.h"
+#include "mozilla/dom/SnappyUtils.h"
+#include "mozilla/dom/StorageDBUpdater.h"
+#include "mozilla/dom/StorageUtils.h"
+#include "mozilla/dom/ipc/IdType.h"
+#include "mozilla/dom/quota/CachingDatabaseConnection.h"
+#include "mozilla/dom/quota/CheckedUnsafePtr.h"
+#include "mozilla/dom/quota/Client.h"
+#include "mozilla/dom/quota/ClientImpl.h"
+#include "mozilla/dom/quota/DirectoryLock.h"
+#include "mozilla/dom/quota/FirstInitializationAttemptsImpl.h"
+#include "mozilla/dom/quota/OriginScope.h"
+#include "mozilla/dom/quota/PersistenceType.h"
+#include "mozilla/dom/quota/QuotaCommon.h"
+#include "mozilla/dom/quota/StorageHelpers.h"
+#include "mozilla/dom/quota/QuotaManager.h"
+#include "mozilla/dom/quota/QuotaObject.h"
+#include "mozilla/dom/quota/ResultExtensions.h"
+#include "mozilla/dom/quota/UsageInfo.h"
+#include "mozilla/ipc/BackgroundChild.h"
+#include "mozilla/ipc/BackgroundParent.h"
+#include "mozilla/ipc/PBackgroundChild.h"
+#include "mozilla/ipc/PBackgroundParent.h"
+#include "mozilla/ipc/PBackgroundSharedTypes.h"
+#include "mozilla/ipc/ProtocolUtils.h"
+#include "mozilla/storage/Variant.h"
+#include "nsBaseHashtable.h"
+#include "nsCOMPtr.h"
+#include "nsClassHashtable.h"
+#include "nsTHashMap.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsHashKeys.h"
+#include "nsIBinaryInputStream.h"
+#include "nsIBinaryOutputStream.h"
+#include "nsIDirectoryEnumerator.h"
+#include "nsIEventTarget.h"
+#include "nsIFile.h"
+#include "nsIInputStream.h"
+#include "nsIObjectInputStream.h"
+#include "nsIObjectOutputStream.h"
+#include "nsIObserver.h"
+#include "nsIObserverService.h"
+#include "nsIOutputStream.h"
+#include "nsIRunnable.h"
+#include "nsISerialEventTarget.h"
+#include "nsISupports.h"
+#include "nsIThread.h"
+#include "nsITimer.h"
+#include "nsIVariant.h"
+#include "nsInterfaceHashtable.h"
+#include "nsLiteralString.h"
+#include "nsNetUtil.h"
+#include "nsPointerHashKeys.h"
+#include "nsPrintfCString.h"
+#include "nsRefPtrHashtable.h"
+#include "nsServiceManagerUtils.h"
+#include "nsString.h"
+#include "nsStringFlags.h"
+#include "nsStringFwd.h"
+#include "nsTArray.h"
+#include "nsTHashSet.h"
+#include "nsTLiteralString.h"
+#include "nsTStringRepr.h"
+#include "nsThreadUtils.h"
+#include "nsVariant.h"
+#include "nsXPCOM.h"
+#include "nsXULAppAPI.h"
+#include "nscore.h"
+#include "prenv.h"
+#include "prtime.h"
+
+#define LS_LOG_TEST() MOZ_LOG_TEST(GetLocalStorageLogger(), LogLevel::Info)
+#define LS_LOG(_args) MOZ_LOG(GetLocalStorageLogger(), LogLevel::Info, _args)
+
+#if defined(MOZ_WIDGET_ANDROID)
+#  define LS_MOBILE
+#endif
+
+namespace mozilla::dom {
+
+using namespace mozilla::dom::quota;
+using namespace mozilla::dom::StorageUtils;
+using namespace mozilla::ipc;
+
+namespace {
+
+struct ArchivedOriginInfo;
+class ArchivedOriginScope;
+class Connection;
+class ConnectionThread;
+class Database;
+class Observer;
+class PrepareDatastoreOp;
+class PreparedDatastore;
+class QuotaClient;
+class Snapshot;
+
+using ArchivedOriginHashtable =
+    nsClassHashtable<nsCStringHashKey, ArchivedOriginInfo>;
+
+/*******************************************************************************
+ * Constants
+ ******************************************************************************/
+
+// Major schema version. Bump for almost everything.
+const uint32_t kMajorSchemaVersion = 5;
+
+// Minor schema version. Should almost always be 0 (maybe bump on release
+// branches if we have to).
+const uint32_t kMinorSchemaVersion = 0;
+
+// The schema version we store in the SQLite database is a (signed) 32-bit
+// integer. The major version is left-shifted 4 bits so the max value is
+// 0xFFFFFFF. The minor version occupies the lower 4 bits and its max is 0xF.
+static_assert(kMajorSchemaVersion <= 0xFFFFFFF,
+              "Major version needs to fit in 28 bits.");
+static_assert(kMinorSchemaVersion <= 0xF,
+              "Minor version needs to fit in 4 bits.");
+
+const int32_t kSQLiteSchemaVersion =
+    int32_t((kMajorSchemaVersion << 4) + kMinorSchemaVersion);
+
+// Changing the value here will override the page size of new databases only.
+// A journal mode change and VACUUM are needed to change existing databases, so
+// the best way to do that is to use the schema version upgrade mechanism.
+const uint32_t kSQLitePageSizeOverride =
+#ifdef LS_MOBILE
+    512;
+#else
+    1024;
+#endif
+
+static_assert(kSQLitePageSizeOverride == /* mozStorage default */ 0 ||
+                  (kSQLitePageSizeOverride % 2 == 0 &&
+                   kSQLitePageSizeOverride >= 512 &&
+                   kSQLitePageSizeOverride <= 65536),
+              "Must be 0 (disabled) or a power of 2 between 512 and 65536!");
+
+// Set to some multiple of the page size to grow the database in larger chunks.
+const uint32_t kSQLiteGrowthIncrement = kSQLitePageSizeOverride * 2;
+
+static_assert(kSQLiteGrowthIncrement >= 0 &&
+                  kSQLiteGrowthIncrement % kSQLitePageSizeOverride == 0 &&
+                  kSQLiteGrowthIncrement < uint32_t(INT32_MAX),
+              "Must be 0 (disabled) or a positive multiple of the page size!");
+
+/**
+ * The database name for LocalStorage data in a per-origin directory.
+ */
+constexpr auto kDataFileName = u"data.sqlite"_ns;
+
+/**
+ * The journal corresponding to kDataFileName.  (We don't use WAL mode.)
+ * Currently only needed in QuotaClient::InitOrigin and only in DEBUG builds.
+ * See the corresponding comment in QuotaClient::InitOrigin.
+ */
+#ifdef DEBUG
+constexpr auto kJournalFileName = u"data.sqlite-journal"_ns;
+#endif
+
+/**
+ * This file contains the current usage of the LocalStorage database as defined
+ * by the mozLength totals of all keys and values for the database, which
+ * differs from the actual size on disk.  We store this value in a separate
+ * file as a cache so that we can initialize the QuotaClient faster.
+ * In the future, this file will be eliminated and the information will be
+ * stored in PROFILE/storage.sqlite or similar QuotaManager-wide storage.
+ *
+ * The file contains a binary verification cookie (32-bits) followed by the
+ * actual usage (64-bits).
+ */
+constexpr auto kUsageFileName = u"usage"_ns;
+
+/**
+ * Following a QuotaManager idiom, this journal file's existence is a marker
+ * that the usage file was in the process of being updated and is currently
+ * invalid.  This file is created prior to updating the usage file and only
+ * deleted after the usage file has been written and closed and any pending
+ * database transactions have been committed.  Note that this idiom is expected
+ * to work if Gecko crashes in the middle of a write, but is not expected to be
+ * foolproof in the face of a system crash, as we do not explicitly attempt to
+ * fsync the directory containing the journal file.
+ *
+ * If the journal file is found to exist at origin initialization time, the
+ * usage will be re-computed from the current state of DATA_FILE_NAME.
+ */
+constexpr auto kUsageJournalFileName = u"usage-journal"_ns;
+
+static const uint32_t kUsageFileSize = 12;
+static const uint32_t kUsageFileCookie = 0x420a420a;
+
+/**
+ * How long between the first moment we know we have data to be written on a
+ * `Connection` and when we should actually perform the write.  This helps
+ * limit disk churn under silly usage patterns and is historically consistent
+ * with the previous, legacy implementation.
+ *
+ * Note that flushing happens downstream of Snapshot checkpointing and its
+ * batch mechanism which helps avoid wasteful IPC in the case of silly content
+ * code.
+ */
+const uint32_t kFlushTimeoutMs = 5000;
+
+const bool kDefaultShadowWrites = false;
+const uint32_t kDefaultSnapshotPrefill = 16384;
+const uint32_t kDefaultSnapshotGradualPrefill = 4096;
+const bool kDefaultClientValidation = true;
+/**
+ * Should all mutations also be reflected in the "shadow" database, which is
+ * the legacy webappsstore.sqlite database.  When this is enabled, users can
+ * downgrade their version of Firefox and/or otherwise fall back to the legacy
+ * implementation without loss of data.  (Older versions of Firefox will
+ * recognize the presence of ls-archive.sqlite and purge it and the other
+ * LocalStorage directories so privacy is maintained.)
+ */
+const char kShadowWritesPref[] = "dom.storage.shadow_writes";
+/**
+ * Byte budget for sending data down to the LSSnapshot instance when it is first
+ * created.  If there is less data than this (measured by tallying the string
+ * length of the keys and values), all data is sent, otherwise partial data is
+ * sent.  See `Snapshot`.
+ */
+const char kSnapshotPrefillPref[] = "dom.storage.snapshot_prefill";
+/**
+ * When a specific value is requested by an LSSnapshot that is not already fully
+ * populated, gradual prefill is used. This preference specifies the number of
+ * bytes to be used to send values beyond the specific value that is requested.
+ * (The size of the explicitly requested value does not impact this preference.)
+ * Setting the value to 0 disables gradual prefill. Tests may set this value to
+ * -1 which is converted to INT_MAX in order to cause gradual prefill to send
+ * all values not previously sent.
+ */
+const char kSnapshotGradualPrefillPref[] =
+    "dom.storage.snapshot_gradual_prefill";
+
+const char kClientValidationPref[] = "dom.storage.client_validation";
+
+/**
+ * The amount of time a PreparedDatastore instance should stick around after a
+ * preload is triggered in order to give time for the page to use LocalStorage
+ * without triggering worst-case synchronous jank.
+ */
+const uint32_t kPreparedDatastoreTimeoutMs = 20000;
+
+/**
+ * Cold storage for LocalStorage data extracted from webappsstore.sqlite at
+ * LSNG first-run that has not yet been migrated to its own per-origin directory
+ * by use.
+ *
+ * In other words, at first run, LSNG copies the contents of webappsstore.sqlite
+ * into this database.  As requests are made for that LocalStorage data, the
+ * contents are removed from this database and placed into per-origin QM
+ * storage.  So the contents of this database are always old, unused
+ * LocalStorage data that we can potentially get rid of at some point in the
+ * future.
+ */
+#define LS_ARCHIVE_FILE_NAME u"ls-archive.sqlite"
+/**
+ * The legacy LocalStorage database.  Its contents are maintained as our
+ * "shadow" database so that LSNG can be disabled without loss of user data.
+ */
+#define WEB_APPS_STORE_FILE_NAME u"webappsstore.sqlite"
+
+// Shadow database Write Ahead Log's maximum size is 512KB
+const uint32_t kShadowMaxWALSize = 512 * 1024;
+
+bool IsOnGlobalConnectionThread();
+
+void AssertIsOnGlobalConnectionThread();
+
+/*******************************************************************************
+ * SQLite functions
+ ******************************************************************************/
+
+int32_t MakeSchemaVersion(uint32_t aMajorSchemaVersion,
+                          uint32_t aMinorSchemaVersion) {
+  return int32_t((aMajorSchemaVersion << 4) + aMinorSchemaVersion);
+}
+
+nsCString GetArchivedOriginHashKey(const nsACString& aOriginSuffix,
+                                   const nsACString& aOriginNoSuffix) {
+  return aOriginSuffix + ":"_ns + aOriginNoSuffix;
+}
+
+nsresult CreateDataTable(mozIStorageConnection* aConnection) {
+  return aConnection->ExecuteSimpleSQL(
+      "CREATE TABLE data"
+      "( key TEXT PRIMARY KEY"
+      ", utf16_length INTEGER NOT NULL"
+      ", conversion_type INTEGER NOT NULL"
+      ", compression_type INTEGER NOT NULL"
+      ", last_access_time INTEGER NOT NULL DEFAULT 0"
+      ", value BLOB NOT NULL"
+      ");"_ns);
+}
+
+nsresult CreateTables(mozIStorageConnection* aConnection) {
+  MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+  MOZ_ASSERT(aConnection);
+
+  // Table `database`
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL(
+      "CREATE TABLE database"
+      "( origin TEXT NOT NULL"
+      ", usage INTEGER NOT NULL DEFAULT 0"
+      ", last_vacuum_time INTEGER NOT NULL DEFAULT 0"
+      ", last_analyze_time INTEGER NOT NULL DEFAULT 0"
+      ", last_vacuum_size INTEGER NOT NULL DEFAULT 0"
+      ");"_ns)));
+
+  // Table `data`
+  QM_TRY(MOZ_TO_RESULT(CreateDataTable(aConnection)));
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->SetSchemaVersion(kSQLiteSchemaVersion)));
+
+  return NS_OK;
+}
+
+nsresult UpgradeSchemaFrom1_0To2_0(mozIStorageConnection* aConnection) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL(
+      "ALTER TABLE database ADD COLUMN usage INTEGER NOT NULL DEFAULT 0;"_ns)));
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL(
+      "UPDATE database "
+      "SET usage = (SELECT total(utf16Length(key) + utf16Length(value)) "
+      "FROM data);"_ns)));
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->SetSchemaVersion(MakeSchemaVersion(2, 0))));
+
+  return NS_OK;
+}
+
+nsresult UpgradeSchemaFrom2_0To3_0(mozIStorageConnection* aConnection) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL(
+      "ALTER TABLE data ADD COLUMN utf16Length INTEGER NOT NULL DEFAULT 0;"_ns)));
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL(
+      "UPDATE data SET utf16Length = utf16Length(value);"_ns)));
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->SetSchemaVersion(MakeSchemaVersion(3, 0))));
+
+  return NS_OK;
+}
+
+nsresult UpgradeSchemaFrom3_0To4_0(mozIStorageConnection* aConnection) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->SetSchemaVersion(MakeSchemaVersion(4, 0))));
+
+  return NS_OK;
+}
+
+nsresult UpgradeSchemaFrom4_0To5_0(mozIStorageConnection* aConnection) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aConnection);
+
+  // Recreate data table in new format following steps at
+  // https://www.sqlite.org/lang_altertable.html
+  // section "Making Other Kinds Of Table Schema Changes"
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL(
+      "CREATE TABLE migrated_data"
+      "( key TEXT PRIMARY KEY"
+      ", utf16_length INTEGER NOT NULL"
+      ", conversion_type INTEGER NOT NULL"
+      ", compression_type INTEGER NOT NULL"
+      ", last_access_time INTEGER NOT NULL DEFAULT 0"
+      ", value BLOB NOT NULL"
+      ");"_ns)));
+
+  // Reinsert old data, all legacy data is UTF8
+  static_assert(1u ==
+                static_cast<uint8_t>(LSValue::ConversionType::UTF16_UTF8));
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL(
+      "INSERT INTO migrated_data (key, utf16_length, conversion_type, "
+      "compression_type, last_access_time, value) "
+      "SELECT key, utf16Length, 1, compressed, lastAccessTime, value "
+      "FROM data;"_ns)));
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL("DROP TABLE data;"_ns)));
+
+  // Rename to data
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL(
+      "ALTER TABLE migrated_data RENAME TO data;"_ns)));
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->SetSchemaVersion(MakeSchemaVersion(5, 0))));
+
+  return NS_OK;
+}
+
+nsresult SetDefaultPragmas(mozIStorageConnection* aConnection) {
+  MOZ_ASSERT(!NS_IsMainThread());
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY(MOZ_TO_RESULT(
+      aConnection->ExecuteSimpleSQL("PRAGMA synchronous = FULL;"_ns)));
+
+#ifndef LS_MOBILE
+  if (kSQLiteGrowthIncrement) {
+    // This is just an optimization so ignore the failure if the disk is
+    // currently too full.
+    QM_TRY(QM_OR_ELSE_WARN_IF(
+        // Expression.
+        MOZ_TO_RESULT(
+            aConnection->SetGrowthIncrement(kSQLiteGrowthIncrement, ""_ns)),
+        // Predicate.
+        IsSpecificError<NS_ERROR_FILE_TOO_BIG>,
+        // Fallback.
+        ErrToDefaultOk<>));
+  }
+#endif  // LS_MOBILE
+
+  return NS_OK;
+}
+
+template <typename CorruptedFileHandler>
+Result<nsCOMPtr<mozIStorageConnection>, nsresult> CreateStorageConnection(
+    nsIFile& aDBFile, nsIFile& aUsageFile, const nsACString& aOrigin,
+    CorruptedFileHandler&& aCorruptedFileHandler) {
+  MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+
+  // XXX Common logic should be refactored out of this method and
+  // cache::DBAction::OpenDBConnection, and maybe other similar functions.
+
+  QM_TRY_INSPECT(const auto& storageService,
+                 MOZ_TO_RESULT_GET_TYPED(nsCOMPtr<mozIStorageService>,
+                                         MOZ_SELECT_OVERLOAD(do_GetService),
+                                         MOZ_STORAGE_SERVICE_CONTRACTID));
+
+  // XXX We can't use QM_OR_ELSE_WARN_IF because base-toolchains builds fail
+  // with: error: use of 'tryResult28' before deduction of 'auto'
+  QM_TRY_UNWRAP(
+      auto connection,
+      OrElseIf(
+          // Expression.
+          MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+              nsCOMPtr<mozIStorageConnection>, storageService, OpenDatabase,
+              &aDBFile, mozIStorageService::CONNECTION_DEFAULT),
+          // Predicate.
+          IsDatabaseCorruptionError,
+          // Fallback.
+          ([&aUsageFile, &aDBFile, &aCorruptedFileHandler,
+            &storageService](const nsresult rv)
+               -> Result<nsCOMPtr<mozIStorageConnection>, nsresult> {
+            // Remove the usage file first (it might not exist at all due
+            // to corrupted state, which is ignored here).
+
+            // Usually we only use QM_OR_ELSE_LOG_VERBOSE(_IF) with Remove and
+            // NS_ERROR_FILE_NOT_FOUND check, but we're already in the rare case
+            // of corruption here, so the use of QM_OR_ELSE_WARN_IF is ok here.
+            QM_TRY(QM_OR_ELSE_WARN_IF(
+                // Expression.
+                MOZ_TO_RESULT(aUsageFile.Remove(false)),
+                // Predicate.
+                ([](const nsresult rv) {
+                  return rv == NS_ERROR_FILE_NOT_FOUND;
+                }),
+                // Fallback.
+                ErrToDefaultOk<>));
+
+            // Call the corrupted file handler before trying to remove the
+            // database file, which might fail.
+            std::forward<CorruptedFileHandler>(aCorruptedFileHandler)();
+
+            // Nuke the database file.
+            QM_TRY(MOZ_TO_RESULT(aDBFile.Remove(false)));
+
+            QM_TRY_RETURN(MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                nsCOMPtr<mozIStorageConnection>, storageService, OpenDatabase,
+                &aDBFile, mozIStorageService::CONNECTION_DEFAULT));
+          })));
+
+  QM_TRY(MOZ_TO_RESULT(SetDefaultPragmas(connection)));
+
+  // Check to make sure that the database schema is correct.
+  // XXX Try to make schemaVersion const.
+  QM_TRY_UNWRAP(int32_t schemaVersion,
+                MOZ_TO_RESULT_INVOKE_MEMBER(connection, GetSchemaVersion));
+
+  QM_TRY(OkIf(schemaVersion <= kSQLiteSchemaVersion), Err(NS_ERROR_FAILURE));
+
+  if (schemaVersion != kSQLiteSchemaVersion) {
+    const bool newDatabase = !schemaVersion;
+
+    if (newDatabase) {
+      // Set the page size first.
+      if (kSQLitePageSizeOverride) {
+        QM_TRY(MOZ_TO_RESULT(connection->ExecuteSimpleSQL(nsPrintfCString(
+            "PRAGMA page_size = %" PRIu32 ";", kSQLitePageSizeOverride))));
+      }
+
+      // We have to set the auto_vacuum mode before opening a transaction.
+      QM_TRY(MOZ_TO_RESULT(connection->ExecuteSimpleSQL(
+#ifdef LS_MOBILE
+          // Turn on full auto_vacuum mode to reclaim disk space on mobile
+          // devices (at the cost of some COMMIT speed).
+          "PRAGMA auto_vacuum = FULL;"_ns
+#else
+          // Turn on incremental auto_vacuum mode on desktop builds.
+          "PRAGMA auto_vacuum = INCREMENTAL;"_ns
+#endif
+          )));
+    }
+
+    bool vacuumNeeded = false;
+
+    mozStorageTransaction transaction(
+        connection, false, mozIStorageConnection::TRANSACTION_IMMEDIATE);
+
+    QM_TRY(MOZ_TO_RESULT(transaction.Start()));
+
+    if (newDatabase) {
+      QM_TRY(MOZ_TO_RESULT(CreateTables(connection)));
+
+#ifdef DEBUG
+      {
+        QM_TRY_INSPECT(
+            const int32_t& schemaVersion,
+            MOZ_TO_RESULT_INVOKE_MEMBER(connection, GetSchemaVersion),
+            QM_ASSERT_UNREACHABLE);
+
+        MOZ_ASSERT(schemaVersion == kSQLiteSchemaVersion);
+      }
+#endif
+
+      QM_TRY_INSPECT(
+          const auto& stmt,
+          MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+              nsCOMPtr<mozIStorageStatement>, connection, CreateStatement,
+              "INSERT INTO database (origin) VALUES (:origin)"_ns));
+
+      QM_TRY(MOZ_TO_RESULT(stmt->BindUTF8StringByName("origin"_ns, aOrigin)));
+
+      QM_TRY(MOZ_TO_RESULT(stmt->Execute()));
+    } else {
+      // This logic needs to change next time we change the schema!
+      static_assert(kSQLiteSchemaVersion == int32_t((5 << 4) + 0),
+                    "Upgrade function needed due to schema version increase.");
+
+      while (schemaVersion != kSQLiteSchemaVersion) {
+        if (schemaVersion == MakeSchemaVersion(1, 0)) {
+          QM_TRY(MOZ_TO_RESULT(UpgradeSchemaFrom1_0To2_0(connection)));
+        } else if (schemaVersion == MakeSchemaVersion(2, 0)) {
+          QM_TRY(MOZ_TO_RESULT(UpgradeSchemaFrom2_0To3_0(connection)));
+        } else if (schemaVersion == MakeSchemaVersion(3, 0)) {
+          QM_TRY(MOZ_TO_RESULT(UpgradeSchemaFrom3_0To4_0(connection)));
+        } else if (schemaVersion == MakeSchemaVersion(4, 0)) {
+          QM_TRY(MOZ_TO_RESULT(UpgradeSchemaFrom4_0To5_0(connection)));
+          vacuumNeeded = true;
+        } else {
+          LS_WARNING(
+              "Unable to open LocalStorage database, no upgrade path is "
+              "available!");
+          return Err(NS_ERROR_FAILURE);
+        }
+
+        QM_TRY_UNWRAP(schemaVersion, MOZ_TO_RESULT_INVOKE_MEMBER(
+                                         connection, GetSchemaVersion));
+      }
+
+      MOZ_ASSERT(schemaVersion == kSQLiteSchemaVersion);
+    }
+
+    QM_TRY(MOZ_TO_RESULT(transaction.Commit()));
+
+    if (vacuumNeeded) {
+      QM_TRY(MOZ_TO_RESULT(connection->ExecuteSimpleSQL("VACUUM;"_ns)));
+    }
+
+    if (newDatabase) {
+      // Windows caches the file size, let's force it to stat the file again.
+      QM_TRY_INSPECT(const bool& exists,
+                     MOZ_TO_RESULT_INVOKE_MEMBER(aDBFile, Exists));
+      Unused << exists;
+
+      QM_TRY_INSPECT(const int64_t& fileSize,
+                     MOZ_TO_RESULT_INVOKE_MEMBER(aDBFile, GetFileSize));
+
+      MOZ_ASSERT(fileSize > 0);
+
+      const PRTime vacuumTime = PR_Now();
+      MOZ_ASSERT(vacuumTime);
+
+      QM_TRY_INSPECT(
+          const auto& vacuumTimeStmt,
+          MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(nsCOMPtr<mozIStorageStatement>,
+                                            connection, CreateStatement,
+                                            "UPDATE database "
+                                            "SET last_vacuum_time = :time"
+                                            ", last_vacuum_size = :size;"_ns));
+
+      QM_TRY(MOZ_TO_RESULT(
+          vacuumTimeStmt->BindInt64ByName("time"_ns, vacuumTime)));
+
+      QM_TRY(
+          MOZ_TO_RESULT(vacuumTimeStmt->BindInt64ByName("size"_ns, fileSize)));
+
+      QM_TRY(MOZ_TO_RESULT(vacuumTimeStmt->Execute()));
+    }
+  }
+
+  return connection;
+}
+
+Result<nsCOMPtr<mozIStorageConnection>, nsresult> GetStorageConnection(
+    const nsAString& aDatabaseFilePath) {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(!aDatabaseFilePath.IsEmpty());
+  MOZ_ASSERT(StringEndsWith(aDatabaseFilePath, u".sqlite"_ns));
+
+  QM_TRY_INSPECT(const auto& databaseFile, QM_NewLocalFile(aDatabaseFilePath));
+
+  QM_TRY_INSPECT(const bool& exists,
+                 MOZ_TO_RESULT_INVOKE_MEMBER(databaseFile, Exists));
+
+  QM_TRY(OkIf(exists), Err(NS_ERROR_FAILURE));
+
+  QM_TRY_INSPECT(const auto& ss,
+                 MOZ_TO_RESULT_GET_TYPED(nsCOMPtr<mozIStorageService>,
+                                         MOZ_SELECT_OVERLOAD(do_GetService),
+                                         MOZ_STORAGE_SERVICE_CONTRACTID));
+
+  QM_TRY_UNWRAP(auto connection,
+                MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                    nsCOMPtr<mozIStorageConnection>, ss, OpenDatabase,
+                    databaseFile, mozIStorageService::CONNECTION_DEFAULT));
+
+  QM_TRY(MOZ_TO_RESULT(SetDefaultPragmas(connection)));
+
+  return connection;
+}
+
+Result<nsCOMPtr<nsIFile>, nsresult> GetArchiveFile(
+    const nsAString& aStoragePath) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(!aStoragePath.IsEmpty());
+
+  QM_TRY_UNWRAP(auto archiveFile, QM_NewLocalFile(aStoragePath));
+
+  QM_TRY(MOZ_TO_RESULT(
+      archiveFile->Append(nsLiteralString(LS_ARCHIVE_FILE_NAME))));
+
+  return archiveFile;
+}
+
+Result<nsCOMPtr<mozIStorageConnection>, nsresult>
+CreateArchiveStorageConnection(const nsAString& aStoragePath) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(!aStoragePath.IsEmpty());
+
+  QM_TRY_INSPECT(const auto& archiveFile, GetArchiveFile(aStoragePath));
+
+  // QuotaManager ensures this file always exists.
+  DebugOnly<bool> exists;
+  MOZ_ASSERT(NS_SUCCEEDED(archiveFile->Exists(&exists)));
+  MOZ_ASSERT(exists);
+
+  QM_TRY_INSPECT(const bool& isDirectory,
+                 MOZ_TO_RESULT_INVOKE_MEMBER(archiveFile, IsDirectory));
+
+  if (isDirectory) {
+    LS_WARNING("ls-archive is not a file!");
+    return nsCOMPtr<mozIStorageConnection>{};
+  }
+
+  QM_TRY_INSPECT(const auto& ss,
+                 MOZ_TO_RESULT_GET_TYPED(nsCOMPtr<mozIStorageService>,
+                                         MOZ_SELECT_OVERLOAD(do_GetService),
+                                         MOZ_STORAGE_SERVICE_CONTRACTID));
+
+  QM_TRY_UNWRAP(
+      auto connection,
+      QM_OR_ELSE_WARN_IF(
+          // Expression.
+          MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+              nsCOMPtr<mozIStorageConnection>, ss, OpenUnsharedDatabase,
+              archiveFile, mozIStorageService::CONNECTION_DEFAULT),
+          // Predicate.
+          IsDatabaseCorruptionError,
+          // Fallback. Don't throw an error, leave a corrupted ls-archive
+          // database as it is.
+          ErrToDefaultOk<nsCOMPtr<mozIStorageConnection>>));
+
+  if (connection) {
+    const nsresult rv = StorageDBUpdater::Update(connection);
+    if (NS_FAILED(rv)) {
+      // Don't throw an error, leave a non-updateable ls-archive database as
+      // it is.
+      return nsCOMPtr<mozIStorageConnection>{};
+    }
+  }
+
+  return connection;
+}
+
+Result<nsCOMPtr<nsIFile>, nsresult> GetShadowFile(const nsAString& aBasePath) {
+  MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+  MOZ_ASSERT(!aBasePath.IsEmpty());
+
+  QM_TRY_UNWRAP(auto archiveFile, QM_NewLocalFile(aBasePath));
+
+  QM_TRY(MOZ_TO_RESULT(
+      archiveFile->Append(nsLiteralString(WEB_APPS_STORE_FILE_NAME))));
+
+  return archiveFile;
+}
+
+nsresult SetShadowJournalMode(mozIStorageConnection* aConnection) {
+  MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+  MOZ_ASSERT(aConnection);
+
+  // Try enabling WAL mode. This can fail in various circumstances so we have to
+  // check the results here.
+  constexpr auto journalModeQueryStart = "PRAGMA journal_mode = "_ns;
+  constexpr auto journalModeWAL = "wal"_ns;
+
+  QM_TRY_INSPECT(const auto& stmt,
+                 CreateAndExecuteSingleStepStatement(
+                     *aConnection, journalModeQueryStart + journalModeWAL));
+
+  QM_TRY_INSPECT(const auto& journalMode,
+                 MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(nsAutoCString, *stmt,
+                                                   GetUTF8String, 0));
+
+  if (journalMode.Equals(journalModeWAL)) {
+    // WAL mode successfully enabled. Set limits on its size here.
+
+    // Set the threshold for auto-checkpointing the WAL. We don't want giant
+    // logs slowing down us.
+    QM_TRY_INSPECT(const auto& stmt, CreateAndExecuteSingleStepStatement(
+                                         *aConnection, "PRAGMA page_size;"_ns));
+
+    QM_TRY_INSPECT(const int32_t& pageSize,
+                   MOZ_TO_RESULT_INVOKE_MEMBER(*stmt, GetInt32, 0));
+
+    MOZ_ASSERT(pageSize >= 512 && pageSize <= 65536);
+
+    // Note there is a default journal_size_limit set by mozStorage.
+    QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteSimpleSQL(
+        "PRAGMA wal_autocheckpoint = "_ns +
+        IntToCString(static_cast<int32_t>(kShadowMaxWALSize / pageSize)))));
+  } else {
+    QM_TRY(MOZ_TO_RESULT(
+        aConnection->ExecuteSimpleSQL(journalModeQueryStart + "truncate"_ns)));
+  }
+
+  return NS_OK;
+}
+
+Result<nsCOMPtr<mozIStorageConnection>, nsresult> CreateShadowStorageConnection(
+    const nsAString& aBasePath) {
+  MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+  MOZ_ASSERT(!aBasePath.IsEmpty());
+
+  QM_TRY_INSPECT(const auto& shadowFile, GetShadowFile(aBasePath));
+
+  QM_TRY_INSPECT(const auto& ss,
+                 MOZ_TO_RESULT_GET_TYPED(nsCOMPtr<mozIStorageService>,
+                                         MOZ_SELECT_OVERLOAD(do_GetService),
+                                         MOZ_STORAGE_SERVICE_CONTRACTID));
+
+  QM_TRY_UNWRAP(
+      auto connection,
+      QM_OR_ELSE_WARN_IF(
+          // Expression.
+          MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+              nsCOMPtr<mozIStorageConnection>, ss, OpenUnsharedDatabase,
+              shadowFile, mozIStorageService::CONNECTION_DEFAULT),
+          // Predicate.
+          IsDatabaseCorruptionError,
+          // Fallback.
+          ([&shadowFile, &ss](const nsresult rv)
+               -> Result<nsCOMPtr<mozIStorageConnection>, nsresult> {
+            QM_TRY(MOZ_TO_RESULT(shadowFile->Remove(false)));
+
+            QM_TRY_RETURN(MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                nsCOMPtr<mozIStorageConnection>, ss, OpenUnsharedDatabase,
+                shadowFile, mozIStorageService::CONNECTION_DEFAULT));
+          })));
+
+  QM_TRY(MOZ_TO_RESULT(SetShadowJournalMode(connection)));
+
+  // XXX Depending on whether the *first* call to OpenUnsharedDatabase above
+  // failed, we (a) might or (b) might not be dealing with a fresh database
+  // here. This is confusing, since in a failure of case (a) we would do the
+  // same thing again. Probably, the control flow should be changed here so that
+  // it's clear we only delete & create a fresh database once. If we still have
+  // a failure then, we better give up. Or, if we really want to handle that,
+  // the number of 2 retries seems arbitrary, and we should better do this in
+  // some loop until a maximum number of retries is reached.
+  //
+  // Compare this with QuotaManager::CreateLocalStorageArchiveConnection, which
+  // actually tracks if the file was removed before, but it's also more
+  // complicated than it should be. Maybe these two methods can be merged (which
+  // would mean that a parameter must be added that indicates whether it's
+  // handling the shadow file or not).
+  QM_TRY(QM_OR_ELSE_WARN(
+      // Expression.
+      MOZ_TO_RESULT(StorageDBUpdater::Update(connection)),
+      // Fallback.
+      ([&connection, &shadowFile, &ss](const nsresult) -> Result<Ok, nsresult> {
+        QM_TRY(MOZ_TO_RESULT(connection->Close()));
+        QM_TRY(MOZ_TO_RESULT(shadowFile->Remove(false)));
+
+        QM_TRY_UNWRAP(connection, MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                                      nsCOMPtr<mozIStorageConnection>, ss,
+                                      OpenUnsharedDatabase, shadowFile,
+                                      mozIStorageService::CONNECTION_DEFAULT));
+
+        QM_TRY(MOZ_TO_RESULT(SetShadowJournalMode(connection)));
+
+        QM_TRY(
+            MOZ_TO_RESULT(StorageDBUpdater::CreateCurrentSchema(connection)));
+
+        return Ok{};
+      })));
+
+  return connection;
+}
+
+Result<nsCOMPtr<mozIStorageConnection>, nsresult> GetShadowStorageConnection(
+    const nsAString& aBasePath) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(!aBasePath.IsEmpty());
+
+  QM_TRY_INSPECT(const auto& shadowFile, GetShadowFile(aBasePath));
+
+  QM_TRY_INSPECT(const bool& exists,
+                 MOZ_TO_RESULT_INVOKE_MEMBER(shadowFile, Exists));
+
+  QM_TRY(OkIf(exists), Err(NS_ERROR_FAILURE));
+
+  QM_TRY_INSPECT(const auto& ss,
+                 MOZ_TO_RESULT_GET_TYPED(nsCOMPtr<mozIStorageService>,
+                                         MOZ_SELECT_OVERLOAD(do_GetService),
+                                         MOZ_STORAGE_SERVICE_CONTRACTID));
+
+  QM_TRY_RETURN(MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+      nsCOMPtr<mozIStorageConnection>, ss, OpenUnsharedDatabase, shadowFile,
+      mozIStorageService::CONNECTION_DEFAULT));
+}
+
+nsresult AttachShadowDatabase(const nsAString& aBasePath,
+                              mozIStorageConnection* aConnection) {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(!aBasePath.IsEmpty());
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY_INSPECT(const auto& shadowFile, GetShadowFile(aBasePath));
+
+#ifdef DEBUG
+  {
+    QM_TRY_INSPECT(const bool& exists,
+                   MOZ_TO_RESULT_INVOKE_MEMBER(shadowFile, Exists));
+
+    MOZ_ASSERT(exists);
+  }
+#endif
+
+  QM_TRY_INSPECT(const auto& path, MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                                       nsString, shadowFile, GetPath));
+
+  QM_TRY_INSPECT(const auto& stmt,
+                 MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                     nsCOMPtr<mozIStorageStatement>, aConnection,
+                     CreateStatement, "ATTACH DATABASE :path AS shadow;"_ns));
+
+  QM_TRY(MOZ_TO_RESULT(stmt->BindStringByName("path"_ns, path)));
+
+  QM_TRY(MOZ_TO_RESULT(stmt->Execute()));
+
+  return NS_OK;
+}
+
+nsresult DetachShadowDatabase(mozIStorageConnection* aConnection) {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY(MOZ_TO_RESULT(
+      aConnection->ExecuteSimpleSQL("DETACH DATABASE shadow"_ns)));
+
+  return NS_OK;
+}
+
+Result<nsCOMPtr<nsIFile>, nsresult> GetUsageFile(
+    const nsAString& aDirectoryPath) {
+  MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+  MOZ_ASSERT(!aDirectoryPath.IsEmpty());
+
+  QM_TRY_UNWRAP(auto usageFile, QM_NewLocalFile(aDirectoryPath));
+
+  QM_TRY(MOZ_TO_RESULT(usageFile->Append(kUsageFileName)));
+
+  return usageFile;
+}
+
+Result<nsCOMPtr<nsIFile>, nsresult> GetUsageJournalFile(
+    const nsAString& aDirectoryPath) {
+  MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+  MOZ_ASSERT(!aDirectoryPath.IsEmpty());
+
+  QM_TRY_UNWRAP(auto usageJournalFile, QM_NewLocalFile(aDirectoryPath));
+
+  QM_TRY(MOZ_TO_RESULT(usageJournalFile->Append(kUsageJournalFileName)));
+
+  return usageJournalFile;
+}
+
+// Checks if aFile exists and is a file. Returns true if it exists and is a
+// file, false if it doesn't exist, and an error if it exists but isn't a file.
+Result<bool, nsresult> ExistsAsFile(nsIFile& aFile) {
+  enum class ExistsAsFileResult { DoesNotExist, IsDirectory, IsFile };
+
+  // This is an optimization to check both properties in one OS case, rather
+  // than calling Exists first, and then IsDirectory. IsDirectory also checks
+  // if the path exists. QM_OR_ELSE_WARN_IF is not used here since we just want
+  // to log NS_ERROR_FILE_NOT_FOUND result and not spam the reports.
+  QM_TRY_INSPECT(
+      const auto& res,
+      QM_OR_ELSE_LOG_VERBOSE_IF(
+          // Expression.
+          MOZ_TO_RESULT_INVOKE_MEMBER(aFile, IsDirectory)
+              .map([](const bool isDirectory) {
+                return isDirectory ? ExistsAsFileResult::IsDirectory
+                                   : ExistsAsFileResult::IsFile;
+              }),
+          // Predicate.
+          ([](const nsresult rv) { return rv == NS_ERROR_FILE_NOT_FOUND; }),
+          // Fallback.
+          ErrToOk<ExistsAsFileResult::DoesNotExist>));
+
+  QM_TRY(OkIf(res != ExistsAsFileResult::IsDirectory), Err(NS_ERROR_FAILURE));
+
+  return res == ExistsAsFileResult::IsFile;
+}
+
+nsresult UpdateUsageFile(nsIFile* aUsageFile, nsIFile* aUsageJournalFile,
+                         int64_t aUsage) {
+  MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+  MOZ_ASSERT(aUsageFile);
+  MOZ_ASSERT(aUsageJournalFile);
+  MOZ_ASSERT(aUsage >= 0);
+
+  QM_TRY_INSPECT(const bool& usageJournalFileExists,
+                 ExistsAsFile(*aUsageJournalFile));
+  if (!usageJournalFileExists) {
+    QM_TRY(MOZ_TO_RESULT(
+        aUsageJournalFile->Create(nsIFile::NORMAL_FILE_TYPE, 0644)));
+  }
+
+  QM_TRY_INSPECT(const auto& stream, NS_NewLocalFileOutputStream(aUsageFile));
+
+  nsCOMPtr<nsIBinaryOutputStream> binaryStream =
+      NS_NewObjectOutputStream(stream);
+
+  QM_TRY(MOZ_TO_RESULT(binaryStream->Write32(kUsageFileCookie)));
+
+  QM_TRY(MOZ_TO_RESULT(binaryStream->Write64(aUsage)));
+
+#if defined(EARLY_BETA_OR_EARLIER) || defined(DEBUG)
+  QM_TRY(MOZ_TO_RESULT(stream->Flush()));
+#endif
+
+  QM_TRY(MOZ_TO_RESULT(stream->Close()));
+
+  return NS_OK;
+}
+
+Result<UsageInfo, nsresult> LoadUsageFile(nsIFile& aUsageFile) {
+  AssertIsOnIOThread();
+
+  QM_TRY_INSPECT(const int64_t& fileSize,
+                 MOZ_TO_RESULT_INVOKE_MEMBER(aUsageFile, GetFileSize));
+
+  QM_TRY(OkIf(fileSize == kUsageFileSize), Err(NS_ERROR_FILE_CORRUPTED));
+
+  QM_TRY_UNWRAP(auto stream, NS_NewLocalFileInputStream(&aUsageFile));
+
+  QM_TRY_INSPECT(const auto& bufferedStream,
+                 NS_NewBufferedInputStream(stream.forget(), 16));
+
+  const nsCOMPtr<nsIBinaryInputStream> binaryStream =
+      NS_NewObjectInputStream(bufferedStream);
+
+  QM_TRY_INSPECT(const uint32_t& cookie,
+                 MOZ_TO_RESULT_INVOKE_MEMBER(binaryStream, Read32));
+
+  QM_TRY(OkIf(cookie == kUsageFileCookie), Err(NS_ERROR_FILE_CORRUPTED));
+
+  QM_TRY_INSPECT(const uint64_t& usage,
+                 MOZ_TO_RESULT_INVOKE_MEMBER(binaryStream, Read64));
+
+  return UsageInfo{DatabaseUsageType(Some(usage))};
+}
+
+/*******************************************************************************
+ * Non-actor class declarations
+ ******************************************************************************/
+
+/**
+ * Coalescing manipulation queue used by `Datastore`.  Used by `Datastore` to
+ * update `Datastore::mOrderedItems` efficiently/for code simplification.
+ * (Datastore does not actually depend on the coalescing, as mutations are
+ * applied atomically when a Snapshot Checkpoints, and with `Datastore::mValues`
+ * being updated at the same time the mutations are applied to Datastore's
+ * mWriteOptimizer.)
+ */
+class DatastoreWriteOptimizer final : public LSWriteOptimizer<LSValue> {
+ public:
+  void ApplyAndReset(nsTArray<LSItemInfo>& aOrderedItems);
+};
+
+/**
+ * Coalescing manipulation queue used by `Connection`.  Used by `Connection` to
+ * buffer and coalesce manipulations applied to the Datastore in batches by
+ * Snapshot Checkpointing until flushed to disk.
+ */
+class ConnectionWriteOptimizer final : public LSWriteOptimizer<LSValue> {
+ public:
+  // Returns the usage as the success value.
+  Result<int64_t, nsresult> Perform(Connection* aConnection,
+                                    bool aShadowWrites);
+
+ private:
+  /**
+   * Handlers for specific mutations.  Each method knows how to `Perform` the
+   * manipulation against a `Connection` and the "shadow" database (legacy
+   * webappsstore.sqlite database that exists so LSNG can be disabled/safely
+   * downgraded from.)
+   */
+  nsresult PerformInsertOrUpdate(Connection* aConnection, bool aShadowWrites,
+                                 const nsAString& aKey, const LSValue& aValue);
+
+  nsresult PerformDelete(Connection* aConnection, bool aShadowWrites,
+                         const nsAString& aKey);
+
+  nsresult PerformTruncate(Connection* aConnection, bool aShadowWrites);
+};
+
+class DatastoreOperationBase : public Runnable {
+  nsCOMPtr<nsIEventTarget> mOwningEventTarget;
+  nsresult mResultCode;
+  Atomic<bool> mMayProceedOnNonOwningThread;
+  bool mMayProceed;
+
+ public:
+  nsIEventTarget* OwningEventTarget() const {
+    MOZ_ASSERT(mOwningEventTarget);
+
+    return mOwningEventTarget;
+  }
+
+  bool IsOnOwningThread() const {
+    MOZ_ASSERT(mOwningEventTarget);
+
+    bool current;
+    return NS_SUCCEEDED(mOwningEventTarget->IsOnCurrentThread(&current)) &&
+           current;
+  }
+
+  void AssertIsOnOwningThread() const {
+    MOZ_ASSERT(IsOnBackgroundThread());
+    MOZ_ASSERT(IsOnOwningThread());
+  }
+
+  nsresult ResultCode() const { return mResultCode; }
+
+  void SetFailureCode(nsresult aErrorCode) {
+    MOZ_ASSERT(NS_SUCCEEDED(mResultCode));
+    MOZ_ASSERT(NS_FAILED(aErrorCode));
+
+    mResultCode = aErrorCode;
+  }
+
+  void MaybeSetFailureCode(nsresult aErrorCode) {
+    MOZ_ASSERT(NS_FAILED(aErrorCode));
+
+    if (NS_SUCCEEDED(mResultCode)) {
+      mResultCode = aErrorCode;
+    }
+  }
+
+  void NoteComplete() {
+    AssertIsOnOwningThread();
+
+    mMayProceed = false;
+    mMayProceedOnNonOwningThread = false;
+  }
+
+  bool MayProceed() const {
+    AssertIsOnOwningThread();
+
+    return mMayProceed;
+  }
+
+  // May be called on any thread, but you should call MayProceed() if you know
+  // you're on the background thread because it is slightly faster.
+  bool MayProceedOnNonOwningThread() const {
+    return mMayProceedOnNonOwningThread;
+  }
+
+ protected:
+  DatastoreOperationBase()
+      : Runnable("dom::DatastoreOperationBase"),
+        mOwningEventTarget(GetCurrentSerialEventTarget()),
+        mResultCode(NS_OK),
+        mMayProceedOnNonOwningThread(true),
+        mMayProceed(true) {}
+
+  ~DatastoreOperationBase() override { MOZ_ASSERT(!mMayProceed); }
+};
+
+class ConnectionDatastoreOperationBase : public DatastoreOperationBase {
+ protected:
+  RefPtr<Connection> mConnection;
+  /**
+   * This boolean flag is used by the CloseOp to avoid creating empty databases.
+   */
+  const bool mEnsureStorageConnection;
+
+ public:
+  // This callback will be called on the background thread before releasing the
+  // final reference to this request object. Subclasses may perform any
+  // additional cleanup here but must always call the base class implementation.
+  virtual void Cleanup();
+
+ protected:
+  ConnectionDatastoreOperationBase(Connection* aConnection,
+                                   bool aEnsureStorageConnection = true);
+
+  ~ConnectionDatastoreOperationBase();
+
+  // Must be overridden in subclasses. Called on the target thread to allow the
+  // subclass to perform necessary datastore operations. A successful return
+  // value will trigger an OnSuccess callback on the background thread while
+  // while a failure value will trigger an OnFailure callback.
+  virtual nsresult DoDatastoreWork() = 0;
+
+  // Methods that subclasses may implement.
+  virtual void OnSuccess();
+
+  virtual void OnFailure(nsresult aResultCode);
+
+ private:
+  void RunOnConnectionThread();
+
+  void RunOnOwningThread();
+
+  // Not to be overridden by subclasses.
+  NS_DECL_NSIRUNNABLE
+};
+
+class Connection final : public CachingDatabaseConnection {
+  friend class ConnectionThread;
+
+  class InitTemporaryOriginHelper;
+
+  class FlushOp;
+  class CloseOp;
+
+  RefPtr<ConnectionThread> mConnectionThread;
+  RefPtr<QuotaClient> mQuotaClient;
+  nsCOMPtr<nsITimer> mFlushTimer;
+  UniquePtr<ArchivedOriginScope> mArchivedOriginScope;
+  ConnectionWriteOptimizer mWriteOptimizer;
+  // XXX Consider changing this to ClientMetadata.
+  const OriginMetadata mOriginMetadata;
+  nsString mDirectoryPath;
+  /**
+   * Propagated from PrepareDatastoreOp. PrepareDatastoreOp may defer the
+   * creation of the localstorage client directory and database on the
+   * QuotaManager IO thread in its DatabaseWork method to
+   * Connection::EnsureStorageConnection, in which case the method needs to know
+   * it is responsible for taking those actions (without redundantly performing
+   * the existence checks).
+   */
+  const bool mDatabaseWasNotAvailable;
+  bool mHasCreatedDatabase;
+  bool mFlushScheduled;
+#ifdef DEBUG
+  bool mInUpdateBatch;
+  bool mFinished;
+#endif
+
+ public:
+  NS_INLINE_DECL_REFCOUNTING(mozilla::dom::Connection)
+
+  void AssertIsOnOwningThread() const { NS_ASSERT_OWNINGTHREAD(Connection); }
+
+  QuotaClient* GetQuotaClient() const {
+    MOZ_ASSERT(mQuotaClient);
+
+    return mQuotaClient;
+  }
+
+  ArchivedOriginScope* GetArchivedOriginScope() const {
+    return mArchivedOriginScope.get();
+  }
+
+  const nsCString& Origin() const { return mOriginMetadata.mOrigin; }
+
+  const nsString& DirectoryPath() const { return mDirectoryPath; }
+
+  void GetFinishInfo(bool& aDatabaseWasNotAvailable,
+                     bool& aHasCreatedDatabase) const {
+    AssertIsOnOwningThread();
+    MOZ_ASSERT(mFinished);
+
+    aDatabaseWasNotAvailable = mDatabaseWasNotAvailable;
+    aHasCreatedDatabase = mHasCreatedDatabase;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Methods which can only be called on the owning thread.
+
+  // This method is used to asynchronously execute a connection datastore
+  // operation on the connection thread.
+  void Dispatch(ConnectionDatastoreOperationBase* aOp);
+
+  // This method is used to asynchronously close the storage connection on the
+  // connection thread.
+  void Close(nsIRunnable* aCallback);
+
+  void SetItem(const nsString& aKey, const LSValue& aValue, int64_t aDelta,
+               bool aIsNewItem);
+
+  void RemoveItem(const nsString& aKey, int64_t aDelta);
+
+  void Clear(int64_t aDelta);
+
+  void BeginUpdateBatch();
+
+  void EndUpdateBatch();
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Methods which can only be called on the connection thread.
+
+  nsresult EnsureStorageConnection();
+
+  mozIStorageConnection* StorageConnection() const {
+    AssertIsOnGlobalConnectionThread();
+
+    return &MutableStorageConnection();
+  }
+
+  void CloseStorageConnection();
+
+  nsresult BeginWriteTransaction();
+
+  nsresult CommitWriteTransaction();
+
+  nsresult RollbackWriteTransaction();
+
+ private:
+  // Only created by ConnectionThread.
+  Connection(ConnectionThread* aConnectionThread,
+             const OriginMetadata& aOriginMetadata,
+             UniquePtr<ArchivedOriginScope>&& aArchivedOriginScope,
+             bool aDatabaseWasNotAvailable);
+
+  ~Connection();
+
+  void ScheduleFlush();
+
+  void Flush();
+
+  static void FlushTimerCallback(nsITimer* aTimer, void* aClosure);
+};
+
+/**
+ * Helper to invoke EnsureTemporaryOriginIsInitialized on the QuotaManager IO
+ * thread from the LocalStorage connection thread when creating a database
+ * connection on demand. This is necessary because we attempt to defer the
+ * creation of the origin directory and the database until absolutely needed,
+ * but the directory creation and origin initialization must happen on the QM
+ * IO thread for invariant reasons. (We can't just use a mutex because there
+ * could be logic on the IO thread that also wants to deal with the same
+ * origin, so we need to queue a runnable and wait our turn.)
+ */
+class Connection::InitTemporaryOriginHelper final : public Runnable {
+  mozilla::Monitor mMonitor MOZ_UNANNOTATED;
+  const OriginMetadata mOriginMetadata;
+  nsString mOriginDirectoryPath;
+  nsresult mIOThreadResultCode;
+  bool mWaiting;
+
+ public:
+  explicit InitTemporaryOriginHelper(const OriginMetadata& aOriginMetadata)
+      : Runnable("dom::localstorage::Connection::InitTemporaryOriginHelper"),
+        mMonitor("InitTemporaryOriginHelper::mMonitor"),
+        mOriginMetadata(aOriginMetadata),
+        mIOThreadResultCode(NS_OK),
+        mWaiting(true) {
+    AssertIsOnGlobalConnectionThread();
+  }
+
+  Result<nsString, nsresult> BlockAndReturnOriginDirectoryPath();
+
+ private:
+  ~InitTemporaryOriginHelper() = default;
+
+  nsresult RunOnIOThread();
+
+  NS_DECL_NSIRUNNABLE
+};
+
+class Connection::FlushOp final : public ConnectionDatastoreOperationBase {
+  ConnectionWriteOptimizer mWriteOptimizer;
+  bool mShadowWrites;
+
+ public:
+  FlushOp(Connection* aConnection, ConnectionWriteOptimizer&& aWriteOptimizer);
+
+ private:
+  nsresult DoDatastoreWork() override;
+
+  void Cleanup() override;
+};
+
+class Connection::CloseOp final : public ConnectionDatastoreOperationBase {
+  nsCOMPtr<nsIRunnable> mCallback;
+
+ public:
+  CloseOp(Connection* aConnection, nsIRunnable* aCallback)
+      : ConnectionDatastoreOperationBase(aConnection,
+                                         /* aEnsureStorageConnection */ false),
+        mCallback(aCallback) {}
+
+ private:
+  nsresult DoDatastoreWork() override;
+
+  void Cleanup() override;
+};
+
+class ConnectionThread final {
+  friend class Connection;
+
+  nsCOMPtr<nsIThread> mThread;
+  nsRefPtrHashtable<nsCStringHashKey, Connection> mConnections;
+
+ public:
+  ConnectionThread();
+
+  void AssertIsOnOwningThread() const {
+    NS_ASSERT_OWNINGTHREAD(ConnectionThread);
+  }
+
+  bool IsOnConnectionThread();
+
+  void AssertIsOnConnectionThread();
+
+  already_AddRefed<Connection> CreateConnection(
+      const OriginMetadata& aOriginMetadata,
+      UniquePtr<ArchivedOriginScope>&& aArchivedOriginScope,
+      bool aDatabaseWasNotAvailable);
+
+  void Shutdown();
+
+  NS_INLINE_DECL_REFCOUNTING(ConnectionThread)
+
+ private:
+  ~ConnectionThread();
+};
+
+/**
+ * Canonical state of Storage for an origin, containing all keys and their
+ * values in the parent process.  Specifically, this is the state that will
+ * be handed out to freshly created Snapshots and that will be persisted to disk
+ * when the Connection's flush completes.  State is mutated in batches as
+ * Snapshot instances Checkpoint their mutations locally accumulated in the
+ * child LSSnapshots.
+ */
+class Datastore final
+    : public SupportsCheckedUnsafePtr<CheckIf<DiagnosticAssertEnabled>> {
+  RefPtr<DirectoryLock> mDirectoryLock;
+  RefPtr<Connection> mConnection;
+  RefPtr<QuotaObject> mQuotaObject;
+  nsCOMPtr<nsIRunnable> mCompleteCallback;
+  /**
+   * PrepareDatastoreOps register themselves with the Datastore at
+   * and unregister in PrepareDatastoreOp::Cleanup.
+   */
+  nsTHashSet<PrepareDatastoreOp*> mPrepareDatastoreOps;
+  /**
+   * PreparedDatastore instances register themselves with their associated
+   * Datastore at construction time and unregister at destruction time.  They
+   * hang around for kPreparedDatastoreTimeoutMs in order to keep the Datastore
+   * from closing itself via MaybeClose(), thereby giving the document enough
+   * time to load and access LocalStorage.
+   */
+  nsTHashSet<PreparedDatastore*> mPreparedDatastores;
+  /**
+   * A database is live (and in this hashtable) if it has a live LSDatabase
+   * actor.  There is at most one Database per origin per content process.  Each
+   * Database corresponds to an LSDatabase in its associated content process.
+   */
+  nsTHashSet<Database*> mDatabases;
+  /**
+   * A database is active if it has a non-null `mSnapshot`.  As long as there
+   * are any active databases final deltas can't be calculated and
+   * `UpdateUsage()` can't be invoked.
+   */
+  nsTHashSet<Database*> mActiveDatabases;
+  /**
+   * Non-authoritative hashtable representation of mOrderedItems for efficient
+   * lookup.
+   */
+  nsTHashMap<nsStringHashKey, LSValue> mValues;
+  /**
+   * The authoritative ordered state of the Datastore; mValue also exists as an
+   * unordered hashtable for efficient lookup.
+   */
+  nsTArray<LSItemInfo> mOrderedItems;
+  nsTArray<int64_t> mPendingUsageDeltas;
+  DatastoreWriteOptimizer mWriteOptimizer;
+  const OriginMetadata mOriginMetadata;
+  const uint32_t mPrivateBrowsingId;
+  int64_t mUsage;
+  int64_t mUpdateBatchUsage;
+  int64_t mSizeOfKeys;
+  int64_t mSizeOfItems;
+  bool mClosed;
+  bool mInUpdateBatch;
+  bool mHasLivePrivateDatastore;
+
+ public:
+  // Created by PrepareDatastoreOp.
+  Datastore(const OriginMetadata& aOriginMetadata, uint32_t aPrivateBrowsingId,
+            int64_t aUsage, int64_t aSizeOfKeys, int64_t aSizeOfItems,
+            RefPtr<DirectoryLock>&& aDirectoryLock,
+            RefPtr<Connection>&& aConnection,
+            RefPtr<QuotaObject>&& aQuotaObject,
+            nsTHashMap<nsStringHashKey, LSValue>& aValues,
+            nsTArray<LSItemInfo>&& aOrderedItems);
+
+  Maybe<DirectoryLock&> MaybeDirectoryLockRef() const {
+    AssertIsOnBackgroundThread();
+
+    return ToMaybeRef(mDirectoryLock.get());
+  }
+
+  const nsCString& Origin() const { return mOriginMetadata.mOrigin; }
+
+  uint32_t PrivateBrowsingId() const { return mPrivateBrowsingId; }
+
+  bool IsPersistent() const {
+    // Private-browsing is forbidden from touching disk, but
+    // StorageAccess::eSessionScoped is allowed to touch disk because
+    // QuotaManager's storage for such origins is wiped at shutdown.
+    return mPrivateBrowsingId == 0;
+  }
+
+  void Close();
+
+  bool IsClosed() const {
+    AssertIsOnBackgroundThread();
+
+    return mClosed;
+  }
+
+  void WaitForConnectionToComplete(nsIRunnable* aCallback);
+
+  void NoteLivePrepareDatastoreOp(PrepareDatastoreOp* aPrepareDatastoreOp);
+
+  void NoteFinishedPrepareDatastoreOp(PrepareDatastoreOp* aPrepareDatastoreOp);
+
+  void NoteLivePrivateDatastore();
+
+  void NoteFinishedPrivateDatastore();
+
+  void NoteLivePreparedDatastore(PreparedDatastore* aPreparedDatastore);
+
+  void NoteFinishedPreparedDatastore(PreparedDatastore* aPreparedDatastore);
+
+  bool HasOtherProcessDatabases(Database* aDatabase);
+
+  void NoteLiveDatabase(Database* aDatabase);
+
+  void NoteFinishedDatabase(Database* aDatabase);
+
+  void NoteActiveDatabase(Database* aDatabase);
+
+  void NoteInactiveDatabase(Database* aDatabase);
+
+  void GetSnapshotLoadInfo(const nsAString& aKey, bool& aAddKeyToUnknownItems,
+                           nsTHashtable<nsStringHashKey>& aLoadedItems,
+                           nsTArray<LSItemInfo>& aItemInfos,
+                           uint32_t& aNextLoadIndex,
+                           LSSnapshot::LoadState& aLoadState);
+
+  uint32_t GetLength() const { return mValues.Count(); }
+
+  const nsTArray<LSItemInfo>& GetOrderedItems() const { return mOrderedItems; }
+
+  void GetItem(const nsAString& aKey, LSValue& aValue) const;
+
+  void GetKeys(nsTArray<nsString>& aKeys) const;
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Mutation Methods
+  //
+  // These are only called during Snapshot::Checkpoint
+
+  /**
+   * Used by Snapshot::Checkpoint to set a key/value pair as part of an
+   * explicit batch.
+   */
+  void SetItem(Database* aDatabase, const nsString& aKey,
+               const LSValue& aValue);
+
+  void RemoveItem(Database* aDatabase, const nsString& aKey);
+
+  void Clear(Database* aDatabase);
+
+  void BeginUpdateBatch(int64_t aSnapshotUsage);
+
+  int64_t EndUpdateBatch(int64_t aSnapshotPeakUsage);
+
+  int64_t GetUsage() const { return mUsage; }
+
+  int64_t AttemptToUpdateUsage(int64_t aMinSize, bool aInitial);
+
+  bool HasOtherProcessObservers(Database* aDatabase);
+
+  void NotifyOtherProcessObservers(Database* aDatabase,
+                                   const nsString& aDocumentURI,
+                                   const nsString& aKey,
+                                   const LSValue& aOldValue,
+                                   const LSValue& aNewValue);
+
+  void NoteChangedObserverArray(const nsTArray<NotNull<Observer*>>& aObservers);
+
+  void Stringify(nsACString& aResult) const;
+
+  NS_INLINE_DECL_REFCOUNTING(Datastore)
+
+ private:
+  // Reference counted.
+  ~Datastore();
+
+  bool UpdateUsage(int64_t aDelta);
+
+  void MaybeClose();
+
+  void ConnectionClosedCallback();
+
+  void CleanupMetadata();
+
+  void NotifySnapshots(Database* aDatabase, const nsAString& aKey,
+                       const LSValue& aOldValue, bool aAffectsOrder);
+
+  void NoteChangedDatabaseMap();
+};
+
+class PrivateDatastore {
+  const NotNull<RefPtr<Datastore>> mDatastore;
+
+ public:
+  explicit PrivateDatastore(MovingNotNull<RefPtr<Datastore>> aDatastore)
+      : mDatastore(std::move(aDatastore)) {
+    AssertIsOnBackgroundThread();
+
+    mDatastore->NoteLivePrivateDatastore();
+  }
+
+  ~PrivateDatastore() { mDatastore->NoteFinishedPrivateDatastore(); }
+
+  const Datastore& DatastoreRef() const {
+    AssertIsOnBackgroundThread();
+
+    return *mDatastore;
+  }
+};
+
+class PreparedDatastore {
+  RefPtr<Datastore> mDatastore;
+  nsCOMPtr<nsITimer> mTimer;
+  const Maybe<ContentParentId> mContentParentId;
+  // Strings share buffers if possible, so it's not a problem to duplicate the
+  // origin here.
+  const nsCString mOrigin;
+  uint64_t mDatastoreId;
+  bool mForPreload;
+  bool mInvalidated;
+
+ public:
+  PreparedDatastore(Datastore* aDatastore,
+                    const Maybe<ContentParentId>& aContentParentId,
+                    const nsACString& aOrigin, uint64_t aDatastoreId,
+                    bool aForPreload)
+      : mDatastore(aDatastore),
+        mTimer(NS_NewTimer()),
+        mContentParentId(aContentParentId),
+        mOrigin(aOrigin),
+        mDatastoreId(aDatastoreId),
+        mForPreload(aForPreload),
+        mInvalidated(false) {
+    AssertIsOnBackgroundThread();
+    MOZ_ASSERT(aDatastore);
+    MOZ_ASSERT(mTimer);
+
+    aDatastore->NoteLivePreparedDatastore(this);
+
+    MOZ_ALWAYS_SUCCEEDS(mTimer->InitWithNamedFuncCallback(
+        TimerCallback, this, kPreparedDatastoreTimeoutMs,
+        nsITimer::TYPE_ONE_SHOT, "PreparedDatastore::TimerCallback"));
+  }
+
+  ~PreparedDatastore() {
+    MOZ_ASSERT(mDatastore);
+    MOZ_ASSERT(mTimer);
+
+    mTimer->Cancel();
+
+    mDatastore->NoteFinishedPreparedDatastore(this);
+  }
+
+  const Datastore& DatastoreRef() const {
+    AssertIsOnBackgroundThread();
+    MOZ_ASSERT(mDatastore);
+
+    return *mDatastore;
+  }
+
+  Datastore& MutableDatastoreRef() const {
+    AssertIsOnBackgroundThread();
+    MOZ_ASSERT(mDatastore);
+
+    return *mDatastore;
+  }
+
+  const Maybe<ContentParentId>& GetContentParentId() const {
+    return mContentParentId;
+  }
+
+  const nsCString& Origin() const { return mOrigin; }
+
+  void Invalidate() {
+    AssertIsOnBackgroundThread();
+
+    mInvalidated = true;
+
+    if (mForPreload) {
+      mTimer->Cancel();
+
+      MOZ_ALWAYS_SUCCEEDS(mTimer->InitWithNamedFuncCallback(
+          TimerCallback, this, 0, nsITimer::TYPE_ONE_SHOT,
+          "PreparedDatastore::TimerCallback"));
+    }
+  }
+
+  bool IsInvalidated() const {
+    AssertIsOnBackgroundThread();
+
+    return mInvalidated;
+  }
+
+ private:
+  void Destroy();
+
+  static void TimerCallback(nsITimer* aTimer, void* aClosure);
+};
+
+/*******************************************************************************
+ * Actor class declarations
+ ******************************************************************************/
+
+class Database final
+    : public PBackgroundLSDatabaseParent,
+      public SupportsCheckedUnsafePtr<CheckIf<DiagnosticAssertEnabled>> {
+  RefPtr<Datastore> mDatastore;
+  Snapshot* mSnapshot;
+  const PrincipalInfo mPrincipalInfo;
+  const Maybe<ContentParentId> mContentParentId;
+  // Strings share buffers if possible, so it's not a problem to duplicate the
+  // origin here.
+  nsCString mOrigin;
+  uint32_t mPrivateBrowsingId;
+  bool mAllowedToClose;
+  bool mActorDestroyed;
+  bool mRequestedAllowToClose;
+#ifdef DEBUG
+  bool mActorWasAlive;
+#endif
+
+ public:
+  // Created in AllocPBackgroundLSDatabaseParent.
+  Database(const PrincipalInfo& aPrincipalInfo,
+           const Maybe<ContentParentId>& aContentParentId,
+           const nsACString& aOrigin, uint32_t aPrivateBrowsingId);
+
+  Datastore* GetDatastore() const {
+    AssertIsOnBackgroundThread();
+    return mDatastore;
+  }
+
+  Maybe<Datastore&> MaybeDatastoreRef() const {
+    AssertIsOnBackgroundThread();
+
+    return ToMaybeRef(mDatastore.get());
+  }
+
+  const PrincipalInfo& GetPrincipalInfo() const { return mPrincipalInfo; }
+
+  bool IsOwnedByProcess(ContentParentId aContentParentId) const {
+    return mContentParentId && mContentParentId.value() == aContentParentId;
+  }
+
+  uint32_t PrivateBrowsingId() const { return mPrivateBrowsingId; }
+
+  const nsCString& Origin() const { return mOrigin; }
+
+  void SetActorAlive(Datastore* aDatastore);
+
+  void RegisterSnapshot(Snapshot* aSnapshot);
+
+  void UnregisterSnapshot(Snapshot* aSnapshot);
+
+  Snapshot* GetSnapshot() const {
+    AssertIsOnBackgroundThread();
+    return mSnapshot;
+  }
+
+  void RequestAllowToClose();
+
+  void ForceKill();
+
+  void Stringify(nsACString& aResult) const;
+
+  NS_INLINE_DECL_REFCOUNTING(mozilla::dom::Database)
+
+ private:
+  // Reference counted.
+  ~Database();
+
+  void AllowToClose();
+
+  // IPDL methods are only called by IPDL.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  mozilla::ipc::IPCResult RecvDeleteMe() override;
+
+  mozilla::ipc::IPCResult RecvAllowToClose() override;
+
+  PBackgroundLSSnapshotParent* AllocPBackgroundLSSnapshotParent(
+      const nsAString& aDocumentURI, const nsAString& aKey,
+      const bool& aIncreasePeakUsage, const int64_t& aMinSize,
+      LSSnapshotInitInfo* aInitInfo) override;
+
+  mozilla::ipc::IPCResult RecvPBackgroundLSSnapshotConstructor(
+      PBackgroundLSSnapshotParent* aActor, const nsAString& aDocumentURI,
+      const nsAString& aKey, const bool& aIncreasePeakUsage,
+      const int64_t& aMinSize, LSSnapshotInitInfo* aInitInfo) override;
+
+  bool DeallocPBackgroundLSSnapshotParent(
+      PBackgroundLSSnapshotParent* aActor) override;
+};
+
+/**
+ * Attempts to capture the state of the underlying Datastore at the time of its
+ * creation so run-to-completion semantics can be honored.
+ *
+ * Rather than simply duplicate the contents of `DataStore::mValues` and
+ * `Datastore::mOrderedItems` at the time of their creation, the Snapshot tracks
+ * mutations to the Datastore as they happen, saving off the state of values as
+ * they existed when the Snapshot was created.  In other words, given an initial
+ * Datastore state of { foo: 'bar', bar: 'baz' }, the Snapshot won't store those
+ * values until it hears via `SaveItem` that "foo" is being over-written.  At
+ * that time, it will save off foo='bar' in mValues.
+ *
+ * ## Quota Allocation ##
+ *
+ * ## States ##
+ *
+ */
+class Snapshot final : public PBackgroundLSSnapshotParent {
+  /**
+   * The Database that owns this snapshot.  There is a 1:1 relationship between
+   * snapshots and databases.
+   */
+  RefPtr<Database> mDatabase;
+  RefPtr<Datastore> mDatastore;
+  /**
+   * The set of keys for which values have been sent to the child LSSnapshot.
+   * Cleared once all values have been sent as indicated by
+   * mLoadedItems.Count()==mTotalLength and therefore mLoadedAllItems should be
+   * true.  No requests should be received for keys already in this set, and
+   * this is enforced by fatal IPC error (unless fuzzing).
+   */
+  nsTHashtable<nsStringHashKey> mLoadedItems;
+  /**
+   * The set of keys for which a RecvLoadValueAndMoreItems request was received
+   * but there was no such key, and so null was returned.  The child LSSnapshot
+   * will also cache these values, so redundant requests are also handled with
+   * fatal process termination just like for mLoadedItems.  Also cleared when
+   * mLoadedAllItems becomes true because then the child can infer that all
+   * other values must be null.  (Note: this could also be done when
+   * mLoadKeysReceived is true as a further optimization, but is not.)
+   */
+  nsTHashSet<nsString> mUnknownItems;
+  /**
+   * Values that have changed in mDatastore as reported by SaveItem
+   * notifications that are not yet known to the child LSSnapshot.
+   *
+   * The naive way to snapshot the state of mDatastore would be to duplicate its
+   * internal mValues at the time of our creation, but that is wasteful if few
+   * changes are made to the Datastore's state.  So we only track values that
+   * are changed/evicted from the Datastore as they happen, as reported to us by
+   * SaveItem notifications.
+   */
+  nsTHashMap<nsStringHashKey, LSValue> mValues;
+  /**
+   * Latched state of mDatastore's keys during a SaveItem notification with
+   * aAffectsOrder=true.  The ordered keys needed to be saved off so that a
+   * consistent ordering could be presented to the child LSSnapshot when it asks
+   * for them via RecvLoadKeys.
+   */
+  nsTArray<nsString> mKeys;
+  nsString mDocumentURI;
+  /**
+   * The index used for restoring iteration over not yet sent key/value pairs to
+   * the child LSSnapshot.
+   */
+  uint32_t mNextLoadIndex;
+  /**
+   * The number of key/value pairs that were present in the Datastore at the
+   * time the snapshot was created.  Once we have sent this many values to the
+   * child LSSnapshot, we can infer that it has received all of the keys/values
+   * and set mLoadedAllItems to true and clear mLoadedItems and mUnknownItems.
+   * Note that knowing the keys/values is not the same as knowing their ordering
+   * and so mKeys may be retained.
+   */
+  uint32_t mTotalLength;
+  int64_t mUsage;
+  int64_t mPeakUsage;
+  /**
+   * True if SaveItem has saved mDatastore's keys into mKeys because a SaveItem
+   * notification with aAffectsOrder=true was received.
+   */
+  bool mSavedKeys;
+  bool mActorDestroyed;
+  bool mFinishReceived;
+  bool mLoadedReceived;
+  /**
+   * True if LSSnapshot's mLoadState should be LoadState::AllOrderedItems or
+   * LoadState::AllUnorderedItems.  It will be AllOrderedItems if the initial
+   * snapshot contained all the data or if the state was AllOrderedKeys and
+   * successive RecvLoadValueAndMoreItems requests have resulted in the
+   * LSSnapshot being told all of the key/value pairs.  It will be
+   * AllUnorderedItems if the state was LoadState::Partial and successive
+   * RecvLoadValueAndMoreItem requests got all the keys/values but the key
+   * ordering was not retrieved.
+   */
+  bool mLoadedAllItems;
+  /**
+   * True if LSSnapshot's mLoadState should be LoadState::AllOrderedItems or
+   * AllOrderedKeys.  This can occur because of the initial snapshot, or because
+   * a RecvLoadKeys request was received.
+   */
+  bool mLoadKeysReceived;
+  bool mSentMarkDirty;
+
+  /**
+   * True if there are Database objects in other content processes. The value
+   * never gets updated, we instead mark snapshots as dirty when Database
+   * objects are added or removed. Marking snapshots as dirty forces creation
+   * of new snapshots for new tasks.
+   */
+  bool mHasOtherProcessDatabases;
+  bool mHasOtherProcessObservers;
+
+ public:
+  // Created in AllocPBackgroundLSSnapshotParent.
+  Snapshot(Database* aDatabase, const nsAString& aDocumentURI);
+
+  void Init(nsTHashtable<nsStringHashKey>& aLoadedItems,
+            nsTHashSet<nsString>&& aUnknownItems, uint32_t aNextLoadIndex,
+            uint32_t aTotalLength, int64_t aUsage, int64_t aPeakUsage,
+            LSSnapshot::LoadState aLoadState, bool aHasOtherProcessDatabases,
+            bool aHasOtherProcessObservers) {
+    AssertIsOnBackgroundThread();
+    MOZ_ASSERT(aUsage >= 0);
+    MOZ_ASSERT(aPeakUsage >= aUsage);
+    MOZ_ASSERT_IF(aLoadState != LSSnapshot::LoadState::AllOrderedItems,
+                  aNextLoadIndex < aTotalLength);
+    MOZ_ASSERT(mTotalLength == 0);
+    MOZ_ASSERT(mUsage == -1);
+    MOZ_ASSERT(mPeakUsage == -1);
+
+    mLoadedItems.SwapElements(aLoadedItems);
+    mUnknownItems = std::move(aUnknownItems);
+    mNextLoadIndex = aNextLoadIndex;
+    mTotalLength = aTotalLength;
+    mUsage = aUsage;
+    mPeakUsage = aPeakUsage;
+    if (aLoadState == LSSnapshot::LoadState::AllOrderedKeys) {
+      MOZ_ASSERT(mUnknownItems.Count() == 0);
+      mLoadKeysReceived = true;
+    } else if (aLoadState == LSSnapshot::LoadState::AllOrderedItems) {
+      MOZ_ASSERT(mLoadedItems.Count() == 0);
+      MOZ_ASSERT(mUnknownItems.Count() == 0);
+      MOZ_ASSERT(mNextLoadIndex == mTotalLength);
+      mLoadedReceived = true;
+      mLoadedAllItems = true;
+      mLoadKeysReceived = true;
+    }
+    mHasOtherProcessDatabases = aHasOtherProcessDatabases;
+    mHasOtherProcessObservers = aHasOtherProcessObservers;
+  }
+
+  /**
+   * Called via NotifySnapshots by Datastore whenever it is updating its
+   * internal state so that snapshots can save off the state of a value at the
+   * time of their creation.
+   */
+  void SaveItem(const nsAString& aKey, const LSValue& aOldValue,
+                bool aAffectsOrder);
+
+  void MarkDirty();
+
+  bool IsDirty() const {
+    AssertIsOnBackgroundThread();
+
+    return mSentMarkDirty;
+  }
+
+  bool HasOtherProcessDatabases() const {
+    AssertIsOnBackgroundThread();
+
+    return mHasOtherProcessDatabases;
+  }
+
+  bool HasOtherProcessObservers() const {
+    AssertIsOnBackgroundThread();
+
+    return mHasOtherProcessObservers;
+  }
+
+  NS_INLINE_DECL_REFCOUNTING(mozilla::dom::Snapshot)
+
+ private:
+  // Reference counted.
+  ~Snapshot();
+
+  mozilla::ipc::IPCResult Checkpoint(nsTArray<LSWriteInfo>&& aWriteInfos);
+
+  mozilla::ipc::IPCResult CheckpointAndNotify(
+      nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos);
+
+  void Finish();
+
+  // IPDL methods are only called by IPDL.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  mozilla::ipc::IPCResult RecvDeleteMe() override;
+
+  mozilla::ipc::IPCResult RecvAsyncCheckpoint(
+      nsTArray<LSWriteInfo>&& aWriteInfos) override;
+
+  mozilla::ipc::IPCResult RecvAsyncCheckpointAndNotify(
+      nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) override;
+
+  mozilla::ipc::IPCResult RecvSyncCheckpoint(
+      nsTArray<LSWriteInfo>&& aWriteInfos) override;
+
+  mozilla::ipc::IPCResult RecvSyncCheckpointAndNotify(
+      nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) override;
+
+  mozilla::ipc::IPCResult RecvAsyncFinish() override;
+
+  mozilla::ipc::IPCResult RecvSyncFinish() override;
+
+  mozilla::ipc::IPCResult RecvLoaded() override;
+
+  mozilla::ipc::IPCResult RecvLoadValueAndMoreItems(
+      const nsAString& aKey, LSValue* aValue,
+      nsTArray<LSItemInfo>* aItemInfos) override;
+
+  mozilla::ipc::IPCResult RecvLoadKeys(nsTArray<nsString>* aKeys) override;
+
+  mozilla::ipc::IPCResult RecvIncreasePeakUsage(const int64_t& aMinSize,
+                                                int64_t* aSize) override;
+};
+
+class Observer final : public PBackgroundLSObserverParent {
+  nsCString mOrigin;
+  bool mActorDestroyed;
+
+ public:
+  // Created in AllocPBackgroundLSObserverParent.
+  explicit Observer(const nsACString& aOrigin);
+
+  const nsCString& Origin() const { return mOrigin; }
+
+  void Observe(Database* aDatabase, const nsString& aDocumentURI,
+               const nsString& aKey, const LSValue& aOldValue,
+               const LSValue& aNewValue);
+
+  NS_INLINE_DECL_REFCOUNTING(mozilla::dom::Observer)
+
+ private:
+  // Reference counted.
+  ~Observer();
+
+  // IPDL methods are only called by IPDL.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  mozilla::ipc::IPCResult RecvDeleteMe() override;
+};
+
+class LSRequestBase : public DatastoreOperationBase,
+                      public PBackgroundLSRequestParent {
+ protected:
+  enum class State {
+    // Just created on the PBackground thread. Next step is StartingRequest.
+    Initial,
+
+    // Waiting to start/starting request on the PBackground thread. Next step is
+    // either Nesting if a subclass needs to process more nested states or
+    // SendingReadyMessage if a subclass doesn't need any nested processing.
+    StartingRequest,
+
+    // Doing nested processing.
+    Nesting,
+
+    // Waiting to send/sending the ready message on the PBackground thread. Next
+    // step is WaitingForFinish.
+    SendingReadyMessage,
+
+    // Waiting for the finish message on the PBackground thread. Next step is
+    // SendingResults.
+    WaitingForFinish,
+
+    // Waiting to send/sending results on the PBackground thread. Next step is
+    // Completed.
+    SendingResults,
+
+    // All done.
+    Completed
+  };
+
+  const LSRequestParams mParams;
+  Maybe<ContentParentId> mContentParentId;
+  State mState;
+  bool mWaitingForFinish;
+
+ public:
+  LSRequestBase(const LSRequestParams& aParams,
+                const Maybe<ContentParentId>& aContentParentId);
+
+  void Dispatch();
+
+  void StringifyState(nsACString& aResult) const;
+
+  virtual void Stringify(nsACString& aResult) const;
+
+  virtual void Log();
+
+ protected:
+  ~LSRequestBase() override;
+
+  virtual nsresult Start() = 0;
+
+  virtual nsresult NestedRun();
+
+  virtual void GetResponse(LSRequestResponse& aResponse) = 0;
+
+  virtual void Cleanup() {}
+
+ private:
+  bool VerifyRequestParams();
+
+  nsresult StartRequest();
+
+  void SendReadyMessage();
+
+  nsresult SendReadyMessageInternal();
+
+  void Finish();
+
+  void FinishInternal();
+
+  void SendResults();
+
+ protected:
+  // Common nsIRunnable implementation that subclasses may not override.
+  NS_IMETHOD
+  Run() final;
+
+  // IPDL methods.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+ private:
+  mozilla::ipc::IPCResult RecvCancel() final;
+
+  mozilla::ipc::IPCResult RecvFinish() final;
+};
+
+class PrepareDatastoreOp
+    : public LSRequestBase,
+      public OpenDirectoryListener,
+      public SupportsCheckedUnsafePtr<CheckIf<DiagnosticAssertEnabled>> {
+  class LoadDataOp;
+
+  class CompressFunction;
+  class CompressionTypeFunction;
+
+  enum class NestedState {
+    // The nesting has not yet taken place. Next step is
+    // CheckExistingOperations.
+    BeforeNesting,
+
+    // Checking if a prepare datastore operation is already running for given
+    // origin on the PBackground thread. Next step is CheckClosingDatastore.
+    CheckExistingOperations,
+
+    // Checking if a datastore is closing the connection for given origin on
+    // the PBackground thread. Next step is PreparationPending.
+    CheckClosingDatastore,
+
+    // Ensuring quota manager is created and opening directory on the
+    // PBackground thread. Next step is either SendingResults if quota manager
+    // is not available or DirectoryOpenPending if quota manager is available.
+    // If a datastore already exists for given origin then the next state is
+    // SendingReadyMessage.
+    PreparationPending,
+
+    // Waiting for directory open allowed on the PBackground thread. The next
+    // step is either SendingReadyMessage if directory lock failed to acquire,
+    // or DatabaseWorkOpen if directory lock is acquired.
+    DirectoryOpenPending,
+
+    // Waiting to do/doing work on the QuotaManager IO thread. Its next step is
+    // BeginLoadData.
+    DatabaseWorkOpen,
+
+    // Starting a load data operation on the PBackground thread. Next step is
+    // DatabaseWorkLoadData.
+    BeginLoadData,
+
+    // Waiting to do/doing work on the connection thread. This involves waiting
+    // for the LoadDataOp to do its work. Eventually the state will transition
+    // to SendingReadyMessage.
+    DatabaseWorkLoadData,
+
+    // The nesting has completed.
+    AfterNesting
+  };
+
+  RefPtr<PrepareDatastoreOp> mDelayedOp;
+  RefPtr<DirectoryLock> mPendingDirectoryLock;
+  RefPtr<DirectoryLock> mDirectoryLock;
+  RefPtr<Connection> mConnection;
+  RefPtr<Datastore> mDatastore;
+  UniquePtr<ArchivedOriginScope> mArchivedOriginScope;
+  LoadDataOp* mLoadDataOp;
+  nsTHashMap<nsStringHashKey, LSValue> mValues;
+  nsTArray<LSItemInfo> mOrderedItems;
+  OriginMetadata mOriginMetadata;
+  nsCString mMainThreadOrigin;
+  nsString mDatabaseFilePath;
+  uint32_t mPrivateBrowsingId;
+  int64_t mUsage;
+  int64_t mSizeOfKeys;
+  int64_t mSizeOfItems;
+  uint64_t mDatastoreId;
+  NestedState mNestedState;
+  const bool mForPreload;
+  bool mDatabaseNotAvailable;
+  // Set when the Datastore has been registered with gPrivateDatastores so that
+  // it can be unregistered if an error is encountered in PrepareDatastoreOp.
+  FlippedOnce<false> mPrivateDatastoreRegistered;
+  // Set when the Datastore has been registered with gPreparedDatastores so
+  // that it can be unregistered if an error is encountered in
+  // PrepareDatastoreOp.
+  FlippedOnce<false> mPreparedDatastoreRegistered;
+  bool mInvalidated;
+
+#ifdef DEBUG
+  int64_t mDEBUGUsage;
+#endif
+
+ public:
+  PrepareDatastoreOp(const LSRequestParams& aParams,
+                     const Maybe<ContentParentId>& aContentParentId);
+
+  Maybe<DirectoryLock&> MaybeDirectoryLockRef() const {
+    AssertIsOnBackgroundThread();
+
+    return ToMaybeRef(mDirectoryLock.get());
+  }
+
+  bool OriginIsKnown() const {
+    MOZ_ASSERT(IsOnOwningThread() || IsOnIOThread());
+
+    return !mOriginMetadata.mOrigin.IsEmpty();
+  }
+
+  const nsCString& Origin() const {
+    MOZ_ASSERT(IsOnOwningThread() || IsOnIOThread());
+    MOZ_ASSERT(OriginIsKnown());
+
+    return mOriginMetadata.mOrigin;
+  }
+
+  void Invalidate() {
+    AssertIsOnOwningThread();
+
+    mInvalidated = true;
+  }
+
+  void StringifyNestedState(nsACString& aResult) const;
+
+  void Stringify(nsACString& aResult) const override;
+
+  void Log() override;
+
+ private:
+  ~PrepareDatastoreOp() override;
+
+  nsresult Start() override;
+
+  nsresult CheckExistingOperations();
+
+  nsresult CheckClosingDatastoreInternal();
+
+  nsresult CheckClosingDatastore();
+
+  nsresult BeginDatastorePreparationInternal();
+
+  nsresult BeginDatastorePreparation();
+
+  void SendToIOThread();
+
+  nsresult DatabaseWork();
+
+  nsresult DatabaseNotAvailable();
+
+  nsresult EnsureDirectoryEntry(nsIFile* aEntry, bool aCreateIfNotExists,
+                                bool aDirectory,
+                                bool* aAlreadyExisted = nullptr);
+
+  nsresult VerifyDatabaseInformation(mozIStorageConnection* aConnection);
+
+  already_AddRefed<QuotaObject> GetQuotaObject();
+
+  nsresult BeginLoadData();
+
+  void FinishNesting();
+
+  nsresult FinishNestingOnNonOwningThread();
+
+  nsresult NestedRun() override;
+
+  void GetResponse(LSRequestResponse& aResponse) override;
+
+  void Cleanup() override;
+
+  void ConnectionClosedCallback();
+
+  void CleanupMetadata();
+
+  NS_DECL_ISUPPORTS_INHERITED
+
+  // IPDL overrides.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+
+  // OpenDirectoryListener overrides.
+  void DirectoryLockAcquired(DirectoryLock* aLock) override;
+
+  void DirectoryLockFailed() override;
+};
+
+class PrepareDatastoreOp::LoadDataOp final
+    : public ConnectionDatastoreOperationBase {
+  RefPtr<PrepareDatastoreOp> mPrepareDatastoreOp;
+
+ public:
+  explicit LoadDataOp(PrepareDatastoreOp* aPrepareDatastoreOp)
+      : ConnectionDatastoreOperationBase(aPrepareDatastoreOp->mConnection),
+        mPrepareDatastoreOp(aPrepareDatastoreOp) {}
+
+ private:
+  ~LoadDataOp() = default;
+
+  nsresult DoDatastoreWork() override;
+
+  void OnSuccess() override;
+
+  void OnFailure(nsresult aResultCode) override;
+
+  void Cleanup() override;
+};
+
+class PrepareDatastoreOp::CompressFunction final : public mozIStorageFunction {
+ private:
+  ~CompressFunction() = default;
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_MOZISTORAGEFUNCTION
+};
+
+class PrepareDatastoreOp::CompressionTypeFunction final
+    : public mozIStorageFunction {
+ private:
+  ~CompressionTypeFunction() = default;
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_MOZISTORAGEFUNCTION
+};
+
+class PrepareObserverOp : public LSRequestBase {
+  nsCString mOrigin;
+
+ public:
+  PrepareObserverOp(const LSRequestParams& aParams,
+                    const Maybe<ContentParentId>& aContentParentId);
+
+ private:
+  nsresult Start() override;
+
+  void GetResponse(LSRequestResponse& aResponse) override;
+};
+
+class LSSimpleRequestBase : public DatastoreOperationBase,
+                            public PBackgroundLSSimpleRequestParent {
+ protected:
+  enum class State {
+    // Just created on the PBackground thread. Next step is StartingRequest.
+    Initial,
+
+    // Waiting to start/starting request on the PBackground thread. Next step is
+    // SendingResults.
+    StartingRequest,
+
+    // Waiting to send/sending results on the PBackground thread. Next step is
+    // Completed.
+    SendingResults,
+
+    // All done.
+    Completed
+  };
+
+  const LSSimpleRequestParams mParams;
+  Maybe<ContentParentId> mContentParentId;
+  State mState;
+
+ public:
+  LSSimpleRequestBase(const LSSimpleRequestParams& aParams,
+                      const Maybe<ContentParentId>& aContentParentId);
+
+  void Dispatch();
+
+ protected:
+  ~LSSimpleRequestBase() override;
+
+  virtual nsresult Start() = 0;
+
+  virtual void GetResponse(LSSimpleRequestResponse& aResponse) = 0;
+
+ private:
+  bool VerifyRequestParams();
+
+  nsresult StartRequest();
+
+  void SendResults();
+
+  // Common nsIRunnable implementation that subclasses may not override.
+  NS_IMETHOD
+  Run() final;
+
+  // IPDL methods.
+  void ActorDestroy(ActorDestroyReason aWhy) override;
+};
+
+class PreloadedOp : public LSSimpleRequestBase {
+  nsCString mOrigin;
+
+ public:
+  PreloadedOp(const LSSimpleRequestParams& aParams,
+              const Maybe<ContentParentId>& aContentParentId);
+
+ private:
+  nsresult Start() override;
+
+  void GetResponse(LSSimpleRequestResponse& aResponse) override;
+};
+
+class GetStateOp : public LSSimpleRequestBase {
+  nsCString mOrigin;
+
+ public:
+  GetStateOp(const LSSimpleRequestParams& aParams,
+             const Maybe<ContentParentId>& aContentParentId);
+
+ private:
+  nsresult Start() override;
+
+  void GetResponse(LSSimpleRequestResponse& aResponse) override;
+};
+
+/*******************************************************************************
+ * Other class declarations
+ ******************************************************************************/
+
+struct ArchivedOriginInfo {
+  OriginAttributes mOriginAttributes;
+  nsCString mOriginNoSuffix;
+
+  ArchivedOriginInfo(const OriginAttributes& aOriginAttributes,
+                     const nsACString& aOriginNoSuffix)
+      : mOriginAttributes(aOriginAttributes),
+        mOriginNoSuffix(aOriginNoSuffix) {}
+};
+
+class ArchivedOriginScope {
+  struct Origin {
+    nsCString mOriginSuffix;
+    nsCString mOriginNoSuffix;
+
+    Origin(const nsACString& aOriginSuffix, const nsACString& aOriginNoSuffix)
+        : mOriginSuffix(aOriginSuffix), mOriginNoSuffix(aOriginNoSuffix) {}
+
+    const nsACString& OriginSuffix() const { return mOriginSuffix; }
+
+    const nsACString& OriginNoSuffix() const { return mOriginNoSuffix; }
+  };
+
+  struct Prefix {
+    nsCString mOriginNoSuffix;
+
+    explicit Prefix(const nsACString& aOriginNoSuffix)
+        : mOriginNoSuffix(aOriginNoSuffix) {}
+
+    const nsACString& OriginNoSuffix() const { return mOriginNoSuffix; }
+  };
+
+  struct Pattern {
+    UniquePtr<OriginAttributesPattern> mPattern;
+
+    explicit Pattern(const OriginAttributesPattern& aPattern)
+        : mPattern(MakeUnique<OriginAttributesPattern>(aPattern)) {}
+
+    Pattern(const Pattern& aOther)
+        : mPattern(MakeUnique<OriginAttributesPattern>(*aOther.mPattern)) {}
+
+    Pattern(Pattern&& aOther) = default;
+
+    const OriginAttributesPattern& GetPattern() const {
+      MOZ_ASSERT(mPattern);
+      return *mPattern;
+    }
+  };
+
+  struct Null {};
+
+  using DataType = Variant<Origin, Pattern, Prefix, Null>;
+
+  DataType mData;
+
+ public:
+  static UniquePtr<ArchivedOriginScope> CreateFromOrigin(
+      const nsACString& aOriginAttrSuffix, const nsACString& aOriginKey);
+
+  static UniquePtr<ArchivedOriginScope> CreateFromPrefix(
+      const nsACString& aOriginKey);
+
+  static UniquePtr<ArchivedOriginScope> CreateFromPattern(
+      const OriginAttributesPattern& aPattern);
+
+  static UniquePtr<ArchivedOriginScope> CreateFromNull();
+
+  bool IsOrigin() const { return mData.is<Origin>(); }
+
+  bool IsPrefix() const { return mData.is<Prefix>(); }
+
+  bool IsPattern() const { return mData.is<Pattern>(); }
+
+  bool IsNull() const { return mData.is<Null>(); }
+
+  const nsACString& OriginSuffix() const {
+    MOZ_ASSERT(IsOrigin());
+
+    return mData.as<Origin>().OriginSuffix();
+  }
+
+  const nsACString& OriginNoSuffix() const {
+    MOZ_ASSERT(IsOrigin() || IsPrefix());
+
+    if (IsOrigin()) {
+      return mData.as<Origin>().OriginNoSuffix();
+    }
+    return mData.as<Prefix>().OriginNoSuffix();
+  }
+
+  const OriginAttributesPattern& GetPattern() const {
+    MOZ_ASSERT(IsPattern());
+
+    return mData.as<Pattern>().GetPattern();
+  }
+
+  nsLiteralCString GetBindingClause() const;
+
+  nsresult BindToStatement(mozIStorageStatement* aStatement) const;
+
+  bool HasMatches(ArchivedOriginHashtable* aHashtable) const;
+
+  void RemoveMatches(ArchivedOriginHashtable* aHashtable) const;
+
+ private:
+  // Move constructors
+  explicit ArchivedOriginScope(const Origin&& aOrigin) : mData(aOrigin) {}
+
+  explicit ArchivedOriginScope(const Pattern&& aPattern) : mData(aPattern) {}
+
+  explicit ArchivedOriginScope(const Prefix&& aPrefix) : mData(aPrefix) {}
+
+  explicit ArchivedOriginScope(const Null&& aNull) : mData(aNull) {}
+};
+
+class QuotaClient final : public mozilla::dom::quota::Client {
+  class MatchFunction;
+
+  static QuotaClient* sInstance;
+
+  Mutex mShadowDatabaseMutex MOZ_UNANNOTATED;
+
+ public:
+  QuotaClient();
+
+  static QuotaClient* GetInstance() {
+    AssertIsOnBackgroundThread();
+
+    return sInstance;
+  }
+
+  mozilla::Mutex& ShadowDatabaseMutex() {
+    MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+
+    return mShadowDatabaseMutex;
+  }
+
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(mozilla::dom::QuotaClient, override)
+
+  Type GetType() override;
+
+  Result<UsageInfo, nsresult> InitOrigin(PersistenceType aPersistenceType,
+                                         const OriginMetadata& aOriginMetadata,
+                                         const AtomicBool& aCanceled) override;
+
+  nsresult InitOriginWithoutTracking(PersistenceType aPersistenceType,
+                                     const OriginMetadata& aOriginMetadata,
+                                     const AtomicBool& aCanceled) override;
+
+  Result<UsageInfo, nsresult> GetUsageForOrigin(
+      PersistenceType aPersistenceType, const OriginMetadata& aOriginMetadata,
+      const AtomicBool& aCanceled) override;
+
+  nsresult AboutToClearOrigins(
+      const Nullable<PersistenceType>& aPersistenceType,
+      const OriginScope& aOriginScope) override;
+
+  void OnOriginClearCompleted(PersistenceType aPersistenceType,
+                              const nsACString& aOrigin) override;
+
+  void OnRepositoryClearCompleted(PersistenceType aPersistenceType) override;
+
+  void ReleaseIOThreadObjects() override;
+
+  void AbortOperationsForLocks(
+      const DirectoryLockIdTable& aDirectoryLockIds) override;
+
+  void AbortOperationsForProcess(ContentParentId aContentParentId) override;
+
+  void AbortAllOperations() override;
+
+  void StartIdleMaintenance() override;
+
+  void StopIdleMaintenance() override;
+
+ private:
+  ~QuotaClient() override;
+
+  void InitiateShutdown() override;
+  bool IsShutdownCompleted() const override;
+  nsCString GetShutdownStatus() const override;
+  void ForceKillActors() override;
+  void FinalizeShutdown() override;
+
+  Result<UniquePtr<ArchivedOriginScope>, nsresult> CreateArchivedOriginScope(
+      const OriginScope& aOriginScope);
+
+  nsresult PerformDelete(mozIStorageConnection* aConnection,
+                         const nsACString& aSchemaName,
+                         ArchivedOriginScope* aArchivedOriginScope) const;
+};
+
+class QuotaClient::MatchFunction final : public mozIStorageFunction {
+  OriginAttributesPattern mPattern;
+
+ public:
+  explicit MatchFunction(const OriginAttributesPattern& aPattern)
+      : mPattern(aPattern) {}
+
+ private:
+  ~MatchFunction() = default;
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_MOZISTORAGEFUNCTION
+};
+
+/*******************************************************************************
+ * Helper classes
+ ******************************************************************************/
+
+class MOZ_STACK_CLASS AutoWriteTransaction final {
+  Connection* mConnection;
+  Maybe<MutexAutoLock> mShadowDatabaseLock;
+  bool mShadowWrites;
+
+ public:
+  explicit AutoWriteTransaction(bool aShadowWrites);
+
+  ~AutoWriteTransaction();
+
+  nsresult Start(Connection* aConnection);
+
+  nsresult Commit();
+
+ private:
+  nsresult LockAndAttachShadowDatabase(Connection* aConnection);
+
+  nsresult DetachShadowDatabaseAndUnlock();
+};
+
+/*******************************************************************************
+ * Globals
+ ******************************************************************************/
+
+#ifdef DEBUG
+bool gLocalStorageInitialized = false;
+#endif
+
+using PrepareDatastoreOpArray =
+    nsTArray<NotNull<CheckedUnsafePtr<PrepareDatastoreOp>>>;
+
+StaticAutoPtr<PrepareDatastoreOpArray> gPrepareDatastoreOps;
+
+// nsCStringHashKey with disabled memmove
+class nsCStringHashKeyDM : public nsCStringHashKey {
+ public:
+  explicit nsCStringHashKeyDM(const nsCStringHashKey::KeyTypePointer aKey)
+      : nsCStringHashKey(aKey) {}
+  enum { ALLOW_MEMMOVE = false };
+};
+
+// When CheckedUnsafePtr's checking is enabled, it's necessary to ensure that
+// the hashtable uses the copy constructor instead of memmove for moving entries
+// since memmove will break CheckedUnsafePtr in a memory-corrupting way.
+using DatastoreHashKey =
+    std::conditional<DiagnosticAssertEnabled::value, nsCStringHashKeyDM,
+                     nsCStringHashKey>::type;
+
+using DatastoreHashtable =
+    nsBaseHashtable<DatastoreHashKey, NotNull<CheckedUnsafePtr<Datastore>>,
+                    MovingNotNull<CheckedUnsafePtr<Datastore>>>;
+
+StaticAutoPtr<DatastoreHashtable> gDatastores;
+
+uint64_t gLastDatastoreId = 0;
+
+using PreparedDatastoreHashtable =
+    nsClassHashtable<nsUint64HashKey, PreparedDatastore>;
+
+StaticAutoPtr<PreparedDatastoreHashtable> gPreparedDatastores;
+
+using PrivateDatastoreHashtable =
+    nsClassHashtable<nsCStringHashKey, PrivateDatastore>;
+
+// Keeps Private Browsing Datastores alive until the private browsing session
+// is closed. This is necessary because LocalStorage Private Browsing data is
+// (currently) not written to disk and therefore needs to explicitly be kept
+// alive in memory so that if a user browses away from a site during a session
+// and then back to it that they will still have their data.
+//
+// The entries are wrapped by PrivateDatastore instances which call
+// NoteLivePrivateDatastore and NoteFinishedPrivateDatastore which set and
+// clear mHasLivePrivateDatastore which inhibits MaybeClose() from closing the
+// datastore (which would discard the data) when there are no active windows
+// using LocalStorage for the origin.
+//
+// The table is cleared when the Private Browsing session is closed, which will
+// cause NoteFinishedPrivateDatastore to be called on each Datastore which will
+// in turn call MaybeClose which should then discard the Datastore. Or in the
+// event of an (unlikely) race where the private browsing windows are still
+// being torn down, will cause the Datastore to be discarded when the last
+// window actually goes away.
+UniquePtr<PrivateDatastoreHashtable> gPrivateDatastores;
+
+using LiveDatabaseArray = nsTArray<NotNull<CheckedUnsafePtr<Database>>>;
+
+StaticAutoPtr<LiveDatabaseArray> gLiveDatabases;
+
+StaticRefPtr<ConnectionThread> gConnectionThread;
+
+uint64_t gLastObserverId = 0;
+
+using PreparedObserverHashtable = nsRefPtrHashtable<nsUint64HashKey, Observer>;
+
+StaticAutoPtr<PreparedObserverHashtable> gPreparedObsevers;
+
+using ObserverHashtable =
+    nsClassHashtable<nsCStringHashKey, nsTArray<NotNull<Observer*>>>;
+
+StaticAutoPtr<ObserverHashtable> gObservers;
+
+Atomic<bool> gShadowWrites(kDefaultShadowWrites);
+Atomic<int32_t, Relaxed> gSnapshotPrefill(kDefaultSnapshotPrefill);
+Atomic<int32_t, Relaxed> gSnapshotGradualPrefill(
+    kDefaultSnapshotGradualPrefill);
+Atomic<bool> gClientValidation(kDefaultClientValidation);
+
+using UsageHashtable = nsTHashMap<nsCStringHashKey, int64_t>;
+
+StaticAutoPtr<ArchivedOriginHashtable> gArchivedOrigins;
+
+// Can only be touched on the Quota Manager I/O thread.
+bool gInitializedShadowStorage = false;
+
+StaticAutoPtr<LSInitializationInfo> gInitializationInfo;
+
+bool IsOnGlobalConnectionThread() {
+  MOZ_ASSERT(gConnectionThread);
+  return gConnectionThread->IsOnConnectionThread();
+}
+
+void AssertIsOnGlobalConnectionThread() {
+  MOZ_ASSERT(gConnectionThread);
+  gConnectionThread->AssertIsOnConnectionThread();
+}
+
+already_AddRefed<Datastore> GetDatastore(const nsACString& aOrigin) {
+  AssertIsOnBackgroundThread();
+
+  if (gDatastores) {
+    auto maybeDatastore = gDatastores->MaybeGet(aOrigin);
+    if (maybeDatastore) {
+      RefPtr<Datastore> result(std::move(*maybeDatastore).unwrapBasePtr());
+      return result.forget();
+    }
+  }
+
+  return nullptr;
+}
+
+nsresult LoadArchivedOrigins() {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(!gArchivedOrigins);
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  // Ensure that the webappsstore.sqlite is moved to new place.
+  QM_TRY(MOZ_TO_RESULT(quotaManager->EnsureStorageIsInitialized()));
+
+  QM_TRY_INSPECT(const auto& connection, CreateArchiveStorageConnection(
+                                             quotaManager->GetStoragePath()));
+
+  if (!connection) {
+    gArchivedOrigins = new ArchivedOriginHashtable();
+    return NS_OK;
+  }
+
+  QM_TRY_INSPECT(
+      const auto& stmt,
+      MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+          nsCOMPtr<mozIStorageStatement>, connection, CreateStatement,
+          "SELECT DISTINCT originAttributes, originKey "
+          "FROM webappsstore2;"_ns));
+
+  auto archivedOrigins = MakeUnique<ArchivedOriginHashtable>();
+
+  // XXX Actually, this could use a hashtable variant of
+  // CollectElementsWhileHasResult
+  QM_TRY(quota::CollectWhileHasResult(
+      *stmt, [&archivedOrigins](auto& stmt) -> Result<Ok, nsresult> {
+        QM_TRY_INSPECT(const auto& originSuffix,
+                       MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(nsCString, stmt,
+                                                         GetUTF8String, 0));
+        QM_TRY_INSPECT(const auto& originNoSuffix,
+                       MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(nsCString, stmt,
+                                                         GetUTF8String, 1));
+
+        const nsCString hashKey =
+            GetArchivedOriginHashKey(originSuffix, originNoSuffix);
+
+        OriginAttributes originAttributes;
+        QM_TRY(OkIf(originAttributes.PopulateFromSuffix(originSuffix)),
+               Err(NS_ERROR_FAILURE));
+
+        archivedOrigins->InsertOrUpdate(
+            hashKey,
+            MakeUnique<ArchivedOriginInfo>(originAttributes, originNoSuffix));
+
+        return Ok{};
+      }));
+
+  gArchivedOrigins = archivedOrigins.release();
+  return NS_OK;
+}
+
+Result<int64_t, nsresult> GetUsage(mozIStorageConnection& aConnection,
+                                   ArchivedOriginScope* aArchivedOriginScope) {
+  AssertIsOnIOThread();
+
+  QM_TRY_INSPECT(
+      const auto& stmt,
+      ([aArchivedOriginScope,
+        &aConnection]() -> Result<nsCOMPtr<mozIStorageStatement>, nsresult> {
+        if (aArchivedOriginScope) {
+          QM_TRY_RETURN(CreateAndExecuteSingleStepStatement<
+                        SingleStepResult::ReturnNullIfNoResult>(
+              aConnection,
+              "SELECT "
+              "total(utf16Length(key) + utf16Length(value)) "
+              "FROM webappsstore2 "
+              "WHERE originKey = :originKey "
+              "AND originAttributes = :originAttributes;"_ns,
+              [aArchivedOriginScope](auto& stmt) -> Result<Ok, nsresult> {
+                QM_TRY(MOZ_TO_RESULT(
+                    aArchivedOriginScope->BindToStatement(&stmt)));
+                return Ok{};
+              }));
+        }
+
+        QM_TRY_RETURN(CreateAndExecuteSingleStepStatement<
+                      SingleStepResult::ReturnNullIfNoResult>(
+            aConnection, "SELECT usage FROM database"_ns));
+      }()));
+
+  QM_TRY(OkIf(stmt), Err(NS_ERROR_FAILURE));
+
+  QM_TRY_RETURN(MOZ_TO_RESULT_INVOKE_MEMBER(stmt, GetInt64, 0));
+}
+
+void ShadowWritesPrefChangedCallback(const char* aPrefName, void* aClosure) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!strcmp(aPrefName, kShadowWritesPref));
+  MOZ_ASSERT(!aClosure);
+
+  gShadowWrites = Preferences::GetBool(aPrefName, kDefaultShadowWrites);
+}
+
+void SnapshotPrefillPrefChangedCallback(const char* aPrefName, void* aClosure) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!strcmp(aPrefName, kSnapshotPrefillPref));
+  MOZ_ASSERT(!aClosure);
+
+  int32_t snapshotPrefill =
+      Preferences::GetInt(aPrefName, kDefaultSnapshotPrefill);
+
+  // The magic -1 is for use only by tests.
+  if (snapshotPrefill == -1) {
+    snapshotPrefill = INT32_MAX;
+  }
+
+  gSnapshotPrefill = snapshotPrefill;
+}
+
+void SnapshotGradualPrefillPrefChangedCallback(const char* aPrefName,
+                                               void* aClosure) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!strcmp(aPrefName, kSnapshotGradualPrefillPref));
+  MOZ_ASSERT(!aClosure);
+
+  int32_t snapshotGradualPrefill =
+      Preferences::GetInt(aPrefName, kDefaultSnapshotGradualPrefill);
+
+  // The magic -1 is for use only by tests.
+  if (snapshotGradualPrefill == -1) {
+    snapshotGradualPrefill = INT32_MAX;
+  }
+
+  gSnapshotGradualPrefill = snapshotGradualPrefill;
+}
+
+int64_t GetSnapshotPeakUsagePreincrement(bool aInitial) {
+  return aInitial ? StaticPrefs::
+                        dom_storage_snapshot_peak_usage_initial_preincrement()
+                  : StaticPrefs::
+                        dom_storage_snapshot_peak_usage_gradual_preincrement();
+}
+
+int64_t GetSnapshotPeakUsageReducedPreincrement(bool aInitial) {
+  return aInitial
+             ? StaticPrefs::
+                   dom_storage_snapshot_peak_usage_reduced_initial_preincrement()
+             : StaticPrefs::
+                   dom_storage_snapshot_peak_usage_reduced_gradual_preincrement();
+}
+
+void ClientValidationPrefChangedCallback(const char* aPrefName,
+                                         void* aClosure) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!strcmp(aPrefName, kClientValidationPref));
+  MOZ_ASSERT(!aClosure);
+
+  gClientValidation = Preferences::GetBool(aPrefName, kDefaultClientValidation);
+}
+
+template <typename Condition>
+void InvalidatePrepareDatastoreOpsMatching(const Condition& aCondition) {
+  if (!gPrepareDatastoreOps) {
+    return;
+  }
+
+  for (const auto& prepareDatastoreOp : *gPrepareDatastoreOps) {
+    if (aCondition(*prepareDatastoreOp)) {
+      prepareDatastoreOp->Invalidate();
+    }
+  }
+}
+
+template <typename Condition>
+void InvalidatePreparedDatastoresMatching(const Condition& aCondition) {
+  if (!gPreparedDatastores) {
+    return;
+  }
+
+  for (const auto& preparedDatastore : gPreparedDatastores->Values()) {
+    MOZ_ASSERT(preparedDatastore);
+
+    if (aCondition(*preparedDatastore)) {
+      preparedDatastore->Invalidate();
+    }
+  }
+}
+
+template <typename Condition>
+nsTArray<RefPtr<Database>> CollectDatabasesMatching(Condition aCondition) {
+  AssertIsOnBackgroundThread();
+
+  if (!gLiveDatabases) {
+    return nsTArray<RefPtr<Database>>{};
+  }
+
+  nsTArray<RefPtr<Database>> databases;
+
+  for (const auto& database : *gLiveDatabases) {
+    if (aCondition(*database)) {
+      databases.AppendElement(database.get());
+    }
+  }
+
+  return databases;
+}
+
+template <typename Condition>
+void RequestAllowToCloseDatabasesMatching(Condition aCondition) {
+  AssertIsOnBackgroundThread();
+
+  nsTArray<RefPtr<Database>> databases = CollectDatabasesMatching(aCondition);
+
+  for (const auto& database : databases) {
+    MOZ_ASSERT(database);
+
+    database->RequestAllowToClose();
+  }
+}
+
+void ForceKillAllDatabases() {
+  AssertIsOnBackgroundThread();
+
+  nsTArray<RefPtr<Database>> databases =
+      CollectDatabasesMatching([](const auto&) { return true; });
+
+  for (const auto& database : databases) {
+    MOZ_ASSERT(database);
+
+    database->ForceKill();
+  }
+}
+
+bool VerifyPrincipalInfo(const PrincipalInfo& aPrincipalInfo,
+                         const PrincipalInfo& aStoragePrincipalInfo,
+                         bool aCheckClientPrincipal) {
+  AssertIsOnBackgroundThread();
+
+  if (NS_WARN_IF(!QuotaManager::IsPrincipalInfoValid(aPrincipalInfo))) {
+    return false;
+  }
+
+  // Note that the client prinicpal could have a different spec than the node
+  // principal but they should have the same origin. It's because the client
+  // could be initialized when opening the initial about:blank document and pass
+  // to the newly opened window and reuse over there if the new window has the
+  // same origin as the initial about:blank document. But, the FilePath could be
+  // different. Therefore, we have to ignore comparing the Spec of the
+  // principals if we are verifying clinet principal here. Also, when
+  // document.domain is set, client principal won't get it. So, we don't compare
+  // domain for client princpal too.
+  bool result = aCheckClientPrincipal
+                    ? StoragePrincipalHelper::
+                          VerifyValidClientPrincipalInfoForPrincipalInfo(
+                              aStoragePrincipalInfo, aPrincipalInfo)
+                    : StoragePrincipalHelper::
+                          VerifyValidStoragePrincipalInfoForPrincipalInfo(
+                              aStoragePrincipalInfo, aPrincipalInfo);
+  if (NS_WARN_IF(!result)) {
+    return false;
+  }
+
+  return true;
+}
+
+bool VerifyClientId(const Maybe<ContentParentId>& aContentParentId,
+                    const Maybe<PrincipalInfo>& aPrincipalInfo,
+                    const Maybe<nsID>& aClientId) {
+  AssertIsOnBackgroundThread();
+
+  if (gClientValidation) {
+    if (NS_WARN_IF(aClientId.isNothing())) {
+      return false;
+    }
+
+    if (NS_WARN_IF(aPrincipalInfo.isNothing())) {
+      return false;
+    }
+
+    RefPtr<ClientManagerService> svc = ClientManagerService::GetInstance();
+    if (svc && NS_WARN_IF(!svc->HasWindow(
+                   aContentParentId, aPrincipalInfo.ref(), aClientId.ref()))) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+bool VerifyOriginKey(const nsACString& aOriginKey,
+                     const PrincipalInfo& aPrincipalInfo) {
+  AssertIsOnBackgroundThread();
+
+  QM_TRY_INSPECT((const auto& [originAttrSuffix, originKey]),
+                 GenerateOriginKey2(aPrincipalInfo), false);
+
+  Unused << originAttrSuffix;
+
+  QM_TRY(OkIf(originKey == aOriginKey), false,
+         ([&originKey = originKey, &aOriginKey](const auto) {
+           LS_WARNING("originKey (%s) doesn't match passed one (%s)!",
+                      originKey.get(), nsCString(aOriginKey).get());
+         }));
+
+  return true;
+}
+
+LSInitializationInfo& MutableInitializationInfoRef(const CreateIfNonExistent&) {
+  if (!gInitializationInfo) {
+    gInitializationInfo = new LSInitializationInfo();
+  }
+  return *gInitializationInfo;
+}
+
+template <typename Func>
+auto ExecuteOriginInitialization(const nsACString& aOrigin,
+                                 const LSOriginInitialization aInitialization,
+                                 const nsACString& aContext, Func&& aFunc)
+    -> std::invoke_result_t<Func, const FirstInitializationAttempt<
+                                      LSOriginInitialization, Nothing>&> {
+  return ExecuteInitialization(
+      MutableInitializationInfoRef(CreateIfNonExistent{})
+          .MutableOriginInitializationInfoRef(aOrigin, CreateIfNonExistent{}),
+      aInitialization, aContext, std::forward<Func>(aFunc));
+}
+
+}  // namespace
+
+/*******************************************************************************
+ * Exported functions
+ ******************************************************************************/
+
+void InitializeLocalStorage() {
+  MOZ_ASSERT(XRE_IsParentProcess());
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!gLocalStorageInitialized);
+
+  // XXX Isn't this redundant? It's already done in InitializeQuotaManager.
+  if (!QuotaManager::IsRunningGTests()) {
+    // This service has to be started on the main thread currently.
+    const nsCOMPtr<mozIStorageService> ss =
+        do_GetService(MOZ_STORAGE_SERVICE_CONTRACTID);
+
+    QM_WARNONLY_TRY(OkIf(ss));
+  }
+
+  Preferences::RegisterCallbackAndCall(ShadowWritesPrefChangedCallback,
+                                       kShadowWritesPref);
+
+  Preferences::RegisterCallbackAndCall(SnapshotPrefillPrefChangedCallback,
+                                       kSnapshotPrefillPref);
+
+  Preferences::RegisterCallbackAndCall(
+      SnapshotGradualPrefillPrefChangedCallback, kSnapshotGradualPrefillPref);
+
+  Preferences::RegisterCallbackAndCall(ClientValidationPrefChangedCallback,
+                                       kClientValidationPref);
+
+#ifdef DEBUG
+  gLocalStorageInitialized = true;
+#endif
+}
+
+PBackgroundLSDatabaseParent* AllocPBackgroundLSDatabaseParent(
+    const PrincipalInfo& aPrincipalInfo, const uint32_t& aPrivateBrowsingId,
+    const uint64_t& aDatastoreId) {
+  AssertIsOnBackgroundThread();
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread())) {
+    return nullptr;
+  }
+
+  if (NS_WARN_IF(!gPreparedDatastores)) {
+    MOZ_ASSERT_UNLESS_FUZZING(false);
+    return nullptr;
+  }
+
+  PreparedDatastore* preparedDatastore = gPreparedDatastores->Get(aDatastoreId);
+  if (NS_WARN_IF(!preparedDatastore)) {
+    MOZ_ASSERT_UNLESS_FUZZING(false);
+    return nullptr;
+  }
+
+  // If we ever decide to return null from this point on, we need to make sure
+  // that the datastore is closed and the prepared datastore is removed from the
+  // gPreparedDatastores hashtable.
+  // We also assume that IPDL must call RecvPBackgroundLSDatabaseConstructor
+  // once we return a valid actor in this method.
+
+  RefPtr<Database> database =
+      new Database(aPrincipalInfo, preparedDatastore->GetContentParentId(),
+                   preparedDatastore->Origin(), aPrivateBrowsingId);
+
+  // Transfer ownership to IPDL.
+  return database.forget().take();
+}
+
+bool RecvPBackgroundLSDatabaseConstructor(PBackgroundLSDatabaseParent* aActor,
+                                          const PrincipalInfo& aPrincipalInfo,
+                                          const uint32_t& aPrivateBrowsingId,
+                                          const uint64_t& aDatastoreId) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aActor);
+  MOZ_ASSERT(gPreparedDatastores);
+  MOZ_ASSERT(gPreparedDatastores->Get(aDatastoreId));
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+
+  // The actor is now completely built (it has a manager, channel and it's
+  // registered as a subprotocol).
+  // ActorDestroy will be called if we fail here.
+
+  mozilla::UniquePtr<PreparedDatastore> preparedDatastore;
+  gPreparedDatastores->Remove(aDatastoreId, &preparedDatastore);
+  MOZ_ASSERT(preparedDatastore);
+
+  auto* database = static_cast<Database*>(aActor);
+
+  database->SetActorAlive(&preparedDatastore->MutableDatastoreRef());
+
+  // It's possible that AbortOperationsForLocks was called before the database
+  // actor was created and became live. Let the child know that the database is
+  // no longer valid.
+  if (preparedDatastore->IsInvalidated()) {
+    database->RequestAllowToClose();
+  }
+
+  return true;
+}
+
+bool DeallocPBackgroundLSDatabaseParent(PBackgroundLSDatabaseParent* aActor) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aActor);
+
+  // Transfer ownership back from IPDL.
+  RefPtr<Database> actor = dont_AddRef(static_cast<Database*>(aActor));
+
+  return true;
+}
+
+PBackgroundLSObserverParent* AllocPBackgroundLSObserverParent(
+    const uint64_t& aObserverId) {
+  AssertIsOnBackgroundThread();
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread())) {
+    return nullptr;
+  }
+
+  if (NS_WARN_IF(!gPreparedObsevers)) {
+    MOZ_ASSERT_UNLESS_FUZZING(false);
+    return nullptr;
+  }
+
+  RefPtr<Observer> observer = gPreparedObsevers->Get(aObserverId);
+  if (NS_WARN_IF(!observer)) {
+    MOZ_ASSERT_UNLESS_FUZZING(false);
+    return nullptr;
+  }
+
+  // observer->SetObject(this);
+
+  // Transfer ownership to IPDL.
+  return observer.forget().take();
+}
+
+bool RecvPBackgroundLSObserverConstructor(PBackgroundLSObserverParent* aActor,
+                                          const uint64_t& aObserverId) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aActor);
+  MOZ_ASSERT(gPreparedObsevers);
+  MOZ_ASSERT(gPreparedObsevers->GetWeak(aObserverId));
+
+  RefPtr<Observer> observer;
+  gPreparedObsevers->Remove(aObserverId, observer.StartAssignment());
+
+  if (!gPreparedObsevers->Count()) {
+    gPreparedObsevers = nullptr;
+  }
+
+  if (!gObservers) {
+    gObservers = new ObserverHashtable();
+  }
+
+  const auto notNullObserver = WrapNotNull(observer.get());
+
+  nsTArray<NotNull<Observer*>>* const array =
+      gObservers->GetOrInsertNew(notNullObserver->Origin());
+  array->AppendElement(notNullObserver);
+
+  if (RefPtr<Datastore> datastore = GetDatastore(observer->Origin())) {
+    datastore->NoteChangedObserverArray(*array);
+  }
+
+  return true;
+}
+
+bool DeallocPBackgroundLSObserverParent(PBackgroundLSObserverParent* aActor) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aActor);
+
+  // Transfer ownership back from IPDL.
+  RefPtr<Observer> actor = dont_AddRef(static_cast<Observer*>(aActor));
+
+  return true;
+}
+
+PBackgroundLSRequestParent* AllocPBackgroundLSRequestParent(
+    PBackgroundParent* aBackgroundActor, const LSRequestParams& aParams) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aParams.type() != LSRequestParams::T__None);
+
+  if (NS_WARN_IF(!NextGenLocalStorageEnabled())) {
+    return nullptr;
+  }
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread())) {
+    return nullptr;
+  }
+
+  Maybe<ContentParentId> contentParentId;
+
+  uint64_t childID = BackgroundParent::GetChildID(aBackgroundActor);
+  if (childID) {
+    contentParentId = Some(ContentParentId(childID));
+  }
+
+  RefPtr<LSRequestBase> actor;
+
+  switch (aParams.type()) {
+    case LSRequestParams::TLSRequestPreloadDatastoreParams:
+    case LSRequestParams::TLSRequestPrepareDatastoreParams: {
+      RefPtr<PrepareDatastoreOp> prepareDatastoreOp =
+          new PrepareDatastoreOp(aParams, contentParentId);
+
+      if (!gPrepareDatastoreOps) {
+        gPrepareDatastoreOps = new PrepareDatastoreOpArray();
+      }
+      gPrepareDatastoreOps->AppendElement(
+          WrapNotNullUnchecked(prepareDatastoreOp.get()));
+
+      actor = std::move(prepareDatastoreOp);
+
+      break;
+    }
+
+    case LSRequestParams::TLSRequestPrepareObserverParams: {
+      RefPtr<PrepareObserverOp> prepareObserverOp =
+          new PrepareObserverOp(aParams, contentParentId);
+
+      actor = std::move(prepareObserverOp);
+
+      break;
+    }
+
+    default:
+      MOZ_CRASH("Should never get here!");
+  }
+
+  // Transfer ownership to IPDL.
+  return actor.forget().take();
+}
+
+bool RecvPBackgroundLSRequestConstructor(PBackgroundLSRequestParent* aActor,
+                                         const LSRequestParams& aParams) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aActor);
+  MOZ_ASSERT(aParams.type() != LSRequestParams::T__None);
+  MOZ_ASSERT(NextGenLocalStorageEnabled());
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+
+  // The actor is now completely built.
+
+  auto* op = static_cast<LSRequestBase*>(aActor);
+
+  op->Dispatch();
+
+  return true;
+}
+
+bool DeallocPBackgroundLSRequestParent(PBackgroundLSRequestParent* aActor) {
+  AssertIsOnBackgroundThread();
+
+  // Transfer ownership back from IPDL.
+  RefPtr<LSRequestBase> actor =
+      dont_AddRef(static_cast<LSRequestBase*>(aActor));
+
+  return true;
+}
+
+PBackgroundLSSimpleRequestParent* AllocPBackgroundLSSimpleRequestParent(
+    PBackgroundParent* aBackgroundActor, const LSSimpleRequestParams& aParams) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aParams.type() != LSSimpleRequestParams::T__None);
+
+  if (NS_WARN_IF(!NextGenLocalStorageEnabled())) {
+    return nullptr;
+  }
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread())) {
+    return nullptr;
+  }
+
+  Maybe<ContentParentId> contentParentId;
+
+  uint64_t childID = BackgroundParent::GetChildID(aBackgroundActor);
+  if (childID) {
+    contentParentId = Some(ContentParentId(childID));
+  }
+
+  RefPtr<LSSimpleRequestBase> actor;
+
+  switch (aParams.type()) {
+    case LSSimpleRequestParams::TLSSimpleRequestPreloadedParams: {
+      RefPtr<PreloadedOp> preloadedOp =
+          new PreloadedOp(aParams, contentParentId);
+
+      actor = std::move(preloadedOp);
+
+      break;
+    }
+
+    case LSSimpleRequestParams::TLSSimpleRequestGetStateParams: {
+      RefPtr<GetStateOp> getStateOp = new GetStateOp(aParams, contentParentId);
+
+      actor = std::move(getStateOp);
+
+      break;
+    }
+
+    default:
+      MOZ_CRASH("Should never get here!");
+  }
+
+  // Transfer ownership to IPDL.
+  return actor.forget().take();
+}
+
+bool RecvPBackgroundLSSimpleRequestConstructor(
+    PBackgroundLSSimpleRequestParent* aActor,
+    const LSSimpleRequestParams& aParams) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aActor);
+  MOZ_ASSERT(aParams.type() != LSSimpleRequestParams::T__None);
+  MOZ_ASSERT(NextGenLocalStorageEnabled());
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+
+  // The actor is now completely built.
+
+  auto* op = static_cast<LSSimpleRequestBase*>(aActor);
+
+  op->Dispatch();
+
+  return true;
+}
+
+bool DeallocPBackgroundLSSimpleRequestParent(
+    PBackgroundLSSimpleRequestParent* aActor) {
+  AssertIsOnBackgroundThread();
+
+  // Transfer ownership back from IPDL.
+  RefPtr<LSSimpleRequestBase> actor =
+      dont_AddRef(static_cast<LSSimpleRequestBase*>(aActor));
+
+  return true;
+}
+
+namespace localstorage {
+
+already_AddRefed<mozilla::dom::quota::Client> CreateQuotaClient() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(CachedNextGenLocalStorageEnabled());
+
+  RefPtr<QuotaClient> client = new QuotaClient();
+  return client.forget();
+}
+
+}  // namespace localstorage
+
+/*******************************************************************************
+ * DatastoreWriteOptimizer
+ ******************************************************************************/
+
+void DatastoreWriteOptimizer::ApplyAndReset(
+    nsTArray<LSItemInfo>& aOrderedItems) {
+  AssertIsOnOwningThread();
+
+  // The mWriteInfos hash table contains all write infos, but it keeps them in
+  // an arbitrary order, which means write infos need to be sorted before being
+  // processed. However, the order is not important for deletions and normal
+  // updates. Usually, filtering out deletions and updates would require extra
+  // work, but we have to check the hash table for each ordered item anyway, so
+  // we can remove the write info if it is a deletion or update without adding
+  // extra overhead. In the end, only insertions need to be sorted before being
+  // processed.
+
+  if (mTruncateInfo) {
+    aOrderedItems.Clear();
+    mTruncateInfo = nullptr;
+  }
+
+  for (int32_t index = aOrderedItems.Length() - 1; index >= 0; index--) {
+    LSItemInfo& item = aOrderedItems[index];
+
+    if (auto entry = mWriteInfos.Lookup(item.key())) {
+      WriteInfo* writeInfo = entry->get();
+
+      switch (writeInfo->GetType()) {
+        case WriteInfo::DeleteItem:
+          aOrderedItems.RemoveElementAt(index);
+          entry.Remove();
+          break;
+
+        case WriteInfo::UpdateItem: {
+          auto updateItemInfo = static_cast<UpdateItemInfo*>(writeInfo);
+          if (updateItemInfo->UpdateWithMove()) {
+            // See the comment in LSWriteOptimizer::InsertItem for more details
+            // about the UpdateWithMove flag.
+
+            aOrderedItems.RemoveElementAt(index);
+            entry.Data() = MakeUnique<InsertItemInfo>(
+                updateItemInfo->SerialNumber(), updateItemInfo->GetKey(),
+                updateItemInfo->GetValue());
+          } else {
+            item.value() = updateItemInfo->GetValue();
+            entry.Remove();
+          }
+          break;
+        }
+
+        case WriteInfo::InsertItem:
+          break;
+
+        default:
+          MOZ_CRASH("Bad type!");
+      }
+    }
+  }
+
+  nsTArray<NotNull<WriteInfo*>> writeInfos;
+  GetSortedWriteInfos(writeInfos);
+
+  for (WriteInfo* writeInfo : writeInfos) {
+    MOZ_ASSERT(writeInfo->GetType() == WriteInfo::InsertItem);
+
+    auto insertItemInfo = static_cast<InsertItemInfo*>(writeInfo);
+
+    LSItemInfo* itemInfo = aOrderedItems.AppendElement();
+    itemInfo->key() = insertItemInfo->GetKey();
+    itemInfo->value() = insertItemInfo->GetValue();
+  }
+
+  mWriteInfos.Clear();
+}
+
+/*******************************************************************************
+ * ConnectionWriteOptimizer
+ ******************************************************************************/
+
+Result<int64_t, nsresult> ConnectionWriteOptimizer::Perform(
+    Connection* aConnection, bool aShadowWrites) {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(aConnection);
+
+  // The order of elements is not stored in the database, so write infos don't
+  // need to be sorted before being processed.
+
+  if (mTruncateInfo) {
+    QM_TRY(MOZ_TO_RESULT(PerformTruncate(aConnection, aShadowWrites)));
+  }
+
+  for (const auto& entry : mWriteInfos) {
+    const WriteInfo* const writeInfo = entry.GetWeak();
+
+    switch (writeInfo->GetType()) {
+      case WriteInfo::InsertItem:
+      case WriteInfo::UpdateItem: {
+        const auto* const insertItemInfo =
+            static_cast<const InsertItemInfo*>(writeInfo);
+
+        QM_TRY(MOZ_TO_RESULT(PerformInsertOrUpdate(
+            aConnection, aShadowWrites, insertItemInfo->GetKey(),
+            insertItemInfo->GetValue())));
+
+        break;
+      }
+
+      case WriteInfo::DeleteItem: {
+        const auto* const deleteItemInfo =
+            static_cast<const DeleteItemInfo*>(writeInfo);
+
+        QM_TRY(MOZ_TO_RESULT(PerformDelete(aConnection, aShadowWrites,
+                                           deleteItemInfo->GetKey())));
+
+        break;
+      }
+
+      default:
+        MOZ_CRASH("Bad type!");
+    }
+  }
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteCachedStatement(
+      "UPDATE database "
+      "SET usage = usage + :delta"_ns,
+      [this](auto& stmt) -> Result<Ok, nsresult> {
+        QM_TRY(MOZ_TO_RESULT(stmt.BindInt64ByName("delta"_ns, mTotalDelta)));
+
+        return Ok{};
+      })));
+
+  QM_TRY_INSPECT(const auto& stmt, CreateAndExecuteSingleStepStatement<
+                                       SingleStepResult::ReturnNullIfNoResult>(
+                                       aConnection->MutableStorageConnection(),
+                                       "SELECT usage FROM database"_ns));
+
+  QM_TRY(OkIf(stmt), Err(NS_ERROR_FAILURE));
+
+  QM_TRY_RETURN(MOZ_TO_RESULT_INVOKE_MEMBER(*stmt, GetInt64, 0));
+}
+
+nsresult ConnectionWriteOptimizer::PerformInsertOrUpdate(
+    Connection* aConnection, bool aShadowWrites, const nsAString& aKey,
+    const LSValue& aValue) {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteCachedStatement(
+      "INSERT OR REPLACE INTO data (key, utf16_length, conversion_type, "
+      "compression_type, value) "
+      "VALUES(:key, :utf16_length, :conversion_type, :compression_type, :value)"_ns,
+      [&aKey, &aValue](auto& stmt) -> Result<Ok, nsresult> {
+        QM_TRY(MOZ_TO_RESULT(stmt.BindStringByName("key"_ns, aKey)));
+        QM_TRY(MOZ_TO_RESULT(
+            stmt.BindInt32ByName("utf16_length"_ns, aValue.UTF16Length())));
+        QM_TRY(MOZ_TO_RESULT(stmt.BindInt32ByName(
+            "conversion_type"_ns,
+            static_cast<int32_t>(aValue.GetConversionType()))));
+        QM_TRY(MOZ_TO_RESULT(stmt.BindInt32ByName(
+            "compression_type"_ns,
+            static_cast<int32_t>(aValue.GetCompressionType()))));
+
+        if (0u == aValue.Length()) {  // Otherwise empty string becomes null
+          QM_TRY(MOZ_TO_RESULT(
+              stmt.BindUTF8StringByName("value"_ns, aValue.AsCString())));
+        } else {
+          QM_TRY(MOZ_TO_RESULT(
+              stmt.BindUTF8StringAsBlobByName("value"_ns, aValue.AsCString())));
+        }
+
+        return Ok{};
+      })));
+
+  if (!aShadowWrites) {
+    return NS_OK;
+  }
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteCachedStatement(
+      "INSERT OR REPLACE INTO shadow.webappsstore2 "
+      "(originAttributes, originKey, scope, key, value) "
+      "VALUES (:originAttributes, :originKey, :scope, :key, :value) "_ns,
+      [&aConnection, &aKey, &aValue](auto& stmt) -> Result<Ok, nsresult> {
+        using ConversionType = LSValue::ConversionType;
+        using CompressionType = LSValue::CompressionType;
+
+        const ArchivedOriginScope* const archivedOriginScope =
+            aConnection->GetArchivedOriginScope();
+
+        QM_TRY(MOZ_TO_RESULT(archivedOriginScope->BindToStatement(&stmt)));
+
+        QM_TRY(MOZ_TO_RESULT(stmt.BindUTF8StringByName(
+            "scope"_ns, Scheme0Scope(archivedOriginScope->OriginSuffix(),
+                                     archivedOriginScope->OriginNoSuffix()))));
+
+        QM_TRY(MOZ_TO_RESULT(stmt.BindStringByName("key"_ns, aKey)));
+
+        bool isCompressed =
+            CompressionType::UNCOMPRESSED != aValue.GetCompressionType();
+        bool isAlreadyConverted =
+            ConversionType::NONE != aValue.GetConversionType();
+
+        nsCString buffer;
+        const nsCString& valueBlob = aValue.AsCString();
+        if (isCompressed) {
+          QM_TRY(OkIf(SnappyUncompress(valueBlob, buffer)),
+                 Err(NS_ERROR_FAILURE));
+        }
+        const nsCString& value = isCompressed ? buffer : valueBlob;
+
+        // For shadow writes, we undo buffer swap and convert destructively
+        nsCString unconverted;
+        if (!isAlreadyConverted) {
+          nsString converted;
+          QM_TRY(OkIf(PutCStringBytesToString(value, converted)),
+                 Err(NS_ERROR_OUT_OF_MEMORY));
+          QM_TRY(OkIf(CopyUTF16toUTF8(converted, unconverted, fallible)),
+                 Err(NS_ERROR_OUT_OF_MEMORY));  // Corrupt invalid data
+        }
+        const nsCString& untransformed =
+            (!isAlreadyConverted) ? unconverted : value;
+
+        QM_TRY(MOZ_TO_RESULT(
+            stmt.BindUTF8StringByName("value"_ns, untransformed)));
+
+        return Ok{};
+      })));
+
+  return NS_OK;
+}
+
+nsresult ConnectionWriteOptimizer::PerformDelete(Connection* aConnection,
+                                                 bool aShadowWrites,
+                                                 const nsAString& aKey) {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteCachedStatement(
+      "DELETE FROM data "
+      "WHERE key = :key;"_ns,
+      [&aKey](auto& stmt) -> Result<Ok, nsresult> {
+        QM_TRY(MOZ_TO_RESULT(stmt.BindStringByName("key"_ns, aKey)));
+
+        return Ok{};
+      })));
+
+  if (!aShadowWrites) {
+    return NS_OK;
+  }
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteCachedStatement(
+      "DELETE FROM shadow.webappsstore2 "
+      "WHERE originAttributes = :originAttributes "
+      "AND originKey = :originKey "
+      "AND key = :key;"_ns,
+      [&aConnection, &aKey](auto& stmt) -> Result<Ok, nsresult> {
+        QM_TRY(MOZ_TO_RESULT(
+            aConnection->GetArchivedOriginScope()->BindToStatement(&stmt)));
+
+        QM_TRY(MOZ_TO_RESULT(stmt.BindStringByName("key"_ns, aKey)));
+
+        return Ok{};
+      })));
+
+  return NS_OK;
+}
+
+nsresult ConnectionWriteOptimizer::PerformTruncate(Connection* aConnection,
+                                                   bool aShadowWrites) {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY(MOZ_TO_RESULT(
+      aConnection->ExecuteCachedStatement("DELETE FROM data;"_ns)));
+
+  if (!aShadowWrites) {
+    return NS_OK;
+  }
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->ExecuteCachedStatement(
+      "DELETE FROM shadow.webappsstore2 "
+      "WHERE originAttributes = :originAttributes "
+      "AND originKey = :originKey;"_ns,
+      [&aConnection](auto& stmt) -> Result<Ok, nsresult> {
+        QM_TRY(MOZ_TO_RESULT(
+            aConnection->GetArchivedOriginScope()->BindToStatement(&stmt)));
+
+        return Ok{};
+      })));
+
+  return NS_OK;
+}
+
+/*******************************************************************************
+ * DatastoreOperationBase
+ ******************************************************************************/
+
+/*******************************************************************************
+ * ConnectionDatastoreOperationBase
+ ******************************************************************************/
+
+ConnectionDatastoreOperationBase::ConnectionDatastoreOperationBase(
+    Connection* aConnection, bool aEnsureStorageConnection)
+    : mConnection(aConnection),
+      mEnsureStorageConnection(aEnsureStorageConnection) {
+  MOZ_ASSERT(aConnection);
+}
+
+ConnectionDatastoreOperationBase::~ConnectionDatastoreOperationBase() {
+  MOZ_ASSERT(!mConnection,
+             "ConnectionDatabaseOperationBase::Cleanup() was not called by a "
+             "subclass!");
+}
+
+void ConnectionDatastoreOperationBase::Cleanup() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mConnection);
+
+  mConnection = nullptr;
+
+  NoteComplete();
+}
+
+void ConnectionDatastoreOperationBase::OnSuccess() { AssertIsOnOwningThread(); }
+
+void ConnectionDatastoreOperationBase::OnFailure(nsresult aResultCode) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(NS_FAILED(aResultCode));
+}
+
+void ConnectionDatastoreOperationBase::RunOnConnectionThread() {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(mConnection);
+  MOZ_ASSERT(NS_SUCCEEDED(ResultCode()));
+
+  if (!MayProceedOnNonOwningThread()) {
+    SetFailureCode(NS_ERROR_ABORT);
+  } else {
+    nsresult rv = NS_OK;
+
+    // The boolean flag is only used by the CloseOp to avoid creating empty
+    // databases.
+    if (mEnsureStorageConnection) {
+      rv = mConnection->EnsureStorageConnection();
+      if (NS_WARN_IF(NS_FAILED(rv))) {
+        SetFailureCode(rv);
+      } else {
+        MOZ_ASSERT(mConnection->HasStorageConnection());
+      }
+    }
+
+    if (NS_SUCCEEDED(rv)) {
+      rv = DoDatastoreWork();
+      if (NS_FAILED(rv)) {
+        SetFailureCode(rv);
+      }
+    }
+  }
+
+  MOZ_ALWAYS_SUCCEEDS(OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL));
+}
+
+void ConnectionDatastoreOperationBase::RunOnOwningThread() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mConnection);
+
+  if (!MayProceed()) {
+    MaybeSetFailureCode(NS_ERROR_ABORT);
+  }
+
+  if (NS_SUCCEEDED(ResultCode())) {
+    OnSuccess();
+  } else {
+    OnFailure(ResultCode());
+  }
+
+  Cleanup();
+}
+
+NS_IMETHODIMP
+ConnectionDatastoreOperationBase::Run() {
+  if (IsOnGlobalConnectionThread()) {
+    RunOnConnectionThread();
+  } else {
+    RunOnOwningThread();
+  }
+
+  return NS_OK;
+}
+
+/*******************************************************************************
+ * Connection implementation
+ ******************************************************************************/
+
+Connection::Connection(ConnectionThread* aConnectionThread,
+                       const OriginMetadata& aOriginMetadata,
+                       UniquePtr<ArchivedOriginScope>&& aArchivedOriginScope,
+                       bool aDatabaseWasNotAvailable)
+    : mConnectionThread(aConnectionThread),
+      mQuotaClient(QuotaClient::GetInstance()),
+      mArchivedOriginScope(std::move(aArchivedOriginScope)),
+      mOriginMetadata(aOriginMetadata),
+      mDatabaseWasNotAvailable(aDatabaseWasNotAvailable),
+      mHasCreatedDatabase(false),
+      mFlushScheduled(false)
+#ifdef DEBUG
+      ,
+      mInUpdateBatch(false),
+      mFinished(false)
+#endif
+{
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!aOriginMetadata.mGroup.IsEmpty());
+  MOZ_ASSERT(!aOriginMetadata.mOrigin.IsEmpty());
+}
+
+Connection::~Connection() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!mFlushScheduled);
+  MOZ_ASSERT(!mInUpdateBatch);
+  MOZ_ASSERT(mFinished);
+}
+
+void Connection::Dispatch(ConnectionDatastoreOperationBase* aOp) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mConnectionThread);
+
+  MOZ_ALWAYS_SUCCEEDS(
+      mConnectionThread->mThread->Dispatch(aOp, NS_DISPATCH_NORMAL));
+}
+
+void Connection::Close(nsIRunnable* aCallback) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aCallback);
+
+  if (mFlushScheduled) {
+    MOZ_ASSERT(mFlushTimer);
+    MOZ_ALWAYS_SUCCEEDS(mFlushTimer->Cancel());
+
+    Flush();
+
+    mFlushTimer = nullptr;
+  }
+
+  RefPtr<CloseOp> op = new CloseOp(this, aCallback);
+
+  Dispatch(op);
+}
+
+void Connection::SetItem(const nsString& aKey, const LSValue& aValue,
+                         int64_t aDelta, bool aIsNewItem) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mInUpdateBatch);
+
+  if (aIsNewItem) {
+    mWriteOptimizer.InsertItem(aKey, aValue, aDelta);
+  } else {
+    mWriteOptimizer.UpdateItem(aKey, aValue, aDelta);
+  }
+}
+
+void Connection::RemoveItem(const nsString& aKey, int64_t aDelta) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mInUpdateBatch);
+
+  mWriteOptimizer.DeleteItem(aKey, aDelta);
+}
+
+void Connection::Clear(int64_t aDelta) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mInUpdateBatch);
+
+  mWriteOptimizer.Truncate(aDelta);
+}
+
+void Connection::BeginUpdateBatch() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!mInUpdateBatch);
+
+#ifdef DEBUG
+  mInUpdateBatch = true;
+#endif
+}
+
+void Connection::EndUpdateBatch() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mInUpdateBatch);
+
+  if (mWriteOptimizer.HasWrites() && !mFlushScheduled) {
+    ScheduleFlush();
+  }
+
+#ifdef DEBUG
+  mInUpdateBatch = false;
+#endif
+}
+
+nsresult Connection::EnsureStorageConnection() {
+  AssertIsOnGlobalConnectionThread();
+
+  if (HasStorageConnection()) {
+    return NS_OK;
+  }
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  if (!mDatabaseWasNotAvailable || mHasCreatedDatabase) {
+    MOZ_ASSERT(mOriginMetadata.mPersistenceType == PERSISTENCE_TYPE_DEFAULT);
+
+    QM_TRY_INSPECT(const auto& directoryEntry,
+                   quotaManager->GetOriginDirectory(mOriginMetadata));
+
+    QM_TRY(MOZ_TO_RESULT(directoryEntry->Append(
+        NS_LITERAL_STRING_FROM_CSTRING(LS_DIRECTORY_NAME))));
+
+    QM_TRY(MOZ_TO_RESULT(directoryEntry->GetPath(mDirectoryPath)));
+    QM_TRY(MOZ_TO_RESULT(directoryEntry->Append(kDataFileName)));
+
+    QM_TRY_INSPECT(
+        const auto& databaseFilePath,
+        MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(nsString, directoryEntry, GetPath));
+
+    QM_TRY_UNWRAP(auto storageConnection,
+                  GetStorageConnection(databaseFilePath));
+    LazyInit(WrapMovingNotNull(std::move(storageConnection)));
+
+    return NS_OK;
+  }
+
+  RefPtr<InitTemporaryOriginHelper> helper =
+      new InitTemporaryOriginHelper(mOriginMetadata);
+
+  QM_TRY_INSPECT(const auto& originDirectoryPath,
+                 helper->BlockAndReturnOriginDirectoryPath());
+
+  QM_TRY_INSPECT(const auto& directoryEntry,
+                 QM_NewLocalFile(originDirectoryPath));
+
+  QM_TRY(MOZ_TO_RESULT(directoryEntry->Append(
+      NS_LITERAL_STRING_FROM_CSTRING(LS_DIRECTORY_NAME))));
+
+  QM_TRY(MOZ_TO_RESULT(directoryEntry->GetPath(mDirectoryPath)));
+
+  QM_TRY_INSPECT(const bool& exists,
+                 MOZ_TO_RESULT_INVOKE_MEMBER(directoryEntry, Exists));
+
+  if (!exists) {
+    QM_TRY(
+        MOZ_TO_RESULT(directoryEntry->Create(nsIFile::DIRECTORY_TYPE, 0755)));
+  }
+
+  QM_TRY(MOZ_TO_RESULT(directoryEntry->Append(kDataFileName)));
+
+#ifdef DEBUG
+  {
+    QM_TRY_INSPECT(const bool& exists,
+                   MOZ_TO_RESULT_INVOKE_MEMBER(directoryEntry, Exists));
+
+    MOZ_ASSERT(!exists);
+  }
+#endif
+
+  QM_TRY_INSPECT(const auto& usageFile, GetUsageFile(mDirectoryPath));
+
+  nsCOMPtr<mozIStorageConnection> storageConnection;
+
+  auto autoRemove = MakeScopeExit([&storageConnection, &directoryEntry] {
+    if (storageConnection) {
+      MOZ_ALWAYS_SUCCEEDS(storageConnection->Close());
+    }
+
+    nsresult rv = directoryEntry->Remove(false);
+    if (rv != NS_ERROR_FILE_NOT_FOUND && NS_FAILED(rv)) {
+      NS_WARNING("Failed to remove database file!");
+    }
+  });
+
+  QM_TRY_UNWRAP(storageConnection,
+                CreateStorageConnection(*directoryEntry, *usageFile, Origin(),
+                                        [] { MOZ_ASSERT_UNREACHABLE(); }));
+
+  MOZ_ASSERT(mQuotaClient);
+
+  MutexAutoLock shadowDatabaseLock(mQuotaClient->ShadowDatabaseMutex());
+
+  nsCOMPtr<mozIStorageConnection> shadowConnection;
+  if (!gInitializedShadowStorage) {
+    QM_TRY_UNWRAP(shadowConnection,
+                  CreateShadowStorageConnection(quotaManager->GetBasePath()));
+
+    gInitializedShadowStorage = true;
+  }
+
+  autoRemove.release();
+
+  if (!mHasCreatedDatabase) {
+    mHasCreatedDatabase = true;
+  }
+
+  LazyInit(WrapMovingNotNull(std::move(storageConnection)));
+
+  return NS_OK;
+}
+
+void Connection::CloseStorageConnection() {
+  AssertIsOnGlobalConnectionThread();
+
+  CachingDatabaseConnection::Close();
+}
+
+nsresult Connection::BeginWriteTransaction() {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(HasStorageConnection());
+
+  QM_TRY(MOZ_TO_RESULT(ExecuteCachedStatement("BEGIN IMMEDIATE;"_ns)));
+
+  return NS_OK;
+}
+
+nsresult Connection::CommitWriteTransaction() {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(HasStorageConnection());
+
+  QM_TRY(MOZ_TO_RESULT(ExecuteCachedStatement("COMMIT;"_ns)));
+
+  return NS_OK;
+}
+
+nsresult Connection::RollbackWriteTransaction() {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(HasStorageConnection());
+
+  QM_TRY_INSPECT(const auto& stmt, BorrowCachedStatement("ROLLBACK;"_ns));
+
+  // This may fail if SQLite already rolled back the transaction so ignore any
+  // errors.
+  Unused << stmt->Execute();
+
+  return NS_OK;
+}
+
+void Connection::ScheduleFlush() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mWriteOptimizer.HasWrites());
+  MOZ_ASSERT(!mFlushScheduled);
+
+  if (!mFlushTimer) {
+    mFlushTimer = NS_NewTimer();
+    MOZ_ASSERT(mFlushTimer);
+  }
+
+  MOZ_ALWAYS_SUCCEEDS(mFlushTimer->InitWithNamedFuncCallback(
+      FlushTimerCallback, this, kFlushTimeoutMs, nsITimer::TYPE_ONE_SHOT,
+      "Connection::FlushTimerCallback"));
+
+  mFlushScheduled = true;
+}
+
+void Connection::Flush() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mFlushScheduled);
+
+  if (mWriteOptimizer.HasWrites()) {
+    RefPtr<FlushOp> op = new FlushOp(this, std::move(mWriteOptimizer));
+
+    Dispatch(op);
+  }
+
+  mFlushScheduled = false;
+}
+
+// static
+void Connection::FlushTimerCallback(nsITimer* aTimer, void* aClosure) {
+  MOZ_ASSERT(aClosure);
+
+  auto* self = static_cast<Connection*>(aClosure);
+  MOZ_ASSERT(self);
+  MOZ_ASSERT(self->mFlushScheduled);
+
+  self->Flush();
+}
+
+Result<nsString, nsresult>
+Connection::InitTemporaryOriginHelper::BlockAndReturnOriginDirectoryPath() {
+  AssertIsOnGlobalConnectionThread();
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  MOZ_ALWAYS_SUCCEEDS(
+      quotaManager->IOThread()->Dispatch(this, NS_DISPATCH_NORMAL));
+
+  mozilla::MonitorAutoLock lock(mMonitor);
+  while (mWaiting) {
+    lock.Wait();
+  }
+
+  QM_TRY(MOZ_TO_RESULT(mIOThreadResultCode));
+
+  return mOriginDirectoryPath;
+}
+
+nsresult Connection::InitTemporaryOriginHelper::RunOnIOThread() {
+  AssertIsOnIOThread();
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  QM_TRY_INSPECT(const auto& directoryEntry,
+                 quotaManager
+                     ->EnsureTemporaryOriginIsInitialized(
+                         PERSISTENCE_TYPE_DEFAULT, mOriginMetadata)
+                     .map([](const auto& res) { return res.first; }));
+
+  QM_TRY(MOZ_TO_RESULT(directoryEntry->GetPath(mOriginDirectoryPath)));
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+Connection::InitTemporaryOriginHelper::Run() {
+  AssertIsOnIOThread();
+
+  nsresult rv = RunOnIOThread();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    mIOThreadResultCode = rv;
+  }
+
+  mozilla::MonitorAutoLock lock(mMonitor);
+  MOZ_ASSERT(mWaiting);
+
+  mWaiting = false;
+  lock.Notify();
+
+  return NS_OK;
+}
+
+Connection::FlushOp::FlushOp(Connection* aConnection,
+                             ConnectionWriteOptimizer&& aWriteOptimizer)
+    : ConnectionDatastoreOperationBase(aConnection),
+      mWriteOptimizer(std::move(aWriteOptimizer)),
+      mShadowWrites(gShadowWrites) {}
+
+nsresult Connection::FlushOp::DoDatastoreWork() {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(mConnection);
+
+  AutoWriteTransaction autoWriteTransaction(mShadowWrites);
+
+  QM_TRY(MOZ_TO_RESULT(autoWriteTransaction.Start(mConnection)));
+
+  QM_TRY_INSPECT(const int64_t& usage,
+                 mWriteOptimizer.Perform(mConnection, mShadowWrites));
+
+  QM_TRY_INSPECT(const auto& usageFile,
+                 GetUsageFile(mConnection->DirectoryPath()));
+
+  QM_TRY_INSPECT(const auto& usageJournalFile,
+                 GetUsageJournalFile(mConnection->DirectoryPath()));
+
+  QM_TRY(MOZ_TO_RESULT(UpdateUsageFile(usageFile, usageJournalFile, usage)));
+
+  QM_TRY(MOZ_TO_RESULT(autoWriteTransaction.Commit()));
+
+  QM_TRY(MOZ_TO_RESULT(usageJournalFile->Remove(false)));
+
+  return NS_OK;
+}
+
+void Connection::FlushOp::Cleanup() {
+  AssertIsOnOwningThread();
+
+  mWriteOptimizer.Reset();
+
+  MOZ_ASSERT(!mWriteOptimizer.HasWrites());
+
+  ConnectionDatastoreOperationBase::Cleanup();
+}
+
+nsresult Connection::CloseOp::DoDatastoreWork() {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(mConnection);
+
+  if (mConnection->HasStorageConnection()) {
+    mConnection->CloseStorageConnection();
+  }
+
+  return NS_OK;
+}
+
+void Connection::CloseOp::Cleanup() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mConnection);
+
+  mConnection->mConnectionThread->mConnections.Remove(mConnection->Origin());
+
+#ifdef DEBUG
+  MOZ_ASSERT(!mConnection->mFinished);
+  mConnection->mFinished = true;
+#endif
+
+  nsCOMPtr<nsIRunnable> callback;
+  mCallback.swap(callback);
+
+  callback->Run();
+
+  ConnectionDatastoreOperationBase::Cleanup();
+}
+
+/*******************************************************************************
+ * ConnectionThread implementation
+ ******************************************************************************/
+
+ConnectionThread::ConnectionThread() {
+  AssertIsOnOwningThread();
+  AssertIsOnBackgroundThread();
+
+  MOZ_ALWAYS_SUCCEEDS(NS_NewNamedThread("LS Thread", getter_AddRefs(mThread)));
+}
+
+ConnectionThread::~ConnectionThread() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!mConnections.Count());
+}
+
+bool ConnectionThread::IsOnConnectionThread() {
+  MOZ_ASSERT(mThread);
+
+  bool current;
+  return NS_SUCCEEDED(mThread->IsOnCurrentThread(&current)) && current;
+}
+
+void ConnectionThread::AssertIsOnConnectionThread() {
+  MOZ_ASSERT(IsOnConnectionThread());
+}
+
+already_AddRefed<Connection> ConnectionThread::CreateConnection(
+    const OriginMetadata& aOriginMetadata,
+    UniquePtr<ArchivedOriginScope>&& aArchivedOriginScope,
+    bool aDatabaseWasNotAvailable) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!aOriginMetadata.mOrigin.IsEmpty());
+  MOZ_ASSERT(!mConnections.Contains(aOriginMetadata.mOrigin));
+
+  RefPtr<Connection> connection =
+      new Connection(this, aOriginMetadata, std::move(aArchivedOriginScope),
+                     aDatabaseWasNotAvailable);
+  mConnections.InsertOrUpdate(aOriginMetadata.mOrigin, RefPtr{connection});
+
+  return connection.forget();
+}
+
+void ConnectionThread::Shutdown() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mThread);
+
+  mThread->Shutdown();
+}
+
+/*******************************************************************************
+ * Datastore
+ ******************************************************************************/
+
+Datastore::Datastore(const OriginMetadata& aOriginMetadata,
+                     uint32_t aPrivateBrowsingId, int64_t aUsage,
+                     int64_t aSizeOfKeys, int64_t aSizeOfItems,
+                     RefPtr<DirectoryLock>&& aDirectoryLock,
+                     RefPtr<Connection>&& aConnection,
+                     RefPtr<QuotaObject>&& aQuotaObject,
+                     nsTHashMap<nsStringHashKey, LSValue>& aValues,
+                     nsTArray<LSItemInfo>&& aOrderedItems)
+    : mDirectoryLock(std::move(aDirectoryLock)),
+      mConnection(std::move(aConnection)),
+      mQuotaObject(std::move(aQuotaObject)),
+      mOrderedItems(std::move(aOrderedItems)),
+      mOriginMetadata(aOriginMetadata),
+      mPrivateBrowsingId(aPrivateBrowsingId),
+      mUsage(aUsage),
+      mUpdateBatchUsage(-1),
+      mSizeOfKeys(aSizeOfKeys),
+      mSizeOfItems(aSizeOfItems),
+      mClosed(false),
+      mInUpdateBatch(false),
+      mHasLivePrivateDatastore(false) {
+  AssertIsOnBackgroundThread();
+
+  mValues.SwapElements(aValues);
+}
+
+Datastore::~Datastore() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(mClosed);
+}
+
+void Datastore::Close() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mClosed);
+  MOZ_ASSERT(!mPrepareDatastoreOps.Count());
+  MOZ_ASSERT(!mPreparedDatastores.Count());
+  MOZ_ASSERT(!mDatabases.Count());
+  MOZ_ASSERT(mDirectoryLock);
+
+  mClosed = true;
+
+  if (IsPersistent()) {
+    MOZ_ASSERT(mConnection);
+    MOZ_ASSERT(mQuotaObject);
+
+    // We can't release the directory lock and unregister itself from the
+    // hashtable until the connection is fully closed.
+    nsCOMPtr<nsIRunnable> callback =
+        NewRunnableMethod("dom::Datastore::ConnectionClosedCallback", this,
+                          &Datastore::ConnectionClosedCallback);
+    mConnection->Close(callback);
+  } else {
+    MOZ_ASSERT(!mConnection);
+    MOZ_ASSERT(!mQuotaObject);
+
+    // There's no connection, so it's safe to release the directory lock and
+    // unregister itself from the hashtable.
+
+    mDirectoryLock = nullptr;
+
+    CleanupMetadata();
+  }
+}
+
+void Datastore::WaitForConnectionToComplete(nsIRunnable* aCallback) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aCallback);
+  MOZ_ASSERT(!mCompleteCallback);
+  MOZ_ASSERT(mClosed);
+
+  mCompleteCallback = aCallback;
+}
+
+void Datastore::NoteLivePrepareDatastoreOp(
+    PrepareDatastoreOp* aPrepareDatastoreOp) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aPrepareDatastoreOp);
+  MOZ_ASSERT(!mPrepareDatastoreOps.Contains(aPrepareDatastoreOp));
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(!mClosed);
+
+  mPrepareDatastoreOps.Insert(aPrepareDatastoreOp);
+}
+
+void Datastore::NoteFinishedPrepareDatastoreOp(
+    PrepareDatastoreOp* aPrepareDatastoreOp) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aPrepareDatastoreOp);
+  MOZ_ASSERT(mPrepareDatastoreOps.Contains(aPrepareDatastoreOp));
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(!mClosed);
+
+  mPrepareDatastoreOps.Remove(aPrepareDatastoreOp);
+
+  QuotaManager::MaybeRecordQuotaClientShutdownStep(
+      quota::Client::LS, "PrepareDatastoreOp finished"_ns);
+
+  MaybeClose();
+}
+
+void Datastore::NoteLivePrivateDatastore() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mHasLivePrivateDatastore);
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(!mClosed);
+
+  mHasLivePrivateDatastore = true;
+}
+
+void Datastore::NoteFinishedPrivateDatastore() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(mHasLivePrivateDatastore);
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(!mClosed);
+
+  mHasLivePrivateDatastore = false;
+
+  QuotaManager::MaybeRecordQuotaClientShutdownStep(
+      quota::Client::LS, "PrivateDatastore finished"_ns);
+
+  MaybeClose();
+}
+
+void Datastore::NoteLivePreparedDatastore(
+    PreparedDatastore* aPreparedDatastore) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aPreparedDatastore);
+  MOZ_ASSERT(!mPreparedDatastores.Contains(aPreparedDatastore));
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(!mClosed);
+
+  mPreparedDatastores.Insert(aPreparedDatastore);
+}
+
+void Datastore::NoteFinishedPreparedDatastore(
+    PreparedDatastore* aPreparedDatastore) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aPreparedDatastore);
+  MOZ_ASSERT(mPreparedDatastores.Contains(aPreparedDatastore));
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(!mClosed);
+
+  mPreparedDatastores.Remove(aPreparedDatastore);
+
+  QuotaManager::MaybeRecordQuotaClientShutdownStep(
+      quota::Client::LS, "PreparedDatastore finished"_ns);
+
+  MaybeClose();
+}
+
+bool Datastore::HasOtherProcessDatabases(Database* aDatabase) {
+  AssertIsOnBackgroundThread();
+
+  PBackgroundParent* databaseBackgroundActor = aDatabase->Manager();
+
+  for (Database* database : mDatabases) {
+    if (database->Manager() != databaseBackgroundActor) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+void Datastore::NoteLiveDatabase(Database* aDatabase) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+  MOZ_ASSERT(!mDatabases.Contains(aDatabase));
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(!mClosed);
+
+  mDatabases.Insert(aDatabase);
+
+  NoteChangedDatabaseMap();
+}
+
+void Datastore::NoteFinishedDatabase(Database* aDatabase) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+  MOZ_ASSERT(mDatabases.Contains(aDatabase));
+  MOZ_ASSERT(!mActiveDatabases.Contains(aDatabase));
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(!mClosed);
+
+  mDatabases.Remove(aDatabase);
+
+  NoteChangedDatabaseMap();
+
+  QuotaManager::MaybeRecordQuotaClientShutdownStep(quota::Client::LS,
+                                                   "Database finished"_ns);
+
+  MaybeClose();
+}
+
+void Datastore::NoteActiveDatabase(Database* aDatabase) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+  MOZ_ASSERT(mDatabases.Contains(aDatabase));
+  MOZ_ASSERT(!mActiveDatabases.Contains(aDatabase));
+  MOZ_ASSERT(!mClosed);
+
+  mActiveDatabases.Insert(aDatabase);
+}
+
+void Datastore::NoteInactiveDatabase(Database* aDatabase) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+  MOZ_ASSERT(mDatabases.Contains(aDatabase));
+  MOZ_ASSERT(mActiveDatabases.Contains(aDatabase));
+  MOZ_ASSERT(!mClosed);
+
+  mActiveDatabases.Remove(aDatabase);
+
+  if (!mActiveDatabases.Count() && mPendingUsageDeltas.Length()) {
+    int64_t finalDelta = 0;
+
+    for (auto delta : mPendingUsageDeltas) {
+      finalDelta += delta;
+    }
+
+    MOZ_ASSERT(finalDelta <= 0);
+
+    if (finalDelta != 0) {
+      DebugOnly<bool> ok = UpdateUsage(finalDelta);
+      MOZ_ASSERT(ok);
+    }
+
+    mPendingUsageDeltas.Clear();
+  }
+}
+
+void Datastore::GetSnapshotLoadInfo(const nsAString& aKey,
+                                    bool& aAddKeyToUnknownItems,
+                                    nsTHashtable<nsStringHashKey>& aLoadedItems,
+                                    nsTArray<LSItemInfo>& aItemInfos,
+                                    uint32_t& aNextLoadIndex,
+                                    LSSnapshot::LoadState& aLoadState) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mClosed);
+  MOZ_ASSERT(!mInUpdateBatch);
+
+#ifdef DEBUG
+  int64_t sizeOfKeys = 0;
+  int64_t sizeOfItems = 0;
+  for (auto item : mOrderedItems) {
+    int64_t sizeOfKey = static_cast<int64_t>(item.key().Length());
+    sizeOfKeys += sizeOfKey;
+    sizeOfItems += sizeOfKey + static_cast<int64_t>(item.value().Length());
+  }
+  MOZ_ASSERT(mSizeOfKeys == sizeOfKeys);
+  MOZ_ASSERT(mSizeOfItems == sizeOfItems);
+#endif
+
+  // Computes load state optimized for current size of keys and items.
+  // Zero key length and value can be passed to do a quick initial estimation.
+  // If computed load state is already AllOrderedItems then excluded key length
+  // and value length can't make it any better.
+  auto GetLoadState = [&](int64_t aKeyLength, int64_t aValueLength) {
+    if (mSizeOfKeys - aKeyLength <= gSnapshotPrefill) {
+      if (mSizeOfItems - aKeyLength - aValueLength <= gSnapshotPrefill) {
+        return LSSnapshot::LoadState::AllOrderedItems;
+      }
+
+      return LSSnapshot::LoadState::AllOrderedKeys;
+    }
+
+    return LSSnapshot::LoadState::Partial;
+  };
+
+  // Value for given aKey if aKey is not void (can be void too if value doesn't
+  // exist for given aKey).
+  LSValue value;
+  // If aKey and value are not void, checkKey will be set to true. Once we find
+  // an item for given aKey in one of the loops below, checkKey is set to false
+  // to prevent additional comparison of strings (string implementation compares
+  // string lengths first to avoid char by char comparison if possible).
+  bool checkKey = false;
+
+  // Avoid additional hash lookup if all ordered items fit into initial prefill
+  // already.
+  LSSnapshot::LoadState loadState = GetLoadState(/* aKeyLength */ 0,
+                                                 /* aValueLength */ 0);
+  if (loadState != LSSnapshot::LoadState::AllOrderedItems && !aKey.IsVoid()) {
+    GetItem(aKey, value);
+    if (!value.IsVoid()) {
+      // Ok, we have a non void aKey and value.
+
+      // We have to watch for aKey during one of the loops below to exclude it
+      // from the size computation. The super fast mode (AllOrderedItems)
+      // doesn't have to do that though.
+      checkKey = true;
+
+      // We have to compute load state again because aKey length and value
+      // length is excluded from the size in this case.
+      loadState = GetLoadState(aKey.Length(), value.Length());
+    }
+  }
+
+  switch (loadState) {
+    case LSSnapshot::LoadState::AllOrderedItems: {
+      // We're sending all ordered items, we don't need to check keys because
+      // mOrderedItems must contain a value for aKey if checkKey is true.
+
+      aItemInfos.AppendElements(mOrderedItems);
+
+      MOZ_ASSERT(aItemInfos.Length() == mValues.Count());
+      aNextLoadIndex = mValues.Count();
+
+      aAddKeyToUnknownItems = false;
+
+      break;
+    }
+
+    case LSSnapshot::LoadState::AllOrderedKeys: {
+      // We don't have enough snapshot budget to send all items, but we do have
+      // enough to send all of the keys and to make a best effort to populate as
+      // many values as possible. We send void string values once we run out of
+      // budget. A complicating factor is that we want to make sure that we send
+      // the value for aKey which is a localStorage read that's triggering this
+      // request. Since that key can happen anywhere in the list of items, we
+      // need to handle it specially.
+      //
+      // The loop is effectively doing 2 things in parallel:
+      //
+      //   1. Looking for the `aKey` to send. This is tracked by `checkKey`
+      //      which is true if there was an `aKey` specified and until we
+      //      populate its value, and false thereafter.
+      //   2. Sending values until we run out of `size` budget and switch to
+      //      sending void values. `doneSendingValues` tracks when we've run out
+      //      of size budget, with `setVoidValue` tracking whether a value
+      //      should be sent for each turn of the event loop but can be
+      //      overridden when `aKey` is found.
+
+      int64_t size = mSizeOfKeys;
+      bool setVoidValue = false;
+      bool doneSendingValues = false;
+      for (uint32_t index = 0; index < mOrderedItems.Length(); index++) {
+        const LSItemInfo& item = mOrderedItems[index];
+
+        const nsString& key = item.key();
+        const LSValue& value = item.value();
+
+        if (checkKey && key == aKey) {
+          checkKey = false;
+          setVoidValue = false;
+        } else if (!setVoidValue) {
+          if (doneSendingValues) {
+            setVoidValue = true;
+          } else {
+            size += static_cast<int64_t>(value.Length());
+
+            if (size > gSnapshotPrefill) {
+              setVoidValue = true;
+              doneSendingValues = true;
+
+              // We set doneSendingValues to true and that will guard against
+              // entering this branch during next iterations. So aNextLoadIndex
+              // is set only once.
+              aNextLoadIndex = index;
+            }
+          }
+        }
+
+        LSItemInfo* itemInfo = aItemInfos.AppendElement();
+        itemInfo->key() = key;
+        if (setVoidValue) {
+          itemInfo->value().SetIsVoid(true);
+        } else {
+          aLoadedItems.PutEntry(key);
+          itemInfo->value() = value;
+        }
+      }
+
+      aAddKeyToUnknownItems = false;
+
+      break;
+    }
+
+    case LSSnapshot::LoadState::Partial: {
+      int64_t size = 0;
+      for (uint32_t index = 0; index < mOrderedItems.Length(); index++) {
+        const LSItemInfo& item = mOrderedItems[index];
+
+        const nsString& key = item.key();
+        const LSValue& value = item.value();
+
+        if (checkKey && key == aKey) {
+          checkKey = false;
+        } else {
+          size += static_cast<int64_t>(key.Length()) +
+                  static_cast<int64_t>(value.Length());
+
+          if (size > gSnapshotPrefill) {
+            aNextLoadIndex = index;
+            break;
+          }
+        }
+
+        aLoadedItems.PutEntry(key);
+
+        LSItemInfo* itemInfo = aItemInfos.AppendElement();
+        itemInfo->key() = key;
+        itemInfo->value() = value;
+      }
+
+      aAddKeyToUnknownItems = false;
+
+      if (!aKey.IsVoid()) {
+        if (value.IsVoid()) {
+          aAddKeyToUnknownItems = true;
+        } else if (checkKey) {
+          // The item wasn't added in the loop above, add it here.
+
+          LSItemInfo* itemInfo = aItemInfos.AppendElement();
+          itemInfo->key() = aKey;
+          itemInfo->value() = value;
+        }
+      }
+
+      MOZ_ASSERT(aItemInfos.Length() < mOrderedItems.Length());
+
+      break;
+    }
+
+    default:
+      MOZ_CRASH("Bad load state value!");
+  }
+
+  aLoadState = loadState;
+}
+
+void Datastore::GetItem(const nsAString& aKey, LSValue& aValue) const {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mClosed);
+
+  if (!mValues.Get(aKey, &aValue)) {
+    aValue.SetIsVoid(true);
+  }
+}
+
+void Datastore::GetKeys(nsTArray<nsString>& aKeys) const {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mClosed);
+
+  for (auto item : mOrderedItems) {
+    aKeys.AppendElement(item.key());
+  }
+}
+
+void Datastore::SetItem(Database* aDatabase, const nsString& aKey,
+                        const LSValue& aValue) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+  MOZ_ASSERT(!mClosed);
+  MOZ_ASSERT(mInUpdateBatch);
+
+  LSValue oldValue;
+  GetItem(aKey, oldValue);
+
+  if (oldValue != aValue) {
+    bool isNewItem = oldValue.IsVoid();
+
+    NotifySnapshots(aDatabase, aKey, oldValue, /* affectsOrder */ isNewItem);
+
+    mValues.InsertOrUpdate(aKey, aValue);
+
+    int64_t delta;
+
+    if (isNewItem) {
+      mWriteOptimizer.InsertItem(aKey, aValue);
+
+      int64_t sizeOfKey = static_cast<int64_t>(aKey.Length());
+
+      delta = sizeOfKey + static_cast<int64_t>(aValue.UTF16Length());
+
+      mUpdateBatchUsage += delta;
+
+      mSizeOfKeys += sizeOfKey;
+      mSizeOfItems += sizeOfKey + static_cast<int64_t>(aValue.Length());
+    } else {
+      mWriteOptimizer.UpdateItem(aKey, aValue);
+
+      delta = static_cast<int64_t>(aValue.UTF16Length()) -
+              static_cast<int64_t>(oldValue.UTF16Length());
+
+      mUpdateBatchUsage += delta;
+
+      mSizeOfItems += static_cast<int64_t>(aValue.Length()) -
+                      static_cast<int64_t>(oldValue.Length());
+    }
+
+    if (IsPersistent()) {
+      mConnection->SetItem(aKey, aValue, delta, isNewItem);
+    }
+  }
+}
+
+void Datastore::RemoveItem(Database* aDatabase, const nsString& aKey) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+  MOZ_ASSERT(!mClosed);
+  MOZ_ASSERT(mInUpdateBatch);
+
+  LSValue oldValue;
+  GetItem(aKey, oldValue);
+
+  if (!oldValue.IsVoid()) {
+    NotifySnapshots(aDatabase, aKey, oldValue, /* aAffectsOrder */ true);
+
+    mValues.Remove(aKey);
+
+    mWriteOptimizer.DeleteItem(aKey);
+
+    int64_t sizeOfKey = static_cast<int64_t>(aKey.Length());
+
+    int64_t delta = -sizeOfKey - static_cast<int64_t>(oldValue.UTF16Length());
+
+    mUpdateBatchUsage += delta;
+
+    mSizeOfKeys -= sizeOfKey;
+    mSizeOfItems -= sizeOfKey + static_cast<int64_t>(oldValue.Length());
+
+    if (IsPersistent()) {
+      mConnection->RemoveItem(aKey, delta);
+    }
+  }
+}
+
+void Datastore::Clear(Database* aDatabase) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mClosed);
+
+  if (mValues.Count()) {
+    int64_t delta = 0;
+    for (const auto& entry : mValues) {
+      const nsAString& key = entry.GetKey();
+      const LSValue& value = entry.GetData();
+
+      delta += -static_cast<int64_t>(key.Length()) -
+               static_cast<int64_t>(value.UTF16Length());
+
+      NotifySnapshots(aDatabase, key, value, /* aAffectsOrder */ true);
+    }
+
+    mValues.Clear();
+
+    if (mInUpdateBatch) {
+      mWriteOptimizer.Truncate();
+
+      mUpdateBatchUsage += delta;
+    } else {
+      mOrderedItems.Clear();
+
+      DebugOnly<bool> ok = UpdateUsage(delta);
+      MOZ_ASSERT(ok);
+    }
+
+    mSizeOfKeys = 0;
+    mSizeOfItems = 0;
+
+    if (IsPersistent()) {
+      mConnection->Clear(delta);
+    }
+  }
+}
+
+void Datastore::BeginUpdateBatch(int64_t aSnapshotUsage) {
+  AssertIsOnBackgroundThread();
+  // Don't assert `aSnapshotUsage >= 0`, it can be negative when multiple
+  // snapshots are operating in parallel.
+  MOZ_ASSERT(!mClosed);
+  MOZ_ASSERT(mUpdateBatchUsage == -1);
+  MOZ_ASSERT(!mInUpdateBatch);
+
+  mUpdateBatchUsage = aSnapshotUsage;
+
+  if (IsPersistent()) {
+    mConnection->BeginUpdateBatch();
+  }
+
+  mInUpdateBatch = true;
+}
+
+int64_t Datastore::EndUpdateBatch(int64_t aSnapshotPeakUsage) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mClosed);
+  MOZ_ASSERT(mInUpdateBatch);
+
+  mWriteOptimizer.ApplyAndReset(mOrderedItems);
+
+  MOZ_ASSERT(!mWriteOptimizer.HasWrites());
+
+  if (aSnapshotPeakUsage >= 0) {
+    int64_t delta = mUpdateBatchUsage - aSnapshotPeakUsage;
+
+    if (mActiveDatabases.Count()) {
+      // We can't apply deltas while other databases are still active.
+      // The final delta must be zero or negative, but individual deltas can
+      // be positive. A positive delta can't be applied asynchronously since
+      // there's no way to fire the quota exceeded error event.
+
+      mPendingUsageDeltas.AppendElement(delta);
+    } else {
+      MOZ_ASSERT(delta <= 0);
+      if (delta != 0) {
+        DebugOnly<bool> ok = UpdateUsage(delta);
+        MOZ_ASSERT(ok);
+      }
+    }
+  }
+
+  int64_t result = mUpdateBatchUsage;
+  mUpdateBatchUsage = -1;
+
+  if (IsPersistent()) {
+    mConnection->EndUpdateBatch();
+  }
+
+  mInUpdateBatch = false;
+
+  return result;
+}
+
+int64_t Datastore::AttemptToUpdateUsage(int64_t aMinSize, bool aInitial) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT_IF(aInitial, aMinSize >= 0);
+  MOZ_ASSERT_IF(!aInitial, aMinSize > 0);
+
+  const int64_t size = aMinSize + GetSnapshotPeakUsagePreincrement(aInitial);
+
+  if (size && UpdateUsage(size)) {
+    return size;
+  }
+
+  const int64_t reducedSize =
+      aMinSize + GetSnapshotPeakUsageReducedPreincrement(aInitial);
+
+  if (reducedSize && UpdateUsage(reducedSize)) {
+    return reducedSize;
+  }
+
+  if (aMinSize > 0 && UpdateUsage(aMinSize)) {
+    return aMinSize;
+  }
+
+  return 0;
+}
+
+bool Datastore::HasOtherProcessObservers(Database* aDatabase) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+
+  if (!gObservers) {
+    return false;
+  }
+
+  nsTArray<NotNull<Observer*>>* array;
+  if (!gObservers->Get(mOriginMetadata.mOrigin, &array)) {
+    return false;
+  }
+
+  MOZ_ASSERT(array);
+
+  PBackgroundParent* databaseBackgroundActor = aDatabase->Manager();
+
+  for (Observer* observer : *array) {
+    if (observer->Manager() != databaseBackgroundActor) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+void Datastore::NotifyOtherProcessObservers(Database* aDatabase,
+                                            const nsString& aDocumentURI,
+                                            const nsString& aKey,
+                                            const LSValue& aOldValue,
+                                            const LSValue& aNewValue) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+
+  if (!gObservers) {
+    return;
+  }
+
+  nsTArray<NotNull<Observer*>>* array;
+  if (!gObservers->Get(mOriginMetadata.mOrigin, &array)) {
+    return;
+  }
+
+  MOZ_ASSERT(array);
+
+  // We do not want to send information about events back to the content process
+  // that caused the change.
+  PBackgroundParent* databaseBackgroundActor = aDatabase->Manager();
+
+  for (Observer* observer : *array) {
+    if (observer->Manager() != databaseBackgroundActor) {
+      observer->Observe(aDatabase, aDocumentURI, aKey, aOldValue, aNewValue);
+    }
+  }
+}
+
+void Datastore::NoteChangedObserverArray(
+    const nsTArray<NotNull<Observer*>>& aObservers) {
+  AssertIsOnBackgroundThread();
+
+  for (Database* database : mActiveDatabases) {
+    Snapshot* snapshot = database->GetSnapshot();
+    MOZ_ASSERT(snapshot);
+
+    if (snapshot->IsDirty()) {
+      continue;
+    }
+
+    bool hasOtherProcessObservers = false;
+
+    PBackgroundParent* databaseBackgroundActor = database->Manager();
+
+    for (Observer* observer : aObservers) {
+      if (observer->Manager() != databaseBackgroundActor) {
+        hasOtherProcessObservers = true;
+        break;
+      }
+    }
+
+    if (snapshot->HasOtherProcessObservers() != hasOtherProcessObservers) {
+      snapshot->MarkDirty();
+    }
+  }
+}
+
+void Datastore::Stringify(nsACString& aResult) const {
+  AssertIsOnBackgroundThread();
+
+  aResult.AppendLiteral("DirectoryLock:");
+  aResult.AppendInt(!!mDirectoryLock);
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("Connection:");
+  aResult.AppendInt(!!mConnection);
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("QuotaObject:");
+  aResult.AppendInt(!!mQuotaObject);
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("PrepareDatastoreOps:");
+  aResult.AppendInt(mPrepareDatastoreOps.Count());
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("PreparedDatastores:");
+  aResult.AppendInt(mPreparedDatastores.Count());
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("Databases:");
+  aResult.AppendInt(mDatabases.Count());
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("ActiveDatabases:");
+  aResult.AppendInt(mActiveDatabases.Count());
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("Origin:");
+  aResult.Append(AnonymizedOriginString(mOriginMetadata.mOrigin));
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("PrivateBrowsingId:");
+  aResult.AppendInt(mPrivateBrowsingId);
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("Closed:");
+  aResult.AppendInt(mClosed);
+}
+
+bool Datastore::UpdateUsage(int64_t aDelta) {
+  AssertIsOnBackgroundThread();
+
+  // Check internal LocalStorage origin limit.
+  int64_t newUsage = mUsage + aDelta;
+
+  MOZ_ASSERT(newUsage >= 0);
+
+  if (newUsage > StaticPrefs::dom_storage_default_quota() * 1024) {
+    return false;
+  }
+
+  // Check QuotaManager limits (group and global limit).
+  if (IsPersistent()) {
+    MOZ_ASSERT(mQuotaObject);
+
+    if (!mQuotaObject->MaybeUpdateSize(newUsage, /* aTruncate */ true)) {
+      return false;
+    }
+  }
+
+  // Quota checks passed, set new usage.
+  mUsage = newUsage;
+
+  return true;
+}
+
+void Datastore::MaybeClose() {
+  AssertIsOnBackgroundThread();
+
+  if (!mPrepareDatastoreOps.Count() && !mHasLivePrivateDatastore &&
+      !mPreparedDatastores.Count() && !mDatabases.Count()) {
+    Close();
+  }
+}
+
+void Datastore::ConnectionClosedCallback() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(mConnection);
+  MOZ_ASSERT(mQuotaObject);
+  MOZ_ASSERT(mClosed);
+
+  // Release the quota object first.
+  mQuotaObject = nullptr;
+
+  bool databaseWasNotAvailable;
+  bool hasCreatedDatabase;
+  mConnection->GetFinishInfo(databaseWasNotAvailable, hasCreatedDatabase);
+
+  if (databaseWasNotAvailable && !hasCreatedDatabase) {
+    MOZ_ASSERT(mUsage == 0);
+
+    QuotaManager* quotaManager = QuotaManager::Get();
+    MOZ_ASSERT(quotaManager);
+
+    quotaManager->ResetUsageForClient(
+        ClientMetadata{mOriginMetadata, mozilla::dom::quota::Client::LS});
+  }
+
+  mConnection = nullptr;
+
+  // Now it's safe to release the directory lock and unregister itself from
+  // the hashtable.
+
+  mDirectoryLock = nullptr;
+
+  CleanupMetadata();
+
+  if (mCompleteCallback) {
+    MOZ_ALWAYS_SUCCEEDS(NS_DispatchToCurrentThread(mCompleteCallback.forget()));
+  }
+}
+
+void Datastore::CleanupMetadata() {
+  AssertIsOnBackgroundThread();
+
+  MOZ_ASSERT(gDatastores);
+  const DebugOnly<bool> removed = gDatastores->Remove(mOriginMetadata.mOrigin);
+  MOZ_ASSERT(removed);
+
+  QuotaManager::MaybeRecordQuotaClientShutdownStep(quota::Client::LS,
+                                                   "Datastore removed"_ns);
+
+  if (!gDatastores->Count()) {
+    gDatastores = nullptr;
+  }
+}
+
+void Datastore::NotifySnapshots(Database* aDatabase, const nsAString& aKey,
+                                const LSValue& aOldValue, bool aAffectsOrder) {
+  AssertIsOnBackgroundThread();
+
+  for (Database* database : mDatabases) {
+    MOZ_ASSERT(database);
+
+    if (database == aDatabase) {
+      continue;
+    }
+
+    Snapshot* snapshot = database->GetSnapshot();
+    if (snapshot) {
+      snapshot->SaveItem(aKey, aOldValue, aAffectsOrder);
+    }
+  }
+}
+
+void Datastore::NoteChangedDatabaseMap() {
+  AssertIsOnBackgroundThread();
+
+  for (Database* database : mActiveDatabases) {
+    Snapshot* snapshot = database->GetSnapshot();
+    MOZ_ASSERT(snapshot);
+
+    if (snapshot->IsDirty()) {
+      continue;
+    }
+
+    if (snapshot->HasOtherProcessDatabases() !=
+        HasOtherProcessDatabases(database)) {
+      snapshot->MarkDirty();
+    }
+  }
+}
+
+/*******************************************************************************
+ * PreparedDatastore
+ ******************************************************************************/
+
+void PreparedDatastore::Destroy() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(gPreparedDatastores);
+  DebugOnly<bool> removed = gPreparedDatastores->Remove(mDatastoreId);
+  MOZ_ASSERT(removed);
+}
+
+// static
+void PreparedDatastore::TimerCallback(nsITimer* aTimer, void* aClosure) {
+  AssertIsOnBackgroundThread();
+
+  auto* self = static_cast<PreparedDatastore*>(aClosure);
+  MOZ_ASSERT(self);
+
+  self->Destroy();
+}
+
+/*******************************************************************************
+ * Database
+ ******************************************************************************/
+
+Database::Database(const PrincipalInfo& aPrincipalInfo,
+                   const Maybe<ContentParentId>& aContentParentId,
+                   const nsACString& aOrigin, uint32_t aPrivateBrowsingId)
+    : mSnapshot(nullptr),
+      mPrincipalInfo(aPrincipalInfo),
+      mContentParentId(aContentParentId),
+      mOrigin(aOrigin),
+      mPrivateBrowsingId(aPrivateBrowsingId),
+      mAllowedToClose(false),
+      mActorDestroyed(false),
+      mRequestedAllowToClose(false)
+#ifdef DEBUG
+      ,
+      mActorWasAlive(false)
+#endif
+{
+  AssertIsOnBackgroundThread();
+}
+
+Database::~Database() {
+  MOZ_ASSERT_IF(mActorWasAlive, mAllowedToClose);
+  MOZ_ASSERT_IF(mActorWasAlive, mActorDestroyed);
+}
+
+void Database::SetActorAlive(Datastore* aDatastore) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mActorWasAlive);
+  MOZ_ASSERT(!mActorDestroyed);
+
+#ifdef DEBUG
+  mActorWasAlive = true;
+#endif
+
+  mDatastore = aDatastore;
+
+  mDatastore->NoteLiveDatabase(this);
+
+  if (!gLiveDatabases) {
+    gLiveDatabases = new LiveDatabaseArray();
+  }
+
+  gLiveDatabases->AppendElement(WrapNotNullUnchecked(this));
+}
+
+void Database::RegisterSnapshot(Snapshot* aSnapshot) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aSnapshot);
+  MOZ_ASSERT(!mSnapshot);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  // Only one snapshot at a time is currently supported.
+  mSnapshot = aSnapshot;
+
+  mDatastore->NoteActiveDatabase(this);
+}
+
+void Database::UnregisterSnapshot(Snapshot* aSnapshot) {
+  MOZ_ASSERT(aSnapshot);
+  MOZ_ASSERT(mSnapshot == aSnapshot);
+
+  mSnapshot = nullptr;
+
+  mDatastore->NoteInactiveDatabase(this);
+}
+
+void Database::RequestAllowToClose() {
+  AssertIsOnBackgroundThread();
+
+  if (mRequestedAllowToClose) {
+    return;
+  }
+
+  mRequestedAllowToClose = true;
+
+  // Send the RequestAllowToClose message to the child to avoid racing with the
+  // child actor. Except the case when the actor was already destroyed.
+  if (mActorDestroyed) {
+    MOZ_ASSERT(mAllowedToClose);
+    return;
+  }
+
+  if (NS_WARN_IF(!SendRequestAllowToClose()) && !mSnapshot) {
+    // This is not necessary, because there should be a runnable scheduled that
+    // will call ActorDestroy which calls AllowToClose. However we can speedup
+    // the shutdown a bit if we do it here directly, but only if there's no
+    // registered snapshot.
+    AllowToClose();
+  }
+}
+
+void Database::ForceKill() {
+  AssertIsOnBackgroundThread();
+
+  if (mActorDestroyed) {
+    MOZ_ASSERT(mAllowedToClose);
+    return;
+  }
+
+  Unused << PBackgroundLSDatabaseParent::Send__delete__(this);
+}
+
+void Database::Stringify(nsACString& aResult) const {
+  AssertIsOnBackgroundThread();
+
+  aResult.AppendLiteral("SnapshotRegistered:");
+  aResult.AppendInt(!!mSnapshot);
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("OtherProcessActor:");
+  aResult.AppendInt(BackgroundParent::IsOtherProcessActor(Manager()));
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("Origin:");
+  aResult.Append(AnonymizedOriginString(mOrigin));
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("PrivateBrowsingId:");
+  aResult.AppendInt(mPrivateBrowsingId);
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("AllowedToClose:");
+  aResult.AppendInt(mAllowedToClose);
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("ActorDestroyed:");
+  aResult.AppendInt(mActorDestroyed);
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("RequestedAllowToClose:");
+  aResult.AppendInt(mRequestedAllowToClose);
+}
+
+void Database::AllowToClose() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mAllowedToClose);
+  MOZ_ASSERT(mDatastore);
+  MOZ_ASSERT(!mSnapshot);
+
+  mAllowedToClose = true;
+
+  mDatastore->NoteFinishedDatabase(this);
+
+  mDatastore = nullptr;
+
+  MOZ_ASSERT(gLiveDatabases);
+  gLiveDatabases->RemoveElement(this);
+
+  QuotaManager::MaybeRecordQuotaClientShutdownStep(quota::Client::LS,
+                                                   "Live database removed"_ns);
+
+  if (gLiveDatabases->IsEmpty()) {
+    gLiveDatabases = nullptr;
+  }
+}
+
+void Database::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mActorDestroyed);
+
+  mActorDestroyed = true;
+
+  if (!mAllowedToClose) {
+    AllowToClose();
+  }
+}
+
+mozilla::ipc::IPCResult Database::RecvDeleteMe() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mActorDestroyed);
+
+  IProtocol* mgr = Manager();
+  if (!PBackgroundLSDatabaseParent::Send__delete__(this)) {
+    return IPC_FAIL(mgr, "Send__delete__ failed!");
+  }
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult Database::RecvAllowToClose() {
+  AssertIsOnBackgroundThread();
+
+  if (NS_WARN_IF(mAllowedToClose)) {
+    return IPC_FAIL(this, "mAllowedToClose already set!");
+  }
+
+  AllowToClose();
+
+  return IPC_OK();
+}
+
+PBackgroundLSSnapshotParent* Database::AllocPBackgroundLSSnapshotParent(
+    const nsAString& aDocumentURI, const nsAString& aKey,
+    const bool& aIncreasePeakUsage, const int64_t& aMinSize,
+    LSSnapshotInitInfo* aInitInfo) {
+  AssertIsOnBackgroundThread();
+
+  if (NS_WARN_IF(aIncreasePeakUsage && aMinSize < 0)) {
+    MOZ_ASSERT_UNLESS_FUZZING(false);
+    return nullptr;
+  }
+
+  if (NS_WARN_IF(mAllowedToClose)) {
+    MOZ_ASSERT_UNLESS_FUZZING(false);
+    return nullptr;
+  }
+
+  RefPtr<Snapshot> snapshot = new Snapshot(this, aDocumentURI);
+
+  // Transfer ownership to IPDL.
+  return snapshot.forget().take();
+}
+
+mozilla::ipc::IPCResult Database::RecvPBackgroundLSSnapshotConstructor(
+    PBackgroundLSSnapshotParent* aActor, const nsAString& aDocumentURI,
+    const nsAString& aKey, const bool& aIncreasePeakUsage,
+    const int64_t& aMinSize, LSSnapshotInitInfo* aInitInfo) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT_IF(aIncreasePeakUsage, aMinSize >= 0);
+  MOZ_ASSERT(aInitInfo);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  auto* snapshot = static_cast<Snapshot*>(aActor);
+
+  bool addKeyToUnknownItems;
+  nsTHashtable<nsStringHashKey> loadedItems;
+  nsTArray<LSItemInfo> itemInfos;
+  uint32_t nextLoadIndex;
+  LSSnapshot::LoadState loadState;
+  mDatastore->GetSnapshotLoadInfo(aKey, addKeyToUnknownItems, loadedItems,
+                                  itemInfos, nextLoadIndex, loadState);
+
+  nsTHashSet<nsString> unknownItems;
+  if (addKeyToUnknownItems) {
+    unknownItems.Insert(aKey);
+  }
+
+  uint32_t totalLength = mDatastore->GetLength();
+
+  int64_t usage = mDatastore->GetUsage();
+
+  int64_t peakUsage = usage;
+
+  if (aIncreasePeakUsage) {
+    int64_t size =
+        mDatastore->AttemptToUpdateUsage(aMinSize, /* aInitial */ true);
+
+    peakUsage += size;
+  }
+
+  bool hasOtherProcessDatabases = mDatastore->HasOtherProcessDatabases(this);
+  bool hasOtherProcessObservers = mDatastore->HasOtherProcessObservers(this);
+
+  snapshot->Init(loadedItems, std::move(unknownItems), nextLoadIndex,
+                 totalLength, usage, peakUsage, loadState,
+                 hasOtherProcessDatabases, hasOtherProcessObservers);
+
+  RegisterSnapshot(snapshot);
+
+  aInitInfo->addKeyToUnknownItems() = addKeyToUnknownItems;
+  aInitInfo->itemInfos() = std::move(itemInfos);
+  aInitInfo->totalLength() = totalLength;
+  aInitInfo->usage() = usage;
+  aInitInfo->peakUsage() = peakUsage;
+  aInitInfo->loadState() = loadState;
+  aInitInfo->hasOtherProcessDatabases() = hasOtherProcessDatabases;
+  aInitInfo->hasOtherProcessObservers() = hasOtherProcessObservers;
+
+  return IPC_OK();
+}
+
+bool Database::DeallocPBackgroundLSSnapshotParent(
+    PBackgroundLSSnapshotParent* aActor) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aActor);
+
+  // Transfer ownership back from IPDL.
+  RefPtr<Snapshot> actor = dont_AddRef(static_cast<Snapshot*>(aActor));
+
+  return true;
+}
+
+/*******************************************************************************
+ * Snapshot
+ ******************************************************************************/
+
+Snapshot::Snapshot(Database* aDatabase, const nsAString& aDocumentURI)
+    : mDatabase(aDatabase),
+      mDatastore(aDatabase->GetDatastore()),
+      mDocumentURI(aDocumentURI),
+      mTotalLength(0),
+      mUsage(-1),
+      mPeakUsage(-1),
+      mSavedKeys(false),
+      mActorDestroyed(false),
+      mFinishReceived(false),
+      mLoadedReceived(false),
+      mLoadedAllItems(false),
+      mLoadKeysReceived(false),
+      mSentMarkDirty(false) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+}
+
+Snapshot::~Snapshot() {
+  MOZ_ASSERT(mActorDestroyed);
+  MOZ_ASSERT(mFinishReceived);
+}
+
+void Snapshot::SaveItem(const nsAString& aKey, const LSValue& aOldValue,
+                        bool aAffectsOrder) {
+  AssertIsOnBackgroundThread();
+
+  MarkDirty();
+
+  if (mLoadedAllItems) {
+    return;
+  }
+
+  if (!mLoadedItems.Contains(aKey) && !mUnknownItems.Contains(aKey)) {
+    mValues.LookupOrInsert(aKey, aOldValue);
+  }
+
+  if (aAffectsOrder && !mSavedKeys) {
+    mDatastore->GetKeys(mKeys);
+    mSavedKeys = true;
+  }
+}
+
+void Snapshot::MarkDirty() {
+  AssertIsOnBackgroundThread();
+
+  if (!mSentMarkDirty) {
+    Unused << SendMarkDirty();
+    mSentMarkDirty = true;
+  }
+}
+
+void Snapshot::Finish() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(mDatabase);
+  MOZ_ASSERT(mDatastore);
+  MOZ_ASSERT(!mFinishReceived);
+
+  mDatastore->BeginUpdateBatch(mUsage);
+
+  mDatastore->EndUpdateBatch(mPeakUsage);
+
+  mDatabase->UnregisterSnapshot(this);
+
+  mFinishReceived = true;
+}
+
+void Snapshot::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mActorDestroyed);
+
+  mActorDestroyed = true;
+
+  if (!mFinishReceived) {
+    Finish();
+  }
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvDeleteMe() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mActorDestroyed);
+
+  IProtocol* mgr = Manager();
+  if (!PBackgroundLSSnapshotParent::Send__delete__(this)) {
+    return IPC_FAIL(mgr, "Send__delete__ failed!");
+  }
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult Snapshot::Checkpoint(
+    nsTArray<LSWriteInfo>&& aWriteInfos) {
+  AssertIsOnBackgroundThread();
+  // Don't assert `mUsage >= 0`, it can be negative when multiple snapshots are
+  // operating in parallel.
+  MOZ_ASSERT(mPeakUsage >= mUsage);
+
+  if (NS_WARN_IF(aWriteInfos.IsEmpty())) {
+    return IPC_FAIL(this, "aWriteInfos is empty!");
+  }
+
+  if (NS_WARN_IF(mHasOtherProcessObservers)) {
+    return IPC_FAIL(this, "mHasOtherProcessObservers already set!");
+  }
+
+  mDatastore->BeginUpdateBatch(mUsage);
+
+  for (uint32_t index = 0; index < aWriteInfos.Length(); index++) {
+    const LSWriteInfo& writeInfo = aWriteInfos[index];
+
+    switch (writeInfo.type()) {
+      case LSWriteInfo::TLSSetItemInfo: {
+        const LSSetItemInfo& info = writeInfo.get_LSSetItemInfo();
+
+        mDatastore->SetItem(mDatabase, info.key(), info.value());
+
+        break;
+      }
+
+      case LSWriteInfo::TLSRemoveItemInfo: {
+        const LSRemoveItemInfo& info = writeInfo.get_LSRemoveItemInfo();
+
+        mDatastore->RemoveItem(mDatabase, info.key());
+
+        break;
+      }
+
+      case LSWriteInfo::TLSClearInfo: {
+        mDatastore->Clear(mDatabase);
+
+        break;
+      }
+
+      default:
+        MOZ_CRASH("Should never get here!");
+    }
+  }
+
+  mUsage = mDatastore->EndUpdateBatch(-1);
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult Snapshot::CheckpointAndNotify(
+    nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) {
+  AssertIsOnBackgroundThread();
+  // Don't assert `mUsage >= 0`, it can be negative when multiple snapshots are
+  // operating in parallel.
+  MOZ_ASSERT(mPeakUsage >= mUsage);
+
+  if (NS_WARN_IF(aWriteAndNotifyInfos.IsEmpty())) {
+    return IPC_FAIL(this, "aWriteAndNotifyInfos is empty!");
+  }
+
+  if (NS_WARN_IF(!mHasOtherProcessObservers)) {
+    return IPC_FAIL(this, "mHasOtherProcessObservers is not set!");
+  }
+
+  mDatastore->BeginUpdateBatch(mUsage);
+
+  for (uint32_t index = 0; index < aWriteAndNotifyInfos.Length(); index++) {
+    const LSWriteAndNotifyInfo& writeAndNotifyInfo =
+        aWriteAndNotifyInfos[index];
+
+    switch (writeAndNotifyInfo.type()) {
+      case LSWriteAndNotifyInfo::TLSSetItemAndNotifyInfo: {
+        const LSSetItemAndNotifyInfo& info =
+            writeAndNotifyInfo.get_LSSetItemAndNotifyInfo();
+
+        mDatastore->SetItem(mDatabase, info.key(), info.value());
+
+        mDatastore->NotifyOtherProcessObservers(
+            mDatabase, mDocumentURI, info.key(), info.oldValue(), info.value());
+
+        break;
+      }
+
+      case LSWriteAndNotifyInfo::TLSRemoveItemAndNotifyInfo: {
+        const LSRemoveItemAndNotifyInfo& info =
+            writeAndNotifyInfo.get_LSRemoveItemAndNotifyInfo();
+
+        mDatastore->RemoveItem(mDatabase, info.key());
+
+        mDatastore->NotifyOtherProcessObservers(mDatabase, mDocumentURI,
+                                                info.key(), info.oldValue(),
+                                                VoidLSValue());
+
+        break;
+      }
+
+      case LSWriteAndNotifyInfo::TLSClearInfo: {
+        mDatastore->Clear(mDatabase);
+
+        mDatastore->NotifyOtherProcessObservers(mDatabase, mDocumentURI,
+                                                VoidString(), VoidLSValue(),
+                                                VoidLSValue());
+
+        break;
+      }
+
+      default:
+        MOZ_CRASH("Should never get here!");
+    }
+  }
+
+  mUsage = mDatastore->EndUpdateBatch(-1);
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvAsyncCheckpoint(
+    nsTArray<LSWriteInfo>&& aWriteInfos) {
+  return Checkpoint(std::move(aWriteInfos));
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvAsyncCheckpointAndNotify(
+    nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) {
+  return CheckpointAndNotify(std::move(aWriteAndNotifyInfos));
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvSyncCheckpoint(
+    nsTArray<LSWriteInfo>&& aWriteInfos) {
+  return Checkpoint(std::move(aWriteInfos));
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvSyncCheckpointAndNotify(
+    nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) {
+  return CheckpointAndNotify(std::move(aWriteAndNotifyInfos));
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvAsyncFinish() {
+  AssertIsOnBackgroundThread();
+
+  if (NS_WARN_IF(mFinishReceived)) {
+    MOZ_ASSERT_UNLESS_FUZZING(false);
+    return IPC_FAIL(this, "Already finished");
+  }
+
+  Finish();
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvSyncFinish() {
+  AssertIsOnBackgroundThread();
+
+  if (NS_WARN_IF(mFinishReceived)) {
+    MOZ_ASSERT_UNLESS_FUZZING(false);
+    return IPC_FAIL(this, "Already finished");
+  }
+
+  Finish();
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvLoaded() {
+  AssertIsOnBackgroundThread();
+
+  if (NS_WARN_IF(mFinishReceived)) {
+    return IPC_FAIL(this, "mFinishReceived already set!");
+  }
+
+  if (NS_WARN_IF(mLoadedReceived)) {
+    return IPC_FAIL(this, "mLoadedReceived already set!");
+  }
+
+  if (NS_WARN_IF(mLoadedAllItems)) {
+    return IPC_FAIL(this, "mLoadedAllItems already set!");
+  }
+
+  if (NS_WARN_IF(mLoadKeysReceived)) {
+    return IPC_FAIL(this, "mLoadKeysReceived already set!");
+  }
+
+  mLoadedReceived = true;
+
+  mLoadedItems.Clear();
+  mUnknownItems.Clear();
+  mValues.Clear();
+  mKeys.Clear();
+  mLoadedAllItems = true;
+  mLoadKeysReceived = true;
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvLoadValueAndMoreItems(
+    const nsAString& aKey, LSValue* aValue, nsTArray<LSItemInfo>* aItemInfos) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aValue);
+  MOZ_ASSERT(aItemInfos);
+  MOZ_ASSERT(mDatastore);
+
+  if (NS_WARN_IF(mFinishReceived)) {
+    return IPC_FAIL(this, "mFinishReceived already set!");
+  }
+
+  if (NS_WARN_IF(mLoadedReceived)) {
+    return IPC_FAIL(this, "mLoadedReceived already set!");
+  }
+
+  if (NS_WARN_IF(mLoadedAllItems)) {
+    return IPC_FAIL(this, "mLoadedAllItems already set!");
+  }
+
+  if (mLoadedItems.Contains(aKey)) {
+    return IPC_FAIL(this, "mLoadedItems already contains aKey!");
+  }
+
+  if (mUnknownItems.Contains(aKey)) {
+    return IPC_FAIL(this, "mUnknownItems already contains aKey!");
+  }
+
+  if (auto entry = mValues.Lookup(aKey)) {
+    *aValue = entry.Data();
+    entry.Remove();
+  } else {
+    mDatastore->GetItem(aKey, *aValue);
+  }
+
+  if (aValue->IsVoid()) {
+    mUnknownItems.Insert(aKey);
+  } else {
+    mLoadedItems.PutEntry(aKey);
+
+    // mLoadedItems.Count()==mTotalLength is checked below.
+  }
+
+  // Load some more key/value pairs (as many as the snapshot gradual prefill
+  // byte budget allows).
+
+  if (gSnapshotGradualPrefill > 0) {
+    const nsTArray<LSItemInfo>& orderedItems = mDatastore->GetOrderedItems();
+
+    uint32_t length;
+    if (mSavedKeys) {
+      length = mKeys.Length();
+    } else {
+      length = orderedItems.Length();
+    }
+
+    int64_t size = 0;
+    while (mNextLoadIndex < length) {
+      // If the datastore's ordering has changed, mSavedKeys will be true and
+      // mKeys contains an ordered list of the keys. Otherwise we can use the
+      // datastore's key ordering which is still the same as when the snapshot
+      // was created.
+
+      nsString key;
+      if (mSavedKeys) {
+        key = mKeys[mNextLoadIndex];
+      } else {
+        key = orderedItems[mNextLoadIndex].key();
+      }
+
+      // Normally we would do this:
+      // if (!mLoadedItems.GetEntry(key)) {
+      //   ...
+      //   mLoadedItems.PutEntry(key);
+      // }
+      // but that requires two hash lookups. We can reduce that to just one
+      // hash lookup if we always call PutEntry and check the number of entries
+      // before and after the put (which is very cheap). However, if we reach
+      // the prefill limit, we need to call RemoveEntry, but that is also cheap
+      // because we pass the entry (not the key).
+
+      uint32_t countBeforePut = mLoadedItems.Count();
+      auto loadedItemEntry = mLoadedItems.PutEntry(key);
+      if (countBeforePut != mLoadedItems.Count()) {
+        // Check mValues first since that contains values as they existed when
+        // our snapshot was created, but have since been changed/removed in the
+        // datastore. If it's not there, then the datastore has the
+        // still-current value. However, if the datastore's key ordering has
+        // changed, we need to do a hash lookup rather than being able to do an
+        // optimized direct access to the index.
+
+        LSValue value;
+        auto valueEntry = mValues.Lookup(key);
+        if (valueEntry) {
+          value = valueEntry.Data();
+        } else if (mSavedKeys) {
+          mDatastore->GetItem(nsString(key), value);
+        } else {
+          value = orderedItems[mNextLoadIndex].value();
+        }
+
+        // All not loaded keys must have a value.
+        MOZ_ASSERT(!value.IsVoid());
+
+        size += static_cast<int64_t>(key.Length()) +
+                static_cast<int64_t>(value.Length());
+
+        if (size > gSnapshotGradualPrefill) {
+          mLoadedItems.RemoveEntry(loadedItemEntry);
+
+          // mNextLoadIndex is not incremented, so we will resume at the same
+          // position next time.
+          break;
+        }
+
+        if (valueEntry) {
+          valueEntry.Remove();
+        }
+
+        LSItemInfo* itemInfo = aItemInfos->AppendElement();
+        itemInfo->key() = key;
+        itemInfo->value() = value;
+      }
+
+      mNextLoadIndex++;
+    }
+  }
+
+  if (mLoadedItems.Count() == mTotalLength) {
+    mLoadedItems.Clear();
+    mUnknownItems.Clear();
+#ifdef DEBUG
+    const bool allValuesVoid =
+        std::all_of(mValues.Values().cbegin(), mValues.Values().cend(),
+                    [](const auto& entry) { return entry.IsVoid(); });
+    MOZ_ASSERT(allValuesVoid);
+#endif
+    mValues.Clear();
+    mLoadedAllItems = true;
+  }
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvLoadKeys(nsTArray<nsString>* aKeys) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aKeys);
+  MOZ_ASSERT(mDatastore);
+
+  if (NS_WARN_IF(mFinishReceived)) {
+    return IPC_FAIL(this, "mFinishReceived already set!");
+  }
+
+  if (NS_WARN_IF(mLoadedReceived)) {
+    return IPC_FAIL(this, "mLoadedReceived already set!");
+  }
+
+  if (NS_WARN_IF(mLoadKeysReceived)) {
+    return IPC_FAIL(this, "mLoadKeysReceived already set!");
+  }
+
+  mLoadKeysReceived = true;
+
+  if (mSavedKeys) {
+    aKeys->AppendElements(std::move(mKeys));
+  } else {
+    mDatastore->GetKeys(*aKeys);
+  }
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvIncreasePeakUsage(const int64_t& aMinSize,
+                                                        int64_t* aSize) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aSize);
+
+  if (NS_WARN_IF(aMinSize <= 0)) {
+    return IPC_FAIL(this, "aMinSize not valid!");
+  }
+
+  if (NS_WARN_IF(mFinishReceived)) {
+    return IPC_FAIL(this, "mFinishReceived already set!");
+  }
+
+  int64_t size =
+      mDatastore->AttemptToUpdateUsage(aMinSize, /* aInitial */ false);
+
+  mPeakUsage += size;
+
+  *aSize = size;
+
+  return IPC_OK();
+}
+
+/*******************************************************************************
+ * Observer
+ ******************************************************************************/
+
+Observer::Observer(const nsACString& aOrigin)
+    : mOrigin(aOrigin), mActorDestroyed(false) {
+  AssertIsOnBackgroundThread();
+}
+
+Observer::~Observer() { MOZ_ASSERT(mActorDestroyed); }
+
+void Observer::Observe(Database* aDatabase, const nsString& aDocumentURI,
+                       const nsString& aKey, const LSValue& aOldValue,
+                       const LSValue& aNewValue) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(aDatabase);
+
+  Unused << SendObserve(aDatabase->GetPrincipalInfo(),
+                        aDatabase->PrivateBrowsingId(), aDocumentURI, aKey,
+                        aOldValue, aNewValue);
+}
+
+void Observer::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mActorDestroyed);
+
+  mActorDestroyed = true;
+
+  MOZ_ASSERT(gObservers);
+
+  nsTArray<NotNull<Observer*>>* array;
+  gObservers->Get(mOrigin, &array);
+  MOZ_ASSERT(array);
+
+  array->RemoveElement(this);
+
+  if (RefPtr<Datastore> datastore = GetDatastore(mOrigin)) {
+    datastore->NoteChangedObserverArray(*array);
+  }
+
+  if (array->IsEmpty()) {
+    gObservers->Remove(mOrigin);
+  }
+
+  if (!gObservers->Count()) {
+    gObservers = nullptr;
+  }
+}
+
+mozilla::ipc::IPCResult Observer::RecvDeleteMe() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!mActorDestroyed);
+
+  IProtocol* mgr = Manager();
+  if (!PBackgroundLSObserverParent::Send__delete__(this)) {
+    return IPC_FAIL(mgr, "Send__delete__ failed!");
+  }
+  return IPC_OK();
+}
+
+/*******************************************************************************
+ * LSRequestBase
+ ******************************************************************************/
+
+LSRequestBase::LSRequestBase(const LSRequestParams& aParams,
+                             const Maybe<ContentParentId>& aContentParentId)
+    : mParams(aParams),
+      mContentParentId(aContentParentId),
+      mState(State::Initial),
+      mWaitingForFinish(false) {}
+
+LSRequestBase::~LSRequestBase() {
+  MOZ_ASSERT_IF(MayProceedOnNonOwningThread(),
+                mState == State::Initial || mState == State::Completed);
+}
+
+void LSRequestBase::Dispatch() {
+  AssertIsOnOwningThread();
+
+  mState = State::StartingRequest;
+
+  MOZ_ALWAYS_SUCCEEDS(NS_DispatchToCurrentThread(this));
+}
+
+void LSRequestBase::StringifyState(nsACString& aResult) const {
+  AssertIsOnOwningThread();
+
+  switch (mState) {
+    case State::Initial:
+      aResult.AppendLiteral("Initial");
+      return;
+
+    case State::StartingRequest:
+      aResult.AppendLiteral("StartingRequest");
+      return;
+
+    case State::Nesting:
+      aResult.AppendLiteral("Nesting");
+      return;
+
+    case State::SendingReadyMessage:
+      aResult.AppendLiteral("SendingReadyMessage");
+      return;
+
+    case State::WaitingForFinish:
+      aResult.AppendLiteral("WaitingForFinish");
+      return;
+
+    case State::SendingResults:
+      aResult.AppendLiteral("SendingResults");
+      return;
+
+    case State::Completed:
+      aResult.AppendLiteral("Completed");
+      return;
+
+    default:
+      MOZ_CRASH("Bad state!");
+  }
+}
+
+void LSRequestBase::Stringify(nsACString& aResult) const {
+  AssertIsOnOwningThread();
+
+  aResult.AppendLiteral("State:");
+  StringifyState(aResult);
+}
+
+void LSRequestBase::Log() {
+  AssertIsOnOwningThread();
+
+  if (!LS_LOG_TEST()) {
+    return;
+  }
+
+  LS_LOG(("LSRequestBase [%p]", this));
+
+  nsCString state;
+  StringifyState(state);
+
+  LS_LOG(("  mState: %s", state.get()));
+}
+
+nsresult LSRequestBase::NestedRun() { return NS_OK; }
+
+bool LSRequestBase::VerifyRequestParams() {
+  AssertIsOnBackgroundThread();
+
+  MOZ_ASSERT(mParams.type() != LSRequestParams::T__None);
+
+  switch (mParams.type()) {
+    case LSRequestParams::TLSRequestPreloadDatastoreParams: {
+      const LSRequestCommonParams& params =
+          mParams.get_LSRequestPreloadDatastoreParams().commonParams();
+
+      if (NS_WARN_IF(!VerifyPrincipalInfo(
+              params.principalInfo(), params.storagePrincipalInfo(), false))) {
+        return false;
+      }
+
+      if (NS_WARN_IF(
+              !VerifyOriginKey(params.originKey(), params.principalInfo()))) {
+        return false;
+      }
+
+      break;
+    }
+
+    case LSRequestParams::TLSRequestPrepareDatastoreParams: {
+      const LSRequestPrepareDatastoreParams& params =
+          mParams.get_LSRequestPrepareDatastoreParams();
+
+      const LSRequestCommonParams& commonParams = params.commonParams();
+
+      if (NS_WARN_IF(!VerifyPrincipalInfo(commonParams.principalInfo(),
+                                          commonParams.storagePrincipalInfo(),
+                                          false))) {
+        return false;
+      }
+
+      if (params.clientPrincipalInfo() &&
+          NS_WARN_IF(!VerifyPrincipalInfo(commonParams.principalInfo(),
+                                          params.clientPrincipalInfo().ref(),
+                                          true))) {
+        return false;
+      }
+
+      if (NS_WARN_IF(!VerifyClientId(mContentParentId,
+                                     params.clientPrincipalInfo(),
+                                     params.clientId()))) {
+        return false;
+      }
+
+      if (NS_WARN_IF(!VerifyOriginKey(commonParams.originKey(),
+                                      commonParams.principalInfo()))) {
+        return false;
+      }
+
+      break;
+    }
+
+    case LSRequestParams::TLSRequestPrepareObserverParams: {
+      const LSRequestPrepareObserverParams& params =
+          mParams.get_LSRequestPrepareObserverParams();
+
+      if (NS_WARN_IF(!VerifyPrincipalInfo(
+              params.principalInfo(), params.storagePrincipalInfo(), false))) {
+        return false;
+      }
+
+      if (params.clientPrincipalInfo() &&
+          NS_WARN_IF(!VerifyPrincipalInfo(params.principalInfo(),
+                                          params.clientPrincipalInfo().ref(),
+                                          true))) {
+        return false;
+      }
+
+      if (NS_WARN_IF(!VerifyClientId(mContentParentId,
+                                     params.clientPrincipalInfo(),
+                                     params.clientId()))) {
+        return false;
+      }
+
+      break;
+    }
+
+    default:
+      MOZ_CRASH("Should never get here!");
+  }
+
+  return true;
+}
+
+nsresult LSRequestBase::StartRequest() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::StartingRequest);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    return NS_ERROR_ABORT;
+  }
+
+#ifdef DEBUG
+  // Always verify parameters in DEBUG builds!
+  bool trustParams = false;
+#else
+  bool trustParams = !BackgroundParent::IsOtherProcessActor(Manager());
+#endif
+
+  if (!trustParams && NS_WARN_IF(!VerifyRequestParams())) {
+    return NS_ERROR_FAILURE;
+  }
+
+  QM_TRY(MOZ_TO_RESULT(Start()));
+
+  return NS_OK;
+}
+
+void LSRequestBase::SendReadyMessage() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingReadyMessage);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    MaybeSetFailureCode(NS_ERROR_ABORT);
+  }
+
+  nsresult rv = SendReadyMessageInternal();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    MaybeSetFailureCode(rv);
+
+    FinishInternal();
+  }
+}
+
+nsresult LSRequestBase::SendReadyMessageInternal() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingReadyMessage);
+
+  if (!MayProceed()) {
+    return NS_ERROR_ABORT;
+  }
+
+  if (NS_WARN_IF(!SendReady())) {
+    return NS_ERROR_FAILURE;
+  }
+
+  mState = State::WaitingForFinish;
+
+  mWaitingForFinish = true;
+
+  return NS_OK;
+}
+
+void LSRequestBase::Finish() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::WaitingForFinish);
+
+  mWaitingForFinish = false;
+
+  FinishInternal();
+}
+
+void LSRequestBase::FinishInternal() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingReadyMessage ||
+             mState == State::WaitingForFinish);
+
+  mState = State::SendingResults;
+
+  // This LSRequestBase can only be held alive by the IPDL. Run() can end up
+  // with clearing that last reference. So we need to add a self reference here.
+  RefPtr<LSRequestBase> kungFuDeathGrip = this;
+
+  MOZ_ALWAYS_SUCCEEDS(this->Run());
+}
+
+void LSRequestBase::SendResults() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingResults);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    MaybeSetFailureCode(NS_ERROR_ABORT);
+  }
+
+  if (MayProceed()) {
+    LSRequestResponse response;
+
+    if (NS_SUCCEEDED(ResultCode())) {
+      GetResponse(response);
+
+      MOZ_ASSERT(response.type() != LSRequestResponse::T__None);
+
+      if (response.type() == LSRequestResponse::Tnsresult) {
+        MOZ_ASSERT(NS_FAILED(response.get_nsresult()));
+
+        SetFailureCode(response.get_nsresult());
+      }
+    } else {
+      response = ResultCode();
+    }
+
+    Unused << PBackgroundLSRequestParent::Send__delete__(this, response);
+  }
+
+  Cleanup();
+
+  mState = State::Completed;
+}
+
+NS_IMETHODIMP
+LSRequestBase::Run() {
+  nsresult rv;
+
+  switch (mState) {
+    case State::StartingRequest:
+      rv = StartRequest();
+      break;
+
+    case State::Nesting:
+      rv = NestedRun();
+      break;
+
+    case State::SendingReadyMessage:
+      SendReadyMessage();
+      return NS_OK;
+
+    case State::SendingResults:
+      SendResults();
+      return NS_OK;
+
+    default:
+      MOZ_CRASH("Bad state!");
+  }
+
+  if (NS_WARN_IF(NS_FAILED(rv)) && mState != State::SendingReadyMessage) {
+    MaybeSetFailureCode(rv);
+
+    // Must set mState before dispatching otherwise we will race with the owning
+    // thread.
+    mState = State::SendingReadyMessage;
+
+    if (IsOnOwningThread()) {
+      SendReadyMessage();
+    } else {
+      MOZ_ALWAYS_SUCCEEDS(
+          OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL));
+    }
+  }
+
+  return NS_OK;
+}
+
+void LSRequestBase::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnOwningThread();
+
+  NoteComplete();
+
+  // Assume ActorDestroy can happen at any time, so we can't probe the current
+  // state since mState can be modified on any thread (only one thread at a time
+  // based on the state machine).  However we can use mWaitingForFinish which is
+  // only touched on the owning thread.  If mWaitingForFinisg is true, we can
+  // also modify mState since we are guaranteed that there are no pending
+  // runnables which would probe mState to decide what code needs to run (there
+  // shouldn't be any running runnables on other threads either).
+
+  if (mWaitingForFinish) {
+    Finish();
+  }
+
+  // We don't have to handle the case when mWaitingForFinish is not true since
+  // it means that either nothing has been initialized yet, so nothing to
+  // cleanup or there are pending runnables that will detect that the actor has
+  // been destroyed and cleanup accordingly.
+}
+
+mozilla::ipc::IPCResult LSRequestBase::RecvCancel() {
+  AssertIsOnOwningThread();
+
+  Log();
+
+  const char* crashOnCancel = PR_GetEnv("LSNG_CRASH_ON_CANCEL");
+  if (crashOnCancel) {
+    MOZ_CRASH("LSNG: Crash on cancel.");
+  }
+
+  IProtocol* mgr = Manager();
+  if (!PBackgroundLSRequestParent::Send__delete__(this, NS_ERROR_ABORT)) {
+    return IPC_FAIL(mgr, "Send__delete__ failed!");
+  }
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult LSRequestBase::RecvFinish() {
+  AssertIsOnOwningThread();
+
+  Finish();
+
+  return IPC_OK();
+}
+
+/*******************************************************************************
+ * PrepareDatastoreOp
+ ******************************************************************************/
+
+PrepareDatastoreOp::PrepareDatastoreOp(
+    const LSRequestParams& aParams,
+    const Maybe<ContentParentId>& aContentParentId)
+    : LSRequestBase(aParams, aContentParentId),
+      mLoadDataOp(nullptr),
+      mPrivateBrowsingId(0),
+      mUsage(0),
+      mSizeOfKeys(0),
+      mSizeOfItems(0),
+      mDatastoreId(0),
+      mNestedState(NestedState::BeforeNesting),
+      mForPreload(aParams.type() ==
+                  LSRequestParams::TLSRequestPreloadDatastoreParams),
+      mDatabaseNotAvailable(false),
+      mInvalidated(false)
+#ifdef DEBUG
+      ,
+      mDEBUGUsage(0)
+#endif
+{
+  MOZ_ASSERT(
+      aParams.type() == LSRequestParams::TLSRequestPreloadDatastoreParams ||
+      aParams.type() == LSRequestParams::TLSRequestPrepareDatastoreParams);
+}
+
+PrepareDatastoreOp::~PrepareDatastoreOp() {
+  MOZ_ASSERT(!mDirectoryLock);
+  MOZ_ASSERT_IF(MayProceedOnNonOwningThread(),
+                mState == State::Initial || mState == State::Completed);
+  MOZ_ASSERT(!mLoadDataOp);
+}
+
+void PrepareDatastoreOp::StringifyNestedState(nsACString& aResult) const {
+  AssertIsOnOwningThread();
+
+  switch (mNestedState) {
+    case NestedState::BeforeNesting:
+      aResult.AppendLiteral("BeforeNesting");
+      return;
+
+    case NestedState::CheckExistingOperations:
+      aResult.AppendLiteral("CheckExistingOperations");
+      return;
+
+    case NestedState::CheckClosingDatastore:
+      aResult.AppendLiteral("CheckClosingDatastore");
+      return;
+
+    case NestedState::PreparationPending:
+      aResult.AppendLiteral("PreparationPending");
+      return;
+
+    case NestedState::DirectoryOpenPending:
+      aResult.AppendLiteral("DirectoryOpenPending");
+      return;
+
+    case NestedState::DatabaseWorkOpen:
+      aResult.AppendLiteral("DatabaseWorkOpen");
+      return;
+
+    case NestedState::BeginLoadData:
+      aResult.AppendLiteral("BeginLoadData");
+      return;
+
+    case NestedState::DatabaseWorkLoadData:
+      aResult.AppendLiteral("DatabaseWorkLoadData");
+      return;
+
+    case NestedState::AfterNesting:
+      aResult.AppendLiteral("AfterNesting");
+      return;
+
+    default:
+      MOZ_CRASH("Bad state!");
+  }
+}
+
+void PrepareDatastoreOp::Stringify(nsACString& aResult) const {
+  AssertIsOnOwningThread();
+
+  LSRequestBase::Stringify(aResult);
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("Origin:");
+  aResult.Append(AnonymizedOriginString(Origin()));
+  aResult.Append(kQuotaGenericDelimiter);
+
+  aResult.AppendLiteral("NestedState:");
+  StringifyNestedState(aResult);
+}
+
+void PrepareDatastoreOp::Log() {
+  AssertIsOnOwningThread();
+
+  LSRequestBase::Log();
+
+  if (!LS_LOG_TEST()) {
+    return;
+  }
+
+  nsCString nestedState;
+  StringifyNestedState(nestedState);
+
+  LS_LOG(("  mNestedState: %s", nestedState.get()));
+
+  switch (mNestedState) {
+    case NestedState::CheckClosingDatastore: {
+      for (uint32_t index = gPrepareDatastoreOps->Length(); index > 0;
+           index--) {
+        const auto& existingOp = (*gPrepareDatastoreOps)[index - 1];
+
+        if (existingOp->mDelayedOp == this) {
+          LS_LOG(("  mDelayedBy: [%p]",
+                  static_cast<PrepareDatastoreOp*>(existingOp.get())));
+
+          existingOp->Log();
+
+          break;
+        }
+      }
+
+      break;
+    }
+
+    case NestedState::DirectoryOpenPending: {
+      MOZ_ASSERT(mPendingDirectoryLock);
+
+      LS_LOG(("  mPendingDirectoryLock: [%p]", mPendingDirectoryLock.get()));
+
+      mPendingDirectoryLock->Log();
+
+      break;
+    }
+
+    default:;
+  }
+}
+
+nsresult PrepareDatastoreOp::Start() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::StartingRequest);
+  MOZ_ASSERT(mNestedState == NestedState::BeforeNesting);
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  QM_TRY(QuotaManager::EnsureCreated());
+
+  const LSRequestCommonParams& commonParams =
+      mForPreload
+          ? mParams.get_LSRequestPreloadDatastoreParams().commonParams()
+          : mParams.get_LSRequestPrepareDatastoreParams().commonParams();
+
+  const PrincipalInfo& storagePrincipalInfo =
+      commonParams.storagePrincipalInfo();
+
+  if (storagePrincipalInfo.type() == PrincipalInfo::TSystemPrincipalInfo) {
+    mOriginMetadata = {QuotaManager::GetInfoForChrome(),
+                       PERSISTENCE_TYPE_DEFAULT};
+  } else {
+    MOZ_ASSERT(storagePrincipalInfo.type() ==
+               PrincipalInfo::TContentPrincipalInfo);
+
+    QM_TRY_UNWRAP(auto principalMetadata,
+                  QuotaManager::Get()->GetInfoFromValidatedPrincipalInfo(
+                      storagePrincipalInfo));
+
+    mOriginMetadata.mSuffix = std::move(principalMetadata.mSuffix);
+    mOriginMetadata.mGroup = std::move(principalMetadata.mGroup);
+    // XXX We can probably get rid of mMainThreadOrigin if we change
+    // LSRequestBase::Dispatch to synchronously run LSRequestBase::StartRequest
+    // through LSRequestBase::Run.
+    mMainThreadOrigin = std::move(principalMetadata.mOrigin);
+    mOriginMetadata.mStorageOrigin =
+        std::move(principalMetadata.mStorageOrigin);
+    mOriginMetadata.mIsPrivate = principalMetadata.mIsPrivate;
+    mOriginMetadata.mPersistenceType = principalMetadata.mIsPrivate
+                                           ? PERSISTENCE_TYPE_PRIVATE
+                                           : PERSISTENCE_TYPE_DEFAULT;
+  }
+
+  mState = State::Nesting;
+  mNestedState = NestedState::CheckExistingOperations;
+
+  MOZ_ALWAYS_SUCCEEDS(OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL));
+
+  return NS_OK;
+}
+
+nsresult PrepareDatastoreOp::CheckExistingOperations() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::CheckExistingOperations);
+  MOZ_ASSERT(gPrepareDatastoreOps);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    return NS_ERROR_ABORT;
+  }
+
+  const LSRequestCommonParams& commonParams =
+      mForPreload
+          ? mParams.get_LSRequestPreloadDatastoreParams().commonParams()
+          : mParams.get_LSRequestPrepareDatastoreParams().commonParams();
+
+  const PrincipalInfo& storagePrincipalInfo =
+      commonParams.storagePrincipalInfo();
+
+  nsCString originAttrSuffix;
+  uint32_t privateBrowsingId;
+
+  if (storagePrincipalInfo.type() == PrincipalInfo::TSystemPrincipalInfo) {
+    privateBrowsingId = 0;
+  } else {
+    MOZ_ASSERT(storagePrincipalInfo.type() ==
+               PrincipalInfo::TContentPrincipalInfo);
+
+    const ContentPrincipalInfo& info =
+        storagePrincipalInfo.get_ContentPrincipalInfo();
+    const OriginAttributes& attrs = info.attrs();
+    attrs.CreateSuffix(originAttrSuffix);
+
+    privateBrowsingId = attrs.mPrivateBrowsingId;
+  }
+
+  mArchivedOriginScope = ArchivedOriginScope::CreateFromOrigin(
+      originAttrSuffix, commonParams.originKey());
+  MOZ_ASSERT(mArchivedOriginScope);
+
+  // Normally it's safe to access member variables without a mutex because even
+  // though we hop between threads, the variables are never accessed by multiple
+  // threads at the same time.
+  // However, the methods OriginIsKnown and Origin can be called at any time.
+  // So we have to make sure the member variable is set on the same thread as
+  // those methods are called.
+  mOriginMetadata.mOrigin = mMainThreadOrigin;
+
+  MOZ_ASSERT(OriginIsKnown());
+
+  mPrivateBrowsingId = privateBrowsingId;
+
+  mNestedState = NestedState::CheckClosingDatastore;
+
+  // See if this PrepareDatastoreOp needs to wait.
+  bool foundThis = false;
+  for (uint32_t index = gPrepareDatastoreOps->Length(); index > 0; index--) {
+    const auto& existingOp = (*gPrepareDatastoreOps)[index - 1];
+
+    if (existingOp == this) {
+      foundThis = true;
+      continue;
+    }
+
+    if (foundThis && existingOp->Origin() == Origin()) {
+      // Only one op can be delayed.
+      MOZ_ASSERT(!existingOp->mDelayedOp);
+      existingOp->mDelayedOp = this;
+
+      return NS_OK;
+    }
+  }
+
+  QM_TRY(MOZ_TO_RESULT(CheckClosingDatastoreInternal()));
+
+  return NS_OK;
+}
+
+nsresult PrepareDatastoreOp::CheckClosingDatastore() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::CheckClosingDatastore);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    return NS_ERROR_ABORT;
+  }
+
+  QM_TRY(MOZ_TO_RESULT(CheckClosingDatastoreInternal()));
+
+  return NS_OK;
+}
+
+nsresult PrepareDatastoreOp::CheckClosingDatastoreInternal() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::CheckClosingDatastore);
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  mNestedState = NestedState::PreparationPending;
+
+  RefPtr<Datastore> datastore;
+  if ((datastore = GetDatastore(Origin())) && datastore->IsClosed()) {
+    datastore->WaitForConnectionToComplete(this);
+
+    return NS_OK;
+  }
+
+  QM_TRY(MOZ_TO_RESULT(BeginDatastorePreparationInternal()));
+
+  return NS_OK;
+}
+
+nsresult PrepareDatastoreOp::BeginDatastorePreparation() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::PreparationPending);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    return NS_ERROR_ABORT;
+  }
+
+  QM_TRY(MOZ_TO_RESULT(BeginDatastorePreparationInternal()));
+
+  return NS_OK;
+}
+
+nsresult PrepareDatastoreOp::BeginDatastorePreparationInternal() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::PreparationPending);
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+  MOZ_ASSERT(OriginIsKnown());
+  MOZ_ASSERT(!mDirectoryLock);
+
+  if ((mDatastore = GetDatastore(Origin()))) {
+    MOZ_ASSERT(!mDatastore->IsClosed());
+
+    mDatastore->NoteLivePrepareDatastoreOp(this);
+
+    FinishNesting();
+
+    return NS_OK;
+  }
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  // Open directory
+  mPendingDirectoryLock = quotaManager->CreateDirectoryLock(
+      mOriginMetadata.mPersistenceType, mOriginMetadata,
+      mozilla::dom::quota::Client::LS,
+      /* aExclusive */ false);
+
+  mNestedState = NestedState::DirectoryOpenPending;
+
+  {
+    // Pin the directory lock, because Acquire might clear mPendingDirectoryLock
+    // during the Acquire call.
+    RefPtr pinnedDirectoryLock = mPendingDirectoryLock;
+    pinnedDirectoryLock->Acquire(this);
+  }
+
+  return NS_OK;
+}
+
+void PrepareDatastoreOp::SendToIOThread() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::DirectoryOpenPending);
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  // Skip all disk related stuff and transition to SendingReadyMessage if we
+  // are preparing a datastore for private browsing.
+  // Note that we do use a directory lock for private browsing even though we
+  // don't do any stuff on disk. The thing is that without a directory lock,
+  // quota manager wouldn't call AbortOperationsForLocks for our private
+  // browsing origin when a clear origin operation is requested.
+  // AbortOperationsForLocks requests all databases to close and the datastore
+  // is destroyed in the end. Any following LocalStorage API call will trigger
+  // preparation of a new (empty) datastore.
+  if (mPrivateBrowsingId) {
+    FinishNesting();
+
+    return;
+  }
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  // Must set this before dispatching otherwise we will race with the IO thread.
+  mNestedState = NestedState::DatabaseWorkOpen;
+
+  MOZ_ALWAYS_SUCCEEDS(
+      quotaManager->IOThread()->Dispatch(this, NS_DISPATCH_NORMAL));
+}
+
+nsresult PrepareDatastoreOp::DatabaseWork() {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(mArchivedOriginScope);
+  MOZ_ASSERT(mUsage == 0);
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::DatabaseWorkOpen);
+
+  const auto innerFunc = [&](const auto&) -> nsresult {
+    // XXX This function is too long, refactor it into helper functions for
+    // readability.
+
+    if (NS_WARN_IF(QuotaClient::IsShuttingDownOnNonBackgroundThread()) ||
+        !MayProceedOnNonOwningThread()) {
+      return NS_ERROR_ABORT;
+    }
+
+    QuotaManager* quotaManager = QuotaManager::Get();
+    MOZ_ASSERT(quotaManager);
+
+    // This must be called before EnsureTemporaryStorageIsInitialized.
+    QM_TRY(MOZ_TO_RESULT(quotaManager->EnsureStorageIsInitialized()));
+
+    // This ensures that usages for existings origin directories are cached in
+    // memory.
+    QM_TRY(MOZ_TO_RESULT(quotaManager->EnsureTemporaryStorageIsInitialized()));
+
+    const UsageInfo usageInfo = quotaManager->GetUsageForClient(
+        PERSISTENCE_TYPE_DEFAULT, mOriginMetadata,
+        mozilla::dom::quota::Client::LS);
+
+    const bool hasUsage = usageInfo.DatabaseUsage().isSome();
+    MOZ_ASSERT(usageInfo.FileUsage().isNothing());
+
+    if (!gArchivedOrigins) {
+      QM_TRY(MOZ_TO_RESULT(LoadArchivedOrigins()));
+      MOZ_ASSERT(gArchivedOrigins);
+    }
+
+    bool hasDataForMigration =
+        mArchivedOriginScope->HasMatches(gArchivedOrigins);
+
+    // If there's nothing to preload (except the case when we want to migrate
+    // data during preloading), then we can finish the operation without
+    // creating a datastore in GetResponse (GetResponse won't create a datastore
+    // if mDatatabaseNotAvailable and mForPreload are both true).
+    if (mForPreload && !hasUsage && !hasDataForMigration) {
+      return DatabaseNotAvailable();
+    }
+
+    // The origin directory doesn't need to be created when we don't have data
+    // for migration. It will be created on the connection thread in
+    // Connection::EnsureStorageConnection.
+    // However, origin quota must be initialized, GetQuotaObject in GetResponse
+    // would fail otherwise.
+    QM_TRY_INSPECT(
+        const auto& directoryEntry,
+        ([hasDataForMigration, &quotaManager,
+          this]() -> mozilla::Result<nsCOMPtr<nsIFile>, nsresult> {
+          if (hasDataForMigration) {
+            QM_TRY_RETURN(quotaManager
+                              ->EnsureTemporaryOriginIsInitialized(
+                                  PERSISTENCE_TYPE_DEFAULT, mOriginMetadata)
+                              .map([](const auto& res) { return res.first; }));
+          }
+
+          MOZ_ASSERT(mOriginMetadata.mPersistenceType ==
+                     PERSISTENCE_TYPE_DEFAULT);
+
+          QM_TRY_UNWRAP(auto directoryEntry,
+                        quotaManager->GetOriginDirectory(mOriginMetadata));
+
+          quotaManager->EnsureQuotaForOrigin(mOriginMetadata);
+
+          return directoryEntry;
+        }()));
+
+    QM_TRY(MOZ_TO_RESULT(directoryEntry->Append(
+        NS_LITERAL_STRING_FROM_CSTRING(LS_DIRECTORY_NAME))));
+
+    QM_TRY_INSPECT(
+        const auto& directoryPath,
+        MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(nsString, directoryEntry, GetPath));
+
+    // The ls directory doesn't need to be created when we don't have data for
+    // migration. It will be created on the connection thread in
+    // Connection::EnsureStorageConnection.
+    QM_TRY(MOZ_TO_RESULT(
+        EnsureDirectoryEntry(directoryEntry,
+                             /* aCreateIfNotExists */ hasDataForMigration,
+                             /* aIsDirectory */ true)));
+
+    QM_TRY(MOZ_TO_RESULT(directoryEntry->Append(kDataFileName)));
+
+    QM_TRY(MOZ_TO_RESULT(directoryEntry->GetPath(mDatabaseFilePath)));
+
+    // The database doesn't need to be created when we don't have data for
+    // migration. It will be created on the connection thread in
+    // Connection::EnsureStorageConnection.
+    bool alreadyExisted;
+    QM_TRY(MOZ_TO_RESULT(
+        EnsureDirectoryEntry(directoryEntry,
+                             /* aCreateIfNotExists */ hasDataForMigration,
+                             /* aIsDirectory */ false, &alreadyExisted)));
+
+    if (alreadyExisted) {
+      // The database does exist.
+      MOZ_ASSERT(hasUsage);
+
+      // XXX Change type of mUsage to UsageInfo or DatabaseUsageType.
+      mUsage = usageInfo.DatabaseUsage().valueOr(0);
+    } else {
+      // The database doesn't exist.
+      MOZ_ASSERT(!hasUsage);
+
+      if (!hasDataForMigration) {
+        // The database doesn't exist and we don't have data for migration.
+        // Finish the operation, but create an empty datastore in GetResponse
+        // (GetResponse will create an empty datastore if mDatabaseNotAvailable
+        // is true and mForPreload is false).
+        return DatabaseNotAvailable();
+      }
+    }
+
+    // We initialized mDatabaseFilePath and mUsage, GetQuotaObject can now be
+    // called.
+    const RefPtr<QuotaObject> quotaObject = GetQuotaObject();
+
+    QM_TRY(OkIf(quotaObject), Err(NS_ERROR_FAILURE));
+
+    QM_TRY_INSPECT(const auto& usageFile, GetUsageFile(directoryPath));
+
+    QM_TRY_INSPECT(const auto& usageJournalFile,
+                   GetUsageJournalFile(directoryPath));
+
+    QM_TRY_INSPECT(
+        const auto& connection,
+        (CreateStorageConnection(
+            *directoryEntry, *usageFile, Origin(), [&quotaObject, this] {
+              // This is called when the usage file was removed or we notice
+              // that the usage file doesn't exist anymore. Adjust the usage
+              // accordingly.
+
+              MOZ_ALWAYS_TRUE(
+                  quotaObject->MaybeUpdateSize(0, /* aTruncate */ true));
+
+              mUsage = 0;
+            })));
+
+    QM_TRY(MOZ_TO_RESULT(VerifyDatabaseInformation(connection)));
+
+    if (hasDataForMigration) {
+      MOZ_ASSERT(mUsage == 0);
+
+      {
+        QM_TRY_INSPECT(const auto& archiveFile,
+                       GetArchiveFile(quotaManager->GetStoragePath()));
+
+        auto autoArchiveDatabaseAttacher =
+            AutoDatabaseAttacher(connection, archiveFile, "archive"_ns);
+
+        QM_TRY(MOZ_TO_RESULT(autoArchiveDatabaseAttacher.Attach()));
+
+        QM_TRY_INSPECT(const int64_t& newUsage,
+                       GetUsage(*connection, mArchivedOriginScope.get()));
+
+        if (!quotaObject->MaybeUpdateSize(newUsage, /* aTruncate */ true)) {
+          return NS_ERROR_FILE_NO_DEVICE_SPACE;
+        }
+
+        auto autoUpdateSize = MakeScopeExit([&quotaObject] {
+          MOZ_ALWAYS_TRUE(
+              quotaObject->MaybeUpdateSize(0, /* aTruncate */ true));
+        });
+
+        mozStorageTransaction transaction(
+            connection, false, mozIStorageConnection::TRANSACTION_IMMEDIATE);
+
+        QM_TRY(MOZ_TO_RESULT(transaction.Start()));
+
+        {
+          nsCOMPtr<mozIStorageFunction> function = new CompressFunction();
+
+          QM_TRY(MOZ_TO_RESULT(
+              connection->CreateFunction("compress"_ns, 1, function)));
+
+          function = new CompressionTypeFunction();
+
+          QM_TRY(MOZ_TO_RESULT(
+              connection->CreateFunction("compressionType"_ns, 1, function)));
+
+          QM_TRY_INSPECT(
+              const auto& stmt,
+              MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                  nsCOMPtr<mozIStorageStatement>, connection, CreateStatement,
+                  "INSERT INTO data (key, utf16_length, conversion_type, "
+                  "compression_type, value) "
+                  "SELECT key, utf16Length(value), :conversionType, "
+                  "compressionType(value), compress(value)"
+                  "FROM webappsstore2 "
+                  "WHERE originKey = :originKey "
+                  "AND originAttributes = :originAttributes;"_ns));
+
+          QM_TRY(MOZ_TO_RESULT(stmt->BindInt32ByName(
+              "conversionType"_ns,
+              static_cast<int32_t>(LSValue::ConversionType::UTF16_UTF8))));
+
+          QM_TRY(MOZ_TO_RESULT(mArchivedOriginScope->BindToStatement(stmt)));
+
+          QM_TRY(MOZ_TO_RESULT(stmt->Execute()));
+
+          QM_TRY(MOZ_TO_RESULT(connection->RemoveFunction("compress"_ns)));
+
+          QM_TRY(
+              MOZ_TO_RESULT(connection->RemoveFunction("compressionType"_ns)));
+        }
+
+        {
+          QM_TRY_INSPECT(
+              const auto& stmt,
+              MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                  nsCOMPtr<mozIStorageStatement>, connection, CreateStatement,
+                  "UPDATE database SET usage = :usage;"_ns));
+
+          QM_TRY(MOZ_TO_RESULT(stmt->BindInt64ByName("usage"_ns, newUsage)));
+
+          QM_TRY(MOZ_TO_RESULT(stmt->Execute()));
+        }
+
+        {
+          QM_TRY_INSPECT(
+              const auto& stmt,
+              MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                  nsCOMPtr<mozIStorageStatement>, connection, CreateStatement,
+                  "DELETE FROM webappsstore2 "
+                  "WHERE originKey = :originKey "
+                  "AND originAttributes = :originAttributes;"_ns));
+
+          QM_TRY(MOZ_TO_RESULT(mArchivedOriginScope->BindToStatement(stmt)));
+          QM_TRY(MOZ_TO_RESULT(stmt->Execute()));
+        }
+
+        QM_TRY(MOZ_TO_RESULT(
+            UpdateUsageFile(usageFile, usageJournalFile, newUsage)));
+        QM_TRY(MOZ_TO_RESULT(transaction.Commit()));
+
+        autoUpdateSize.release();
+
+        QM_TRY(MOZ_TO_RESULT(usageJournalFile->Remove(false)));
+
+        mUsage = newUsage;
+
+        QM_TRY(MOZ_TO_RESULT(autoArchiveDatabaseAttacher.Detach()));
+      }
+
+      MOZ_ASSERT(gArchivedOrigins);
+      MOZ_ASSERT(mArchivedOriginScope->HasMatches(gArchivedOrigins));
+      mArchivedOriginScope->RemoveMatches(gArchivedOrigins);
+    }
+
+    nsCOMPtr<mozIStorageConnection> shadowConnection;
+    if (!gInitializedShadowStorage) {
+      QM_TRY_UNWRAP(shadowConnection,
+                    CreateShadowStorageConnection(quotaManager->GetBasePath()));
+
+      gInitializedShadowStorage = true;
+    }
+
+    // Must close connections before dispatching otherwise we might race with
+    // the connection thread which needs to open the same databases.
+    MOZ_ALWAYS_SUCCEEDS(connection->Close());
+
+    if (shadowConnection) {
+      MOZ_ALWAYS_SUCCEEDS(shadowConnection->Close());
+    }
+
+    // Must set this before dispatching otherwise we will race with the owning
+    // thread.
+    mNestedState = NestedState::BeginLoadData;
+
+    QM_TRY(
+        MOZ_TO_RESULT(OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL)));
+
+    return NS_OK;
+  };
+
+  return ExecuteOriginInitialization(
+      mOriginMetadata.mOrigin, LSOriginInitialization::Datastore,
+      "dom::localstorage::FirstOriginInitializationAttempt::Datastore"_ns,
+      innerFunc);
+}
+
+nsresult PrepareDatastoreOp::DatabaseNotAvailable() {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::DatabaseWorkOpen);
+
+  mDatabaseNotAvailable = true;
+
+  nsresult rv = FinishNestingOnNonOwningThread();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult PrepareDatastoreOp::EnsureDirectoryEntry(nsIFile* aEntry,
+                                                  bool aCreateIfNotExists,
+                                                  bool aIsDirectory,
+                                                  bool* aAlreadyExisted) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aEntry);
+
+  QM_TRY_INSPECT(const bool& exists,
+                 MOZ_TO_RESULT_INVOKE_MEMBER(aEntry, Exists));
+
+  if (!exists) {
+    if (!aCreateIfNotExists) {
+      if (aAlreadyExisted) {
+        *aAlreadyExisted = false;
+      }
+      return NS_OK;
+    }
+
+    if (aIsDirectory) {
+      QM_TRY(MOZ_TO_RESULT(aEntry->Create(nsIFile::DIRECTORY_TYPE, 0755)));
+    }
+  }
+#ifdef DEBUG
+  else {
+    bool isDirectory;
+    MOZ_ASSERT(NS_SUCCEEDED(aEntry->IsDirectory(&isDirectory)));
+    MOZ_ASSERT(isDirectory == aIsDirectory);
+  }
+#endif
+
+  if (aAlreadyExisted) {
+    *aAlreadyExisted = exists;
+  }
+  return NS_OK;
+}
+
+nsresult PrepareDatastoreOp::VerifyDatabaseInformation(
+    mozIStorageConnection* aConnection) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aConnection);
+
+  QM_TRY_INSPECT(const auto& stmt,
+                 CreateAndExecuteSingleStepStatement<
+                     SingleStepResult::ReturnNullIfNoResult>(
+                     *aConnection, "SELECT origin FROM database"_ns));
+
+  QM_TRY(OkIf(stmt), NS_ERROR_FILE_CORRUPTED);
+
+  QM_TRY_INSPECT(const auto& origin, MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                                         nsCString, stmt, GetUTF8String, 0));
+
+  QM_TRY(OkIf(QuotaManager::AreOriginsEqualOnDisk(Origin(), origin)),
+         NS_ERROR_FILE_CORRUPTED);
+
+  return NS_OK;
+}
+
+already_AddRefed<QuotaObject> PrepareDatastoreOp::GetQuotaObject() {
+  MOZ_ASSERT(IsOnOwningThread() || IsOnIOThread());
+  MOZ_ASSERT(!mOriginMetadata.mGroup.IsEmpty());
+  MOZ_ASSERT(OriginIsKnown());
+  MOZ_ASSERT(!mDatabaseFilePath.IsEmpty());
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  RefPtr<QuotaObject> quotaObject = quotaManager->GetQuotaObject(
+      PERSISTENCE_TYPE_DEFAULT, mOriginMetadata,
+      mozilla::dom::quota::Client::LS, mDatabaseFilePath, mUsage);
+
+  if (!quotaObject) {
+    LS_WARNING("Failed to get quota object for group (%s) and origin (%s)!",
+               mOriginMetadata.mGroup.get(), Origin().get());
+  }
+
+  return quotaObject.forget();
+}
+
+nsresult PrepareDatastoreOp::BeginLoadData() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::BeginLoadData);
+  MOZ_ASSERT(!mConnection);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    return NS_ERROR_ABORT;
+  }
+
+  if (!gConnectionThread) {
+    gConnectionThread = new ConnectionThread();
+  }
+
+  mConnection = gConnectionThread->CreateConnection(
+      mOriginMetadata, std::move(mArchivedOriginScope),
+      /* aDatabaseWasNotAvailable */ false);
+  MOZ_ASSERT(mConnection);
+
+  // Must set this before dispatching otherwise we will race with the
+  // connection thread.
+  mNestedState = NestedState::DatabaseWorkLoadData;
+
+  // Can't assign to mLoadDataOp directly since that's a weak reference and
+  // LoadDataOp is reference counted.
+  RefPtr<LoadDataOp> loadDataOp = new LoadDataOp(this);
+
+  // This add refs loadDataOp.
+  mConnection->Dispatch(loadDataOp);
+
+  // This is cleared in LoadDataOp::Cleanup() before the load data op is
+  // destroyed.
+  mLoadDataOp = loadDataOp;
+
+  return NS_OK;
+}
+
+void PrepareDatastoreOp::FinishNesting() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+
+  // The caller holds a strong reference to us, no need for a self reference
+  // before calling Run().
+
+  mState = State::SendingReadyMessage;
+  mNestedState = NestedState::AfterNesting;
+
+  MOZ_ALWAYS_SUCCEEDS(Run());
+}
+
+nsresult PrepareDatastoreOp::FinishNestingOnNonOwningThread() {
+  MOZ_ASSERT(!IsOnOwningThread());
+  MOZ_ASSERT(mState == State::Nesting);
+
+  // Must set mState before dispatching otherwise we will race with the owning
+  // thread.
+  mState = State::SendingReadyMessage;
+  mNestedState = NestedState::AfterNesting;
+
+  QM_TRY(
+      MOZ_TO_RESULT(OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL)));
+
+  return NS_OK;
+}
+
+nsresult PrepareDatastoreOp::NestedRun() {
+  nsresult rv;
+
+  switch (mNestedState) {
+    case NestedState::CheckExistingOperations:
+      rv = CheckExistingOperations();
+      break;
+
+    case NestedState::CheckClosingDatastore:
+      rv = CheckClosingDatastore();
+      break;
+
+    case NestedState::PreparationPending:
+      rv = BeginDatastorePreparation();
+      break;
+
+    case NestedState::DatabaseWorkOpen:
+      rv = DatabaseWork();
+      break;
+
+    case NestedState::BeginLoadData:
+      rv = BeginLoadData();
+      break;
+
+    default:
+      MOZ_CRASH("Bad state!");
+  }
+
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    mNestedState = NestedState::AfterNesting;
+
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+void PrepareDatastoreOp::GetResponse(LSRequestResponse& aResponse) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingResults);
+  MOZ_ASSERT(NS_SUCCEEDED(ResultCode()));
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  // A datastore is not created when we are just trying to preload data and
+  // there's no database file.
+  if (mDatabaseNotAvailable && mForPreload) {
+    LSRequestPreloadDatastoreResponse preloadDatastoreResponse;
+
+    aResponse = preloadDatastoreResponse;
+
+    return;
+  }
+
+  if (!mDatastore) {
+    MOZ_ASSERT(mUsage == mDEBUGUsage);
+
+    RefPtr<QuotaObject> quotaObject;
+
+    if (mPrivateBrowsingId == 0) {
+      if (!mConnection) {
+        // This can happen when there's no database file.
+        MOZ_ASSERT(mDatabaseNotAvailable);
+
+        // Even though there's no database file, we need to create a connection
+        // and pass it to datastore.
+        if (!gConnectionThread) {
+          gConnectionThread = new ConnectionThread();
+        }
+
+        mConnection = gConnectionThread->CreateConnection(
+            mOriginMetadata, std::move(mArchivedOriginScope),
+            /* aDatabaseWasNotAvailable */ true);
+        MOZ_ASSERT(mConnection);
+      }
+
+      quotaObject = GetQuotaObject();
+      if (!quotaObject) {
+        aResponse = NS_ERROR_FAILURE;
+        return;
+      }
+    }
+
+    mDatastore = new Datastore(
+        mOriginMetadata, mPrivateBrowsingId, mUsage, mSizeOfKeys, mSizeOfItems,
+        std::move(mDirectoryLock), std::move(mConnection),
+        std::move(quotaObject), mValues, std::move(mOrderedItems));
+
+    mDatastore->NoteLivePrepareDatastoreOp(this);
+
+    if (!gDatastores) {
+      gDatastores = new DatastoreHashtable();
+    }
+
+    MOZ_ASSERT(!gDatastores->Contains(Origin()));
+    gDatastores->InsertOrUpdate(Origin(),
+                                WrapMovingNotNullUnchecked(mDatastore));
+  }
+
+  if (mPrivateBrowsingId && !mInvalidated) {
+    if (!gPrivateDatastores) {
+      gPrivateDatastores = MakeUnique<PrivateDatastoreHashtable>();
+    }
+
+    gPrivateDatastores->LookupOrInsertWith(Origin(), [&] {
+      auto privateDatastore =
+          MakeUnique<PrivateDatastore>(WrapMovingNotNull(mDatastore));
+
+      mPrivateDatastoreRegistered.Flip();
+
+      return privateDatastore;
+    });
+  }
+
+  mDatastoreId = ++gLastDatastoreId;
+
+  if (!gPreparedDatastores) {
+    gPreparedDatastores = new PreparedDatastoreHashtable();
+  }
+  const auto& preparedDatastore = gPreparedDatastores->InsertOrUpdate(
+      mDatastoreId, MakeUnique<PreparedDatastore>(
+                        mDatastore, mContentParentId, Origin(), mDatastoreId,
+                        /* aForPreload */ mForPreload));
+
+  if (mInvalidated) {
+    preparedDatastore->Invalidate();
+  }
+
+  mPreparedDatastoreRegistered.Flip();
+
+  if (mForPreload) {
+    LSRequestPreloadDatastoreResponse preloadDatastoreResponse;
+
+    aResponse = preloadDatastoreResponse;
+  } else {
+    LSRequestPrepareDatastoreResponse prepareDatastoreResponse;
+    prepareDatastoreResponse.datastoreId() = mDatastoreId;
+
+    aResponse = prepareDatastoreResponse;
+  }
+}
+
+void PrepareDatastoreOp::Cleanup() {
+  AssertIsOnOwningThread();
+
+  if (mDatastore) {
+    MOZ_ASSERT(!mDirectoryLock);
+    MOZ_ASSERT(!mConnection);
+
+    if (NS_FAILED(ResultCode())) {
+      if (mPrivateDatastoreRegistered) {
+        MOZ_ASSERT(gPrivateDatastores);
+        DebugOnly<bool> removed = gPrivateDatastores->Remove(Origin());
+        MOZ_ASSERT(removed);
+
+        if (!gPrivateDatastores->Count()) {
+          gPrivateDatastores = nullptr;
+        }
+      }
+
+      if (mPreparedDatastoreRegistered) {
+        // Just in case we failed to send datastoreId to the child, we need to
+        // destroy prepared datastore, otherwise it won't be destroyed until
+        // the timer fires (after 20 seconds).
+        MOZ_ASSERT(gPreparedDatastores);
+        MOZ_ASSERT(mDatastoreId > 0);
+        DebugOnly<bool> removed = gPreparedDatastores->Remove(mDatastoreId);
+        MOZ_ASSERT(removed);
+
+        if (!gPreparedDatastores->Count()) {
+          gPreparedDatastores = nullptr;
+        }
+      }
+    }
+
+    // Make sure to release the datastore on this thread.
+
+    mDatastore->NoteFinishedPrepareDatastoreOp(this);
+
+    mDatastore = nullptr;
+
+    CleanupMetadata();
+  } else if (mConnection) {
+    // If we have a connection then the operation must have failed and there
+    // must be a directory lock too.
+    MOZ_ASSERT(NS_FAILED(ResultCode()));
+    MOZ_ASSERT(mDirectoryLock);
+
+    // We must close the connection on the connection thread before releasing
+    // it on this thread. The directory lock can't be released either.
+    nsCOMPtr<nsIRunnable> callback =
+        NewRunnableMethod("dom::OpenDatabaseOp::ConnectionClosedCallback", this,
+                          &PrepareDatastoreOp::ConnectionClosedCallback);
+
+    mConnection->Close(callback);
+  } else {
+    // If we don't have a connection, but we do have a directory lock then the
+    // operation must have failed or we were preloading a datastore and there
+    // was no physical database on disk.
+    MOZ_ASSERT_IF(mDirectoryLock,
+                  NS_FAILED(ResultCode()) || mDatabaseNotAvailable);
+
+    // There's no connection, so it's safe to release the directory lock and
+    // unregister itself from the array.
+
+    mDirectoryLock = nullptr;
+
+    CleanupMetadata();
+  }
+}
+
+void PrepareDatastoreOp::ConnectionClosedCallback() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(NS_FAILED(ResultCode()));
+  MOZ_ASSERT(mDirectoryLock);
+  MOZ_ASSERT(mConnection);
+
+  mConnection = nullptr;
+  mDirectoryLock = nullptr;
+
+  CleanupMetadata();
+}
+
+void PrepareDatastoreOp::CleanupMetadata() {
+  AssertIsOnOwningThread();
+
+  if (mDelayedOp) {
+    MOZ_ALWAYS_SUCCEEDS(NS_DispatchToCurrentThread(mDelayedOp.forget()));
+  }
+
+  MOZ_ASSERT(gPrepareDatastoreOps);
+  gPrepareDatastoreOps->RemoveElement(this);
+
+  QuotaManager::MaybeRecordQuotaClientShutdownStep(
+      quota::Client::LS, "PrepareDatastoreOp completed"_ns);
+
+  if (gPrepareDatastoreOps->IsEmpty()) {
+    gPrepareDatastoreOps = nullptr;
+  }
+}
+
+NS_IMPL_ISUPPORTS_INHERITED0(PrepareDatastoreOp, LSRequestBase)
+
+void PrepareDatastoreOp::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnOwningThread();
+
+  LSRequestBase::ActorDestroy(aWhy);
+
+  if (mLoadDataOp) {
+    mLoadDataOp->NoteComplete();
+  }
+}
+
+void PrepareDatastoreOp::DirectoryLockAcquired(DirectoryLock* aLock) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::DirectoryOpenPending);
+  MOZ_ASSERT(!mDirectoryLock);
+
+  mPendingDirectoryLock = nullptr;
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    MaybeSetFailureCode(NS_ERROR_ABORT);
+
+    FinishNesting();
+
+    return;
+  }
+
+  mDirectoryLock = aLock;
+
+  SendToIOThread();
+}
+
+void PrepareDatastoreOp::DirectoryLockFailed() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Nesting);
+  MOZ_ASSERT(mNestedState == NestedState::DirectoryOpenPending);
+  MOZ_ASSERT(!mDirectoryLock);
+
+  mPendingDirectoryLock = nullptr;
+
+  MaybeSetFailureCode(NS_ERROR_FAILURE);
+
+  FinishNesting();
+}
+
+nsresult PrepareDatastoreOp::LoadDataOp::DoDatastoreWork() {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(mConnection);
+  MOZ_ASSERT(mPrepareDatastoreOp);
+  MOZ_ASSERT(mPrepareDatastoreOp->mState == State::Nesting);
+  MOZ_ASSERT(mPrepareDatastoreOp->mNestedState ==
+             NestedState::DatabaseWorkLoadData);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnNonBackgroundThread()) ||
+      !MayProceedOnNonOwningThread()) {
+    return NS_ERROR_ABORT;
+  }
+
+  QM_TRY_INSPECT(
+      const auto& stmt,
+      mConnection->BorrowCachedStatement(
+          "SELECT key, utf16_length, conversion_type, compression_type, value "
+          "FROM data;"_ns));
+
+  QM_TRY(quota::CollectWhileHasResult(
+      *stmt, [this](auto& stmt) -> mozilla::Result<Ok, nsresult> {
+        QM_TRY_UNWRAP(auto key, MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                                    nsString, stmt, GetString, 0));
+
+        LSValue value;
+        QM_TRY(MOZ_TO_RESULT(value.InitFromStatement(&stmt, 1)));
+
+        mPrepareDatastoreOp->mValues.InsertOrUpdate(key, value);
+        mPrepareDatastoreOp->mSizeOfKeys += key.Length();
+        mPrepareDatastoreOp->mSizeOfItems += key.Length() + value.Length();
+#ifdef DEBUG
+        mPrepareDatastoreOp->mDEBUGUsage += key.Length() + value.UTF16Length();
+#endif
+
+        auto item = mPrepareDatastoreOp->mOrderedItems.AppendElement();
+        item->key() = std::move(key);
+        item->value() = std::move(value);
+
+        return Ok{};
+      }));
+
+  return NS_OK;
+}
+
+void PrepareDatastoreOp::LoadDataOp::OnSuccess() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mPrepareDatastoreOp);
+  MOZ_ASSERT(mPrepareDatastoreOp->mState == State::Nesting);
+  MOZ_ASSERT(mPrepareDatastoreOp->mNestedState ==
+             NestedState::DatabaseWorkLoadData);
+  MOZ_ASSERT(mPrepareDatastoreOp->mLoadDataOp == this);
+
+  mPrepareDatastoreOp->FinishNesting();
+}
+
+void PrepareDatastoreOp::LoadDataOp::OnFailure(nsresult aResultCode) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mPrepareDatastoreOp);
+  MOZ_ASSERT(mPrepareDatastoreOp->mState == State::Nesting);
+  MOZ_ASSERT(mPrepareDatastoreOp->mNestedState ==
+             NestedState::DatabaseWorkLoadData);
+  MOZ_ASSERT(mPrepareDatastoreOp->mLoadDataOp == this);
+
+  mPrepareDatastoreOp->SetFailureCode(aResultCode);
+
+  mPrepareDatastoreOp->FinishNesting();
+}
+
+void PrepareDatastoreOp::LoadDataOp::Cleanup() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mPrepareDatastoreOp);
+  MOZ_ASSERT(mPrepareDatastoreOp->mLoadDataOp == this);
+
+  mPrepareDatastoreOp->mLoadDataOp = nullptr;
+  mPrepareDatastoreOp = nullptr;
+
+  ConnectionDatastoreOperationBase::Cleanup();
+}
+
+NS_IMPL_ISUPPORTS(PrepareDatastoreOp::CompressFunction, mozIStorageFunction)
+
+NS_IMETHODIMP
+PrepareDatastoreOp::CompressFunction::OnFunctionCall(
+    mozIStorageValueArray* aFunctionArguments, nsIVariant** aResult) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aFunctionArguments);
+  MOZ_ASSERT(aResult);
+
+#ifdef DEBUG
+  {
+    uint32_t argCount;
+    MOZ_ALWAYS_SUCCEEDS(aFunctionArguments->GetNumEntries(&argCount));
+    MOZ_ASSERT(argCount == 1);
+
+    int32_t type;
+    MOZ_ALWAYS_SUCCEEDS(aFunctionArguments->GetTypeOfIndex(0, &type));
+    MOZ_ASSERT(type == mozIStorageValueArray::VALUE_TYPE_TEXT);
+  }
+#endif
+
+  QM_TRY_INSPECT(const auto& value,
+                 MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                     nsCString, aFunctionArguments, GetUTF8String, 0));
+
+  nsCString compressed;
+  QM_TRY(OkIf(SnappyCompress(value, compressed)), NS_ERROR_OUT_OF_MEMORY);
+
+  const nsCString& buffer = compressed.IsVoid() ? value : compressed;
+
+  // mozStorage transforms empty blobs into null values, but our database
+  // schema doesn't allow null values. We can workaround this by storing
+  // empty buffers as UTF8 text (SQLite supports the type affinity, so the type
+  // of the column is not fixed).
+  nsCOMPtr<nsIVariant> result;
+  if (0u == buffer.Length()) {  // Otherwise empty string becomes null
+    result = new storage::UTF8TextVariant(buffer);
+  } else {
+    result = new storage::BlobVariant(std::make_pair(
+        static_cast<const void*>(buffer.get()), int(buffer.Length())));
+  }
+
+  result.forget(aResult);
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(PrepareDatastoreOp::CompressionTypeFunction,
+                  mozIStorageFunction)
+
+NS_IMETHODIMP
+PrepareDatastoreOp::CompressionTypeFunction::OnFunctionCall(
+    mozIStorageValueArray* aFunctionArguments, nsIVariant** aResult) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aFunctionArguments);
+  MOZ_ASSERT(aResult);
+
+#ifdef DEBUG
+  {
+    uint32_t argCount;
+    MOZ_ALWAYS_SUCCEEDS(aFunctionArguments->GetNumEntries(&argCount));
+    MOZ_ASSERT(argCount == 1);
+
+    int32_t type;
+    MOZ_ALWAYS_SUCCEEDS(aFunctionArguments->GetTypeOfIndex(0, &type));
+    MOZ_ASSERT(type == mozIStorageValueArray::VALUE_TYPE_TEXT);
+  }
+#endif
+
+  QM_TRY_INSPECT(const auto& value,
+                 MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                     nsCString, aFunctionArguments, GetUTF8String, 0));
+
+  nsCString compressed;
+  QM_TRY(OkIf(SnappyCompress(value, compressed)), NS_ERROR_OUT_OF_MEMORY);
+
+  const int32_t compression = static_cast<int32_t>(
+      compressed.IsVoid() ? LSValue::CompressionType::UNCOMPRESSED
+                          : LSValue::CompressionType::SNAPPY);
+
+  nsCOMPtr<nsIVariant> result = new storage::IntegerVariant(compression);
+
+  result.forget(aResult);
+  return NS_OK;
+}
+
+/*******************************************************************************
+ * PrepareObserverOp
+ ******************************************************************************/
+
+PrepareObserverOp::PrepareObserverOp(
+    const LSRequestParams& aParams,
+    const Maybe<ContentParentId>& aContentParentId)
+    : LSRequestBase(aParams, aContentParentId) {
+  MOZ_ASSERT(aParams.type() ==
+             LSRequestParams::TLSRequestPrepareObserverParams);
+}
+
+nsresult PrepareObserverOp::Start() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::StartingRequest);
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  const LSRequestPrepareObserverParams params =
+      mParams.get_LSRequestPrepareObserverParams();
+
+  const PrincipalInfo& storagePrincipalInfo = params.storagePrincipalInfo();
+
+  if (storagePrincipalInfo.type() == PrincipalInfo::TSystemPrincipalInfo) {
+    mOrigin = QuotaManager::GetOriginForChrome();
+  } else {
+    MOZ_ASSERT(storagePrincipalInfo.type() ==
+               PrincipalInfo::TContentPrincipalInfo);
+
+    mOrigin =
+        QuotaManager::GetOriginFromValidatedPrincipalInfo(storagePrincipalInfo);
+  }
+
+  mState = State::SendingReadyMessage;
+  MOZ_ALWAYS_SUCCEEDS(OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL));
+
+  return NS_OK;
+}
+
+void PrepareObserverOp::GetResponse(LSRequestResponse& aResponse) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingResults);
+  MOZ_ASSERT(NS_SUCCEEDED(ResultCode()));
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  uint64_t observerId = ++gLastObserverId;
+
+  RefPtr<Observer> observer = new Observer(mOrigin);
+
+  if (!gPreparedObsevers) {
+    gPreparedObsevers = new PreparedObserverHashtable();
+  }
+  gPreparedObsevers->InsertOrUpdate(observerId, std::move(observer));
+
+  LSRequestPrepareObserverResponse prepareObserverResponse;
+  prepareObserverResponse.observerId() = observerId;
+
+  aResponse = prepareObserverResponse;
+}
+
+/*******************************************************************************
++ * LSSimpleRequestBase
++
+******************************************************************************/
+
+LSSimpleRequestBase::LSSimpleRequestBase(
+    const LSSimpleRequestParams& aParams,
+    const Maybe<ContentParentId>& aContentParentId)
+    : mParams(aParams),
+      mContentParentId(aContentParentId),
+      mState(State::Initial) {}
+
+LSSimpleRequestBase::~LSSimpleRequestBase() {
+  MOZ_ASSERT_IF(MayProceedOnNonOwningThread(),
+                mState == State::Initial || mState == State::Completed);
+}
+
+void LSSimpleRequestBase::Dispatch() {
+  AssertIsOnOwningThread();
+
+  mState = State::StartingRequest;
+
+  MOZ_ALWAYS_SUCCEEDS(NS_DispatchToCurrentThread(this));
+}
+
+bool LSSimpleRequestBase::VerifyRequestParams() {
+  AssertIsOnBackgroundThread();
+
+  MOZ_ASSERT(mParams.type() != LSSimpleRequestParams::T__None);
+
+  switch (mParams.type()) {
+    case LSSimpleRequestParams::TLSSimpleRequestPreloadedParams: {
+      const LSSimpleRequestPreloadedParams& params =
+          mParams.get_LSSimpleRequestPreloadedParams();
+
+      if (NS_WARN_IF(!VerifyPrincipalInfo(
+              params.principalInfo(), params.storagePrincipalInfo(), false))) {
+        return false;
+      }
+
+      break;
+    }
+
+    case LSSimpleRequestParams::TLSSimpleRequestGetStateParams: {
+      const LSSimpleRequestGetStateParams& params =
+          mParams.get_LSSimpleRequestGetStateParams();
+
+      if (NS_WARN_IF(!VerifyPrincipalInfo(
+              params.principalInfo(), params.storagePrincipalInfo(), false))) {
+        return false;
+      }
+
+      break;
+    }
+
+    default:
+      MOZ_CRASH("Should never get here!");
+  }
+
+  return true;
+}
+
+nsresult LSSimpleRequestBase::StartRequest() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::StartingRequest);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    return NS_ERROR_ABORT;
+  }
+
+#ifdef DEBUG
+  // Always verify parameters in DEBUG builds!
+  bool trustParams = false;
+#else
+  bool trustParams = !BackgroundParent::IsOtherProcessActor(Manager());
+#endif
+
+  if (!trustParams && NS_WARN_IF(!VerifyRequestParams())) {
+    return NS_ERROR_FAILURE;
+  }
+
+  QM_TRY(MOZ_TO_RESULT(Start()));
+
+  return NS_OK;
+}
+
+void LSSimpleRequestBase::SendResults() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingResults);
+
+  if (NS_WARN_IF(QuotaClient::IsShuttingDownOnBackgroundThread()) ||
+      !MayProceed()) {
+    MaybeSetFailureCode(NS_ERROR_ABORT);
+  }
+
+  if (MayProceed()) {
+    LSSimpleRequestResponse response;
+
+    if (NS_SUCCEEDED(ResultCode())) {
+      GetResponse(response);
+    } else {
+      response = ResultCode();
+    }
+
+    Unused << PBackgroundLSSimpleRequestParent::Send__delete__(this, response);
+  }
+
+  mState = State::Completed;
+}
+
+NS_IMETHODIMP
+LSSimpleRequestBase::Run() {
+  nsresult rv;
+
+  switch (mState) {
+    case State::StartingRequest:
+      rv = StartRequest();
+      break;
+
+    case State::SendingResults:
+      SendResults();
+      return NS_OK;
+
+    default:
+      MOZ_CRASH("Bad state!");
+  }
+
+  if (NS_WARN_IF(NS_FAILED(rv)) && mState != State::SendingResults) {
+    MaybeSetFailureCode(rv);
+
+    // Must set mState before dispatching otherwise we will race with the owning
+    // thread.
+    mState = State::SendingResults;
+
+    if (IsOnOwningThread()) {
+      SendResults();
+    } else {
+      MOZ_ALWAYS_SUCCEEDS(
+          OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL));
+    }
+  }
+
+  return NS_OK;
+}
+
+void LSSimpleRequestBase::ActorDestroy(ActorDestroyReason aWhy) {
+  AssertIsOnOwningThread();
+
+  NoteComplete();
+}
+
+/*******************************************************************************
+ * PreloadedOp
+ ******************************************************************************/
+
+PreloadedOp::PreloadedOp(const LSSimpleRequestParams& aParams,
+                         const Maybe<ContentParentId>& aContentParentId)
+    : LSSimpleRequestBase(aParams, aContentParentId) {
+  MOZ_ASSERT(aParams.type() ==
+             LSSimpleRequestParams::TLSSimpleRequestPreloadedParams);
+}
+
+nsresult PreloadedOp::Start() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::StartingRequest);
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  const LSSimpleRequestPreloadedParams& params =
+      mParams.get_LSSimpleRequestPreloadedParams();
+
+  const PrincipalInfo& storagePrincipalInfo = params.storagePrincipalInfo();
+
+  MOZ_ASSERT(
+      storagePrincipalInfo.type() == PrincipalInfo::TSystemPrincipalInfo ||
+      storagePrincipalInfo.type() == PrincipalInfo::TContentPrincipalInfo);
+  mOrigin = storagePrincipalInfo.type() == PrincipalInfo::TSystemPrincipalInfo
+                ? nsCString{QuotaManager::GetOriginForChrome()}
+                : QuotaManager::GetOriginFromValidatedPrincipalInfo(
+                      storagePrincipalInfo);
+
+  mState = State::SendingResults;
+  MOZ_ALWAYS_SUCCEEDS(OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL));
+
+  return NS_OK;
+}
+
+void PreloadedOp::GetResponse(LSSimpleRequestResponse& aResponse) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingResults);
+  MOZ_ASSERT(NS_SUCCEEDED(ResultCode()));
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  bool preloaded;
+  RefPtr<Datastore> datastore;
+  if ((datastore = GetDatastore(mOrigin)) && !datastore->IsClosed()) {
+    preloaded = true;
+  } else {
+    preloaded = false;
+  }
+
+  LSSimpleRequestPreloadedResponse preloadedResponse;
+  preloadedResponse.preloaded() = preloaded;
+
+  aResponse = preloadedResponse;
+}
+
+/*******************************************************************************
+ * GetStateOp
+ ******************************************************************************/
+
+GetStateOp::GetStateOp(const LSSimpleRequestParams& aParams,
+                       const Maybe<ContentParentId>& aContentParentId)
+    : LSSimpleRequestBase(aParams, aContentParentId) {
+  MOZ_ASSERT(aParams.type() ==
+             LSSimpleRequestParams::TLSSimpleRequestGetStateParams);
+}
+
+nsresult GetStateOp::Start() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::StartingRequest);
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  const LSSimpleRequestGetStateParams& params =
+      mParams.get_LSSimpleRequestGetStateParams();
+
+  const PrincipalInfo& storagePrincipalInfo = params.storagePrincipalInfo();
+
+  MOZ_ASSERT(
+      storagePrincipalInfo.type() == PrincipalInfo::TSystemPrincipalInfo ||
+      storagePrincipalInfo.type() == PrincipalInfo::TContentPrincipalInfo);
+  mOrigin = storagePrincipalInfo.type() == PrincipalInfo::TSystemPrincipalInfo
+                ? nsCString{QuotaManager::GetOriginForChrome()}
+                : QuotaManager::GetOriginFromValidatedPrincipalInfo(
+                      storagePrincipalInfo);
+
+  mState = State::SendingResults;
+  MOZ_ALWAYS_SUCCEEDS(OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL));
+
+  return NS_OK;
+}
+
+void GetStateOp::GetResponse(LSSimpleRequestResponse& aResponse) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingResults);
+  MOZ_ASSERT(NS_SUCCEEDED(ResultCode()));
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  LSSimpleRequestGetStateResponse getStateResponse;
+
+  if (RefPtr<Datastore> datastore = GetDatastore(mOrigin)) {
+    if (!datastore->IsClosed()) {
+      getStateResponse.itemInfos() = datastore->GetOrderedItems().Clone();
+    }
+  }
+
+  aResponse = getStateResponse;
+}
+
+/*******************************************************************************
+ * ArchivedOriginScope
+ ******************************************************************************/
+
+// static
+UniquePtr<ArchivedOriginScope> ArchivedOriginScope::CreateFromOrigin(
+    const nsACString& aOriginAttrSuffix, const nsACString& aOriginKey) {
+  return WrapUnique(
+      new ArchivedOriginScope(Origin(aOriginAttrSuffix, aOriginKey)));
+}
+
+// static
+UniquePtr<ArchivedOriginScope> ArchivedOriginScope::CreateFromPrefix(
+    const nsACString& aOriginKey) {
+  return WrapUnique(new ArchivedOriginScope(Prefix(aOriginKey)));
+}
+
+// static
+UniquePtr<ArchivedOriginScope> ArchivedOriginScope::CreateFromPattern(
+    const OriginAttributesPattern& aPattern) {
+  return WrapUnique(new ArchivedOriginScope(Pattern(aPattern)));
+}
+
+// static
+UniquePtr<ArchivedOriginScope> ArchivedOriginScope::CreateFromNull() {
+  return WrapUnique(new ArchivedOriginScope(Null()));
+}
+
+nsLiteralCString ArchivedOriginScope::GetBindingClause() const {
+  return mData.match(
+      [](const Origin&) {
+        return " WHERE originKey = :originKey "
+               "AND originAttributes = :originAttributes"_ns;
+      },
+      [](const Pattern&) {
+        return " WHERE originAttributes MATCH :originAttributesPattern"_ns;
+      },
+      [](const Prefix&) { return " WHERE originKey = :originKey"_ns; },
+      [](const Null&) { return ""_ns; });
+}
+
+nsresult ArchivedOriginScope::BindToStatement(
+    mozIStorageStatement* aStmt) const {
+  MOZ_ASSERT(IsOnIOThread() || IsOnGlobalConnectionThread());
+  MOZ_ASSERT(aStmt);
+
+  struct Matcher {
+    mozIStorageStatement* mStmt;
+
+    explicit Matcher(mozIStorageStatement* aStmt) : mStmt(aStmt) {}
+
+    nsresult operator()(const Origin& aOrigin) {
+      QM_TRY(MOZ_TO_RESULT(mStmt->BindUTF8StringByName(
+          "originKey"_ns, aOrigin.OriginNoSuffix())));
+
+      QM_TRY(MOZ_TO_RESULT(mStmt->BindUTF8StringByName(
+          "originAttributes"_ns, aOrigin.OriginSuffix())));
+
+      return NS_OK;
+    }
+
+    nsresult operator()(const Prefix& aPrefix) {
+      QM_TRY(MOZ_TO_RESULT(mStmt->BindUTF8StringByName(
+          "originKey"_ns, aPrefix.OriginNoSuffix())));
+
+      return NS_OK;
+    }
+
+    nsresult operator()(const Pattern& aPattern) {
+      QM_TRY(MOZ_TO_RESULT(mStmt->BindUTF8StringByName(
+          "originAttributesPattern"_ns, "pattern1"_ns)));
+
+      return NS_OK;
+    }
+
+    nsresult operator()(const Null& aNull) { return NS_OK; }
+  };
+
+  QM_TRY(MOZ_TO_RESULT(mData.match(Matcher(aStmt))));
+
+  return NS_OK;
+}
+
+bool ArchivedOriginScope::HasMatches(
+    ArchivedOriginHashtable* aHashtable) const {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aHashtable);
+
+  return mData.match(
+      [aHashtable](const Origin& aOrigin) {
+        const nsCString hashKey = GetArchivedOriginHashKey(
+            aOrigin.OriginSuffix(), aOrigin.OriginNoSuffix());
+
+        return aHashtable->Contains(hashKey);
+      },
+      [aHashtable](const Pattern& aPattern) {
+        return std::any_of(
+            aHashtable->Values().cbegin(), aHashtable->Values().cend(),
+            [&aPattern](const auto& entry) {
+              return aPattern.GetPattern().Matches(entry->mOriginAttributes);
+            });
+      },
+      [aHashtable](const Prefix& aPrefix) {
+        return std::any_of(
+            aHashtable->Values().cbegin(), aHashtable->Values().cend(),
+            [&aPrefix](const auto& entry) {
+              return entry->mOriginNoSuffix == aPrefix.OriginNoSuffix();
+            });
+      },
+      [aHashtable](const Null& aNull) { return !aHashtable->IsEmpty(); });
+}
+
+void ArchivedOriginScope::RemoveMatches(
+    ArchivedOriginHashtable* aHashtable) const {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aHashtable);
+
+  struct Matcher {
+    ArchivedOriginHashtable* mHashtable;
+
+    explicit Matcher(ArchivedOriginHashtable* aHashtable)
+        : mHashtable(aHashtable) {}
+
+    void operator()(const Origin& aOrigin) {
+      nsCString hashKey = GetArchivedOriginHashKey(aOrigin.OriginSuffix(),
+                                                   aOrigin.OriginNoSuffix());
+
+      mHashtable->Remove(hashKey);
+    }
+
+    void operator()(const Prefix& aPrefix) {
+      for (auto iter = mHashtable->Iter(); !iter.Done(); iter.Next()) {
+        const auto& archivedOriginInfo = iter.Data();
+
+        if (archivedOriginInfo->mOriginNoSuffix == aPrefix.OriginNoSuffix()) {
+          iter.Remove();
+        }
+      }
+    }
+
+    void operator()(const Pattern& aPattern) {
+      for (auto iter = mHashtable->Iter(); !iter.Done(); iter.Next()) {
+        const auto& archivedOriginInfo = iter.Data();
+
+        if (aPattern.GetPattern().Matches(
+                archivedOriginInfo->mOriginAttributes)) {
+          iter.Remove();
+        }
+      }
+    }
+
+    void operator()(const Null& aNull) { mHashtable->Clear(); }
+  };
+
+  mData.match(Matcher(aHashtable));
+}
+
+/*******************************************************************************
+ * QuotaClient
+ ******************************************************************************/
+
+QuotaClient* QuotaClient::sInstance = nullptr;
+
+QuotaClient::QuotaClient()
+    : mShadowDatabaseMutex("LocalStorage mShadowDatabaseMutex") {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(!sInstance, "We expect this to be a singleton!");
+
+  sInstance = this;
+}
+
+QuotaClient::~QuotaClient() {
+  AssertIsOnBackgroundThread();
+  MOZ_ASSERT(sInstance == this, "We expect this to be a singleton!");
+
+  sInstance = nullptr;
+}
+
+mozilla::dom::quota::Client::Type QuotaClient::GetType() {
+  return QuotaClient::LS;
+}
+
+Result<UsageInfo, nsresult> QuotaClient::InitOrigin(
+    PersistenceType aPersistenceType, const OriginMetadata& aOriginMetadata,
+    const AtomicBool& aCanceled) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aPersistenceType == PERSISTENCE_TYPE_DEFAULT);
+  MOZ_ASSERT(aOriginMetadata.mPersistenceType == aPersistenceType);
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  QM_TRY_INSPECT(const auto& directory,
+                 quotaManager->GetOriginDirectory(aOriginMetadata));
+
+  MOZ_ASSERT(directory);
+
+  QM_TRY(MOZ_TO_RESULT(
+      directory->Append(NS_LITERAL_STRING_FROM_CSTRING(LS_DIRECTORY_NAME))));
+
+#ifdef DEBUG
+  {
+    QM_TRY_INSPECT(const bool& exists,
+                   MOZ_TO_RESULT_INVOKE_MEMBER(directory, Exists));
+    MOZ_ASSERT(exists);
+  }
+#endif
+
+  QM_TRY_INSPECT(const auto& directoryPath, MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                                                nsString, directory, GetPath));
+
+  QM_TRY_INSPECT(const auto& usageFile, GetUsageFile(directoryPath));
+
+  // XXX Try to make usageFileExists const
+  QM_TRY_UNWRAP(bool usageFileExists, ExistsAsFile(*usageFile));
+
+  QM_TRY_INSPECT(const auto& usageJournalFile,
+                 GetUsageJournalFile(directoryPath));
+
+  QM_TRY_INSPECT(const bool& usageJournalFileExists,
+                 ExistsAsFile(*usageJournalFile));
+
+  if (usageJournalFileExists) {
+    if (usageFileExists) {
+      QM_TRY(MOZ_TO_RESULT(usageFile->Remove(false)));
+
+      usageFileExists = false;
+    }
+
+    QM_TRY(MOZ_TO_RESULT(usageJournalFile->Remove(false)));
+  }
+
+  QM_TRY_INSPECT(const auto& file,
+                 CloneFileAndAppend(*directory, kDataFileName));
+
+  QM_TRY_INSPECT(const bool& fileExists, ExistsAsFile(*file));
+
+  QM_TRY_INSPECT(
+      const UsageInfo& res,
+      ([fileExists, usageFileExists, &file, &usageFile, &usageJournalFile,
+        &aOriginMetadata]() -> Result<UsageInfo, nsresult> {
+        if (fileExists) {
+          QM_TRY_RETURN(QM_OR_ELSE_WARN(
+              // Expression. To simplify control flow, we call LoadUsageFile
+              // unconditionally here, even though it will necessarily fail if
+              // usageFileExists is false.
+              LoadUsageFile(*usageFile),
+              // Fallback.
+              ([&file, &usageFile, &usageJournalFile, &aOriginMetadata](
+                   const nsresult) -> Result<UsageInfo, nsresult> {
+                QM_TRY_INSPECT(
+                    const auto& connection,
+                    CreateStorageConnection(*file, *usageFile,
+                                            aOriginMetadata.mOrigin, [] {}));
+
+                QM_TRY_INSPECT(const int64_t& usage,
+                               GetUsage(*connection,
+                                        /* aArchivedOriginScope */ nullptr));
+
+                QM_TRY(MOZ_TO_RESULT(
+                    UpdateUsageFile(usageFile, usageJournalFile, usage)));
+
+                QM_TRY(MOZ_TO_RESULT(usageJournalFile->Remove(false)));
+
+                MOZ_ASSERT(usage >= 0);
+                return UsageInfo{DatabaseUsageType(Some(uint64_t(usage)))};
+              })));
+        }
+
+        if (usageFileExists) {
+          QM_TRY(MOZ_TO_RESULT(usageFile->Remove(false)));
+        }
+
+        return UsageInfo{};
+      }()));
+
+  // Report unknown files in debug builds, but don't fail, just warn (we don't
+  // report unknown files in release builds because that requires extra
+  // scanning of the directory which would slow down entire initialization for
+  // little benefit).
+
+#ifdef DEBUG
+  QM_TRY(CollectEachFileAtomicCancelable(
+      *directory, aCanceled,
+      [](const nsCOMPtr<nsIFile>& file) -> Result<Ok, nsresult> {
+        QM_TRY_INSPECT(const auto& dirEntryKind, GetDirEntryKind(*file));
+
+        switch (dirEntryKind) {
+          case nsIFileKind::ExistsAsDirectory:
+            Unused << WARN_IF_FILE_IS_UNKNOWN(*file);
+            break;
+
+          case nsIFileKind::ExistsAsFile: {
+            QM_TRY_INSPECT(
+                const auto& leafName,
+                MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(nsString, file, GetLeafName));
+
+            if (leafName.Equals(kDataFileName) ||
+                leafName.Equals(kJournalFileName) ||
+                leafName.Equals(kUsageFileName) ||
+                leafName.Equals(kUsageJournalFileName)) {
+              return Ok{};
+            }
+
+            Unused << WARN_IF_FILE_IS_UNKNOWN(*file);
+
+            break;
+          }
+
+          case nsIFileKind::DoesNotExist:
+            // Ignore files that got removed externally while iterating.
+            break;
+        }
+        return Ok{};
+      }));
+#endif
+
+  return res;
+}
+
+nsresult QuotaClient::InitOriginWithoutTracking(
+    PersistenceType aPersistenceType, const OriginMetadata& aOriginMetadata,
+    const AtomicBool& aCanceled) {
+  AssertIsOnIOThread();
+
+  // This is called when a storage/permanent/${origin}/ls directory exists. Even
+  // though this shouldn't happen with a "good" profile, we shouldn't return an
+  // error here, since that would cause origin initialization to fail. We just
+  // warn and otherwise ignore that.
+  UNKNOWN_FILE_WARNING(NS_LITERAL_STRING_FROM_CSTRING(LS_DIRECTORY_NAME));
+  return NS_OK;
+}
+
+Result<UsageInfo, nsresult> QuotaClient::GetUsageForOrigin(
+    PersistenceType aPersistenceType, const OriginMetadata& aOriginMetadata,
+    const AtomicBool& aCanceled) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aPersistenceType == PERSISTENCE_TYPE_DEFAULT);
+
+  // We can't open the database at this point, since it can be already used
+  // by the connection thread. Use the cached value instead.
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  return quotaManager->GetUsageForClient(PERSISTENCE_TYPE_DEFAULT,
+                                         aOriginMetadata, Client::LS);
+}
+
+nsresult QuotaClient::AboutToClearOrigins(
+    const Nullable<PersistenceType>& aPersistenceType,
+    const OriginScope& aOriginScope) {
+  AssertIsOnIOThread();
+
+  // This method is not called when the clearing is triggered by the eviction
+  // process. It's on purpose to avoid a problem with the origin access time
+  // which can be described as follows:
+  // When there's a storage pressure condition and quota manager starts
+  // collecting origins for eviction, there can be an origin that hasn't been
+  // touched for long time. However, the old implementation of local storage
+  // could have touched the origin only recently and the new implementation
+  // hasn't had a chance to create a new per origin database for it yet (the
+  // data is still in the archive database), so the origin access time hasn't
+  // been updated either. In the end, the origin would be evicted despite the
+  // fact that there was recent local storage activity.
+  // So this method clears the archived data and shadow database entries for
+  // given origin scope, but only if it's a privacy-related origin clearing.
+
+  if (!aPersistenceType.IsNull() &&
+      aPersistenceType.Value() != PERSISTENCE_TYPE_DEFAULT) {
+    return NS_OK;
+  }
+
+  // There can be no data for the system principal in the archive or the shadow
+  // database. This early return silences potential warnings caused by failed
+  // `CreateAerchivedOriginScope` because it calls `GenerateOriginKey2` which
+  // doesn't support the system principal.
+  if (aOriginScope.IsOrigin() &&
+      aOriginScope.GetOrigin() == QuotaManager::GetOriginForChrome()) {
+    return NS_OK;
+  }
+
+  const bool shadowWrites = gShadowWrites;
+
+  QM_TRY_INSPECT(const auto& archivedOriginScope,
+                 CreateArchivedOriginScope(aOriginScope));
+
+  if (!gArchivedOrigins) {
+    QM_TRY(MOZ_TO_RESULT(LoadArchivedOrigins()));
+    MOZ_ASSERT(gArchivedOrigins);
+  }
+
+  const bool hasDataForRemoval =
+      archivedOriginScope->HasMatches(gArchivedOrigins);
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  const nsString& basePath = quotaManager->GetBasePath();
+
+  {
+    MutexAutoLock shadowDatabaseLock(mShadowDatabaseMutex);
+
+    QM_TRY_INSPECT(
+        const auto& connection,
+        ([&basePath]() -> Result<nsCOMPtr<mozIStorageConnection>, nsresult> {
+          if (gInitializedShadowStorage) {
+            QM_TRY_RETURN(GetShadowStorageConnection(basePath));
+          }
+
+          QM_TRY_UNWRAP(auto connection,
+                        CreateShadowStorageConnection(basePath));
+
+          gInitializedShadowStorage = true;
+
+          return connection;
+        }()));
+
+    {
+      Maybe<AutoDatabaseAttacher> maybeAutoArchiveDatabaseAttacher;
+
+      if (hasDataForRemoval) {
+        QM_TRY_INSPECT(const auto& archiveFile,
+                       GetArchiveFile(quotaManager->GetStoragePath()));
+
+        maybeAutoArchiveDatabaseAttacher.emplace(
+            AutoDatabaseAttacher(connection, archiveFile, "archive"_ns));
+
+        QM_TRY(MOZ_TO_RESULT(maybeAutoArchiveDatabaseAttacher->Attach()));
+      }
+
+      if (archivedOriginScope->IsPattern()) {
+        nsCOMPtr<mozIStorageFunction> function(
+            new MatchFunction(archivedOriginScope->GetPattern()));
+
+        QM_TRY(
+            MOZ_TO_RESULT(connection->CreateFunction("match"_ns, 2, function)));
+      }
+
+      {
+        QM_TRY_INSPECT(const auto& stmt,
+                       MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                           nsCOMPtr<mozIStorageStatement>, connection,
+                           CreateStatement, "BEGIN IMMEDIATE;"_ns));
+
+        QM_TRY(MOZ_TO_RESULT(stmt->Execute()));
+      }
+
+      if (shadowWrites) {
+        QM_TRY(MOZ_TO_RESULT(
+            PerformDelete(connection, "main"_ns, archivedOriginScope.get())));
+      }
+
+      if (hasDataForRemoval) {
+        QM_TRY(MOZ_TO_RESULT(PerformDelete(connection, "archive"_ns,
+                                           archivedOriginScope.get())));
+      }
+
+      {
+        QM_TRY_INSPECT(const auto& stmt,
+                       MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                           nsCOMPtr<mozIStorageStatement>, connection,
+                           CreateStatement, "COMMIT;"_ns));
+
+        QM_TRY(MOZ_TO_RESULT(stmt->Execute()));
+      }
+
+      if (archivedOriginScope->IsPattern()) {
+        QM_TRY(MOZ_TO_RESULT(connection->RemoveFunction("match"_ns)));
+      }
+
+      if (hasDataForRemoval) {
+        MOZ_ASSERT(maybeAutoArchiveDatabaseAttacher.isSome());
+        QM_TRY(MOZ_TO_RESULT(maybeAutoArchiveDatabaseAttacher->Detach()));
+
+        maybeAutoArchiveDatabaseAttacher.reset();
+
+        MOZ_ASSERT(gArchivedOrigins);
+        MOZ_ASSERT(archivedOriginScope->HasMatches(gArchivedOrigins));
+        archivedOriginScope->RemoveMatches(gArchivedOrigins);
+      }
+    }
+    QM_TRY(MOZ_TO_RESULT(connection->Close()));
+  }
+
+  if (aOriginScope.IsNull()) {
+    QM_TRY_INSPECT(const auto& shadowFile, GetShadowFile(basePath));
+
+    QM_TRY(MOZ_TO_RESULT(shadowFile->Remove(false)));
+
+    gInitializedShadowStorage = false;
+  }
+
+  return NS_OK;
+}
+
+void QuotaClient::OnOriginClearCompleted(PersistenceType aPersistenceType,
+                                         const nsACString& aOrigin) {
+  AssertIsOnIOThread();
+}
+
+void QuotaClient::OnRepositoryClearCompleted(PersistenceType aPersistenceType) {
+  AssertIsOnIOThread();
+}
+
+void QuotaClient::ReleaseIOThreadObjects() {
+  AssertIsOnIOThread();
+
+  gInitializationInfo = nullptr;
+
+  // Delete archived origins hashtable since QuotaManager clears the whole
+  // storage directory including ls-archive.sqlite.
+
+  gArchivedOrigins = nullptr;
+}
+
+void QuotaClient::AbortOperationsForLocks(
+    const DirectoryLockIdTable& aDirectoryLockIds) {
+  AssertIsOnBackgroundThread();
+
+  // A PrepareDatastoreOp object could already acquire a directory lock for
+  // the given origin. Its last step is creation of a Datastore object (which
+  // will take ownership of the directory lock) and a PreparedDatastore object
+  // which keeps the Datastore alive until a database actor is created.
+  // We need to invalidate the PreparedDatastore object when it's created,
+  // otherwise the Datastore object can block the origin clear operation for
+  // long time. It's not a problem that we don't fail the PrepareDatastoreOp
+  // immediatelly (avoiding the creation of the Datastore and PreparedDatastore
+  // object). We will call RequestAllowToClose on the database actor once it's
+  // created and the child actor will respond by sending AllowToClose which
+  // will close the Datastore on the parent side (the closing releases the
+  // directory lock).
+
+  InvalidatePrepareDatastoreOpsMatching(
+      [&aDirectoryLockIds](const auto& prepareDatastoreOp) {
+        // Check if the PrepareDatastoreOp holds an acquired DirectoryLock.
+        // Origin clearing can't be blocked by this PrepareDatastoreOp if there
+        // is no acquired DirectoryLock. If there is an acquired DirectoryLock,
+        // check if the table contains the lock for the PrepareDatastoreOp.
+        return IsLockForObjectAcquiredAndContainedInLockTable(
+            prepareDatastoreOp, aDirectoryLockIds);
+      });
+
+  if (gPrivateDatastores) {
+    gPrivateDatastores->RemoveIf([&aDirectoryLockIds](const auto& iter) {
+      const auto& privateDatastore = iter.Data();
+
+      // The PrivateDatastore::mDatastore member is not cleared until the
+      // PrivateDatastore is destroyed.
+      const auto& datastore = privateDatastore->DatastoreRef();
+
+      // If the PrivateDatastore exists then it must be registered in
+      // Datastore::mHasLivePrivateDatastore as well. The Datastore must have
+      // a DirectoryLock if there is a registered PrivateDatastore.
+      return IsLockForObjectContainedInLockTable(datastore, aDirectoryLockIds);
+    });
+
+    if (!gPrivateDatastores->Count()) {
+      gPrivateDatastores = nullptr;
+    }
+  }
+
+  InvalidatePreparedDatastoresMatching([&aDirectoryLockIds](
+                                           const auto& preparedDatastore) {
+    // The PreparedDatastore::mDatastore member is not cleared until the
+    // PreparedDatastore is destroyed.
+    const auto& datastore = preparedDatastore.DatastoreRef();
+
+    // If the PreparedDatastore exists then it must be registered in
+    // Datastore::mPreparedDatastores as well. The Datastore must have a
+    // DirectoryLock if there are registered PreparedDatastore objects.
+    return IsLockForObjectContainedInLockTable(datastore, aDirectoryLockIds);
+  });
+
+  RequestAllowToCloseDatabasesMatching(
+      [&aDirectoryLockIds](const auto& database) {
+        const auto& maybeDatastore = database.MaybeDatastoreRef();
+
+        // If the Database is registered in gLiveDatabases then it must have a
+        // Datastore.
+        MOZ_ASSERT(maybeDatastore.isSome());
+
+        // If the Database is registered in gLiveDatabases then it must be
+        // registered in Datastore::mDatabases as well. The Datastore must have
+        // a DirectoryLock if there are registered Database objects.
+        return IsLockForObjectContainedInLockTable(*maybeDatastore,
+                                                   aDirectoryLockIds);
+      });
+}
+
+void QuotaClient::AbortOperationsForProcess(ContentParentId aContentParentId) {
+  AssertIsOnBackgroundThread();
+
+  RequestAllowToCloseDatabasesMatching(
+      [&aContentParentId](const auto& database) {
+        return database.IsOwnedByProcess(aContentParentId);
+      });
+}
+
+void QuotaClient::AbortAllOperations() {
+  AssertIsOnBackgroundThread();
+
+  InvalidatePrepareDatastoreOpsMatching([](const auto& prepareDatastoreOp) {
+    return prepareDatastoreOp.MaybeDirectoryLockRef();
+  });
+
+  if (gPrivateDatastores) {
+    gPrivateDatastores = nullptr;
+  }
+
+  InvalidatePreparedDatastoresMatching([](const auto&) { return true; });
+
+  RequestAllowToCloseDatabasesMatching([](const auto&) { return true; });
+}
+
+void QuotaClient::StartIdleMaintenance() { AssertIsOnBackgroundThread(); }
+
+void QuotaClient::StopIdleMaintenance() { AssertIsOnBackgroundThread(); }
+
+void QuotaClient::InitiateShutdown() {
+  // gPrepareDatastoreOps are short lived objects running a state machine.
+  // The shutdown flag is checked between states, so we don't have to notify
+  // all the objects here.
+  // Allocation of a new PrepareDatastoreOp object is prevented once the
+  // shutdown flag is set.
+  // When the last PrepareDatastoreOp finishes, the gPrepareDatastoreOps array
+  // is destroyed.
+
+  if (gPreparedDatastores) {
+    gPreparedDatastores = nullptr;
+  }
+
+  if (gPrivateDatastores) {
+    gPrivateDatastores = nullptr;
+  }
+
+  RequestAllowToCloseDatabasesMatching([](const auto&) { return true; });
+
+  if (gPreparedObsevers) {
+    gPreparedObsevers = nullptr;
+  }
+}
+
+bool QuotaClient::IsShutdownCompleted() const {
+  // Don't have to check gPrivateDatastores and gPreparedDatastores since we
+  // nulled it out in InitiateShutdown.
+  return !gPrepareDatastoreOps && !gDatastores && !gLiveDatabases;
+}
+
+void QuotaClient::ForceKillActors() { ForceKillAllDatabases(); }
+
+nsCString QuotaClient::GetShutdownStatus() const {
+  AssertIsOnBackgroundThread();
+
+  nsCString data;
+
+  if (gPrepareDatastoreOps) {
+    data.Append("PrepareDatastoreOperations: ");
+    data.AppendInt(static_cast<uint32_t>(gPrepareDatastoreOps->Length()));
+    data.Append(" (");
+
+    // XXX What's the purpose of adding these to a hashtable before joining them
+    // to the string? (Maybe this used to be an ordered container before???)
+    nsTHashSet<nsCString> ids;
+    std::transform(gPrepareDatastoreOps->cbegin(), gPrepareDatastoreOps->cend(),
+                   MakeInserter(ids), [](const auto& prepareDatastoreOp) {
+                     nsCString id;
+                     prepareDatastoreOp->Stringify(id);
+                     return id;
+                   });
+
+    StringJoinAppend(data, ", "_ns, ids);
+
+    data.Append(")\n");
+  }
+
+  if (gDatastores) {
+    data.Append("Datastores: ");
+    data.AppendInt(gDatastores->Count());
+    data.Append(" (");
+
+    // XXX It might be confusing to remove duplicates here, as the actual list
+    // won't match the count then.
+    nsTHashSet<nsCString> ids;
+    std::transform(gDatastores->Values().cbegin(), gDatastores->Values().cend(),
+                   MakeInserter(ids), [](const auto& entry) {
+                     nsCString id;
+                     entry->Stringify(id);
+                     return id;
+                   });
+
+    StringJoinAppend(data, ", "_ns, ids);
+
+    data.Append(")\n");
+  }
+
+  if (gLiveDatabases) {
+    data.Append("LiveDatabases: ");
+    data.AppendInt(static_cast<uint32_t>(gLiveDatabases->Length()));
+    data.Append(" (");
+
+    // XXX It might be confusing to remove duplicates here, as the actual list
+    // won't match the count then.
+    nsTHashSet<nsCString> ids;
+    std::transform(gLiveDatabases->cbegin(), gLiveDatabases->cend(),
+                   MakeInserter(ids), [](const auto& database) {
+                     nsCString id;
+                     database->Stringify(id);
+                     return id;
+                   });
+
+    StringJoinAppend(data, ", "_ns, ids);
+
+    data.Append(")\n");
+  }
+
+  return data;
+}
+
+void QuotaClient::FinalizeShutdown() {
+  // And finally, shutdown the connection thread.
+  if (gConnectionThread) {
+    gConnectionThread->Shutdown();
+
+    gConnectionThread = nullptr;
+  }
+}
+
+Result<UniquePtr<ArchivedOriginScope>, nsresult>
+QuotaClient::CreateArchivedOriginScope(const OriginScope& aOriginScope) {
+  AssertIsOnIOThread();
+
+  if (aOriginScope.IsOrigin()) {
+    QM_TRY_INSPECT(const auto& principalInfo,
+                   QuotaManager::ParseOrigin(aOriginScope.GetOrigin()));
+
+    QM_TRY_INSPECT((const auto& [originAttrSuffix, originKey]),
+                   GenerateOriginKey2(principalInfo));
+
+    return ArchivedOriginScope::CreateFromOrigin(originAttrSuffix, originKey);
+  }
+
+  if (aOriginScope.IsPrefix()) {
+    QM_TRY_INSPECT(const auto& principalInfo,
+                   QuotaManager::ParseOrigin(aOriginScope.GetOriginNoSuffix()));
+
+    QM_TRY_INSPECT((const auto& [originAttrSuffix, originKey]),
+                   GenerateOriginKey2(principalInfo));
+
+    Unused << originAttrSuffix;
+
+    return ArchivedOriginScope::CreateFromPrefix(originKey);
+  }
+
+  if (aOriginScope.IsPattern()) {
+    return ArchivedOriginScope::CreateFromPattern(aOriginScope.GetPattern());
+  }
+
+  MOZ_ASSERT(aOriginScope.IsNull());
+
+  return ArchivedOriginScope::CreateFromNull();
+}
+
+nsresult QuotaClient::PerformDelete(
+    mozIStorageConnection* aConnection, const nsACString& aSchemaName,
+    ArchivedOriginScope* aArchivedOriginScope) const {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aConnection);
+  MOZ_ASSERT(aArchivedOriginScope);
+
+  QM_TRY_INSPECT(
+      const auto& stmt,
+      MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+          nsCOMPtr<mozIStorageStatement>, aConnection, CreateStatement,
+          "DELETE FROM "_ns + aSchemaName + ".webappsstore2"_ns +
+              aArchivedOriginScope->GetBindingClause() + ";"_ns));
+
+  QM_TRY(MOZ_TO_RESULT(aArchivedOriginScope->BindToStatement(stmt)));
+
+  QM_TRY(MOZ_TO_RESULT(stmt->Execute()));
+
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(QuotaClient::MatchFunction, mozIStorageFunction)
+
+NS_IMETHODIMP
+QuotaClient::MatchFunction::OnFunctionCall(
+    mozIStorageValueArray* aFunctionArguments, nsIVariant** aResult) {
+  AssertIsOnIOThread();
+  MOZ_ASSERT(aFunctionArguments);
+  MOZ_ASSERT(aResult);
+
+  QM_TRY_INSPECT(const auto& suffix,
+                 MOZ_TO_RESULT_INVOKE_MEMBER_TYPED(
+                     nsAutoCString, aFunctionArguments, GetUTF8String, 1));
+
+  OriginAttributes oa;
+  QM_TRY(OkIf(oa.PopulateFromSuffix(suffix)), NS_ERROR_FAILURE);
+
+  const bool result = mPattern.Matches(oa);
+
+  RefPtr<nsVariant> outVar(new nsVariant());
+  QM_TRY(MOZ_TO_RESULT(outVar->SetAsBool(result)));
+
+  outVar.forget(aResult);
+  return NS_OK;
+}
+
+/*******************************************************************************
+ * AutoWriteTransaction
+ ******************************************************************************/
+
+AutoWriteTransaction::AutoWriteTransaction(bool aShadowWrites)
+    : mConnection(nullptr), mShadowWrites(aShadowWrites) {
+  AssertIsOnGlobalConnectionThread();
+
+  MOZ_COUNT_CTOR(mozilla::dom::AutoWriteTransaction);
+}
+
+AutoWriteTransaction::~AutoWriteTransaction() {
+  AssertIsOnGlobalConnectionThread();
+
+  MOZ_COUNT_DTOR(mozilla::dom::AutoWriteTransaction);
+
+  if (mConnection) {
+    QM_WARNONLY_TRY(QM_TO_RESULT(mConnection->RollbackWriteTransaction()));
+
+    if (mShadowWrites) {
+      QM_WARNONLY_TRY(QM_TO_RESULT(DetachShadowDatabaseAndUnlock()));
+    }
+  }
+}
+
+nsresult AutoWriteTransaction::Start(Connection* aConnection) {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(aConnection);
+  MOZ_ASSERT(!mConnection);
+
+  if (mShadowWrites) {
+    QM_TRY(MOZ_TO_RESULT(LockAndAttachShadowDatabase(aConnection)));
+  }
+
+  QM_TRY(MOZ_TO_RESULT(aConnection->BeginWriteTransaction()));
+
+  mConnection = aConnection;
+
+  return NS_OK;
+}
+
+nsresult AutoWriteTransaction::Commit() {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(mConnection);
+
+  QM_TRY(MOZ_TO_RESULT(mConnection->CommitWriteTransaction()));
+
+  if (mShadowWrites) {
+    QM_TRY(MOZ_TO_RESULT(DetachShadowDatabaseAndUnlock()));
+  }
+
+  mConnection = nullptr;
+
+  return NS_OK;
+}
+
+nsresult AutoWriteTransaction::LockAndAttachShadowDatabase(
+    Connection* aConnection) {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(aConnection);
+  MOZ_ASSERT(!mConnection);
+  MOZ_ASSERT(mShadowDatabaseLock.isNothing());
+  MOZ_ASSERT(mShadowWrites);
+
+  QuotaManager* quotaManager = QuotaManager::Get();
+  MOZ_ASSERT(quotaManager);
+
+  mShadowDatabaseLock.emplace(
+      aConnection->GetQuotaClient()->ShadowDatabaseMutex());
+
+  QM_TRY(MOZ_TO_RESULT(AttachShadowDatabase(
+      quotaManager->GetBasePath(), &aConnection->MutableStorageConnection())));
+
+  return NS_OK;
+}
+
+nsresult AutoWriteTransaction::DetachShadowDatabaseAndUnlock() {
+  AssertIsOnGlobalConnectionThread();
+  MOZ_ASSERT(mConnection);
+  MOZ_ASSERT(mShadowDatabaseLock.isSome());
+  MOZ_ASSERT(mShadowWrites);
+
+  nsCOMPtr<mozIStorageConnection> storageConnection =
+      mConnection->StorageConnection();
+  MOZ_ASSERT(storageConnection);
+
+  QM_TRY(MOZ_TO_RESULT(DetachShadowDatabase(storageConnection)));
+
+  mShadowDatabaseLock.reset();
+
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/ActorsParent.h b/dom/localstorage/ActorsParent.h
new file mode 100644
index 0000000000..a7f50ac61a
--- /dev/null
+++ b/dom/localstorage/ActorsParent.h
@@ -0,0 +1,87 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_ActorsParent_h
+#define mozilla_dom_localstorage_ActorsParent_h
+
+#include <cstdint>
+#include "mozilla/AlreadyAddRefed.h"
+
+namespace mozilla {
+
+namespace ipc {
+
+class PBackgroundParent;
+class PrincipalInfo;
+
+}  // namespace ipc
+
+namespace dom {
+
+class LSRequestParams;
+class LSSimpleRequestParams;
+class PBackgroundLSDatabaseParent;
+class PBackgroundLSObserverParent;
+class PBackgroundLSRequestParent;
+class PBackgroundLSSimpleRequestParent;
+
+namespace quota {
+
+class Client;
+
+}  // namespace quota
+
+void InitializeLocalStorage();
+
+PBackgroundLSDatabaseParent* AllocPBackgroundLSDatabaseParent(
+    const mozilla::ipc::PrincipalInfo& aPrincipalInfo,
+    const uint32_t& aPrivateBrowsingId, const uint64_t& aDatastoreId);
+
+bool RecvPBackgroundLSDatabaseConstructor(
+    PBackgroundLSDatabaseParent* aActor,
+    const mozilla::ipc::PrincipalInfo& aPrincipalInfo,
+    const uint32_t& aPrivateBrowsingId, const uint64_t& aDatastoreId);
+
+bool DeallocPBackgroundLSDatabaseParent(PBackgroundLSDatabaseParent* aActor);
+
+PBackgroundLSObserverParent* AllocPBackgroundLSObserverParent(
+    const uint64_t& aObserverId);
+
+bool RecvPBackgroundLSObserverConstructor(PBackgroundLSObserverParent* aActor,
+                                          const uint64_t& aObservereId);
+
+bool DeallocPBackgroundLSObserverParent(PBackgroundLSObserverParent* aActor);
+
+PBackgroundLSRequestParent* AllocPBackgroundLSRequestParent(
+    mozilla::ipc::PBackgroundParent* aBackgroundActor,
+    const LSRequestParams& aParams);
+
+bool RecvPBackgroundLSRequestConstructor(PBackgroundLSRequestParent* aActor,
+                                         const LSRequestParams& aParams);
+
+bool DeallocPBackgroundLSRequestParent(PBackgroundLSRequestParent* aActor);
+
+PBackgroundLSSimpleRequestParent* AllocPBackgroundLSSimpleRequestParent(
+    mozilla::ipc::PBackgroundParent* aBackgroundActor,
+    const LSSimpleRequestParams& aParams);
+
+bool RecvPBackgroundLSSimpleRequestConstructor(
+    PBackgroundLSSimpleRequestParent* aActor,
+    const LSSimpleRequestParams& aParams);
+
+bool DeallocPBackgroundLSSimpleRequestParent(
+    PBackgroundLSSimpleRequestParent* aActor);
+
+namespace localstorage {
+
+already_AddRefed<mozilla::dom::quota::Client> CreateQuotaClient();
+
+}  // namespace localstorage
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_localstorage_ActorsParent_h
diff --git a/dom/localstorage/LSDatabase.cpp b/dom/localstorage/LSDatabase.cpp
new file mode 100644
index 0000000000..2dccdaad26
--- /dev/null
+++ b/dom/localstorage/LSDatabase.cpp
@@ -0,0 +1,448 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "LSDatabase.h"
+
+// Local includes
+#include "ActorsChild.h"
+#include "LSObject.h"
+#include "LSSnapshot.h"
+
+// Global includes
+#include <cstring>
+#include <new>
+#include <utility>
+#include "MainThreadUtils.h"
+#include "mozilla/MacroForEach.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/Services.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/dom/PBackgroundLSDatabase.h"
+#include "nsBaseHashtable.h"
+#include "nsCOMPtr.h"
+#include "nsTHashMap.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsHashKeys.h"
+#include "nsIObserver.h"
+#include "nsIObserverService.h"
+#include "nsString.h"
+#include "nsTArray.h"
+#include "nscore.h"
+
+namespace mozilla::dom {
+
+namespace {
+
+#define XPCOM_SHUTDOWN_OBSERVER_TOPIC "xpcom-shutdown"
+
+using LSDatabaseHashtable = nsTHashMap<nsCStringHashKey, LSDatabase*>;
+
+StaticAutoPtr<LSDatabaseHashtable> gLSDatabases;
+
+}  // namespace
+
+StaticRefPtr<LSDatabase::Observer> LSDatabase::sObserver;
+
+class LSDatabase::Observer final : public nsIObserver {
+  bool mInvalidated;
+
+ public:
+  Observer() : mInvalidated(false) { MOZ_ASSERT(NS_IsMainThread()); }
+
+  void Invalidate() { mInvalidated = true; }
+
+ private:
+  ~Observer() { MOZ_ASSERT(NS_IsMainThread()); }
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+};
+
+LSDatabase::LSDatabase(const nsACString& aOrigin)
+    : mActor(nullptr),
+      mSnapshot(nullptr),
+      mOrigin(aOrigin),
+      mAllowedToClose(false),
+      mRequestedAllowToClose(false) {
+  AssertIsOnOwningThread();
+
+  if (!gLSDatabases) {
+    gLSDatabases = new LSDatabaseHashtable();
+
+    MOZ_ASSERT(!sObserver);
+
+    sObserver = new Observer();
+
+    nsCOMPtr<nsIObserverService> obsSvc =
+        mozilla::services::GetObserverService();
+    MOZ_ASSERT(obsSvc);
+
+    MOZ_ALWAYS_SUCCEEDS(
+        obsSvc->AddObserver(sObserver, XPCOM_SHUTDOWN_OBSERVER_TOPIC, false));
+  }
+
+  MOZ_ASSERT(!gLSDatabases->Contains(mOrigin));
+  gLSDatabases->InsertOrUpdate(mOrigin, this);
+}
+
+LSDatabase::~LSDatabase() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!mSnapshot);
+
+  if (!mAllowedToClose) {
+    AllowToClose();
+  }
+
+  if (mActor) {
+    mActor->SendDeleteMeInternal();
+    MOZ_ASSERT(!mActor, "SendDeleteMeInternal should have cleared!");
+  }
+}
+
+// static
+LSDatabase* LSDatabase::Get(const nsACString& aOrigin) {
+  return gLSDatabases ? gLSDatabases->Get(aOrigin) : nullptr;
+}
+
+void LSDatabase::SetActor(LSDatabaseChild* aActor) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aActor);
+  MOZ_ASSERT(!mActor);
+
+  mActor = aActor;
+}
+
+void LSDatabase::RequestAllowToClose() {
+  AssertIsOnOwningThread();
+
+  if (mRequestedAllowToClose) {
+    return;
+  }
+
+  mRequestedAllowToClose = true;
+
+  if (mSnapshot) {
+    mSnapshot->MarkDirty();
+  } else {
+    AllowToClose();
+  }
+}
+
+void LSDatabase::NoteFinishedSnapshot(LSSnapshot* aSnapshot) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aSnapshot == mSnapshot);
+
+  mSnapshot = nullptr;
+
+  if (mRequestedAllowToClose) {
+    AllowToClose();
+  }
+}
+
+// All these methods assert `!mAllowedToClose` because they shoudn't be called
+// if the database is being closed. Callers should first check the state by
+// calling `IsAlloweToClose` and eventually obtain a new database.
+
+nsresult LSDatabase::GetLength(LSObject* aObject, uint32_t* aResult) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aObject);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  nsresult rv = EnsureSnapshot(aObject, VoidString());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = mSnapshot->GetLength(aResult);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSDatabase::GetKey(LSObject* aObject, uint32_t aIndex,
+                            nsAString& aResult) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aObject);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  nsresult rv = EnsureSnapshot(aObject, VoidString());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = mSnapshot->GetKey(aIndex, aResult);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSDatabase::GetItem(LSObject* aObject, const nsAString& aKey,
+                             nsAString& aResult) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aObject);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  nsresult rv = EnsureSnapshot(aObject, aKey);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = mSnapshot->GetItem(aKey, aResult);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSDatabase::GetKeys(LSObject* aObject, nsTArray<nsString>& aKeys) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aObject);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  nsresult rv = EnsureSnapshot(aObject, VoidString());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = mSnapshot->GetKeys(aKeys);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSDatabase::SetItem(LSObject* aObject, const nsAString& aKey,
+                             const nsAString& aValue,
+                             LSNotifyInfo& aNotifyInfo) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aObject);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  nsresult rv = EnsureSnapshot(aObject, aKey);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = mSnapshot->SetItem(aKey, aValue, aNotifyInfo);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSDatabase::RemoveItem(LSObject* aObject, const nsAString& aKey,
+                                LSNotifyInfo& aNotifyInfo) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aObject);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  nsresult rv = EnsureSnapshot(aObject, aKey);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = mSnapshot->RemoveItem(aKey, aNotifyInfo);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSDatabase::Clear(LSObject* aObject, LSNotifyInfo& aNotifyInfo) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aObject);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  nsresult rv = EnsureSnapshot(aObject, VoidString());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = mSnapshot->Clear(aNotifyInfo);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSDatabase::BeginExplicitSnapshot(LSObject* aObject) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aObject);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+  MOZ_ASSERT(!mSnapshot);
+
+  nsresult rv = EnsureSnapshot(aObject, VoidString(), /* aExplicit */ true);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSDatabase::CheckpointExplicitSnapshot() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+  MOZ_ASSERT(mSnapshot);
+  MOZ_ASSERT(mSnapshot->Explicit());
+
+  nsresult rv = mSnapshot->ExplicitCheckpoint();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSDatabase::EndExplicitSnapshot() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+  MOZ_ASSERT(mSnapshot);
+  MOZ_ASSERT(mSnapshot->Explicit());
+
+  nsresult rv = mSnapshot->ExplicitEnd();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+bool LSDatabase::HasSnapshot() const {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  return !!mSnapshot;
+}
+
+int64_t LSDatabase::GetSnapshotUsage() const {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+  MOZ_ASSERT(mSnapshot);
+
+  return mSnapshot->GetUsage();
+}
+
+nsresult LSDatabase::EnsureSnapshot(LSObject* aObject, const nsAString& aKey,
+                                    bool aExplicit) {
+  MOZ_ASSERT(aObject);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT_IF(mSnapshot, !aExplicit);
+  MOZ_ASSERT(!mAllowedToClose);
+
+  if (mSnapshot) {
+    return NS_OK;
+  }
+
+  RefPtr<LSSnapshot> snapshot = new LSSnapshot(this);
+
+  LSSnapshotChild* actor = new LSSnapshotChild(snapshot);
+
+  LSSnapshotInitInfo initInfo;
+  bool ok = mActor->SendPBackgroundLSSnapshotConstructor(
+      actor, aObject->DocumentURI(), nsString(aKey),
+      /* increasePeakUsage */ true,
+      /* minSize */ 0, &initInfo);
+  if (NS_WARN_IF(!ok)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  snapshot->SetActor(actor);
+
+  // This add refs snapshot.
+  nsresult rv = snapshot->Init(aKey, initInfo, aExplicit);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  // This is cleared in LSSnapshot::Run() before the snapshot is destroyed.
+  mSnapshot = snapshot;
+
+  return NS_OK;
+}
+
+void LSDatabase::AllowToClose() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!mAllowedToClose);
+  MOZ_ASSERT(!mSnapshot);
+
+  mAllowedToClose = true;
+
+  if (mActor) {
+    mActor->SendAllowToClose();
+  }
+
+  MOZ_ASSERT(gLSDatabases);
+  MOZ_ASSERT(gLSDatabases->Get(mOrigin));
+  gLSDatabases->Remove(mOrigin);
+
+  if (!gLSDatabases->Count()) {
+    gLSDatabases = nullptr;
+
+    MOZ_ASSERT(sObserver);
+
+    nsCOMPtr<nsIObserverService> obsSvc =
+        mozilla::services::GetObserverService();
+    MOZ_ASSERT(obsSvc);
+
+    MOZ_ALWAYS_SUCCEEDS(
+        obsSvc->RemoveObserver(sObserver, XPCOM_SHUTDOWN_OBSERVER_TOPIC));
+
+    // We also need to invalidate the observer because AllowToClose can be
+    // triggered by an indirectly related observer, so the observer service
+    // may still keep our observer alive and call Observe on it. This is
+    // possible because observer service snapshots the observer list for given
+    // subject before looping over the list.
+    sObserver->Invalidate();
+
+    sObserver = nullptr;
+  }
+}
+
+NS_IMPL_ISUPPORTS(LSDatabase::Observer, nsIObserver)
+
+NS_IMETHODIMP
+LSDatabase::Observer::Observe(nsISupports* aSubject, const char* aTopic,
+                              const char16_t* aData) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!strcmp(aTopic, XPCOM_SHUTDOWN_OBSERVER_TOPIC));
+
+  if (mInvalidated) {
+    return NS_OK;
+  }
+
+  MOZ_ASSERT(gLSDatabases);
+
+  for (const RefPtr<LSDatabase>& database :
+       ToTArray<nsTArray<RefPtr<LSDatabase>>>(gLSDatabases->Values())) {
+    database->RequestAllowToClose();
+  }
+
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/LSDatabase.h b/dom/localstorage/LSDatabase.h
new file mode 100644
index 0000000000..b85798eeac
--- /dev/null
+++ b/dom/localstorage/LSDatabase.h
@@ -0,0 +1,105 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LSDatabase_h
+#define mozilla_dom_localstorage_LSDatabase_h
+
+#include <cstdint>
+#include "ErrorList.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/StaticPtr.h"
+#include "nsISupports.h"
+#include "nsString.h"
+#include "nsTArrayForwardDeclare.h"
+
+namespace mozilla::dom {
+
+class LSDatabaseChild;
+class LSNotifyInfo;
+class LSObject;
+class LSSnapshot;
+
+class LSDatabase final {
+  class Observer;
+
+  LSDatabaseChild* mActor;
+
+  LSSnapshot* mSnapshot;
+
+  const nsCString mOrigin;
+
+  bool mAllowedToClose;
+  bool mRequestedAllowToClose;
+
+  static StaticRefPtr<Observer> sObserver;
+
+ public:
+  explicit LSDatabase(const nsACString& aOrigin);
+
+  static LSDatabase* Get(const nsACString& aOrigin);
+
+  NS_INLINE_DECL_REFCOUNTING(LSDatabase)
+
+  void AssertIsOnOwningThread() const { NS_ASSERT_OWNINGTHREAD(LSDatabase); }
+
+  void SetActor(LSDatabaseChild* aActor);
+
+  void ClearActor() {
+    AssertIsOnOwningThread();
+    MOZ_ASSERT(mActor);
+
+    mActor = nullptr;
+  }
+
+  bool IsAllowedToClose() const {
+    AssertIsOnOwningThread();
+
+    return mAllowedToClose;
+  }
+
+  void RequestAllowToClose();
+
+  void NoteFinishedSnapshot(LSSnapshot* aSnapshot);
+
+  nsresult GetLength(LSObject* aObject, uint32_t* aResult);
+
+  nsresult GetKey(LSObject* aObject, uint32_t aIndex, nsAString& aResult);
+
+  nsresult GetItem(LSObject* aObject, const nsAString& aKey,
+                   nsAString& aResult);
+
+  nsresult GetKeys(LSObject* aObject, nsTArray<nsString>& aKeys);
+
+  nsresult SetItem(LSObject* aObject, const nsAString& aKey,
+                   const nsAString& aValue, LSNotifyInfo& aNotifyInfo);
+
+  nsresult RemoveItem(LSObject* aObject, const nsAString& aKey,
+                      LSNotifyInfo& aNotifyInfo);
+
+  nsresult Clear(LSObject* aObject, LSNotifyInfo& aNotifyInfo);
+
+  nsresult BeginExplicitSnapshot(LSObject* aObject);
+
+  nsresult CheckpointExplicitSnapshot();
+
+  nsresult EndExplicitSnapshot();
+
+  bool HasSnapshot() const;
+
+  int64_t GetSnapshotUsage() const;
+
+ private:
+  ~LSDatabase();
+
+  nsresult EnsureSnapshot(LSObject* aObject, const nsAString& aKey,
+                          bool aExplicit = false);
+
+  void AllowToClose();
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_localstorage_LSDatabase_h
diff --git a/dom/localstorage/LSInitializationTypes.h b/dom/localstorage/LSInitializationTypes.h
new file mode 100644
index 0000000000..63090066da
--- /dev/null
+++ b/dom/localstorage/LSInitializationTypes.h
@@ -0,0 +1,48 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef DOM_LOCALSTORAGE_INITIALIZATIONTYPES_H_
+#define DOM_LOCALSTORAGE_INITIALIZATIONTYPES_H_
+
+#include "mozilla/TypedEnumBits.h"
+#include <mozilla/dom/quota/FirstInitializationAttempts.h>
+#include "nsTHashMap.h"
+
+namespace mozilla {
+struct CreateIfNonExistent;
+}
+
+namespace mozilla::dom {
+
+enum class LSOriginInitialization {
+  None = 0,
+  Datastore = 1 << 0,
+};
+
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(LSOriginInitialization)
+
+using LSOriginInitializationInfo =
+    quota::FirstInitializationAttempts<LSOriginInitialization, Nothing>;
+
+class LSInitializationInfo final {
+  nsTHashMap<nsCStringHashKey, LSOriginInitializationInfo>
+      mOriginInitializationInfos;
+
+ public:
+  LSOriginInitializationInfo& MutableOriginInitializationInfoRef(
+      const nsACString& aOrigin) {
+    return *mOriginInitializationInfos.Lookup(aOrigin);
+  }
+
+  LSOriginInitializationInfo& MutableOriginInitializationInfoRef(
+      const nsACString& aOrigin, const CreateIfNonExistent&) {
+    return mOriginInitializationInfos.LookupOrInsert(aOrigin);
+  }
+};
+
+}  // namespace mozilla::dom
+
+#endif  // DOM_LOCALSTORAGE_INITIALIZATIONTYPES_H_
diff --git a/dom/localstorage/LSObject.cpp b/dom/localstorage/LSObject.cpp
new file mode 100644
index 0000000000..196b401e21
--- /dev/null
+++ b/dom/localstorage/LSObject.cpp
@@ -0,0 +1,1167 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "LSObject.h"
+
+// Local includes
+#include "ActorsChild.h"
+#include "LSDatabase.h"
+#include "LSObserver.h"
+
+// Global includes
+#include <utility>
+#include "MainThreadUtils.h"
+#include "mozilla/BasePrincipal.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/MacroForEach.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/OriginAttributes.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/RemoteLazyInputStreamThread.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/SpinEventLoopUntil.h"
+#include "mozilla/StaticMutex.h"
+#include "mozilla/StorageAccess.h"
+#include "mozilla/Unused.h"
+#include "mozilla/dom/ClientInfo.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/LocalStorageCommon.h"
+#include "mozilla/dom/PBackgroundLSRequest.h"
+#include "mozilla/dom/PBackgroundLSSharedTypes.h"
+#include "mozilla/dom/quota/QuotaManager.h"
+#include "mozilla/ipc/BackgroundChild.h"
+#include "mozilla/ipc/BackgroundUtils.h"
+#include "mozilla/ipc/PBackgroundChild.h"
+#include "mozilla/ipc/ProcessChild.h"
+#include "nsCOMPtr.h"
+#include "nsContentUtils.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsIEventTarget.h"
+#include "nsIPrincipal.h"
+#include "nsIRunnable.h"
+#include "nsIScriptObjectPrincipal.h"
+#include "nsISerialEventTarget.h"
+#include "nsITimer.h"
+#include "nsPIDOMWindow.h"
+#include "nsString.h"
+#include "nsTArray.h"
+#include "nsTStringRepr.h"
+#include "nsThread.h"
+#include "nsThreadUtils.h"
+#include "nsXULAppAPI.h"
+#include "nscore.h"
+
+/**
+ * Automatically cancel and abort synchronous LocalStorage requests (for example
+ * datastore preparation) if they take this long.  We've chosen a value that is
+ * long enough that it is unlikely for the problem to be falsely triggered by
+ * slow system I/O.  We've also chosen a value long enough so that automated
+ * tests should time out and fail if LocalStorage hangs.  Also, this value is
+ * long enough so that testers can notice the (content process) hang; we want to
+ * know about the hangs, not hide them.  On the other hand this value is less
+ * than 60 seconds which is used by nsTerminator to crash a hung main process.
+ */
+#define FAILSAFE_CANCEL_SYNC_OP_MS 50000
+
+/**
+ * Interval with which to wake up while waiting for the sync op to complete to
+ * check ExpectingShutdown().
+ */
+#define SYNC_OP_WAKE_INTERVAL_MS 500
+
+namespace mozilla::dom {
+
+namespace {
+
+class RequestHelper;
+
+/**
+ * Main-thread helper that implements the blocking logic required by
+ * LocalStorage's synchronous semantics.  StartAndReturnResponse blocks on a
+ * monitor until a result is received. See StartAndReturnResponse() for info on
+ * this choice.
+ *
+ * The normal life-cycle of this method looks like:
+ * - Main Thread: LSObject::DoRequestSynchronously creates a RequestHelper and
+ *   invokes StartAndReturnResponse().  It Dispatches the RequestHelper to the
+ *   RemoteLazyInputStream thread, and waits on mMonitor.
+ * - RemoteLazyInputStream Thread: RequestHelper::Run is called, invoking
+ *   Start() which invokes LSObject::StartRequest, which gets-or-creates the
+ *   PBackground actor if necessary, sends LSRequest constructor which is
+ *   provided with a callback reference to the RequestHelper. State advances to
+ *   ResponsePending.
+ * - RemoteLazyInputStreamThread: LSRequestChild::Recv__delete__ is received,
+ *   which invokes RequestHelepr::OnResponse, advancing the state to Complete
+ *   and notifying mMonitor.
+ * - Main Thread: The main thread wakes up after waiting on the monitor,
+ *   returning the received response.
+ *
+ * See LocalStorageCommon.h for high-level context and method comments for
+ * low-level details.
+ */
+class RequestHelper final : public Runnable, public LSRequestChildCallback {
+  enum class State {
+    /**
+     * The RequestHelper has been created and dispatched to the
+     * RemoteLazyInputStream Thread.
+     */
+    Initial,
+    /**
+     * Start() has been invoked on the RemoteLazyInputStream Thread and
+     * LSObject::StartRequest has been invoked from there, sending an IPC
+     * message to PBackground to service the request.  We stay in this state
+     * until a response is received or a timeout occurs.
+     */
+    ResponsePending,
+    /**
+     * The request timed out, or failed in some fashion, and needs to be
+     * cancelled. A runnable has been dispatched to the DOM File thread to
+     * notify the parent actor, and the main thread will continue to block until
+     * we receive a reponse.
+     */
+    Canceling,
+    /**
+     * The request is complete, either successfully or due to being cancelled.
+     * The main thread can stop waiting and immediately return to the caller of
+     * StartAndReturnResponse.
+     */
+    Complete
+  };
+
+  // The object we are issuing a request on behalf of.  Present because of the
+  // need to invoke LSObject::StartRequest off the main thread.  Dropped on
+  // return to the main-thread in Finish().
+  RefPtr<LSObject> mObject;
+  // The thread the RequestHelper was created on.  This should be the main
+  // thread.
+  nsCOMPtr<nsIEventTarget> mOwningEventTarget;
+  // The IPC actor handling the request with standard IPC allocation rules.
+  // Our reference is nulled in OnResponse which corresponds to the actor's
+  // __destroy__ method.
+  LSRequestChild* mActor;
+  const LSRequestParams mParams;
+  Monitor mMonitor;
+  LSRequestResponse mResponse MOZ_GUARDED_BY(mMonitor);
+  nsresult mResultCode MOZ_GUARDED_BY(mMonitor);
+  State mState MOZ_GUARDED_BY(mMonitor);
+
+ public:
+  RequestHelper(LSObject* aObject, const LSRequestParams& aParams)
+      : Runnable("dom::RequestHelper"),
+        mObject(aObject),
+        mOwningEventTarget(GetCurrentSerialEventTarget()),
+        mActor(nullptr),
+        mParams(aParams),
+        mMonitor("dom::RequestHelper::mMonitor"),
+        mResultCode(NS_OK),
+        mState(State::Initial) {}
+
+  bool IsOnOwningThread() const {
+    MOZ_ASSERT(mOwningEventTarget);
+
+    bool current;
+    return NS_SUCCEEDED(mOwningEventTarget->IsOnCurrentThread(&current)) &&
+           current;
+  }
+
+  void AssertIsOnOwningThread() const {
+    MOZ_ASSERT(NS_IsMainThread());
+    MOZ_ASSERT(IsOnOwningThread());
+  }
+
+  nsresult StartAndReturnResponse(LSRequestResponse& aResponse);
+
+ private:
+  ~RequestHelper() = default;
+
+  NS_DECL_ISUPPORTS_INHERITED
+
+  NS_DECL_NSIRUNNABLE
+
+  // LSRequestChildCallback
+  void OnResponse(const LSRequestResponse& aResponse) override;
+};
+
+void AssertExplicitSnapshotInvariants(const LSObject& aObject) {
+  // Can be only called if the mInExplicitSnapshot flag is true.
+  // An explicit snapshot must have been created.
+  MOZ_ASSERT(aObject.InExplicitSnapshot());
+
+  // If an explicit snapshot has been created then mDatabase must be not null.
+  // DropDatabase could be called in the meatime, but that must be preceded by
+  // Disconnect which sets mInExplicitSnapshot to false. EnsureDatabase could
+  // be called in the meantime too, but that can't set mDatabase to null or to
+  // a new value. See the comment below.
+  MOZ_ASSERT(aObject.DatabaseStrongRef());
+
+  // Existence of a snapshot prevents the database from allowing to close. See
+  // LSDatabase::RequestAllowToClose and LSDatabase::NoteFinishedSnapshot.
+  // If the database is not allowed to close then mDatabase could not have been
+  // nulled out or set to a new value. See EnsureDatabase.
+  MOZ_ASSERT(!aObject.DatabaseStrongRef()->IsAllowedToClose());
+}
+
+}  // namespace
+
+LSObject::LSObject(nsPIDOMWindowInner* aWindow, nsIPrincipal* aPrincipal,
+                   nsIPrincipal* aStoragePrincipal)
+    : Storage(aWindow, aPrincipal, aStoragePrincipal),
+      mPrivateBrowsingId(0),
+      mInExplicitSnapshot(false) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(NextGenLocalStorageEnabled());
+}
+
+LSObject::~LSObject() {
+  AssertIsOnOwningThread();
+
+  DropObserver();
+}
+
+// static
+nsresult LSObject::CreateForWindow(nsPIDOMWindowInner* aWindow,
+                                   Storage** aStorage) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aWindow);
+  MOZ_ASSERT(aStorage);
+  MOZ_ASSERT(NextGenLocalStorageEnabled());
+  MOZ_ASSERT(StorageAllowedForWindow(aWindow) != StorageAccess::eDeny);
+
+  nsCOMPtr<nsIScriptObjectPrincipal> sop = do_QueryInterface(aWindow);
+  MOZ_ASSERT(sop);
+
+  nsCOMPtr<nsIPrincipal> principal = sop->GetPrincipal();
+  if (NS_WARN_IF(!principal)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<nsIPrincipal> storagePrincipal = sop->GetEffectiveStoragePrincipal();
+  if (NS_WARN_IF(!storagePrincipal)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  if (principal->IsSystemPrincipal()) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  // localStorage is not available on some pages on purpose, for example
+  // about:home. Match the old implementation by using GenerateOriginKey
+  // for the check.
+  nsCString originAttrSuffix;
+  nsCString originKey;
+  nsresult rv = storagePrincipal->GetStorageOriginKey(originKey);
+  storagePrincipal->OriginAttributesRef().CreateSuffix(originAttrSuffix);
+
+  if (NS_FAILED(rv)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  auto principalInfo = MakeUnique<PrincipalInfo>();
+  rv = PrincipalToPrincipalInfo(principal, principalInfo.get());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  MOZ_ASSERT(principalInfo->type() == PrincipalInfo::TContentPrincipalInfo);
+
+  auto storagePrincipalInfo = MakeUnique<PrincipalInfo>();
+  rv = PrincipalToPrincipalInfo(storagePrincipal, storagePrincipalInfo.get());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  MOZ_ASSERT(storagePrincipalInfo->type() ==
+             PrincipalInfo::TContentPrincipalInfo);
+
+  if (NS_WARN_IF(
+          !quota::QuotaManager::IsPrincipalInfoValid(*storagePrincipalInfo))) {
+    return NS_ERROR_FAILURE;
+  }
+
+#ifdef DEBUG
+  QM_TRY_INSPECT(
+      const auto& principalMetadata,
+      quota::QuotaManager::GetInfoFromPrincipal(storagePrincipal.get()));
+
+  MOZ_ASSERT(originAttrSuffix == principalMetadata.mSuffix);
+
+  const auto& origin = principalMetadata.mOrigin;
+#else
+  QM_TRY_INSPECT(
+      const auto& origin,
+      quota::QuotaManager::GetOriginFromPrincipal(storagePrincipal.get()));
+#endif
+
+  uint32_t privateBrowsingId;
+  rv = storagePrincipal->GetPrivateBrowsingId(&privateBrowsingId);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  Maybe<ClientInfo> clientInfo = aWindow->GetClientInfo();
+  if (clientInfo.isNothing()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  Maybe<nsID> clientId = Some(clientInfo.ref().Id());
+
+  Maybe<PrincipalInfo> clientPrincipalInfo =
+      Some(clientInfo.ref().PrincipalInfo());
+
+  nsString documentURI;
+  if (nsCOMPtr<Document> doc = aWindow->GetExtantDoc()) {
+    rv = doc->GetDocumentURI(documentURI);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+  }
+
+  RefPtr<LSObject> object = new LSObject(aWindow, principal, storagePrincipal);
+  object->mPrincipalInfo = std::move(principalInfo);
+  object->mStoragePrincipalInfo = std::move(storagePrincipalInfo);
+  object->mPrivateBrowsingId = privateBrowsingId;
+  object->mClientId = clientId;
+  object->mClientPrincipalInfo = clientPrincipalInfo;
+  object->mOrigin = origin;
+  object->mOriginKey = originKey;
+  object->mDocumentURI = documentURI;
+
+  object.forget(aStorage);
+  return NS_OK;
+}
+
+// static
+nsresult LSObject::CreateForPrincipal(nsPIDOMWindowInner* aWindow,
+                                      nsIPrincipal* aPrincipal,
+                                      nsIPrincipal* aStoragePrincipal,
+                                      const nsAString& aDocumentURI,
+                                      bool aPrivate, LSObject** aObject) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(aStoragePrincipal);
+  MOZ_ASSERT(aObject);
+
+  nsCString originAttrSuffix;
+  nsCString originKey;
+  nsresult rv = aStoragePrincipal->GetStorageOriginKey(originKey);
+  aStoragePrincipal->OriginAttributesRef().CreateSuffix(originAttrSuffix);
+  if (NS_FAILED(rv)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  auto principalInfo = MakeUnique<PrincipalInfo>();
+  rv = PrincipalToPrincipalInfo(aPrincipal, principalInfo.get());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  MOZ_ASSERT(principalInfo->type() == PrincipalInfo::TContentPrincipalInfo ||
+             principalInfo->type() == PrincipalInfo::TSystemPrincipalInfo);
+
+  auto storagePrincipalInfo = MakeUnique<PrincipalInfo>();
+  rv = PrincipalToPrincipalInfo(aStoragePrincipal, storagePrincipalInfo.get());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  MOZ_ASSERT(
+      storagePrincipalInfo->type() == PrincipalInfo::TContentPrincipalInfo ||
+      storagePrincipalInfo->type() == PrincipalInfo::TSystemPrincipalInfo);
+
+  if (NS_WARN_IF(
+          !quota::QuotaManager::IsPrincipalInfoValid(*storagePrincipalInfo))) {
+    return NS_ERROR_FAILURE;
+  }
+
+#ifdef DEBUG
+  QM_TRY_INSPECT(
+      const auto& principalMetadata,
+      ([&storagePrincipalInfo,
+        &aPrincipal]() -> Result<quota::PrincipalMetadata, nsresult> {
+        if (storagePrincipalInfo->type() ==
+            PrincipalInfo::TSystemPrincipalInfo) {
+          return quota::QuotaManager::GetInfoForChrome();
+        }
+
+        QM_TRY_RETURN(quota::QuotaManager::GetInfoFromPrincipal(aPrincipal));
+      }()));
+
+  MOZ_ASSERT(originAttrSuffix == principalMetadata.mSuffix);
+
+  const auto& origin = principalMetadata.mOrigin;
+#else
+  QM_TRY_INSPECT(
+      const auto& origin, ([&storagePrincipalInfo,
+                            &aPrincipal]() -> Result<nsAutoCString, nsresult> {
+        if (storagePrincipalInfo->type() ==
+            PrincipalInfo::TSystemPrincipalInfo) {
+          return nsAutoCString{quota::QuotaManager::GetOriginForChrome()};
+        }
+
+        QM_TRY_RETURN(quota::QuotaManager::GetOriginFromPrincipal(aPrincipal));
+      }()));
+#endif
+
+  Maybe<nsID> clientId;
+  if (aWindow) {
+    Maybe<ClientInfo> clientInfo = aWindow->GetClientInfo();
+    if (clientInfo.isNothing()) {
+      return NS_ERROR_FAILURE;
+    }
+
+    clientId = Some(clientInfo.ref().Id());
+  } else if (Preferences::GetBool("dom.storage.client_validation")) {
+    return NS_ERROR_FAILURE;
+  }
+
+  RefPtr<LSObject> object =
+      new LSObject(aWindow, aPrincipal, aStoragePrincipal);
+  object->mPrincipalInfo = std::move(principalInfo);
+  object->mStoragePrincipalInfo = std::move(storagePrincipalInfo);
+  object->mPrivateBrowsingId = aPrivate ? 1 : 0;
+  object->mClientId = clientId;
+  object->mOrigin = origin;
+  object->mOriginKey = originKey;
+  object->mDocumentURI = aDocumentURI;
+
+  object.forget(aObject);
+  return NS_OK;
+}  // namespace dom
+
+LSRequestChild* LSObject::StartRequest(const LSRequestParams& aParams,
+                                       LSRequestChildCallback* aCallback) {
+  AssertIsOnDOMFileThread();
+
+  mozilla::ipc::PBackgroundChild* backgroundActor =
+      mozilla::ipc::BackgroundChild::GetOrCreateForCurrentThread();
+  if (NS_WARN_IF(!backgroundActor)) {
+    return nullptr;
+  }
+
+  LSRequestChild* actor = new LSRequestChild();
+
+  if (!backgroundActor->SendPBackgroundLSRequestConstructor(actor, aParams)) {
+    return nullptr;
+  }
+
+  // Must set callback after calling SendPBackgroundLSRequestConstructor since
+  // it can be called synchronously when SendPBackgroundLSRequestConstructor
+  // fails.
+  actor->SetCallback(aCallback);
+
+  return actor;
+}
+
+Storage::StorageType LSObject::Type() const {
+  AssertIsOnOwningThread();
+
+  return eLocalStorage;
+}
+
+bool LSObject::IsForkOf(const Storage* aStorage) const {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aStorage);
+
+  if (aStorage->Type() != eLocalStorage) {
+    return false;
+  }
+
+  return static_cast<const LSObject*>(aStorage)->mOrigin == mOrigin;
+}
+
+int64_t LSObject::GetOriginQuotaUsage() const {
+  AssertIsOnOwningThread();
+
+  // It's not necessary to return an actual value here.  This method is
+  // implemented only because the SessionStore currently needs it to cap the
+  // amount of data it persists to disk (via nsIDOMWindowUtils.getStorageUsage).
+  // Any callers that want to know about storage usage should be asking
+  // QuotaManager directly.
+  //
+  // Note: This may change as LocalStorage is repurposed to be the new
+  // SessionStorage backend.
+  return 0;
+}
+
+void LSObject::Disconnect() {
+  // Explicit snapshots which were not ended in JS, must be ended here while
+  // IPC is still available. We can't do that in DropDatabase because actors
+  // may have been destroyed already at that point.
+  if (mInExplicitSnapshot) {
+    AssertExplicitSnapshotInvariants(*this);
+
+    nsresult rv = mDatabase->EndExplicitSnapshot();
+    Unused << NS_WARN_IF(NS_FAILED(rv));
+
+    mInExplicitSnapshot = false;
+  }
+}
+
+uint32_t LSObject::GetLength(nsIPrincipal& aSubjectPrincipal,
+                             ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return 0;
+  }
+
+  nsresult rv = EnsureDatabase();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return 0;
+  }
+
+  uint32_t result;
+  rv = mDatabase->GetLength(this, &result);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return 0;
+  }
+
+  return result;
+}
+
+void LSObject::Key(uint32_t aIndex, nsAString& aResult,
+                   nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  nsresult rv = EnsureDatabase();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  nsString result;
+  rv = mDatabase->GetKey(this, aIndex, result);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  aResult = result;
+}
+
+void LSObject::GetItem(const nsAString& aKey, nsAString& aResult,
+                       nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  nsresult rv = EnsureDatabase();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  nsString result;
+  rv = mDatabase->GetItem(this, aKey, result);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  aResult = result;
+}
+
+void LSObject::GetSupportedNames(nsTArray<nsString>& aNames) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(*nsContentUtils::SubjectPrincipal())) {
+    // Return just an empty array.
+    aNames.Clear();
+    return;
+  }
+
+  nsresult rv = EnsureDatabase();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return;
+  }
+
+  rv = mDatabase->GetKeys(this, aNames);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return;
+  }
+}
+
+void LSObject::SetItem(const nsAString& aKey, const nsAString& aValue,
+                       nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  nsresult rv = EnsureDatabase();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  LSNotifyInfo info;
+  rv = mDatabase->SetItem(this, aKey, aValue, info);
+  if (rv == NS_ERROR_FILE_NO_DEVICE_SPACE) {
+    rv = NS_ERROR_DOM_QUOTA_EXCEEDED_ERR;
+  }
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  if (info.changed()) {
+    OnChange(aKey, info.oldValue(), aValue);
+  }
+}
+
+void LSObject::RemoveItem(const nsAString& aKey,
+                          nsIPrincipal& aSubjectPrincipal,
+                          ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  nsresult rv = EnsureDatabase();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  LSNotifyInfo info;
+  rv = mDatabase->RemoveItem(this, aKey, info);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  if (info.changed()) {
+    OnChange(aKey, info.oldValue(), VoidString());
+  }
+}
+
+void LSObject::Clear(nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  nsresult rv = EnsureDatabase();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  LSNotifyInfo info;
+  rv = mDatabase->Clear(this, info);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  if (info.changed()) {
+    OnChange(VoidString(), VoidString(), VoidString());
+  }
+}
+
+void LSObject::Open(nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  nsresult rv = EnsureDatabase();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+}
+
+void LSObject::Close(nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  DropDatabase();
+}
+
+void LSObject::BeginExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                                     ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  if (mInExplicitSnapshot) {
+    aError.Throw(NS_ERROR_ALREADY_INITIALIZED);
+    return;
+  }
+
+  nsresult rv = EnsureDatabase();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  rv = mDatabase->BeginExplicitSnapshot(this);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  mInExplicitSnapshot = true;
+}
+
+void LSObject::CheckpointExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                                          ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  if (!mInExplicitSnapshot) {
+    aError.Throw(NS_ERROR_NOT_INITIALIZED);
+    return;
+  }
+
+  AssertExplicitSnapshotInvariants(*this);
+
+  nsresult rv = mDatabase->CheckpointExplicitSnapshot();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+}
+
+void LSObject::EndExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                                   ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  if (!mInExplicitSnapshot) {
+    aError.Throw(NS_ERROR_NOT_INITIALIZED);
+    return;
+  }
+
+  AssertExplicitSnapshotInvariants(*this);
+
+  nsresult rv = mDatabase->EndExplicitSnapshot();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+
+  mInExplicitSnapshot = false;
+}
+
+bool LSObject::GetHasSnapshot(nsIPrincipal& aSubjectPrincipal,
+                              ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return false;
+  }
+
+  // We can't call `HasSnapshot` on the database if it's being closed, but we
+  // know that a database which is being closed can't have a snapshot, so we
+  // return false in that case directly here.
+  if (!mDatabase || mDatabase->IsAllowedToClose()) {
+    return false;
+  }
+
+  return mDatabase->HasSnapshot();
+}
+
+int64_t LSObject::GetSnapshotUsage(nsIPrincipal& aSubjectPrincipal,
+                                   ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return 0;
+  }
+
+  if (!mDatabase || mDatabase->IsAllowedToClose()) {
+    aError.Throw(NS_ERROR_NOT_AVAILABLE);
+    return 0;
+  }
+
+  if (!mDatabase->HasSnapshot()) {
+    aError.Throw(NS_ERROR_NOT_AVAILABLE);
+    return 0;
+  }
+
+  return mDatabase->GetSnapshotUsage();
+}
+
+NS_IMPL_ADDREF_INHERITED(LSObject, Storage)
+NS_IMPL_RELEASE_INHERITED(LSObject, Storage)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(LSObject)
+NS_INTERFACE_MAP_END_INHERITING(Storage)
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(LSObject)
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(LSObject, Storage)
+  tmp->AssertIsOnOwningThread();
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(LSObject, Storage)
+  tmp->AssertIsOnOwningThread();
+  tmp->DropDatabase();
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+nsresult LSObject::DoRequestSynchronously(const LSRequestParams& aParams,
+                                          LSRequestResponse& aResponse) {
+  // We don't need this yet, but once the request successfully finishes, it's
+  // too late to initialize PBackground child on the owning thread, because
+  // it can fail and parent would keep an extra strong ref to the datastore or
+  // observer.
+  mozilla::ipc::PBackgroundChild* backgroundActor =
+      mozilla::ipc::BackgroundChild::GetOrCreateForCurrentThread();
+  if (NS_WARN_IF(!backgroundActor)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  RefPtr<RequestHelper> helper = new RequestHelper(this, aParams);
+
+  // This will start and finish the request on the RemoteLazyInputStream thread.
+  // The owning thread is synchronously blocked while the request is
+  // asynchronously processed on the RemoteLazyInputStream thread.
+  nsresult rv = helper->StartAndReturnResponse(aResponse);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  if (aResponse.type() == LSRequestResponse::Tnsresult) {
+    nsresult errorCode = aResponse.get_nsresult();
+
+    if (errorCode == NS_ERROR_FILE_NO_DEVICE_SPACE) {
+      errorCode = NS_ERROR_DOM_QUOTA_EXCEEDED_ERR;
+    }
+
+    return errorCode;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSObject::EnsureDatabase() {
+  AssertIsOnOwningThread();
+
+  if (mDatabase && !mDatabase->IsAllowedToClose()) {
+    return NS_OK;
+  }
+
+  mDatabase = LSDatabase::Get(mOrigin);
+
+  if (mDatabase) {
+    MOZ_ASSERT(!mDatabase->IsAllowedToClose());
+    return NS_OK;
+  }
+
+  // We don't need this yet, but once the request successfully finishes, it's
+  // too late to initialize PBackground child on the owning thread, because
+  // it can fail and parent would keep an extra strong ref to the datastore.
+  mozilla::ipc::PBackgroundChild* backgroundActor =
+      mozilla::ipc::BackgroundChild::GetOrCreateForCurrentThread();
+  if (NS_WARN_IF(!backgroundActor)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  LSRequestCommonParams commonParams;
+  commonParams.principalInfo() = *mPrincipalInfo;
+  commonParams.storagePrincipalInfo() = *mStoragePrincipalInfo;
+  commonParams.originKey() = mOriginKey;
+
+  LSRequestPrepareDatastoreParams params;
+  params.commonParams() = commonParams;
+  params.clientId() = mClientId;
+  params.clientPrincipalInfo() = mClientPrincipalInfo;
+
+  LSRequestResponse response;
+
+  nsresult rv = DoRequestSynchronously(params, response);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  MOZ_ASSERT(response.type() ==
+             LSRequestResponse::TLSRequestPrepareDatastoreResponse);
+
+  const LSRequestPrepareDatastoreResponse& prepareDatastoreResponse =
+      response.get_LSRequestPrepareDatastoreResponse();
+
+  uint64_t datastoreId = prepareDatastoreResponse.datastoreId();
+
+  // The datastore is now ready on the parent side (prepared by the asynchronous
+  // request on the RemoteLazyInputStream thread).
+  // Let's create a direct connection to the datastore (through a database
+  // actor) from the owning thread.
+  // Note that we now can't error out, otherwise parent will keep an extra
+  // strong reference to the datastore.
+
+  RefPtr<LSDatabase> database = new LSDatabase(mOrigin);
+
+  LSDatabaseChild* actor = new LSDatabaseChild(database);
+
+  MOZ_ALWAYS_TRUE(backgroundActor->SendPBackgroundLSDatabaseConstructor(
+      actor, *mStoragePrincipalInfo, mPrivateBrowsingId, datastoreId));
+
+  database->SetActor(actor);
+
+  mDatabase = std::move(database);
+
+  return NS_OK;
+}
+
+void LSObject::DropDatabase() {
+  AssertIsOnOwningThread();
+
+  mDatabase = nullptr;
+}
+
+nsresult LSObject::EnsureObserver() {
+  AssertIsOnOwningThread();
+
+  if (mObserver) {
+    return NS_OK;
+  }
+
+  mObserver = LSObserver::Get(mOrigin);
+
+  if (mObserver) {
+    return NS_OK;
+  }
+
+  LSRequestPrepareObserverParams params;
+  params.principalInfo() = *mPrincipalInfo;
+  params.storagePrincipalInfo() = *mStoragePrincipalInfo;
+  params.clientId() = mClientId;
+  params.clientPrincipalInfo() = mClientPrincipalInfo;
+
+  LSRequestResponse response;
+
+  nsresult rv = DoRequestSynchronously(params, response);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  MOZ_ASSERT(response.type() ==
+             LSRequestResponse::TLSRequestPrepareObserverResponse);
+
+  const LSRequestPrepareObserverResponse& prepareObserverResponse =
+      response.get_LSRequestPrepareObserverResponse();
+
+  uint64_t observerId = prepareObserverResponse.observerId();
+
+  // The obsserver is now ready on the parent side (prepared by the asynchronous
+  // request on the RemoteLazyInputStream thread).
+  // Let's create a direct connection to the observer (through an observer
+  // actor) from the owning thread.
+  // Note that we now can't error out, otherwise parent will keep an extra
+  // strong reference to the observer.
+
+  mozilla::ipc::PBackgroundChild* backgroundActor =
+      mozilla::ipc::BackgroundChild::GetForCurrentThread();
+  MOZ_ASSERT(backgroundActor);
+
+  RefPtr<LSObserver> observer = new LSObserver(mOrigin);
+
+  LSObserverChild* actor = new LSObserverChild(observer);
+
+  MOZ_ALWAYS_TRUE(
+      backgroundActor->SendPBackgroundLSObserverConstructor(actor, observerId));
+
+  observer->SetActor(actor);
+
+  mObserver = std::move(observer);
+
+  return NS_OK;
+}
+
+void LSObject::DropObserver() {
+  AssertIsOnOwningThread();
+
+  if (mObserver) {
+    mObserver = nullptr;
+  }
+}
+
+void LSObject::OnChange(const nsAString& aKey, const nsAString& aOldValue,
+                        const nsAString& aNewValue) {
+  AssertIsOnOwningThread();
+
+  NotifyChange(/* aStorage */ this, StoragePrincipal(), aKey, aOldValue,
+               aNewValue, /* aStorageType */ kLocalStorageType, mDocumentURI,
+               /* aIsPrivate */ !!mPrivateBrowsingId,
+               /* aImmediateDispatch */ false);
+}
+
+void LSObject::LastRelease() {
+  AssertIsOnOwningThread();
+
+  DropDatabase();
+}
+
+nsresult RequestHelper::StartAndReturnResponse(LSRequestResponse& aResponse) {
+  AssertIsOnOwningThread();
+
+  nsCOMPtr<nsIEventTarget> domFileThread =
+      RemoteLazyInputStreamThread::GetOrCreate();
+  if (NS_WARN_IF(!domFileThread)) {
+    return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
+  }
+
+  nsresult rv = domFileThread->Dispatch(this, NS_DISPATCH_NORMAL);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  TimeStamp deadline = TimeStamp::Now() + TimeDuration::FromMilliseconds(
+                                              FAILSAFE_CANCEL_SYNC_OP_MS);
+
+  MonitorAutoLock lock(mMonitor);
+  while (mState != State::Complete) {
+    TimeStamp now = TimeStamp::Now();
+    // If we are expecting shutdown or have passed our deadline, immediately
+    // dispatch ourselves to the DOM File thread to cancel the operation.  We
+    // don't abort until the cancellation has gone through, as otherwise we
+    // could race with the DOM File thread.
+    if (mozilla::ipc::ProcessChild::ExpectingShutdown() || now >= deadline) {
+      switch (mState) {
+        case State::Initial:
+          // The DOM File thread never even woke before ExpectingShutdown() or a
+          // timeout - skip even creating the actor and just report an error.
+          mResultCode = NS_ERROR_FAILURE;
+          mState = State::Complete;
+          continue;
+        case State::ResponsePending:
+          // The DOM File thread is currently waiting for a reply, switch to a
+          // canceling state, and notify it to cancel by dispatching a runnable.
+          mState = State::Canceling;
+          MOZ_ALWAYS_SUCCEEDS(
+              domFileThread->Dispatch(this, NS_DISPATCH_NORMAL));
+          [[fallthrough]];
+        case State::Canceling:
+          // We've cancelled the request, so just need to wait indefinitely for
+          // it to complete.
+          lock.Wait();
+          continue;
+        default:
+          MOZ_ASSERT_UNREACHABLE("unexpected state");
+      }
+    }
+
+    // Wait until either we reach out deadline or for SYNC_OP_WAIT_INTERVAL_MS.
+    lock.Wait(TimeDuration::Min(
+        TimeDuration::FromMilliseconds(SYNC_OP_WAKE_INTERVAL_MS),
+        deadline - now));
+  }
+
+  // The operation is complete, clear our reference to the LSObject.
+  mObject = nullptr;
+
+  if (NS_WARN_IF(NS_FAILED(mResultCode))) {
+    return mResultCode;
+  }
+
+  aResponse = std::move(mResponse);
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS_INHERITED0(RequestHelper, Runnable)
+
+NS_IMETHODIMP
+RequestHelper::Run() {
+  AssertIsOnDOMFileThread();
+
+  MonitorAutoLock lock(mMonitor);
+
+  switch (mState) {
+    case State::Initial: {
+      mState = State::ResponsePending;
+      {
+        MonitorAutoUnlock unlock(mMonitor);
+        mActor = mObject->StartRequest(mParams, this);
+      }
+      if (NS_WARN_IF(!mActor) && mState != State::Complete) {
+        // If we fail to even create the actor, instantly fail and notify our
+        // caller of the error. Otherwise we'll notify from OnResponse as called
+        // by the actor.
+        mResultCode = NS_ERROR_FAILURE;
+        mState = State::Complete;
+        lock.Notify();
+      }
+      return NS_OK;
+    }
+
+    case State::Canceling: {
+      // StartRequest() could fail or OnResponse was already called, so we need
+      // to check if actor is not null. The actor can also be in the final
+      // (finishing) state, in that case we are not allowed to send the cancel
+      // message and it wouldn't make any sense because the request is about to
+      // be destroyed anyway.
+      if (mActor && !mActor->Finishing()) {
+        mActor->SendCancel();
+      }
+      return NS_OK;
+    }
+
+    case State::Complete: {
+      // The operation was cancelled before we ran, do nothing.
+      return NS_OK;
+    }
+
+    default:
+      MOZ_CRASH("Bad state!");
+  }
+}
+
+void RequestHelper::OnResponse(const LSRequestResponse& aResponse) {
+  AssertIsOnDOMFileThread();
+
+  MonitorAutoLock lock(mMonitor);
+
+  MOZ_ASSERT(mState == State::ResponsePending || mState == State::Canceling);
+
+  mActor = nullptr;
+
+  mResponse = aResponse;
+
+  mState = State::Complete;
+
+  lock.Notify();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/LSObject.h b/dom/localstorage/LSObject.h
new file mode 100644
index 0000000000..d394816fa4
--- /dev/null
+++ b/dom/localstorage/LSObject.h
@@ -0,0 +1,220 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LSObject_h
+#define mozilla_dom_localstorage_LSObject_h
+
+#include <cstdint>
+#include "ErrorList.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/dom/Storage.h"
+#include "mozilla/ipc/PBackgroundSharedTypes.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsID.h"
+#include "nsISupports.h"
+#include "nsStringFwd.h"
+#include "nsTArrayForwardDeclare.h"
+
+class nsGlobalWindowInner;
+class nsIEventTarget;
+class nsIPrincipal;
+class nsISerialEventTarget;
+class nsPIDOMWindowInner;
+
+namespace mozilla {
+
+class ErrorResult;
+
+namespace dom {
+
+class LSDatabase;
+class LSObjectChild;
+class LSObserver;
+class LSRequestChild;
+class LSRequestChildCallback;
+class LSRequestParams;
+class LSRequestResponse;
+
+/**
+ * Backs the WebIDL `Storage` binding; all content LocalStorage calls are
+ * handled by this class.
+ *
+ * ## Semantics under e10s / multi-process ##
+ *
+ * A snapshot mechanism used in conjuction with stable points ensures that JS
+ * run-to-completion semantics are experienced even if the same origin is
+ * concurrently accessing LocalStorage across multiple content processes.
+ *
+ * ### Snapshot Consistency ###
+ *
+ * An LSSnapshot is created locally whenever the contents of LocalStorage are
+ * about to be read or written (including length).  This synchronously
+ * establishes a corresponding Snapshot in PBackground in the parent process.
+ * An effort is made to send as much data from the parent process as possible,
+ * so sites using a small/reasonable amount of LocalStorage data will have it
+ * sent to the content process for immediate access.  Sites with greater
+ * LocalStorage usage may only have some of the information relayed.  In that
+ * case, the parent Snapshot will ensure that it retains the exact state of the
+ * parent Datastore at the moment the Snapshot was created.
+ */
+class LSObject final : public Storage {
+  using PrincipalInfo = mozilla::ipc::PrincipalInfo;
+
+  friend nsGlobalWindowInner;
+
+  UniquePtr<PrincipalInfo> mPrincipalInfo;
+  UniquePtr<PrincipalInfo> mStoragePrincipalInfo;
+
+  RefPtr<LSDatabase> mDatabase;
+  RefPtr<LSObserver> mObserver;
+
+  uint32_t mPrivateBrowsingId;
+  Maybe<nsID> mClientId;
+  Maybe<PrincipalInfo> mClientPrincipalInfo;
+  nsCString mOrigin;
+  nsCString mOriginKey;
+  nsString mDocumentURI;
+
+  bool mInExplicitSnapshot;
+
+ public:
+  /**
+   * The normal creation path invoked by nsGlobalWindowInner.
+   */
+  static nsresult CreateForWindow(nsPIDOMWindowInner* aWindow,
+                                  Storage** aStorage);
+
+  /**
+   * nsIDOMStorageManager creation path for use in testing logic.  Supports the
+   * system principal where CreateForWindow does not.  This is also why aPrivate
+   * exists separate from the principal; because the system principal can never
+   * be mutated to have a private browsing id even though it can be used in a
+   * window/document marked as private browsing.  That's a legacy issue that is
+   * being dealt with, but it's why it exists here.
+   */
+  static nsresult CreateForPrincipal(nsPIDOMWindowInner* aWindow,
+                                     nsIPrincipal* aPrincipal,
+                                     nsIPrincipal* aStoragePrincipal,
+                                     const nsAString& aDocumentURI,
+                                     bool aPrivate, LSObject** aObject);
+
+  void AssertIsOnOwningThread() const { NS_ASSERT_OWNINGTHREAD(LSObject); }
+
+  const RefPtr<LSDatabase>& DatabaseStrongRef() const { return mDatabase; }
+
+  const nsString& DocumentURI() const { return mDocumentURI; }
+
+  bool InExplicitSnapshot() const { return mInExplicitSnapshot; }
+
+  LSRequestChild* StartRequest(const LSRequestParams& aParams,
+                               LSRequestChildCallback* aCallback);
+
+  // Storage overrides.
+  StorageType Type() const override;
+
+  bool IsForkOf(const Storage* aStorage) const override;
+
+  int64_t GetOriginQuotaUsage() const override;
+
+  void Disconnect() override;
+
+  uint32_t GetLength(nsIPrincipal& aSubjectPrincipal,
+                     ErrorResult& aError) override;
+
+  void Key(uint32_t aIndex, nsAString& aResult, nsIPrincipal& aSubjectPrincipal,
+           ErrorResult& aError) override;
+
+  void GetItem(const nsAString& aKey, nsAString& aResult,
+               nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) override;
+
+  void GetSupportedNames(nsTArray<nsString>& aNames) override;
+
+  void SetItem(const nsAString& aKey, const nsAString& aValue,
+               nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) override;
+
+  void RemoveItem(const nsAString& aKey, nsIPrincipal& aSubjectPrincipal,
+                  ErrorResult& aError) override;
+
+  void Clear(nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) override;
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Testing Methods: See Storage.h
+  void Open(nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) override;
+
+  void Close(nsIPrincipal& aSubjectPrincipal, ErrorResult& aError) override;
+
+  void BeginExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                             ErrorResult& aError) override;
+
+  void CheckpointExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                                  ErrorResult& aError) override;
+
+  void EndExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                           ErrorResult& aError) override;
+
+  bool GetHasSnapshot(nsIPrincipal& aSubjectPrincipal,
+                      ErrorResult& aError) override;
+
+  int64_t GetSnapshotUsage(nsIPrincipal& aSubjectPrincipal,
+                           ErrorResult& aError) override;
+
+  //////////////////////////////////////////////////////////////////////////////
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(LSObject, Storage)
+
+ private:
+  LSObject(nsPIDOMWindowInner* aWindow, nsIPrincipal* aPrincipal,
+           nsIPrincipal* aStoragePrincipal);
+
+  ~LSObject();
+
+  nsresult DoRequestSynchronously(const LSRequestParams& aParams,
+                                  LSRequestResponse& aResponse);
+
+  nsresult EnsureDatabase();
+
+  void DropDatabase();
+
+  /**
+   * Invoked by nsGlobalWindowInner whenever a new "storage" event listener is
+   * added to the window in order to ensure that "storage" events are received
+   * from other processes.  (`LSObject::OnChange` directly invokes
+   * `Storage::NotifyChange` to notify in-process listeners.)
+   *
+   * If this is the first request in the process for an observer for this
+   * origin, this will trigger a RequestHelper-mediated synchronous LSRequest
+   * to prepare a new observer in the parent process and also construction of
+   * corresponding actors, which will result in the observer being fully
+   * registered in the parent process.
+   */
+  nsresult EnsureObserver();
+
+  /**
+   * Invoked by nsGlobalWindowInner whenever its last "storage" event listener
+   * is removed.
+   */
+  void DropObserver();
+
+  /**
+   * Internal helper method used by mutation methods that wraps the call to
+   * Storage::NotifyChange to generate same-process "storage" events.
+   */
+  void OnChange(const nsAString& aKey, const nsAString& aOldValue,
+                const nsAString& aNewValue);
+
+  // Storage overrides.
+  void LastRelease() override;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_localstorage_LSObject_h
diff --git a/dom/localstorage/LSObserver.cpp b/dom/localstorage/LSObserver.cpp
new file mode 100644
index 0000000000..fda56bd1ed
--- /dev/null
+++ b/dom/localstorage/LSObserver.cpp
@@ -0,0 +1,70 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "LSObserver.h"
+
+// Local includes
+#include "ActorsChild.h"
+
+// Global includes
+#include <utility>
+#include "mozilla/StaticPtr.h"
+#include "nsTHashMap.h"
+#include "nsHashKeys.h"
+
+namespace mozilla::dom {
+
+namespace {
+
+using LSObserverHashtable = nsTHashMap<nsCStringHashKey, LSObserver*>;
+
+StaticAutoPtr<LSObserverHashtable> gLSObservers;
+
+}  // namespace
+
+LSObserver::LSObserver(const nsACString& aOrigin)
+    : mActor(nullptr), mOrigin(aOrigin) {
+  AssertIsOnOwningThread();
+
+  if (!gLSObservers) {
+    gLSObservers = new LSObserverHashtable();
+  }
+
+  MOZ_ASSERT(!gLSObservers->Contains(mOrigin));
+  gLSObservers->InsertOrUpdate(mOrigin, this);
+}
+
+LSObserver::~LSObserver() {
+  AssertIsOnOwningThread();
+
+  if (mActor) {
+    mActor->SendDeleteMeInternal();
+    MOZ_ASSERT(!mActor, "SendDeleteMeInternal should have cleared!");
+  }
+
+  MOZ_ASSERT(gLSObservers);
+  MOZ_ASSERT(gLSObservers->Get(mOrigin));
+  gLSObservers->Remove(mOrigin);
+
+  if (!gLSObservers->Count()) {
+    gLSObservers = nullptr;
+  }
+}
+
+// static
+LSObserver* LSObserver::Get(const nsACString& aOrigin) {
+  return gLSObservers ? gLSObservers->Get(aOrigin) : nullptr;
+}
+
+void LSObserver::SetActor(LSObserverChild* aActor) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aActor);
+  MOZ_ASSERT(!mActor);
+
+  mActor = aActor;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/LSObserver.h b/dom/localstorage/LSObserver.h
new file mode 100644
index 0000000000..55107bc6c4
--- /dev/null
+++ b/dom/localstorage/LSObserver.h
@@ -0,0 +1,68 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LSObserver_h
+#define mozilla_dom_localstorage_LSObserver_h
+
+#include "mozilla/Assertions.h"
+#include "nsISupports.h"
+#include "nsString.h"
+
+namespace mozilla::dom {
+
+class LSObserverChild;
+
+/**
+ * Effectively just a refcounted life-cycle management wrapper around
+ * LSObserverChild which exists to receive "storage" event information from
+ * other processes.  (Same-process events are handled within the process, see
+ * `LSObject::OnChange`.)
+ *
+ * ## Lifecycle ##
+ * - Created by LSObject::EnsureObserver via synchronous LSRequest idiom
+ *   whenever the first window's origin adds a "storage" event.  Placed in the
+ *   gLSObservers LSObserverHashtable for subsequent LSObject's via
+ *   LSObserver::Get lookup.
+ * - The LSObserverChild directly handles "Observe" messages, shunting them
+ *   directly to Storage::NotifyChange which does all the legwork of notifying
+ *   windows about "storage" events.
+ * - Destroyed when refcount goes to zero due to all owning LSObjects being
+ *   destroyed or having their `LSObject::DropObserver` methods invoked due to
+ *   the last "storage" event listener being removed from the owning window.
+ */
+class LSObserver final {
+  friend class LSObject;
+
+  LSObserverChild* mActor;
+
+  const nsCString mOrigin;
+
+ public:
+  static LSObserver* Get(const nsACString& aOrigin);
+
+  NS_INLINE_DECL_REFCOUNTING(LSObserver)
+
+  void AssertIsOnOwningThread() const { NS_ASSERT_OWNINGTHREAD(LSObserver); }
+
+  void SetActor(LSObserverChild* aActor);
+
+  void ClearActor() {
+    AssertIsOnOwningThread();
+    MOZ_ASSERT(mActor);
+
+    mActor = nullptr;
+  }
+
+ private:
+  // Only created by LSObject.
+  explicit LSObserver(const nsACString& aOrigin);
+
+  ~LSObserver();
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_localstorage_LSObserver_h
diff --git a/dom/localstorage/LSSnapshot.cpp b/dom/localstorage/LSSnapshot.cpp
new file mode 100644
index 0000000000..e41a682ae4
--- /dev/null
+++ b/dom/localstorage/LSSnapshot.cpp
@@ -0,0 +1,1078 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/dom/LSSnapshot.h"
+
+// Local includes
+#include "ActorsChild.h"
+#include "LSDatabase.h"
+#include "LSWriteOptimizer.h"
+#include "LSWriteOptimizerImpl.h"
+#include "LocalStorageCommon.h"
+
+// Global includes
+#include <cstdint>
+#include <cstdlib>
+#include <new>
+#include <type_traits>
+#include <utility>
+#include "ErrorList.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/MacroForEach.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/LSValue.h"
+#include "mozilla/dom/PBackgroundLSDatabase.h"
+#include "mozilla/dom/PBackgroundLSSharedTypes.h"
+#include "mozilla/dom/PBackgroundLSSnapshot.h"
+#include "nsBaseHashtable.h"
+#include "nsCOMPtr.h"
+#include "nsContentUtils.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsITimer.h"
+#include "nsString.h"
+#include "nsStringFlags.h"
+#include "nsStringFwd.h"
+#include "nsTArray.h"
+#include "nsTStringRepr.h"
+#include "nscore.h"
+
+namespace mozilla::dom {
+
+/**
+ * Coalescing manipulation queue used by `LSSnapshot`.  Used by `LSSnapshot` to
+ * buffer and coalesce manipulations before they are sent to the parent process,
+ * when a Snapshot Checkpoints. (This can only be done when there are no
+ * observers for other content processes.)
+ */
+class SnapshotWriteOptimizer final : public LSWriteOptimizer<LSValue> {
+ public:
+  void Enumerate(nsTArray<LSWriteInfo>& aWriteInfos);
+};
+
+void SnapshotWriteOptimizer::Enumerate(nsTArray<LSWriteInfo>& aWriteInfos) {
+  AssertIsOnOwningThread();
+
+  // The mWriteInfos hash table contains all write infos, but it keeps them in
+  // an arbitrary order, which means write infos need to be sorted before being
+  // processed.
+
+  nsTArray<NotNull<WriteInfo*>> writeInfos;
+  GetSortedWriteInfos(writeInfos);
+
+  for (WriteInfo* writeInfo : writeInfos) {
+    switch (writeInfo->GetType()) {
+      case WriteInfo::InsertItem: {
+        auto insertItemInfo = static_cast<InsertItemInfo*>(writeInfo);
+
+        LSSetItemInfo setItemInfo;
+        setItemInfo.key() = insertItemInfo->GetKey();
+        setItemInfo.value() = insertItemInfo->GetValue();
+
+        aWriteInfos.AppendElement(std::move(setItemInfo));
+
+        break;
+      }
+
+      case WriteInfo::UpdateItem: {
+        auto updateItemInfo = static_cast<UpdateItemInfo*>(writeInfo);
+
+        if (updateItemInfo->UpdateWithMove()) {
+          // See the comment in LSWriteOptimizer::InsertItem for more details
+          // about the UpdateWithMove flag.
+
+          LSRemoveItemInfo removeItemInfo;
+          removeItemInfo.key() = updateItemInfo->GetKey();
+
+          aWriteInfos.AppendElement(std::move(removeItemInfo));
+        }
+
+        LSSetItemInfo setItemInfo;
+        setItemInfo.key() = updateItemInfo->GetKey();
+        setItemInfo.value() = updateItemInfo->GetValue();
+
+        aWriteInfos.AppendElement(std::move(setItemInfo));
+
+        break;
+      }
+
+      case WriteInfo::DeleteItem: {
+        auto deleteItemInfo = static_cast<DeleteItemInfo*>(writeInfo);
+
+        LSRemoveItemInfo removeItemInfo;
+        removeItemInfo.key() = deleteItemInfo->GetKey();
+
+        aWriteInfos.AppendElement(std::move(removeItemInfo));
+
+        break;
+      }
+
+      case WriteInfo::Truncate: {
+        LSClearInfo clearInfo;
+
+        aWriteInfos.AppendElement(std::move(clearInfo));
+
+        break;
+      }
+
+      default:
+        MOZ_CRASH("Bad type!");
+    }
+  }
+}
+
+LSSnapshot::LSSnapshot(LSDatabase* aDatabase)
+    : mDatabase(aDatabase),
+      mActor(nullptr),
+      mInitLength(0),
+      mLength(0),
+      mUsage(0),
+      mPeakUsage(0),
+      mLoadState(LoadState::Initial),
+      mHasOtherProcessDatabases(false),
+      mHasOtherProcessObservers(false),
+      mExplicit(false),
+      mHasPendingStableStateCallback(false),
+      mHasPendingIdleTimerCallback(false),
+      mDirty(false)
+#ifdef DEBUG
+      ,
+      mInitialized(false),
+      mSentFinish(false)
+#endif
+{
+  AssertIsOnOwningThread();
+}
+
+LSSnapshot::~LSSnapshot() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mDatabase);
+  MOZ_ASSERT(!mHasPendingStableStateCallback);
+  MOZ_ASSERT(!mHasPendingIdleTimerCallback);
+  MOZ_ASSERT_IF(mInitialized, mSentFinish);
+
+  if (mActor) {
+    mActor->SendDeleteMeInternal();
+    MOZ_ASSERT(!mActor, "SendDeleteMeInternal should have cleared!");
+  }
+}
+
+void LSSnapshot::SetActor(LSSnapshotChild* aActor) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(aActor);
+  MOZ_ASSERT(!mActor);
+
+  mActor = aActor;
+}
+
+nsresult LSSnapshot::Init(const nsAString& aKey,
+                          const LSSnapshotInitInfo& aInitInfo, bool aExplicit) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!mSelfRef);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mLoadState == LoadState::Initial);
+  MOZ_ASSERT(!mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  mSelfRef = this;
+
+  LoadState loadState = aInitInfo.loadState();
+
+  const nsTArray<LSItemInfo>& itemInfos = aInitInfo.itemInfos();
+  for (uint32_t i = 0; i < itemInfos.Length(); i++) {
+    const LSItemInfo& itemInfo = itemInfos[i];
+
+    const LSValue& value = itemInfo.value();
+
+    if (loadState != LoadState::AllOrderedItems && !value.IsVoid()) {
+      mLoadedItems.Insert(itemInfo.key());
+    }
+
+    mValues.InsertOrUpdate(itemInfo.key(), value.AsString());
+  }
+
+  if (loadState == LoadState::Partial) {
+    if (aInitInfo.addKeyToUnknownItems()) {
+      mUnknownItems.Insert(aKey);
+    }
+    mInitLength = aInitInfo.totalLength();
+    mLength = mInitLength;
+  } else if (loadState == LoadState::AllOrderedKeys) {
+    mInitLength = aInitInfo.totalLength();
+  } else {
+    MOZ_ASSERT(loadState == LoadState::AllOrderedItems);
+  }
+
+  mUsage = aInitInfo.usage();
+  mPeakUsage = aInitInfo.peakUsage();
+
+  mLoadState = aInitInfo.loadState();
+
+  mHasOtherProcessDatabases = aInitInfo.hasOtherProcessDatabases();
+  mHasOtherProcessObservers = aInitInfo.hasOtherProcessObservers();
+
+  mExplicit = aExplicit;
+
+#ifdef DEBUG
+  mInitialized = true;
+#endif
+
+  if (mHasOtherProcessObservers) {
+    mWriteAndNotifyInfos = MakeUnique<nsTArray<LSWriteAndNotifyInfo>>();
+  } else {
+    mWriteOptimizer = MakeUnique<SnapshotWriteOptimizer>();
+  }
+
+  if (!mExplicit) {
+    mIdleTimer = NS_NewTimer();
+    MOZ_ASSERT(mIdleTimer);
+
+    ScheduleStableStateCallback();
+  }
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::GetLength(uint32_t* aResult) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  MaybeScheduleStableStateCallback();
+
+  if (mLoadState == LoadState::Partial) {
+    *aResult = mLength;
+  } else {
+    *aResult = mValues.Count();
+  }
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::GetKey(uint32_t aIndex, nsAString& aResult) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  MaybeScheduleStableStateCallback();
+
+  nsresult rv = EnsureAllKeys();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  aResult.SetIsVoid(true);
+  for (auto iter = mValues.ConstIter(); !iter.Done(); iter.Next()) {
+    if (aIndex == 0) {
+      aResult = iter.Key();
+      return NS_OK;
+    }
+    aIndex--;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::GetItem(const nsAString& aKey, nsAString& aResult) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  MaybeScheduleStableStateCallback();
+
+  nsString result;
+  nsresult rv = GetItemInternal(aKey, Optional<nsString>(), result);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  aResult = result;
+  return NS_OK;
+}
+
+nsresult LSSnapshot::GetKeys(nsTArray<nsString>& aKeys) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  MaybeScheduleStableStateCallback();
+
+  nsresult rv = EnsureAllKeys();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  AppendToArray(aKeys, mValues.Keys());
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::SetItem(const nsAString& aKey, const nsAString& aValue,
+                             LSNotifyInfo& aNotifyInfo) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  MaybeScheduleStableStateCallback();
+
+  nsString oldValue;
+  nsresult rv =
+      GetItemInternal(aKey, Optional<nsString>(nsString(aValue)), oldValue);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  bool changed;
+  if (oldValue == aValue && oldValue.IsVoid() == aValue.IsVoid()) {
+    changed = false;
+  } else {
+    changed = true;
+
+    auto autoRevertValue = MakeScopeExit([&] {
+      if (oldValue.IsVoid()) {
+        mValues.Remove(aKey);
+      } else {
+        mValues.InsertOrUpdate(aKey, oldValue);
+      }
+    });
+
+    // Anything that can fail must be done early before we start modifying the
+    // state.
+
+    Maybe<LSValue> oldValueFromString;
+    if (mHasOtherProcessObservers) {
+      oldValueFromString.emplace();
+      if (NS_WARN_IF(!oldValueFromString->InitFromString(oldValue))) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+
+    LSValue valueFromString;
+    if (NS_WARN_IF(!valueFromString.InitFromString(aValue))) {
+      return NS_ERROR_FAILURE;
+    }
+
+    int64_t delta = static_cast<int64_t>(aValue.Length()) -
+                    static_cast<int64_t>(oldValue.Length());
+
+    if (oldValue.IsVoid()) {
+      delta += static_cast<int64_t>(aKey.Length());
+    }
+
+    rv = UpdateUsage(delta);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+
+    if (oldValue.IsVoid() && mLoadState == LoadState::Partial) {
+      mLength++;
+    }
+
+    if (mHasOtherProcessObservers) {
+      MOZ_ASSERT(mWriteAndNotifyInfos);
+      MOZ_ASSERT(oldValueFromString.isSome());
+
+      LSSetItemAndNotifyInfo setItemAndNotifyInfo;
+      setItemAndNotifyInfo.key() = aKey;
+      setItemAndNotifyInfo.oldValue() = oldValueFromString.value();
+      setItemAndNotifyInfo.value() = valueFromString;
+
+      mWriteAndNotifyInfos->AppendElement(std::move(setItemAndNotifyInfo));
+    } else {
+      MOZ_ASSERT(mWriteOptimizer);
+
+      if (oldValue.IsVoid()) {
+        mWriteOptimizer->InsertItem(aKey, valueFromString);
+      } else {
+        mWriteOptimizer->UpdateItem(aKey, valueFromString);
+      }
+    }
+
+    autoRevertValue.release();
+  }
+
+  aNotifyInfo.changed() = changed;
+  aNotifyInfo.oldValue() = oldValue;
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::RemoveItem(const nsAString& aKey,
+                                LSNotifyInfo& aNotifyInfo) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  MaybeScheduleStableStateCallback();
+
+  nsString oldValue;
+  nsresult rv =
+      GetItemInternal(aKey, Optional<nsString>(VoidString()), oldValue);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  bool changed;
+  if (oldValue.IsVoid()) {
+    changed = false;
+  } else {
+    changed = true;
+
+    auto autoRevertValue = MakeScopeExit([&] {
+      MOZ_ASSERT(!oldValue.IsVoid());
+      mValues.InsertOrUpdate(aKey, oldValue);
+    });
+
+    // Anything that can fail must be done early before we start modifying the
+    // state.
+
+    Maybe<LSValue> oldValueFromString;
+    if (mHasOtherProcessObservers) {
+      oldValueFromString.emplace();
+      if (NS_WARN_IF(!oldValueFromString->InitFromString(oldValue))) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+
+    int64_t delta = -(static_cast<int64_t>(aKey.Length()) +
+                      static_cast<int64_t>(oldValue.Length()));
+
+    DebugOnly<nsresult> rv = UpdateUsage(delta);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+
+    if (mLoadState == LoadState::Partial) {
+      mLength--;
+    }
+
+    if (mHasOtherProcessObservers) {
+      MOZ_ASSERT(mWriteAndNotifyInfos);
+      MOZ_ASSERT(oldValueFromString.isSome());
+
+      LSRemoveItemAndNotifyInfo removeItemAndNotifyInfo;
+      removeItemAndNotifyInfo.key() = aKey;
+      removeItemAndNotifyInfo.oldValue() = oldValueFromString.value();
+
+      mWriteAndNotifyInfos->AppendElement(std::move(removeItemAndNotifyInfo));
+    } else {
+      MOZ_ASSERT(mWriteOptimizer);
+
+      mWriteOptimizer->DeleteItem(aKey);
+    }
+
+    autoRevertValue.release();
+  }
+
+  aNotifyInfo.changed() = changed;
+  aNotifyInfo.oldValue() = oldValue;
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::Clear(LSNotifyInfo& aNotifyInfo) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  MaybeScheduleStableStateCallback();
+
+  uint32_t length;
+  if (mLoadState == LoadState::Partial) {
+    length = mLength;
+    MOZ_ASSERT(length);
+
+    MOZ_ALWAYS_TRUE(mActor->SendLoaded());
+
+    mLoadedItems.Clear();
+    mUnknownItems.Clear();
+    mLength = 0;
+    mLoadState = LoadState::AllOrderedItems;
+  } else {
+    length = mValues.Count();
+  }
+
+  bool changed;
+  if (!length) {
+    changed = false;
+  } else {
+    changed = true;
+
+    int64_t delta = 0;
+    for (const auto& entry : mValues) {
+      const nsAString& key = entry.GetKey();
+      const nsString& value = entry.GetData();
+
+      delta += -static_cast<int64_t>(key.Length()) -
+               static_cast<int64_t>(value.Length());
+    }
+
+    DebugOnly<nsresult> rv = UpdateUsage(delta);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+
+    mValues.Clear();
+
+    if (mHasOtherProcessObservers) {
+      MOZ_ASSERT(mWriteAndNotifyInfos);
+
+      LSClearInfo clearInfo;
+
+      mWriteAndNotifyInfos->AppendElement(std::move(clearInfo));
+    } else {
+      MOZ_ASSERT(mWriteOptimizer);
+
+      mWriteOptimizer->Truncate();
+    }
+  }
+
+  aNotifyInfo.changed() = changed;
+
+  return NS_OK;
+}
+
+void LSSnapshot::MarkDirty() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  if (mDirty) {
+    return;
+  }
+
+  mDirty = true;
+
+  if (!mExplicit && !mHasPendingStableStateCallback) {
+    CancelIdleTimer();
+
+    MOZ_ALWAYS_SUCCEEDS(Checkpoint());
+
+    MOZ_ALWAYS_SUCCEEDS(Finish());
+  } else {
+    MOZ_ASSERT(!mHasPendingIdleTimerCallback);
+  }
+}
+
+nsresult LSSnapshot::ExplicitCheckpoint() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mExplicit);
+  MOZ_ASSERT(!mHasPendingStableStateCallback);
+  MOZ_ASSERT(!mHasPendingIdleTimerCallback);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  nsresult rv = Checkpoint(/* aSync */ true);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::ExplicitEnd() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mExplicit);
+  MOZ_ASSERT(!mHasPendingStableStateCallback);
+  MOZ_ASSERT(!mHasPendingIdleTimerCallback);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  nsresult rv = Checkpoint();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  RefPtr<LSSnapshot> kungFuDeathGrip = this;
+
+  rv = Finish(/* aSync */ true);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+int64_t LSSnapshot::GetUsage() const {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  return mUsage;
+}
+
+void LSSnapshot::ScheduleStableStateCallback() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mIdleTimer);
+  MOZ_ASSERT(!mExplicit);
+  MOZ_ASSERT(!mHasPendingStableStateCallback);
+
+  CancelIdleTimer();
+
+  nsCOMPtr<nsIRunnable> runnable = this;
+  nsContentUtils::RunInStableState(runnable.forget());
+
+  mHasPendingStableStateCallback = true;
+}
+
+void LSSnapshot::MaybeScheduleStableStateCallback() {
+  AssertIsOnOwningThread();
+
+  if (!mExplicit && !mHasPendingStableStateCallback) {
+    ScheduleStableStateCallback();
+  } else {
+    MOZ_ASSERT(!mHasPendingIdleTimerCallback);
+  }
+}
+
+nsresult LSSnapshot::GetItemInternal(const nsAString& aKey,
+                                     const Optional<nsString>& aValue,
+                                     nsAString& aResult) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  nsString result;
+
+  switch (mLoadState) {
+    case LoadState::Partial: {
+      if (mValues.Get(aKey, &result)) {
+        MOZ_ASSERT(!result.IsVoid());
+      } else if (mLoadedItems.Contains(aKey) || mUnknownItems.Contains(aKey)) {
+        result.SetIsVoid(true);
+      } else {
+        LSValue value;
+        nsTArray<LSItemInfo> itemInfos;
+        if (NS_WARN_IF(!mActor->SendLoadValueAndMoreItems(
+                nsString(aKey), &value, &itemInfos))) {
+          return NS_ERROR_FAILURE;
+        }
+
+        result = value.AsString();
+
+        if (result.IsVoid()) {
+          mUnknownItems.Insert(aKey);
+        } else {
+          mLoadedItems.Insert(aKey);
+          mValues.InsertOrUpdate(aKey, result);
+
+          // mLoadedItems.Count()==mInitLength is checked below.
+        }
+
+        for (uint32_t i = 0; i < itemInfos.Length(); i++) {
+          const LSItemInfo& itemInfo = itemInfos[i];
+
+          mLoadedItems.Insert(itemInfo.key());
+          mValues.InsertOrUpdate(itemInfo.key(), itemInfo.value().AsString());
+        }
+
+        if (mLoadedItems.Count() == mInitLength) {
+          mLoadedItems.Clear();
+          mUnknownItems.Clear();
+          mLength = 0;
+          mLoadState = LoadState::AllUnorderedItems;
+        }
+      }
+
+      if (aValue.WasPassed()) {
+        const nsString& value = aValue.Value();
+        if (!value.IsVoid()) {
+          mValues.InsertOrUpdate(aKey, value);
+        } else if (!result.IsVoid()) {
+          mValues.Remove(aKey);
+        }
+      }
+
+      break;
+    }
+
+    case LoadState::AllOrderedKeys: {
+      if (mValues.Get(aKey, &result)) {
+        if (result.IsVoid()) {
+          LSValue value;
+          nsTArray<LSItemInfo> itemInfos;
+          if (NS_WARN_IF(!mActor->SendLoadValueAndMoreItems(
+                  nsString(aKey), &value, &itemInfos))) {
+            return NS_ERROR_FAILURE;
+          }
+
+          result = value.AsString();
+
+          MOZ_ASSERT(!result.IsVoid());
+
+          mLoadedItems.Insert(aKey);
+          mValues.InsertOrUpdate(aKey, result);
+
+          // mLoadedItems.Count()==mInitLength is checked below.
+
+          for (uint32_t i = 0; i < itemInfos.Length(); i++) {
+            const LSItemInfo& itemInfo = itemInfos[i];
+
+            mLoadedItems.Insert(itemInfo.key());
+            mValues.InsertOrUpdate(itemInfo.key(), itemInfo.value().AsString());
+          }
+
+          if (mLoadedItems.Count() == mInitLength) {
+            mLoadedItems.Clear();
+            MOZ_ASSERT(mLength == 0);
+            mLoadState = LoadState::AllOrderedItems;
+          }
+        }
+      } else {
+        result.SetIsVoid(true);
+      }
+
+      if (aValue.WasPassed()) {
+        const nsString& value = aValue.Value();
+        if (!value.IsVoid()) {
+          mValues.InsertOrUpdate(aKey, value);
+        } else if (!result.IsVoid()) {
+          mValues.Remove(aKey);
+        }
+      }
+
+      break;
+    }
+
+    case LoadState::AllUnorderedItems:
+    case LoadState::AllOrderedItems: {
+      if (aValue.WasPassed()) {
+        const nsString& value = aValue.Value();
+        if (!value.IsVoid()) {
+          mValues.WithEntryHandle(aKey, [&](auto&& entry) {
+            if (entry) {
+              result = std::exchange(entry.Data(), value);
+            } else {
+              result.SetIsVoid(true);
+              entry.Insert(value);
+            }
+          });
+        } else {
+          if (auto entry = mValues.Lookup(aKey)) {
+            result = entry.Data();
+            MOZ_ASSERT(!result.IsVoid());
+            entry.Remove();
+          } else {
+            result.SetIsVoid(true);
+          }
+        }
+      } else {
+        if (mValues.Get(aKey, &result)) {
+          MOZ_ASSERT(!result.IsVoid());
+        } else {
+          result.SetIsVoid(true);
+        }
+      }
+
+      break;
+    }
+
+    default:
+      MOZ_CRASH("Bad state!");
+  }
+
+  aResult = result;
+  return NS_OK;
+}
+
+nsresult LSSnapshot::EnsureAllKeys() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+  MOZ_ASSERT(mLoadState != LoadState::Initial);
+
+  if (mLoadState == LoadState::AllOrderedKeys ||
+      mLoadState == LoadState::AllOrderedItems) {
+    return NS_OK;
+  }
+
+  nsTArray<nsString> keys;
+  if (NS_WARN_IF(!mActor->SendLoadKeys(&keys))) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsTHashMap<nsStringHashKey, nsString> newValues;
+
+  for (auto key : keys) {
+    newValues.InsertOrUpdate(key, VoidString());
+  }
+
+  if (mHasOtherProcessObservers) {
+    MOZ_ASSERT(mWriteAndNotifyInfos);
+
+    if (!mWriteAndNotifyInfos->IsEmpty()) {
+      for (uint32_t index = 0; index < mWriteAndNotifyInfos->Length();
+           index++) {
+        const LSWriteAndNotifyInfo& writeAndNotifyInfo =
+            mWriteAndNotifyInfos->ElementAt(index);
+
+        switch (writeAndNotifyInfo.type()) {
+          case LSWriteAndNotifyInfo::TLSSetItemAndNotifyInfo: {
+            newValues.InsertOrUpdate(
+                writeAndNotifyInfo.get_LSSetItemAndNotifyInfo().key(),
+                VoidString());
+            break;
+          }
+          case LSWriteAndNotifyInfo::TLSRemoveItemAndNotifyInfo: {
+            newValues.Remove(
+                writeAndNotifyInfo.get_LSRemoveItemAndNotifyInfo().key());
+            break;
+          }
+          case LSWriteAndNotifyInfo::TLSClearInfo: {
+            newValues.Clear();
+            break;
+          }
+
+          default:
+            MOZ_CRASH("Should never get here!");
+        }
+      }
+    }
+  } else {
+    MOZ_ASSERT(mWriteOptimizer);
+
+    if (mWriteOptimizer->HasWrites()) {
+      nsTArray<LSWriteInfo> writeInfos;
+      mWriteOptimizer->Enumerate(writeInfos);
+
+      MOZ_ASSERT(!writeInfos.IsEmpty());
+
+      for (uint32_t index = 0; index < writeInfos.Length(); index++) {
+        const LSWriteInfo& writeInfo = writeInfos[index];
+
+        switch (writeInfo.type()) {
+          case LSWriteInfo::TLSSetItemInfo: {
+            newValues.InsertOrUpdate(writeInfo.get_LSSetItemInfo().key(),
+                                     VoidString());
+            break;
+          }
+          case LSWriteInfo::TLSRemoveItemInfo: {
+            newValues.Remove(writeInfo.get_LSRemoveItemInfo().key());
+            break;
+          }
+          case LSWriteInfo::TLSClearInfo: {
+            newValues.Clear();
+            break;
+          }
+
+          default:
+            MOZ_CRASH("Should never get here!");
+        }
+      }
+    }
+  }
+
+  MOZ_ASSERT_IF(mLoadState == LoadState::AllUnorderedItems,
+                newValues.Count() == mValues.Count());
+
+  for (auto iter = newValues.Iter(); !iter.Done(); iter.Next()) {
+    nsString value;
+    if (mValues.Get(iter.Key(), &value)) {
+      iter.Data() = value;
+    }
+  }
+
+  mValues.SwapElements(newValues);
+
+  if (mLoadState == LoadState::Partial) {
+    mUnknownItems.Clear();
+    mLength = 0;
+    mLoadState = LoadState::AllOrderedKeys;
+  } else {
+    MOZ_ASSERT(mLoadState == LoadState::AllUnorderedItems);
+
+    MOZ_ASSERT(mUnknownItems.Count() == 0);
+    MOZ_ASSERT(mLength == 0);
+    mLoadState = LoadState::AllOrderedItems;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::UpdateUsage(int64_t aDelta) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mDatabase);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mPeakUsage >= mUsage);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  int64_t newUsage = mUsage + aDelta;
+  if (newUsage > mPeakUsage) {
+    const int64_t minSize = newUsage - mPeakUsage;
+
+    int64_t size;
+    if (NS_WARN_IF(!mActor->SendIncreasePeakUsage(minSize, &size))) {
+      return NS_ERROR_FAILURE;
+    }
+
+    MOZ_ASSERT(size >= 0);
+
+    if (size == 0) {
+      return NS_ERROR_FILE_NO_DEVICE_SPACE;
+    }
+
+    mPeakUsage += size;
+  }
+
+  mUsage = newUsage;
+  return NS_OK;
+}
+
+nsresult LSSnapshot::Checkpoint(bool aSync) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  if (mHasOtherProcessObservers) {
+    MOZ_ASSERT(mWriteAndNotifyInfos);
+
+    if (!mWriteAndNotifyInfos->IsEmpty()) {
+      if (aSync) {
+        MOZ_ALWAYS_TRUE(
+            mActor->SendSyncCheckpointAndNotify(*mWriteAndNotifyInfos));
+      } else {
+        MOZ_ALWAYS_TRUE(
+            mActor->SendAsyncCheckpointAndNotify(*mWriteAndNotifyInfos));
+      }
+
+      mWriteAndNotifyInfos->Clear();
+    }
+  } else {
+    MOZ_ASSERT(mWriteOptimizer);
+
+    if (mWriteOptimizer->HasWrites()) {
+      nsTArray<LSWriteInfo> writeInfos;
+      mWriteOptimizer->Enumerate(writeInfos);
+
+      MOZ_ASSERT(!writeInfos.IsEmpty());
+
+      if (aSync) {
+        MOZ_ALWAYS_TRUE(mActor->SendSyncCheckpoint(writeInfos));
+      } else {
+        MOZ_ALWAYS_TRUE(mActor->SendAsyncCheckpoint(writeInfos));
+      }
+
+      mWriteOptimizer->Reset();
+    }
+  }
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::Finish(bool aSync) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mDatabase);
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  if (aSync) {
+    MOZ_ALWAYS_TRUE(mActor->SendSyncFinish());
+  } else {
+    MOZ_ALWAYS_TRUE(mActor->SendAsyncFinish());
+  }
+
+  mDatabase->NoteFinishedSnapshot(this);
+
+#ifdef DEBUG
+  mSentFinish = true;
+#endif
+
+  // Clear the self reference added in Init method.
+  MOZ_ASSERT(mSelfRef);
+  mSelfRef = nullptr;
+
+  return NS_OK;
+}
+
+void LSSnapshot::CancelIdleTimer() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mIdleTimer);
+
+  if (mHasPendingIdleTimerCallback) {
+    MOZ_ALWAYS_SUCCEEDS(mIdleTimer->Cancel());
+    mHasPendingIdleTimerCallback = false;
+  }
+}
+
+// static
+void LSSnapshot::IdleTimerCallback(nsITimer* aTimer, void* aClosure) {
+  MOZ_ASSERT(aTimer);
+
+  auto* self = static_cast<LSSnapshot*>(aClosure);
+  MOZ_ASSERT(self);
+  MOZ_ASSERT(self->mIdleTimer);
+  MOZ_ASSERT(SameCOMIdentity(self->mIdleTimer, aTimer));
+  MOZ_ASSERT(!self->mHasPendingStableStateCallback);
+  MOZ_ASSERT(self->mHasPendingIdleTimerCallback);
+
+  self->mHasPendingIdleTimerCallback = false;
+
+  MOZ_ALWAYS_SUCCEEDS(self->Finish());
+}
+
+NS_IMPL_ISUPPORTS(LSSnapshot, nsIRunnable)
+
+NS_IMETHODIMP
+LSSnapshot::Run() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(!mExplicit);
+  MOZ_ASSERT(mHasPendingStableStateCallback);
+  MOZ_ASSERT(!mHasPendingIdleTimerCallback);
+
+  mHasPendingStableStateCallback = false;
+
+  MOZ_ALWAYS_SUCCEEDS(Checkpoint());
+
+  // 1. The unused pre-incremented snapshot peak usage can't be undone when
+  //    there are other snapshots for the same database. We only add a pending
+  //    usage delta when a snapshot finishes and usage deltas are then applied
+  //    when the last database becomes inactive.
+  // 2. If there's a snapshot with pre-incremented peak usage, the next
+  //    snapshot will use that as a base for its usage.
+  // 3. When a task for given snapshot finishes, we try to reuse the snapshot
+  //    by only checkpointing the snapshot and delaying the finish by a timer.
+  // 4. If two or more tabs for the same origin use localStorage periodically
+  //    at the same time the usage gradually grows until it hits the quota
+  //    limit.
+  // 5. We prevent that from happening by finishing the snapshot immediatelly
+  //    if there are databases in other processess.
+
+  if (mDirty || mHasOtherProcessDatabases ||
+      !Preferences::GetBool("dom.storage.snapshot_reusing")) {
+    MOZ_ALWAYS_SUCCEEDS(Finish());
+  } else {
+    MOZ_ASSERT(mIdleTimer);
+
+    MOZ_ALWAYS_SUCCEEDS(mIdleTimer->InitWithNamedFuncCallback(
+        IdleTimerCallback, this,
+        StaticPrefs::dom_storage_snapshot_idle_timeout_ms(),
+        nsITimer::TYPE_ONE_SHOT, "LSSnapshot::IdleTimerCallback"));
+
+    mHasPendingIdleTimerCallback = true;
+  }
+
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/LSSnapshot.h b/dom/localstorage/LSSnapshot.h
new file mode 100644
index 0000000000..c12a4efce2
--- /dev/null
+++ b/dom/localstorage/LSSnapshot.h
@@ -0,0 +1,193 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LSSnapshot_h
+#define mozilla_dom_localstorage_LSSnapshot_h
+
+#include <cstdint>
+#include <cstdlib>
+#include "ErrorList.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
+#include "nsCOMPtr.h"
+#include "nsTHashMap.h"
+#include "nsHashKeys.h"
+#include "nsIRunnable.h"
+#include "nsISupports.h"
+#include "nsStringFwd.h"
+#include "nsTArrayForwardDeclare.h"
+#include "nsTHashSet.h"
+
+class nsITimer;
+
+namespace mozilla::dom {
+
+class LSDatabase;
+class LSNotifyInfo;
+class LSSnapshotChild;
+class LSSnapshotInitInfo;
+class LSWriteAndNotifyInfo;
+class SnapshotWriteOptimizer;
+
+template <typename>
+class Optional;
+
+class LSSnapshot final : public nsIRunnable {
+ public:
+  /**
+   * The LoadState expresses what subset of information a snapshot has from the
+   * authoritative Datastore in the parent process.  The initial snapshot is
+   * populated heuristically based on the size of the keys and size of the items
+   * (inclusive of the key value; item is key+value, not just value) of the
+   * entire datastore relative to the configured prefill limit (via pref
+   * "dom.storage.snapshot_prefill" exposed as gSnapshotPrefill in bytes).
+   *
+   * If there's less data than the limit, we send both keys and values and end
+   * up as AllOrderedItems.  If there's enough room for all the keys but not
+   * all the values, we end up as AllOrderedKeys with as many values present as
+   * would fit.  If there's not enough room for all the keys, then we end up as
+   * Partial with as many key-value pairs as will fit.
+   *
+   * The state AllUnorderedItems can only be reached by code getting items one
+   * by one.
+   */
+  enum class LoadState {
+    /**
+     * Class constructed, Init(LSSnapshotInitInfo) has not been invoked yet.
+     */
+    Initial,
+    /**
+     * Some keys and their values are known.
+     */
+    Partial,
+    /**
+     * All the keys are known in order, but some values are unknown.
+     */
+    AllOrderedKeys,
+    /**
+     * All keys and their values are known, but in an arbitrary order.
+     */
+    AllUnorderedItems,
+    /**
+     * All keys and their values are known and are present in their canonical
+     * order.  This is everything, and is the preferred case.  The initial
+     * population will send this info when the size of all items is less than
+     * the prefill threshold.
+     *
+     * mValues will contain all keys and values, mLoadedItems and mUnknownItems
+     * are unused.
+     */
+    AllOrderedItems,
+    EndGuard
+  };
+
+ private:
+  RefPtr<LSSnapshot> mSelfRef;
+
+  RefPtr<LSDatabase> mDatabase;
+
+  nsCOMPtr<nsITimer> mIdleTimer;
+
+  LSSnapshotChild* mActor;
+
+  nsTHashSet<nsString> mLoadedItems;
+  nsTHashSet<nsString> mUnknownItems;
+  nsTHashMap<nsStringHashKey, nsString> mValues;
+  UniquePtr<SnapshotWriteOptimizer> mWriteOptimizer;
+  UniquePtr<nsTArray<LSWriteAndNotifyInfo>> mWriteAndNotifyInfos;
+
+  uint32_t mInitLength;
+  uint32_t mLength;
+  int64_t mUsage;
+  int64_t mPeakUsage;
+
+  LoadState mLoadState;
+
+  bool mHasOtherProcessDatabases;
+  bool mHasOtherProcessObservers;
+  bool mExplicit;
+  bool mHasPendingStableStateCallback;
+  bool mHasPendingIdleTimerCallback;
+  bool mDirty;
+
+#ifdef DEBUG
+  bool mInitialized;
+  bool mSentFinish;
+#endif
+
+ public:
+  explicit LSSnapshot(LSDatabase* aDatabase);
+
+  void AssertIsOnOwningThread() const { NS_ASSERT_OWNINGTHREAD(LSSnapshot); }
+
+  void SetActor(LSSnapshotChild* aActor);
+
+  void ClearActor() {
+    AssertIsOnOwningThread();
+    MOZ_ASSERT(mActor);
+
+    mActor = nullptr;
+  }
+
+  bool Explicit() const { return mExplicit; }
+
+  nsresult Init(const nsAString& aKey, const LSSnapshotInitInfo& aInitInfo,
+                bool aExplicit);
+
+  nsresult GetLength(uint32_t* aResult);
+
+  nsresult GetKey(uint32_t aIndex, nsAString& aResult);
+
+  nsresult GetItem(const nsAString& aKey, nsAString& aResult);
+
+  nsresult GetKeys(nsTArray<nsString>& aKeys);
+
+  nsresult SetItem(const nsAString& aKey, const nsAString& aValue,
+                   LSNotifyInfo& aNotifyInfo);
+
+  nsresult RemoveItem(const nsAString& aKey, LSNotifyInfo& aNotifyInfo);
+
+  nsresult Clear(LSNotifyInfo& aNotifyInfo);
+
+  void MarkDirty();
+
+  nsresult ExplicitCheckpoint();
+
+  nsresult ExplicitEnd();
+
+  int64_t GetUsage() const;
+
+ private:
+  ~LSSnapshot();
+
+  void ScheduleStableStateCallback();
+
+  void MaybeScheduleStableStateCallback();
+
+  nsresult GetItemInternal(const nsAString& aKey,
+                           const Optional<nsString>& aValue,
+                           nsAString& aResult);
+
+  nsresult EnsureAllKeys();
+
+  nsresult UpdateUsage(int64_t aDelta);
+
+  nsresult Checkpoint(bool aSync = false);
+
+  nsresult Finish(bool aSync = false);
+
+  void CancelIdleTimer();
+
+  static void IdleTimerCallback(nsITimer* aTimer, void* aClosure);
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIRUNNABLE
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_localstorage_LSSnapshot_h
diff --git a/dom/localstorage/LSValue.cpp b/dom/localstorage/LSValue.cpp
new file mode 100644
index 0000000000..03637fdeba
--- /dev/null
+++ b/dom/localstorage/LSValue.cpp
@@ -0,0 +1,202 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/dom/LSValue.h"
+
+#include "mozIStorageStatement.h"
+#include "mozilla/dom/SnappyUtils.h"
+#include "mozilla/fallible.h"
+#include "mozilla/TextUtils.h"
+#include "nsDebug.h"
+#include "nsError.h"
+
+namespace mozilla::dom {
+
+namespace {
+
+bool PutStringBytesToCString(const nsAString& aSrc, nsCString& aDest) {
+  const char16_t* bufferData;
+  const size_t byteLength = sizeof(char16_t) * aSrc.GetData(&bufferData);
+
+  char* destDataPtr;
+  const auto newLength = aDest.GetMutableData(&destDataPtr, byteLength);
+  if (newLength != byteLength) {
+    return false;
+  }
+  std::memcpy(static_cast<void*>(destDataPtr),
+              static_cast<const void*>(bufferData), byteLength);
+
+  return true;
+}
+
+template <class T>
+using TypeBufferResult = Result<std::pair<T, nsCString>, nsresult>;
+
+}  // namespace
+
+bool PutCStringBytesToString(const nsACString& aSrc, nsString& aDest) {
+  const char* bufferData;
+  const size_t byteLength = aSrc.GetData(&bufferData);
+  const size_t shortLength = byteLength / sizeof(char16_t);
+
+  char16_t* destDataPtr;
+  const auto newLength = aDest.GetMutableData(&destDataPtr, shortLength);
+  if (newLength != shortLength) {
+    return false;
+  }
+
+  std::memcpy(static_cast<void*>(destDataPtr),
+              static_cast<const void*>(bufferData), byteLength);
+  return true;
+}
+
+LSValue::Converter::Converter(const LSValue& aValue) {
+  using ConversionType = LSValue::ConversionType;
+  using CompressionType = LSValue::CompressionType;
+
+  if (aValue.mBuffer.IsVoid()) {
+    mBuffer.SetIsVoid(true);
+    return;
+  }
+
+  const CompressionType compressionType = aValue.GetCompressionType();
+  const ConversionType conversionType = aValue.GetConversionType();
+
+  const nsCString uncompressed = [compressionType, &aValue]() {
+    if (CompressionType::UNCOMPRESSED != compressionType) {
+      nsCString buffer;
+      MOZ_ASSERT(CompressionType::SNAPPY == compressionType);
+      if (NS_WARN_IF(!SnappyUncompress(aValue.mBuffer, buffer))) {
+        buffer.Truncate();
+      }
+      return buffer;
+    }
+
+    return aValue.mBuffer;
+  }();
+
+  if (ConversionType::NONE != conversionType) {
+    MOZ_ASSERT(ConversionType::UTF16_UTF8 == conversionType);
+    if (NS_WARN_IF(!CopyUTF8toUTF16(uncompressed, mBuffer, fallible))) {
+      mBuffer.SetIsVoid(true);
+    }
+    return;
+  }
+
+  if (NS_WARN_IF(!PutCStringBytesToString(uncompressed, mBuffer))) {
+    mBuffer.SetIsVoid(true);
+  }
+}
+
+bool LSValue::InitFromString(const nsAString& aBuffer) {
+  MOZ_ASSERT(mBuffer.IsVoid());
+  MOZ_ASSERT(!mUTF16Length);
+  MOZ_ASSERT(ConversionType::NONE == mConversionType);
+  MOZ_ASSERT(CompressionType::UNCOMPRESSED == mCompressionType);
+
+  if (aBuffer.IsVoid()) {
+    return true;
+  }
+
+  const uint32_t utf16Length = aBuffer.Length();
+
+  const auto conversionRes = [&aBuffer]() -> TypeBufferResult<ConversionType> {
+    nsCString converted;
+
+    if (Utf16ValidUpTo(aBuffer) == aBuffer.Length()) {
+      if (NS_WARN_IF(!CopyUTF16toUTF8(aBuffer, converted, fallible))) {
+        return Err(NS_ERROR_OUT_OF_MEMORY);
+      }
+      return std::pair{ConversionType::UTF16_UTF8, std::move(converted)};
+    }
+
+    if (NS_WARN_IF(!PutStringBytesToCString(aBuffer, converted))) {
+      return Err(NS_ERROR_OUT_OF_MEMORY);
+    }
+    return std::pair{ConversionType::NONE, std::move(converted)};
+  }();
+
+  if (conversionRes.isErr()) {
+    return false;
+  }
+
+  const auto& [conversionType, converted] = conversionRes.inspect();
+
+  const auto compressionRes =
+      [&converted = converted]() -> TypeBufferResult<CompressionType> {
+    nsCString compressed;
+    if (NS_WARN_IF(!SnappyCompress(converted, compressed))) {
+      return Err(NS_ERROR_OUT_OF_MEMORY);
+    }
+    if (!compressed.IsVoid()) {
+      return std::pair{CompressionType::SNAPPY, std::move(compressed)};
+    }
+
+    compressed = converted;
+    return std::pair{CompressionType::UNCOMPRESSED, std::move(compressed)};
+  }();
+
+  if (compressionRes.isErr()) {
+    return false;
+  }
+
+  const auto& [compressionType, compressed] = compressionRes.inspect();
+
+  mBuffer = compressed;
+  mUTF16Length = utf16Length;
+  mConversionType = conversionType;
+  mCompressionType = compressionType;
+
+  return true;
+}
+
+nsresult LSValue::InitFromStatement(mozIStorageStatement* aStatement,
+                                    uint32_t aIndex) {
+  MOZ_ASSERT(aStatement);
+  MOZ_ASSERT(mBuffer.IsVoid());
+  MOZ_ASSERT(!mUTF16Length);
+  MOZ_ASSERT(ConversionType::NONE == mConversionType);
+  MOZ_ASSERT(CompressionType::UNCOMPRESSED == mCompressionType);
+
+  int32_t utf16Length;
+  nsresult rv = aStatement->GetInt32(aIndex, &utf16Length);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  int32_t conversionType;
+  rv = aStatement->GetInt32(aIndex + 1, &conversionType);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  int32_t compressionType;
+  rv = aStatement->GetInt32(aIndex + 2, &compressionType);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  nsCString buffer;
+  rv = aStatement->GetBlobAsUTF8String(aIndex + 3, buffer);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  mBuffer = buffer;
+  mUTF16Length = static_cast<uint32_t>(utf16Length);
+  mConversionType = static_cast<decltype(mConversionType)>(conversionType);
+  mCompressionType = static_cast<decltype(mCompressionType)>(compressionType);
+
+  return NS_OK;
+}
+
+const LSValue& VoidLSValue() {
+  static const LSValue sVoidLSValue;
+
+  return sVoidLSValue;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/LSValue.h b/dom/localstorage/LSValue.h
new file mode 100644
index 0000000000..c1b362a50f
--- /dev/null
+++ b/dom/localstorage/LSValue.h
@@ -0,0 +1,135 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LSValue_h
+#define mozilla_dom_localstorage_LSValue_h
+
+#include <cstdint>
+#include "ErrorList.h"
+#include "SnappyUtils.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Span.h"
+#include "nsString.h"
+#include "nsStringFwd.h"
+#include "nsTStringRepr.h"
+
+class mozIStorageStatement;
+
+namespace IPC {
+template <typename>
+struct ParamTraits;
+}
+
+namespace mozilla::dom {
+
+/**
+ * Represents a LocalStorage value. From content's perspective, values (if
+ * present) are always DOMStrings. This is also true from a quota-tracking
+ * perspective. However, for memory and disk efficiency it's preferable to store
+ * the value in alternate compressed or utf-8 encoding representations. The
+ * LSValue type exists to support these alternate representations, dynamically
+ * decompressing/re-encoding to utf-16 while still tracking value size on a
+ * utf-16 basis for quota purposes.
+ */
+class LSValue final {
+  friend struct IPC::ParamTraits<LSValue>;
+
+ public:
+  enum class ConversionType : uint8_t {
+    NONE = 0u,
+    UTF16_UTF8 = 1u,
+    NUM_TYPES = 2u
+  };
+
+  enum class CompressionType : uint8_t {
+    UNCOMPRESSED = 0u,
+    SNAPPY = 1u,
+    NUM_TYPES = 2u
+  };
+
+  nsCString mBuffer;
+  uint32_t mUTF16Length;
+  ConversionType mConversionType;
+  CompressionType mCompressionType;
+
+  explicit LSValue()
+      : mUTF16Length(0u),
+        mConversionType(ConversionType::NONE),
+        mCompressionType(CompressionType::UNCOMPRESSED) {
+    SetIsVoid(true);
+  }
+
+  bool InitFromString(const nsAString& aBuffer);
+
+  nsresult InitFromStatement(mozIStorageStatement* aStatement, uint32_t aIndex);
+
+  bool IsVoid() const { return mBuffer.IsVoid(); }
+
+  void SetIsVoid(bool aVal) { mBuffer.SetIsVoid(aVal); }
+
+  /**
+   * This represents the "physical" length that the parent process uses for
+   * the size of value/item computation. This can also be used to see how much
+   * memory the value is using at rest or what the cost is for sending the value
+   * over IPC.
+   */
+  uint32_t Length() const { return mBuffer.Length(); }
+
+  /*
+   * This represents the "logical" length that content sees and that is also
+   * used for quota management purposes.
+   */
+  uint32_t UTF16Length() const { return mUTF16Length; }
+
+  ConversionType GetConversionType() const { return mConversionType; }
+
+  CompressionType GetCompressionType() const { return mCompressionType; }
+
+  bool Equals(const LSValue& aOther) const {
+    return mBuffer == aOther.mBuffer &&
+           mBuffer.IsVoid() == aOther.mBuffer.IsVoid() &&
+           mUTF16Length == aOther.mUTF16Length &&
+           mConversionType == aOther.mConversionType &&
+           mCompressionType == aOther.mCompressionType;
+  }
+
+  bool operator==(const LSValue& aOther) const { return Equals(aOther); }
+
+  bool operator!=(const LSValue& aOther) const { return !Equals(aOther); }
+
+  constexpr const nsCString& AsCString() const { return mBuffer; }
+
+  class Converter {
+    nsString mBuffer;
+
+   public:
+    explicit Converter(const LSValue& aValue);
+    Converter(Converter&& aOther) = default;
+    ~Converter() = default;
+
+    operator const nsString&() const { return mBuffer; }
+
+   private:
+    Converter() = delete;
+    Converter(const Converter&) = delete;
+    Converter& operator=(const Converter&) = delete;
+    Converter& operator=(const Converter&&) = delete;
+  };
+
+  Converter AsString() const { return Converter{*this}; }
+};
+
+const LSValue& VoidLSValue();
+
+/**
+ * XXX: This function doesn't have to be public
+ * once the support for shadow writes is removed.
+ */
+bool PutCStringBytesToString(const nsACString& aSrc, nsString& aDest);
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_localstorage_LSValue_h
diff --git a/dom/localstorage/LSWriteOptimizer.cpp b/dom/localstorage/LSWriteOptimizer.cpp
new file mode 100644
index 0000000000..0928dca35e
--- /dev/null
+++ b/dom/localstorage/LSWriteOptimizer.cpp
@@ -0,0 +1,72 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "LSWriteOptimizer.h"
+
+#include <new>
+#include "nsBaseHashtable.h"
+#include "nsTArray.h"
+
+namespace mozilla::dom {
+
+class LSWriteOptimizerBase::WriteInfoComparator {
+ public:
+  bool Equals(const WriteInfo* a, const WriteInfo* b) const {
+    MOZ_ASSERT(a && b);
+    return a->SerialNumber() == b->SerialNumber();
+  }
+
+  bool LessThan(const WriteInfo* a, const WriteInfo* b) const {
+    MOZ_ASSERT(a && b);
+    return a->SerialNumber() < b->SerialNumber();
+  }
+};
+
+void LSWriteOptimizerBase::DeleteItem(const nsAString& aKey, int64_t aDelta) {
+  AssertIsOnOwningThread();
+
+  mWriteInfos.WithEntryHandle(aKey, [&](auto&& entry) {
+    if (entry && entry.Data()->GetType() == WriteInfo::InsertItem) {
+      entry.Remove();
+    } else {
+      entry.InsertOrUpdate(
+          MakeUnique<DeleteItemInfo>(NextSerialNumber(), aKey));
+    }
+  });
+
+  mTotalDelta += aDelta;
+}
+
+void LSWriteOptimizerBase::Truncate(int64_t aDelta) {
+  AssertIsOnOwningThread();
+
+  mWriteInfos.Clear();
+
+  if (!mTruncateInfo) {
+    mTruncateInfo = MakeUnique<TruncateInfo>(NextSerialNumber());
+  }
+
+  mTotalDelta += aDelta;
+}
+
+void LSWriteOptimizerBase::GetSortedWriteInfos(
+    nsTArray<NotNull<WriteInfo*>>& aWriteInfos) {
+  AssertIsOnOwningThread();
+
+  if (mTruncateInfo) {
+    aWriteInfos.InsertElementSorted(WrapNotNullUnchecked(mTruncateInfo.get()),
+                                    WriteInfoComparator());
+  }
+
+  for (const auto& entry : mWriteInfos) {
+    WriteInfo* writeInfo = entry.GetWeak();
+
+    aWriteInfos.InsertElementSorted(WrapNotNull(writeInfo),
+                                    WriteInfoComparator());
+  }
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/LSWriteOptimizer.h b/dom/localstorage/LSWriteOptimizer.h
new file mode 100644
index 0000000000..353f8c17b5
--- /dev/null
+++ b/dom/localstorage/LSWriteOptimizer.h
@@ -0,0 +1,194 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LSWriteOptimizer_h
+#define mozilla_dom_localstorage_LSWriteOptimizer_h
+
+#include <cstdint>
+#include <utility>
+#include "mozilla/Assertions.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/UniquePtr.h"
+#include "nsClassHashtable.h"
+#include "nsHashKeys.h"
+#include "nsISupports.h"
+#include "nsStringFwd.h"
+#include "nsTArrayForwardDeclare.h"
+
+namespace mozilla::dom {
+
+/**
+ * Base class for coalescing manipulation queue.
+ */
+class LSWriteOptimizerBase {
+  class WriteInfoComparator;
+
+ protected:
+  class WriteInfo;
+  class DeleteItemInfo;
+  class TruncateInfo;
+
+  UniquePtr<WriteInfo> mTruncateInfo;
+  nsClassHashtable<nsStringHashKey, WriteInfo> mWriteInfos;
+  CheckedUint64 mLastSerialNumber;
+  int64_t mTotalDelta;
+
+  NS_DECL_OWNINGTHREAD
+
+ public:
+  LSWriteOptimizerBase() : mLastSerialNumber(0), mTotalDelta(0) {}
+
+  LSWriteOptimizerBase(LSWriteOptimizerBase&& aWriteOptimizer)
+      : mTruncateInfo(std::move(aWriteOptimizer.mTruncateInfo)) {
+    AssertIsOnOwningThread();
+    MOZ_ASSERT(&aWriteOptimizer != this);
+
+    mWriteInfos.SwapElements(aWriteOptimizer.mWriteInfos);
+    mTotalDelta = aWriteOptimizer.mTotalDelta;
+    aWriteOptimizer.mTotalDelta = 0;
+  }
+
+  void AssertIsOnOwningThread() const {
+    NS_ASSERT_OWNINGTHREAD(LSWriteOptimizerBase);
+  }
+
+  void DeleteItem(const nsAString& aKey, int64_t aDelta = 0);
+
+  void Truncate(int64_t aDelta = 0);
+
+  bool HasWrites() const {
+    AssertIsOnOwningThread();
+
+    return mTruncateInfo || !mWriteInfos.IsEmpty();
+  }
+
+  void Reset() {
+    AssertIsOnOwningThread();
+
+    mTruncateInfo = nullptr;
+    mWriteInfos.Clear();
+  }
+
+ protected:
+  uint64_t NextSerialNumber() {
+    AssertIsOnOwningThread();
+
+    mLastSerialNumber++;
+
+    MOZ_ASSERT(mLastSerialNumber.isValid());
+
+    return mLastSerialNumber.value();
+  }
+
+  /**
+   * This method can be used by derived classes to get a sorted list of write
+   * infos. Write infos are sorted by the serial number.
+   */
+  void GetSortedWriteInfos(nsTArray<NotNull<WriteInfo*>>& aWriteInfos);
+};
+
+/**
+ * Base class for specific mutations.
+ */
+class LSWriteOptimizerBase::WriteInfo {
+  uint64_t mSerialNumber;
+
+ public:
+  WriteInfo(uint64_t aSerialNumber) : mSerialNumber(aSerialNumber) {}
+
+  virtual ~WriteInfo() = default;
+
+  uint64_t SerialNumber() const { return mSerialNumber; }
+
+  enum Type { InsertItem = 0, UpdateItem, DeleteItem, Truncate };
+
+  virtual Type GetType() const = 0;
+};
+
+class LSWriteOptimizerBase::DeleteItemInfo final : public WriteInfo {
+  nsString mKey;
+
+ public:
+  DeleteItemInfo(uint64_t aSerialNumber, const nsAString& aKey)
+      : WriteInfo(aSerialNumber), mKey(aKey) {}
+
+  const nsAString& GetKey() const { return mKey; }
+
+ private:
+  Type GetType() const override { return DeleteItem; }
+};
+
+/**
+ * Truncate mutation.
+ */
+class LSWriteOptimizerBase::TruncateInfo final : public WriteInfo {
+ public:
+  explicit TruncateInfo(uint64_t aSerialNumber) : WriteInfo(aSerialNumber) {}
+
+ private:
+  Type GetType() const override { return Truncate; }
+};
+
+/**
+ * Coalescing manipulation queue.
+ */
+template <typename T, typename U = T>
+class LSWriteOptimizer;
+
+template <typename T, typename U>
+class LSWriteOptimizer : public LSWriteOptimizerBase {
+ protected:
+  class InsertItemInfo;
+  class UpdateItemInfo;
+
+ public:
+  void InsertItem(const nsAString& aKey, const T& aValue, int64_t aDelta = 0);
+
+  void UpdateItem(const nsAString& aKey, const T& aValue, int64_t aDelta = 0);
+};
+
+/**
+ * Insert mutation (the key did not previously exist).
+ */
+template <typename T, typename U>
+class LSWriteOptimizer<T, U>::InsertItemInfo : public WriteInfo {
+  nsString mKey;
+  U mValue;
+
+ public:
+  InsertItemInfo(uint64_t aSerialNumber, const nsAString& aKey, const T& aValue)
+      : WriteInfo(aSerialNumber), mKey(aKey), mValue(aValue) {}
+
+  const nsAString& GetKey() const { return mKey; }
+
+  const T& GetValue() const { return mValue; }
+
+ private:
+  WriteInfo::Type GetType() const override { return InsertItem; }
+};
+
+/**
+ * Update mutation (the key already existed).
+ */
+template <typename T, typename U>
+class LSWriteOptimizer<T, U>::UpdateItemInfo final : public InsertItemInfo {
+  bool mUpdateWithMove;
+
+ public:
+  UpdateItemInfo(uint64_t aSerialNumber, const nsAString& aKey, const T& aValue,
+                 bool aUpdateWithMove)
+      : InsertItemInfo(aSerialNumber, aKey, aValue),
+        mUpdateWithMove(aUpdateWithMove) {}
+
+  bool UpdateWithMove() const { return mUpdateWithMove; }
+
+ private:
+  WriteInfo::Type GetType() const override { return WriteInfo::UpdateItem; }
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_localstorage_LSWriteOptimizer_h
diff --git a/dom/localstorage/LSWriteOptimizerImpl.h b/dom/localstorage/LSWriteOptimizerImpl.h
new file mode 100644
index 0000000000..32ccff6c63
--- /dev/null
+++ b/dom/localstorage/LSWriteOptimizerImpl.h
@@ -0,0 +1,62 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LSWriteOptimizerImpl_h
+#define mozilla_dom_localstorage_LSWriteOptimizerImpl_h
+
+#include "LSWriteOptimizer.h"
+
+namespace mozilla::dom {
+
+template <typename T, typename U>
+void LSWriteOptimizer<T, U>::InsertItem(const nsAString& aKey, const T& aValue,
+                                        int64_t aDelta) {
+  AssertIsOnOwningThread();
+
+  mWriteInfos.WithEntryHandle(aKey, [&](auto&& entry) {
+    if (entry && entry.Data()->GetType() == WriteInfo::DeleteItem) {
+      // We could just simply replace the deletion with ordinary update, but
+      // that would preserve item's original position/index. Imagine a case when
+      // we have only one existing key k1. Now let's create a new optimizer and
+      // remove k1, add k2 and add k1 back. The final order should be k2, k1
+      // (ordinary update would produce k1, k2). So we need to differentiate
+      // between normal update and "optimized" update which resulted from a
+      // deletion followed by an insertion. We use the UpdateWithMove flag for
+      // this.
+
+      entry.Update(MakeUnique<UpdateItemInfo>(NextSerialNumber(), aKey, aValue,
+                                              /* aUpdateWithMove */ true));
+    } else {
+      entry.InsertOrUpdate(
+          MakeUnique<InsertItemInfo>(NextSerialNumber(), aKey, aValue));
+    }
+  });
+
+  mTotalDelta += aDelta;
+}
+
+template <typename T, typename U>
+void LSWriteOptimizer<T, U>::UpdateItem(const nsAString& aKey, const T& aValue,
+                                        int64_t aDelta) {
+  AssertIsOnOwningThread();
+
+  mWriteInfos.WithEntryHandle(aKey, [&](auto&& entry) {
+    if (entry && entry.Data()->GetType() == WriteInfo::InsertItem) {
+      entry.Update(
+          MakeUnique<InsertItemInfo>(NextSerialNumber(), aKey, aValue));
+    } else {
+      entry.InsertOrUpdate(
+          MakeUnique<UpdateItemInfo>(NextSerialNumber(), aKey, aValue,
+                                     /* aUpdateWithMove */ false));
+    }
+  });
+
+  mTotalDelta += aDelta;
+}
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_localstorage_LSWriteOptimizerImpl_h
diff --git a/dom/localstorage/LocalStorageCommon.cpp b/dom/localstorage/LocalStorageCommon.cpp
new file mode 100644
index 0000000000..a5b01e0146
--- /dev/null
+++ b/dom/localstorage/LocalStorageCommon.cpp
@@ -0,0 +1,160 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "LocalStorageCommon.h"
+
+#include <cstdint>
+#include "MainThreadUtils.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Logging.h"
+#include "mozilla/OriginAttributes.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/StaticMutex.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/dom/StorageUtils.h"
+#include "mozilla/dom/quota/ResultExtensions.h"
+#include "mozilla/ipc/PBackgroundSharedTypes.h"
+#include "mozilla/net/MozURL.h"
+#include "mozilla/net/WebSocketFrame.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsPrintfCString.h"
+#include "nsString.h"
+#include "nsStringFlags.h"
+#include "nsXULAppAPI.h"
+
+namespace mozilla::dom {
+
+using namespace mozilla::net;
+
+namespace {
+
+StaticMutex gNextGenLocalStorageMutex;
+Atomic<int32_t> gNextGenLocalStorageEnabled(-1);
+LazyLogModule gLogger("LocalStorage");
+
+}  // namespace
+
+const char16_t* kLocalStorageType = u"localStorage";
+
+bool NextGenLocalStorageEnabled() {
+  if (XRE_IsParentProcess()) {
+    StaticMutexAutoLock lock(gNextGenLocalStorageMutex);
+
+    if (gNextGenLocalStorageEnabled == -1) {
+      // Ideally all this Mutex stuff would be replaced with just using
+      // an AtStartup StaticPref, but there are concerns about this causing
+      // deadlocks if this access needs to init the AtStartup cache.
+      bool enabled =
+          !StaticPrefs::
+              dom_storage_enable_unsupported_legacy_implementation_DoNotUseDirectly();
+
+      gNextGenLocalStorageEnabled = enabled ? 1 : 0;
+    }
+
+    return !!gNextGenLocalStorageEnabled;
+  }
+
+  return CachedNextGenLocalStorageEnabled();
+}
+
+void RecvInitNextGenLocalStorageEnabled(const bool aEnabled) {
+  MOZ_ASSERT(!XRE_IsParentProcess());
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(gNextGenLocalStorageEnabled == -1);
+
+  gNextGenLocalStorageEnabled = aEnabled ? 1 : 0;
+}
+
+bool CachedNextGenLocalStorageEnabled() {
+  MOZ_ASSERT(gNextGenLocalStorageEnabled != -1);
+
+  return !!gNextGenLocalStorageEnabled;
+}
+
+Result<std::pair<nsCString, nsCString>, nsresult> GenerateOriginKey2(
+    const mozilla::ipc::PrincipalInfo& aPrincipalInfo) {
+  OriginAttributes attrs;
+  nsCString spec;
+
+  switch (aPrincipalInfo.type()) {
+    case mozilla::ipc::PrincipalInfo::TNullPrincipalInfo: {
+      const auto& info = aPrincipalInfo.get_NullPrincipalInfo();
+
+      attrs = info.attrs();
+      spec = info.spec();
+
+      break;
+    }
+
+    case mozilla::ipc::PrincipalInfo::TContentPrincipalInfo: {
+      const auto& info = aPrincipalInfo.get_ContentPrincipalInfo();
+
+      attrs = info.attrs();
+      spec = info.spec();
+
+      break;
+    }
+
+    default: {
+      spec.SetIsVoid(true);
+
+      break;
+    }
+  }
+
+  if (spec.IsVoid()) {
+    return Err(NS_ERROR_UNEXPECTED);
+  }
+
+  nsCString originAttrSuffix;
+  attrs.CreateSuffix(originAttrSuffix);
+
+  RefPtr<MozURL> specURL;
+  QM_TRY(MOZ_TO_RESULT(MozURL::Init(getter_AddRefs(specURL), spec)));
+
+  nsCString host(specURL->Host());
+  uint32_t length = host.Length();
+  if (length > 0 && host.CharAt(0) == '[' && host.CharAt(length - 1) == ']') {
+    host = Substring(host, 1, length - 2);
+  }
+
+  nsCString domainOrigin(host);
+
+  if (domainOrigin.IsEmpty()) {
+    // For the file:/// protocol use the exact directory as domain.
+    if (specURL->Scheme().EqualsLiteral("file")) {
+      domainOrigin.Assign(specURL->Directory());
+    }
+  }
+
+  // Append reversed domain
+  nsAutoCString reverseDomain;
+  nsresult rv = StorageUtils::CreateReversedDomain(domainOrigin, reverseDomain);
+  if (NS_FAILED(rv)) {
+    return Err(rv);
+  }
+
+  nsCString originKey = reverseDomain;
+
+  // Append scheme
+  originKey.Append(':');
+  originKey.Append(specURL->Scheme());
+
+  // Append port if any
+  int32_t port = specURL->RealPort();
+  if (port != -1) {
+    originKey.AppendPrintf(":%d", port);
+  }
+
+  return std::make_pair(std::move(originAttrSuffix), std::move(originKey));
+}
+
+LogModule* GetLocalStorageLogger() { return gLogger; }
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/LocalStorageCommon.h b/dom/localstorage/LocalStorageCommon.h
new file mode 100644
index 0000000000..408f7106a2
--- /dev/null
+++ b/dom/localstorage/LocalStorageCommon.h
@@ -0,0 +1,263 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LocalStorageCommon_h
+#define mozilla_dom_localstorage_LocalStorageCommon_h
+
+#include <cstdint>
+#include "ErrorList.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/quota/QuotaCommon.h"
+#include "nsLiteralString.h"
+#include "nsStringFwd.h"
+
+/*
+ * Local storage
+ * ~~~~~~~~~~~~~
+ *
+ * Implementation overview
+ * ~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * The implementation is based on a per principal/origin cache (datastore)
+ * living in the main process and synchronous calls initiated from content
+ * processes.
+ * The IPC communication is managed by database actors which link to the
+ * datastore.
+ * The synchronous blocking of the main thread is done by using a special
+ * technique or by using standard synchronous IPC calls.
+ *
+ * General architecture
+ * ~~~~~~~~~~~~~~~~~~~~
+ * The current browser architecture consists of one main process and multiple
+ * content processes (there are other processes but for simplicity's sake, they
+ * are not mentioned here). The processes use the IPC communication to talk to
+ * each other. Local storage implementation uses the client-server model, so
+ * the main process manages all the data and content processes then request
+ * particular data from the main process. The main process is also called the
+ * parent or the parent side, the content process is then called the child or
+ * the child side.
+ *
+ * Datastores
+ * ~~~~~~~~~~
+ *
+ * A datastore provides a convenient way to access data for given origin. The
+ * data is always preloaded into memory and indexed using a hash table. This
+ * enables very fast access to particular stored items. There can be only one
+ * datastore per origin and exists solely on the parent side. It is represented
+ * by the "Datastore" class. A datastore instance is a ref counted object and
+ * lives on the PBackground thread, it is kept alive by database objects. When
+ * the last database object for given origin is destroyed, the associated
+ * datastore object is destroyed too.
+ *
+ * Databases
+ * ~~~~~~~~~
+ *
+ * A database allows direct access to a datastore from a content process. There
+ * can be multiple databases for the same origin, but they all share the same
+ * datastore.
+ * Databases use the PBackgroundLSDatabase IPDL protocol for IPC communication.
+ * Given the nature of local storage, most of PBackgroundLSDatabase messages
+ * are synchronous.
+ *
+ * On the parent side, the database is represented by the "Database" class that
+ * is a parent actor as well (implements the "PBackgroundLSDatabaseParent"
+ * interface). A database instance is a ref counted object and lives on the
+ * PBackground thread.
+ * All live database actors are tracked in an array.
+ *
+ * On the child side, the database is represented by the "LSDatabase" class
+ * that provides indirect access to a child actor. An LSDatabase instance is a
+ * ref counted object and lives on the main thread.
+ * The actual child actor is represented by the "LSDatabaseChild" class that
+ * implements the "PBackgroundLSDatabaseChild" interface. An "LSDatabaseChild"
+ * instance is not ref counted and lives on the main thread too.
+ *
+ * Synchronous blocking
+ * ~~~~~~~~~~~~~~~~~~~~
+ *
+ * Local storage is synchronous in nature which means the execution can't move
+ * forward until there's a reply for given method call.
+ * Since we have to use IPC anyway, we could just always use synchronous IPC
+ * messages for all local storage method calls. Well, there's a problem with
+ * that approach.
+ * If the main process needs to do some off PBackground thread stuff like
+ * getting info from principals on the main thread or some asynchronous stuff
+ * like directory locking before sending a reply to a synchronous message, then
+ * we would have to block the thread or spin the event loop which is usually a
+ * bad idea, especially in the main process.
+ * Instead, we can use a special thread in the content process called
+ * RemoteLazyInputStream thread for communication with the main process using
+ * asynchronous messages and synchronously block the main thread until the DOM
+ * File thread is done (the main thread blocking is a bit more complicated, see
+ * the comment in RequestHelper::StartAndReturnResponse for more details).
+ * Anyway, the extra hop to the RemoteLazyInputStream thread brings another
+ * overhead and latency. The final solution is to use a combination of the
+ * special thread for complex stuff like datastore preparation and synchronous
+ * IPC messages sent directly from the main thread for database access when data
+ * is already loaded from disk into memory.
+ *
+ * Requests
+ * ~~~~~~~~
+ *
+ * Requests are used to handle asynchronous high level datastore operations
+ * which are initiated in a content process and then processed in the parent
+ * process (for example, preparation of a datastore).
+ * Requests use the "PBackgroundLSRequest" IPDL protocol for IPC communication.
+ *
+ * On the parent side, the request is represented by the "LSRequestBase" class
+ * that is a parent actor as well (implements the "PBackgroundLSRequestParent"
+ * interface). It's an abstract class (contains pure virtual functions) so it
+ * can't be used to create instances.
+ * It also inherits from the "DatastoreOperationBase" class which is a generic
+ * base class for all datastore operations. The "DatastoreOperationsBase" class
+ * inherits from the "Runnable" class, so derived class instances are ref
+ * counted, can be dispatched to multiple threads and thus they are used on
+ * multiple threads. However, derived class instances can be created on the
+ * PBackground thread only.
+ *
+ * On the child side, the request is represented by the "RequestHelper" class
+ * that covers all the complexity needed to start a new request, handle
+ * responses and do safe main thread blocking at the same time.
+ * It inherits from the "Runnable" class, so instances are ref counted and
+ * they are internally used on multiple threads (specifically on the main
+ * thread and on the RemoteLazyInputStream thread). Anyway, users should create
+ * and use instances of this class only on the main thread.
+ * The actual child actor is represented by the "LSRequestChild" class that
+ * implements the "PBackgroundLSRequestChild" interface. An "LSRequestChild"
+ * instance is not ref counted and lives on the RemoteLazyInputStream thread.
+ * Request responses are passed using the "LSRequestChildCallback" interface.
+ *
+ * Preparation of a datastore
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * The datastore preparation is needed to make sure a datastore is fully loaded
+ * into memory. Every datastore preparation produces a unique id (even if the
+ * datastore for given origin already exists).
+ * On the parent side, the preparation is handled by the "PrepareDatastoreOp"
+ * class which inherits from the "LSRequestBase" class. The preparation process
+ * on the parent side is quite complicated, it happens sequentially on multiple
+ * threads and is managed by a state machine.
+ * On the child side, the preparation is done in the LSObject::EnsureDatabase
+ * method using the "RequestHelper" class. The method starts a new preparation
+ * request and obtains a unique id produced by the parent (or an error code if
+ * the requested failed to complete).
+ *
+ * Linking databases to a datastore
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * A datastore exists only on the parent side, but it can be accessed from the
+ * content via database actors. Database actors are initiated on the child side
+ * and they need to be linked to a datastore on the parent side via an id. The
+ * datastore preparation process gives us the required id.
+ * The linking is initiated on the child side in the LSObject::EnsureDatabase
+ * method by calling SendPBackgroundLSDatabaseConstructor and finished in
+ * RecvPBackgroundLSDatabaseConstructor on the parent side.
+ *
+ * Actor migration
+ * ~~~~~~~~~~~~~~~
+ *
+ * In theory, the datastore preparation request could return a database actor
+ * directly (instead of returning an id intended for database linking to a
+ * datastore). However, as it was explained above, the preparation must be done
+ * on the RemoteLazyInputStream thread and database objects are used on the main
+ * thread. The returned actor would have to be migrated from the
+ * RemoteLazyInputStream thread to the main thread and that's something which
+ * our IPDL doesn't support yet.
+ *
+ * Exposing local storage
+ * ~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * The implementation is exposed to the DOM via window.localStorage attribute.
+ * Local storage's sibling, session storage shares the same WebIDL interface
+ * for exposing it to web content, therefore there's an abstract class called
+ * "Storage" that handles some of the common DOM bindings stuff. Local storage
+ * specific functionality is defined in the "LSObject" derived class.
+ * The "LSObject" class is also a starting point for the datastore preparation
+ * and database linking.
+ *
+ * Local storage manager
+ * ~~~~~~~~~~~~~~~~~~~~~
+ *
+ * The local storage manager exposes some of the features that need to be
+ * available only in the chrome code or tests. The manager is represented by
+ * the "LocalStorageManager2" class that implements the "nsIDOMStorageManager"
+ * interface.
+ */
+
+namespace mozilla {
+
+class LogModule;
+
+namespace ipc {
+
+class PrincipalInfo;
+
+}  // namespace ipc
+
+namespace dom {
+
+extern const char16_t* kLocalStorageType;
+
+/**
+ * Convenience data-structure to make it easier to track whether a value has
+ * changed and what its previous value was for notification purposes.  Instances
+ * are created on the stack by LSObject and passed to LSDatabase which in turn
+ * passes them onto LSSnapshot for final updating/population.  LSObject then
+ * generates an event, if appropriate.
+ */
+class MOZ_STACK_CLASS LSNotifyInfo {
+  bool mChanged;
+  nsString mOldValue;
+
+ public:
+  LSNotifyInfo() : mChanged(false) {}
+
+  bool changed() const { return mChanged; }
+
+  bool& changed() { return mChanged; }
+
+  const nsString& oldValue() const { return mOldValue; }
+
+  nsString& oldValue() { return mOldValue; }
+};
+
+/**
+ * A check of LSNG being enabled, the value is latched once initialized so
+ * changing the preference during runtime has no effect. May be called on any
+ * thread in the parent process, but you should call
+ * CachedNextGenLocalStorageEnabled if you know that NextGenLocalStorageEnabled
+ * was already called because it is faster. May be called on any thread in
+ * content processes, but you should call CachedNextGenLocalStorageEnabled
+ * directly if you know you are in a content process because it is slightly
+ * faster.
+ */
+bool NextGenLocalStorageEnabled();
+
+/**
+ * Called by ContentChild during content process initialization to initialize
+ * the global variable in the content process with the latched value in the
+ * parent process."
+ */
+void RecvInitNextGenLocalStorageEnabled(const bool aEnabled);
+
+/**
+ * Cached any-thread version of NextGenLocalStorageEnabled().
+ */
+bool CachedNextGenLocalStorageEnabled();
+
+/**
+ * Returns a success value containing a pair of origin attribute suffix and
+ * origin key.
+ */
+Result<std::pair<nsCString, nsCString>, nsresult> GenerateOriginKey2(
+    const mozilla::ipc::PrincipalInfo& aPrincipalInfo);
+
+LogModule* GetLocalStorageLogger();
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_localstorage_LocalStorageCommon_h
diff --git a/dom/localstorage/LocalStorageManager2.cpp b/dom/localstorage/LocalStorageManager2.cpp
new file mode 100644
index 0000000000..5e19cf2254
--- /dev/null
+++ b/dom/localstorage/LocalStorageManager2.cpp
@@ -0,0 +1,661 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "LocalStorageManager2.h"
+
+// Local includes
+#include "ActorsChild.h"
+#include "LSObject.h"
+
+// Global includes
+#include <utility>
+#include "MainThreadUtils.h"
+#include "jsapi.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/MacroForEach.h"
+#include "mozilla/OriginAttributes.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/RemoteLazyInputStreamThread.h"
+#include "mozilla/dom/LocalStorageCommon.h"
+#include "mozilla/dom/PBackgroundLSRequest.h"
+#include "mozilla/dom/PBackgroundLSSharedTypes.h"
+#include "mozilla/dom/PBackgroundLSSimpleRequest.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/quota/QuotaManager.h"
+#include "mozilla/ipc/BackgroundChild.h"
+#include "mozilla/ipc/BackgroundUtils.h"
+#include "mozilla/ipc/PBackgroundChild.h"
+#include "mozilla/ipc/PBackgroundSharedTypes.h"
+#include "nsCOMPtr.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsIEventTarget.h"
+#include "nsILocalStorageManager.h"
+#include "nsIPrincipal.h"
+#include "nsIRunnable.h"
+#include "nsPIDOMWindow.h"
+#include "nsStringFwd.h"
+#include "nsThreadUtils.h"
+#include "nscore.h"
+#include "xpcpublic.h"
+
+namespace mozilla::dom {
+
+namespace {
+
+class AsyncRequestHelper final : public Runnable,
+                                 public LSRequestChildCallback {
+  enum class State {
+    /**
+     * The AsyncRequestHelper has been created and dispatched to the
+     * RemoteLazyInputStream Thread.
+     */
+    Initial,
+    /**
+     * Start() has been invoked on the RemoteLazyInputStream Thread and
+     * LocalStorageManager2::StartRequest has been invoked from there, sending
+     * an IPC message to PBackground to service the request.  We stay in this
+     * state until a response is received.
+     */
+    ResponsePending,
+    /**
+     * A response has been received and AsyncRequestHelper has been dispatched
+     * back to the owning event target to call Finish().
+     */
+    Finishing,
+    /**
+     * Finish() has been called on the main thread. The promise will be resolved
+     * according to the received response.
+     */
+    Complete
+  };
+
+  // The object we are issuing a request on behalf of.  Present because of the
+  // need to invoke LocalStorageManager2::StartRequest off the main thread.
+  // Dropped on return to the main-thread in Finish().
+  RefPtr<LocalStorageManager2> mManager;
+  // The thread the AsyncRequestHelper was created on.  This should be the main
+  // thread.
+  nsCOMPtr<nsIEventTarget> mOwningEventTarget;
+  // The IPC actor handling the request with standard IPC allocation rules.
+  // Our reference is nulled in OnResponse which corresponds to the actor's
+  // __destroy__ method.
+  LSRequestChild* mActor;
+  RefPtr<Promise> mPromise;
+  const LSRequestParams mParams;
+  LSRequestResponse mResponse;
+  nsresult mResultCode;
+  State mState;
+
+ public:
+  AsyncRequestHelper(LocalStorageManager2* aManager, Promise* aPromise,
+                     const LSRequestParams& aParams)
+      : Runnable("dom::LocalStorageManager2::AsyncRequestHelper"),
+        mManager(aManager),
+        mOwningEventTarget(GetCurrentSerialEventTarget()),
+        mActor(nullptr),
+        mPromise(aPromise),
+        mParams(aParams),
+        mResultCode(NS_OK),
+        mState(State::Initial) {}
+
+  bool IsOnOwningThread() const {
+    MOZ_ASSERT(mOwningEventTarget);
+
+    bool current;
+    return NS_SUCCEEDED(mOwningEventTarget->IsOnCurrentThread(&current)) &&
+           current;
+  }
+
+  void AssertIsOnOwningThread() const {
+    MOZ_ASSERT(NS_IsMainThread());
+    MOZ_ASSERT(IsOnOwningThread());
+  }
+
+  nsresult Dispatch();
+
+ private:
+  ~AsyncRequestHelper() = default;
+
+  nsresult Start();
+
+  void Finish();
+
+  NS_DECL_ISUPPORTS_INHERITED
+
+  NS_DECL_NSIRUNNABLE
+
+  // LSRequestChildCallback
+  void OnResponse(const LSRequestResponse& aResponse) override;
+};
+
+class SimpleRequestResolver final : public LSSimpleRequestChildCallback {
+  RefPtr<Promise> mPromise;
+
+ public:
+  explicit SimpleRequestResolver(Promise* aPromise) : mPromise(aPromise) {}
+
+  NS_INLINE_DECL_REFCOUNTING(SimpleRequestResolver, override);
+
+ private:
+  ~SimpleRequestResolver() = default;
+
+  void HandleResponse(nsresult aResponse);
+
+  void HandleResponse(bool aResponse);
+
+  void HandleResponse(const nsTArray<LSItemInfo>& aResponse);
+
+  // LSRequestChildCallback
+  void OnResponse(const LSSimpleRequestResponse& aResponse) override;
+};
+
+nsresult CreatePromise(JSContext* aContext, Promise** aPromise) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aContext);
+
+  nsIGlobalObject* global =
+      xpc::NativeGlobal(JS::CurrentGlobalOrNull(aContext));
+  if (NS_WARN_IF(!global)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  ErrorResult result;
+  RefPtr<Promise> promise = Promise::Create(global, result);
+  if (result.Failed()) {
+    return result.StealNSResult();
+  }
+
+  promise.forget(aPromise);
+  return NS_OK;
+}
+
+nsresult CheckedPrincipalToPrincipalInfo(
+    nsIPrincipal* aPrincipal, mozilla::ipc::PrincipalInfo& aPrincipalInfo) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+
+  nsresult rv = PrincipalToPrincipalInfo(aPrincipal, &aPrincipalInfo);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  if (NS_WARN_IF(!quota::QuotaManager::IsPrincipalInfoValid(aPrincipalInfo))) {
+    return NS_ERROR_FAILURE;
+  }
+
+  if (aPrincipalInfo.type() !=
+          mozilla::ipc::PrincipalInfo::TContentPrincipalInfo &&
+      aPrincipalInfo.type() !=
+          mozilla::ipc::PrincipalInfo::TSystemPrincipalInfo) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  return NS_OK;
+}
+
+}  // namespace
+
+LocalStorageManager2::LocalStorageManager2() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(NextGenLocalStorageEnabled());
+}
+
+LocalStorageManager2::~LocalStorageManager2() { MOZ_ASSERT(NS_IsMainThread()); }
+
+NS_IMPL_ISUPPORTS(LocalStorageManager2, nsIDOMStorageManager,
+                  nsILocalStorageManager)
+
+NS_IMETHODIMP
+LocalStorageManager2::PrecacheStorage(nsIPrincipal* aPrincipal,
+                                      nsIPrincipal* aStoragePrincipal,
+                                      Storage** _retval) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(aStoragePrincipal);
+  MOZ_ASSERT(_retval);
+
+  // This method was created as part of the e10s-ification of the old LS
+  // implementation to perform a preload in the content/current process.  That's
+  // not how things work in LSNG.  Instead everything happens in the parent
+  // process, triggered by the official preloading spot,
+  // ContentParent::AboutToLoadHttpFtpDocumentForChild.
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LocalStorageManager2::CreateStorage(mozIDOMWindow* aWindow,
+                                    nsIPrincipal* aPrincipal,
+                                    nsIPrincipal* aStoragePrincipal,
+                                    const nsAString& aDocumentURI,
+                                    bool aPrivate, Storage** _retval) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(aStoragePrincipal);
+  MOZ_ASSERT(_retval);
+
+  nsCOMPtr<nsPIDOMWindowInner> inner = nsPIDOMWindowInner::From(aWindow);
+
+  RefPtr<LSObject> object;
+  nsresult rv = LSObject::CreateForPrincipal(inner, aPrincipal,
+                                             aStoragePrincipal, aDocumentURI,
+                                             aPrivate, getter_AddRefs(object));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  object.forget(_retval);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocalStorageManager2::GetStorage(mozIDOMWindow* aWindow,
+                                 nsIPrincipal* aPrincipal,
+                                 nsIPrincipal* aStoragePrincipal, bool aPrivate,
+                                 Storage** _retval) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(aStoragePrincipal);
+  MOZ_ASSERT(_retval);
+
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LocalStorageManager2::CloneStorage(Storage* aStorageToCloneFrom) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aStorageToCloneFrom);
+
+  // Cloning is specific to sessionStorage; state is forked when a new tab is
+  // opened from an existing tab.
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LocalStorageManager2::CheckStorage(nsIPrincipal* aPrincipal, Storage* aStorage,
+                                   bool* _retval) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(aStorage);
+  MOZ_ASSERT(_retval);
+
+  // Only used by sessionStorage.
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LocalStorageManager2::GetNextGenLocalStorageEnabled(bool* aResult) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aResult);
+
+  *aResult = NextGenLocalStorageEnabled();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocalStorageManager2::Preload(nsIPrincipal* aPrincipal, JSContext* aContext,
+                              Promise** _retval) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(_retval);
+
+  nsCString originAttrSuffix;
+  nsCString originKey;
+  nsresult rv = aPrincipal->GetStorageOriginKey(originKey);
+  aPrincipal->OriginAttributesRef().CreateSuffix(originAttrSuffix);
+  if (NS_FAILED(rv)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  mozilla::ipc::PrincipalInfo principalInfo;
+  rv = CheckedPrincipalToPrincipalInfo(aPrincipal, principalInfo);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  RefPtr<Promise> promise;
+
+  if (aContext) {
+    rv = CreatePromise(aContext, getter_AddRefs(promise));
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+  }
+
+  LSRequestCommonParams commonParams;
+  commonParams.principalInfo() = principalInfo;
+  commonParams.storagePrincipalInfo() = principalInfo;
+  commonParams.originKey() = originKey;
+
+  LSRequestPreloadDatastoreParams params(commonParams);
+
+  RefPtr<AsyncRequestHelper> helper =
+      new AsyncRequestHelper(this, promise, params);
+
+  // This will start and finish the async request on the RemoteLazyInputStream
+  // thread.
+  // This must be done on RemoteLazyInputStream Thread because it's very likely
+  // that a content process will issue a prepare datastore request for the same
+  // principal while blocking the content process on the main thread.
+  // There would be a potential for deadlock if the preloading was initialized
+  // from the main thread of the parent process and a11y issued a synchronous
+  // message from the parent process to the content process (approximately at
+  // the same time) because the preload request wouldn't be able to respond
+  // to the Ready message by sending the Finish message which is needed to
+  // finish the preload request and unblock the prepare datastore request.
+  rv = helper->Dispatch();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  promise.forget(_retval);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocalStorageManager2::IsPreloaded(nsIPrincipal* aPrincipal, JSContext* aContext,
+                                  Promise** _retval) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(_retval);
+
+  RefPtr<Promise> promise;
+  nsresult rv = CreatePromise(aContext, getter_AddRefs(promise));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  LSSimpleRequestPreloadedParams params;
+
+  rv = CheckedPrincipalToPrincipalInfo(aPrincipal, params.principalInfo());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  params.storagePrincipalInfo() = params.principalInfo();
+
+  rv = StartSimpleRequest(promise, params);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  promise.forget(_retval);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocalStorageManager2::GetState(nsIPrincipal* aPrincipal, JSContext* aContext,
+                               Promise** _retval) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(_retval);
+
+  RefPtr<Promise> promise;
+  nsresult rv = CreatePromise(aContext, getter_AddRefs(promise));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  LSSimpleRequestGetStateParams params;
+
+  rv = CheckedPrincipalToPrincipalInfo(aPrincipal, params.principalInfo());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  params.storagePrincipalInfo() = params.principalInfo();
+
+  rv = StartSimpleRequest(promise, params);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  promise.forget(_retval);
+  return NS_OK;
+}
+
+LSRequestChild* LocalStorageManager2::StartRequest(
+    const LSRequestParams& aParams, LSRequestChildCallback* aCallback) {
+  AssertIsOnDOMFileThread();
+
+  mozilla::ipc::PBackgroundChild* backgroundActor =
+      mozilla::ipc::BackgroundChild::GetOrCreateForCurrentThread();
+  if (NS_WARN_IF(!backgroundActor)) {
+    return nullptr;
+  }
+
+  auto actor = new LSRequestChild();
+
+  if (!backgroundActor->SendPBackgroundLSRequestConstructor(actor, aParams)) {
+    return nullptr;
+  }
+
+  // Must set callback after calling SendPBackgroundLSRequestConstructor since
+  // it can be called synchronously when SendPBackgroundLSRequestConstructor
+  // fails.
+  actor->SetCallback(aCallback);
+
+  return actor;
+}
+
+nsresult LocalStorageManager2::StartSimpleRequest(
+    Promise* aPromise, const LSSimpleRequestParams& aParams) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPromise);
+
+  mozilla::ipc::PBackgroundChild* backgroundActor =
+      mozilla::ipc::BackgroundChild::GetOrCreateForCurrentThread();
+  if (NS_WARN_IF(!backgroundActor)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  auto actor = new LSSimpleRequestChild();
+
+  if (!backgroundActor->SendPBackgroundLSSimpleRequestConstructor(actor,
+                                                                  aParams)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  RefPtr<SimpleRequestResolver> resolver = new SimpleRequestResolver(aPromise);
+
+  // Must set callback after calling SendPBackgroundLSRequestConstructor since
+  // it can be called synchronously when SendPBackgroundLSRequestConstructor
+  // fails.
+  actor->SetCallback(resolver);
+
+  return NS_OK;
+}
+
+nsresult AsyncRequestHelper::Dispatch() {
+  AssertIsOnOwningThread();
+
+  nsCOMPtr<nsIEventTarget> domFileThread =
+      RemoteLazyInputStreamThread::GetOrCreate();
+  if (NS_WARN_IF(!domFileThread)) {
+    return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
+  }
+
+  nsresult rv = domFileThread->Dispatch(this, NS_DISPATCH_NORMAL);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult AsyncRequestHelper::Start() {
+  AssertIsOnDOMFileThread();
+  MOZ_ASSERT(mState == State::Initial);
+
+  mState = State::ResponsePending;
+
+  LSRequestChild* actor = mManager->StartRequest(mParams, this);
+  if (NS_WARN_IF(!actor)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  mActor = actor;
+
+  return NS_OK;
+}
+
+void AsyncRequestHelper::Finish() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::Finishing);
+
+  if (NS_WARN_IF(NS_FAILED(mResultCode))) {
+    if (mPromise) {
+      mPromise->MaybeReject(mResultCode);
+    }
+  } else {
+    switch (mResponse.type()) {
+      case LSRequestResponse::Tnsresult:
+        if (mPromise) {
+          mPromise->MaybeReject(mResponse.get_nsresult());
+        }
+        break;
+
+      case LSRequestResponse::TLSRequestPreloadDatastoreResponse:
+        if (mPromise) {
+          mPromise->MaybeResolveWithUndefined();
+        }
+        break;
+      default:
+        MOZ_CRASH("Unknown response type!");
+    }
+  }
+
+  mManager = nullptr;
+
+  mState = State::Complete;
+}
+
+NS_IMPL_ISUPPORTS_INHERITED0(AsyncRequestHelper, Runnable)
+
+NS_IMETHODIMP
+AsyncRequestHelper::Run() {
+  nsresult rv;
+
+  switch (mState) {
+    case State::Initial:
+      rv = Start();
+      break;
+
+    case State::Finishing:
+      Finish();
+      return NS_OK;
+
+    default:
+      MOZ_CRASH("Bad state!");
+  }
+
+  if (NS_WARN_IF(NS_FAILED(rv)) && mState != State::Finishing) {
+    if (NS_SUCCEEDED(mResultCode)) {
+      mResultCode = rv;
+    }
+
+    mState = State::Finishing;
+
+    if (IsOnOwningThread()) {
+      Finish();
+    } else {
+      MOZ_ALWAYS_SUCCEEDS(
+          mOwningEventTarget->Dispatch(this, NS_DISPATCH_NORMAL));
+    }
+  }
+
+  return NS_OK;
+}
+
+void AsyncRequestHelper::OnResponse(const LSRequestResponse& aResponse) {
+  AssertIsOnDOMFileThread();
+  MOZ_ASSERT(mState == State::ResponsePending);
+
+  mActor = nullptr;
+
+  mResponse = aResponse;
+
+  mState = State::Finishing;
+
+  MOZ_ALWAYS_SUCCEEDS(mOwningEventTarget->Dispatch(this, NS_DISPATCH_NORMAL));
+}
+
+void SimpleRequestResolver::HandleResponse(nsresult aResponse) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mPromise);
+
+  mPromise->MaybeReject(aResponse);
+}
+
+void SimpleRequestResolver::HandleResponse(bool aResponse) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mPromise);
+
+  mPromise->MaybeResolve(aResponse);
+}
+
+[[nodiscard]] static bool ToJSValue(JSContext* aCx,
+                                    const nsTArray<LSItemInfo>& aArgument,
+                                    JS::MutableHandle<JS::Value> aValue) {
+  JS::Rooted<JSObject*> obj(aCx, JS_NewPlainObject(aCx));
+  if (!obj) {
+    return false;
+  }
+
+  for (size_t i = 0; i < aArgument.Length(); ++i) {
+    const LSItemInfo& itemInfo = aArgument[i];
+
+    const nsString& key = itemInfo.key();
+
+    JS::Rooted<JS::Value> value(aCx);
+    if (!ToJSValue(aCx, itemInfo.value().AsString(), &value)) {
+      return false;
+    }
+
+    if (!JS_DefineUCProperty(aCx, obj, key.BeginReading(), key.Length(), value,
+                             JSPROP_ENUMERATE)) {
+      return false;
+    }
+  }
+
+  aValue.setObject(*obj);
+  return true;
+}
+
+void SimpleRequestResolver::HandleResponse(
+    const nsTArray<LSItemInfo>& aResponse) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mPromise);
+
+  mPromise->MaybeResolve(aResponse);
+}
+
+void SimpleRequestResolver::OnResponse(
+    const LSSimpleRequestResponse& aResponse) {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  switch (aResponse.type()) {
+    case LSSimpleRequestResponse::Tnsresult:
+      HandleResponse(aResponse.get_nsresult());
+      break;
+
+    case LSSimpleRequestResponse::TLSSimpleRequestPreloadedResponse:
+      HandleResponse(
+          aResponse.get_LSSimpleRequestPreloadedResponse().preloaded());
+      break;
+
+    case LSSimpleRequestResponse::TLSSimpleRequestGetStateResponse:
+      HandleResponse(
+          aResponse.get_LSSimpleRequestGetStateResponse().itemInfos());
+      break;
+
+    default:
+      MOZ_CRASH("Unknown response type!");
+  }
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/LocalStorageManager2.h b/dom/localstorage/LocalStorageManager2.h
new file mode 100644
index 0000000000..63cc9a9a15
--- /dev/null
+++ b/dom/localstorage/LocalStorageManager2.h
@@ -0,0 +1,68 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LocalStorageManager2_h
+#define mozilla_dom_localstorage_LocalStorageManager2_h
+
+#include <cstdint>
+#include "ErrorList.h"
+#include "nsIDOMStorageManager.h"
+#include "nsILocalStorageManager.h"
+#include "nsISupports.h"
+
+namespace mozilla::dom {
+
+class LSRequestChild;
+class LSRequestChildCallback;
+class LSRequestParams;
+class LSSimpleRequestParams;
+class Promise;
+
+/**
+ * Under LSNG this exposes nsILocalStorageManager::Preload to ContentParent to
+ * trigger preloading.  Otherwise, this is basically just a place for test logic
+ * that doesn't make sense to put directly on the Storage WebIDL interface.
+ *
+ * Previously, the nsIDOMStorageManager XPCOM interface was also used by
+ * nsGlobalWindowInner to interact with LocalStorage, but in these de-XPCOM
+ * days, we've moved to just directly reference the relevant concrete classes
+ * (ex: LSObject) directly.
+ *
+ * Note that testing methods are now also directly exposed on the Storage WebIDL
+ * interface for simplicity/sanity.
+ */
+class LocalStorageManager2 final : public nsIDOMStorageManager,
+                                   public nsILocalStorageManager {
+ public:
+  LocalStorageManager2();
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIDOMSTORAGEMANAGER
+  NS_DECL_NSILOCALSTORAGEMANAGER
+
+  /**
+   * Helper to trigger an LSRequest and resolve/reject the provided promise when
+   * the result comes in.  This routine is notable because the LSRequest
+   * mechanism is normally used synchronously from content, but here it's
+   * exposed asynchronously.
+   */
+  LSRequestChild* StartRequest(const LSRequestParams& aParams,
+                               LSRequestChildCallback* aCallback);
+
+ private:
+  ~LocalStorageManager2();
+
+  /**
+   * Helper to trigger an LSSimpleRequst and resolve/reject the provided promise
+   * when the result comes in.
+   */
+  nsresult StartSimpleRequest(Promise* aPromise,
+                              const LSSimpleRequestParams& aParams);
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_localstorage_LocalStorageManager2_h
diff --git a/dom/localstorage/PBackgroundLSDatabase.ipdl b/dom/localstorage/PBackgroundLSDatabase.ipdl
new file mode 100644
index 0000000000..9a4402c97e
--- /dev/null
+++ b/dom/localstorage/PBackgroundLSDatabase.ipdl
@@ -0,0 +1,175 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+include protocol PBackground;
+include protocol PBackgroundLSSnapshot;
+
+include PBackgroundLSSharedTypes;
+
+include "mozilla/dom/localstorage/SerializationHelpers.h";
+
+using mozilla::dom::LSSnapshot::LoadState
+  from "mozilla/dom/LSSnapshot.h";
+
+namespace mozilla {
+namespace dom {
+
+/**
+ * Initial LSSnapshot state as produced by Datastore::GetSnapshotLoadInfo.  See
+ * `LSSnapshot::LoadState` for more details about the possible states and a
+ * high level overview.
+ */
+struct LSSnapshotInitInfo
+{
+  /**
+   * Boolean indicating whether the `key` provided as an argument to the
+   * PBackgroundLSSnapshot constructor did not exist in the Datastore and should
+   * be treated as an unknown and therefore undefined value. Note that `key` may
+   * have been provided as a void string, in which case this value is forced to
+   * be false.
+   */
+  bool addKeyToUnknownItems;
+
+  /**
+   * As many key/value or key/void pairs as the snapshot prefill byte budget
+   * allowed.
+   */
+  LSItemInfo[] itemInfos;
+
+  /**
+   * The total number of key/value pairs in LocalStorage for this origin at the
+   * time the snapshot was created.  (And the point of the snapshot is to
+   * conceptually freeze the state of the Datastore in time, so this value does
+   * not change despite what other LSDatabase objects get up to in other
+   * processes.)
+   */
+  uint32_t totalLength;
+
+  /**
+   * The current amount of LocalStorage usage as measured by the summing the
+   * nsString Length() of both the key and the value over all stored pairs.
+   */
+  int64_t usage;
+
+  /**
+   * The amount of storage allowed to be used by the Snapshot without requesting
+   * more storage space via IncreasePeakUsage.  This is the `usage` plus 0 or
+   * more bytes of space.  If space was available, the increase will be the
+   * `minSize` from the PBackgroundLSSnapshot constructor plus the configured
+   * pre-increment (via "dom.storage.snapshot_peak_usage.initial_preincrement").
+   * If the LocalStorage total usage was already close to the limit, then the
+   * fallback is either the `minSize` plus the configured reduced pre-increment
+   * (via "dom.storage.snapshot_peak_usage.reduced_initial_preincrement"), or
+   * `minSize`, or 0 depending on remaining available space.
+   */
+  int64_t peakUsage;
+
+  // See `LSSnapshot::LoadState` in `LSSnapshot.h`
+  LoadState loadState;
+
+  /**
+   * Boolean indicating whether there where cross-process databases registered
+   * for this origin at the time the snapshot was created.
+   */
+  bool hasOtherProcessDatabases;
+
+  /**
+   * Boolean indicating whether there where cross-process observers registered
+   * for this origin at the time the snapshot was created.
+   */
+  bool hasOtherProcessObservers;
+};
+
+/**
+ * This protocol is asynchronously created via constructor on PBackground but
+ * has synchronous semantics from the perspective of content on the main thread.
+ * The construction potentially involves waiting for disk I/O to load the
+ * LocalStorage data from disk as well as related QuotaManager checks, so async
+ * calls to PBackground are the only viable mechanism because blocking
+ * PBackground is not acceptable.  (Note that an attempt is made to minimize any
+ * I/O latency by triggering preloading from
+ * ContentParent::AboutToLoadHttpFtpDocumentForChild, the central place
+ * for pre-loading.)
+ */
+[ManualDealloc, ChildImpl=virtual, ParentImpl=virtual]
+sync protocol PBackgroundLSDatabase
+{
+  manager PBackground;
+  manages PBackgroundLSSnapshot;
+
+parent:
+  // The DeleteMe message is used to avoid a race condition between the parent
+  // actor and the child actor. The PBackgroundLSDatabase protocol could be
+  // simply destroyed by sending the __delete__ message from the child side.
+  // However, that would destroy the child actor immediatelly and the parent
+  // could be sending a message to the child at the same time resulting in a
+  // routing error since the child actor wouldn't exist anymore. A routing
+  // error typically causes a crash. The race can be prevented by doing the
+  // teardown in two steps. First, we send the DeleteMe message to the parent
+  // and the parent then sends the __delete__ message to the child.
+  async DeleteMe();
+
+  /**
+   * Sent in response to a `RequestAllowToClose` message once the snapshot
+   * cleanup has happened OR from LSDatabase's destructor if AllowToClose has
+   * not already been reported.
+   */
+  async AllowToClose();
+
+  /**
+   * Invoked to create an LSSnapshot backed by a Snapshot in PBackground that
+   * presents an atomic and consistent view of the state of the authoritative
+   * Datastore state in the parent.
+   *
+   * This needs to be synchronous because LocalStorage's semantics are
+   * synchronous.  Note that the Datastore in the PBackground parent already
+   * has the answers to this request immediately available without needing to
+   * consult any other threads or perform any I/O.  Additionally, the response
+   * is explicitly bounded in size by the tunable snapshot prefill byte limit.
+   *
+   * @param key
+   *   If key is non-void, then the snapshot is being triggered by a direct
+   *   access to a localStorage key (get, set, or removal, with set/removal
+   *   requiring the old value in order to properly populate the "storage"
+   *   event), the key being requested. It's possible the key is not present in
+   *   localStorage, in which case LSSnapshotInitInfo::addKeyToUnknownItems will
+   *   be true indicating that there is no such key/value pair, otherwise it
+   *   will be false.
+   * @param increasePeakUsage
+   *   Whether the parent should increase initial peak uage of the Snapshot.
+   *   See also the comment for LSSnapshotInitInfo::peakUsage above.
+   */
+  sync PBackgroundLSSnapshot(nsString documentURI,
+                             nsString key,
+                             bool increasePeakUsage,
+                             int64_t minSize)
+    returns (LSSnapshotInitInfo initInfo);
+
+child:
+  /**
+   * Only sent by the parent in response to the child's DeleteMe request.
+   */
+  async __delete__();
+
+  /**
+   * Request to close the LSDatabase, checkpointing and finishing any
+   * outstanding snapshots so no state is lost.  This request is issued when
+   * QuotaManager is shutting down or is aborting operations for an origin or
+   * process.  Once the snapshot has cleaned up, AllowToClose will be sent to
+   * the parent.
+   *
+   * Note that the QuotaManager shutdown process is more likely to happen in
+   * unit tests where we explicitly reset the QuotaManager.  At runtime, we
+   * expect windows to be closed and content processes terminated well before
+   * QuotaManager shutdown would actually occur.
+   *
+   * Also, Operations are usually aborted for an origin due to privacy API's
+   * clearing data for an origin.  Operations are aborted for a process by
+   * ContentParent::ShutDownProcess.
+   */
+  async RequestAllowToClose();
+};
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/localstorage/PBackgroundLSObserver.ipdl b/dom/localstorage/PBackgroundLSObserver.ipdl
new file mode 100644
index 0000000000..c800828da3
--- /dev/null
+++ b/dom/localstorage/PBackgroundLSObserver.ipdl
@@ -0,0 +1,63 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+include protocol PBackground;
+
+include PBackgroundSharedTypes;
+
+include "mozilla/dom/localstorage/SerializationHelpers.h";
+
+using mozilla::dom::LSValue
+  from "mozilla/dom/LSValue.h";
+
+namespace mozilla {
+namespace dom {
+
+/**
+ * The observer protocol sends "storage" event notifications for changes to
+ * LocalStorage that take place in other processes as their Snapshots are
+ * Checkpointed to the canonical Datastore in the parent process.  Same-process
+ * notifications are generated as mutations happen.
+ *
+ * Note that mutations are never generated for redundant mutations.  Setting the
+ * key "foo" to have value "bar" when it already has value "bar" will never
+ * result in a "storage" event.
+ */
+[ManualDealloc, ChildImpl=virtual, ParentImpl=virtual]
+async protocol PBackgroundLSObserver
+{
+  manager PBackground;
+
+parent:
+  /**
+   * Sent by the LSObserver's destructor when it's going away.  Any Observe
+   * messages received after this is sent will be ignored.  Which is fine,
+   * because there should be nothing around left to hear.  In the event a new
+   * page came into existence, its Observer creation will happen (effectively)
+   * synchronously.
+   */
+  async DeleteMe();
+
+child:
+  /**
+   * Only sent by the parent in response to a deletion request.
+   */
+  async __delete__();
+
+  /**
+   * Sent by the parent process as Snapshots from other processes are
+   * Checkpointed, applying their mutations.  The child actor currently directly
+   * shunts these to Storage::NotifyChange to generate "storage" events for
+   * immediate dispatch.
+   */
+  async Observe(PrincipalInfo principalInfo,
+                uint32_t privateBrowsingId,
+                nsString documentURI,
+                nsString key,
+                LSValue oldValue,
+                LSValue newValue);
+};
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/localstorage/PBackgroundLSRequest.ipdl b/dom/localstorage/PBackgroundLSRequest.ipdl
new file mode 100644
index 0000000000..1ced0a9a17
--- /dev/null
+++ b/dom/localstorage/PBackgroundLSRequest.ipdl
@@ -0,0 +1,103 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+include protocol PBackground;
+
+using struct mozilla::null_t from "mozilla/ipc/IPCCore.h";
+
+namespace mozilla {
+namespace dom {
+
+struct LSRequestPreloadDatastoreResponse
+{
+};
+
+struct LSRequestPrepareDatastoreResponse
+{
+  uint64_t datastoreId;
+};
+
+struct LSRequestPrepareObserverResponse
+{
+  uint64_t observerId;
+};
+
+/**
+ * Discriminated union which can contain an error code (`nsresult`) or
+ * particular request response.
+ */
+union LSRequestResponse
+{
+  nsresult;
+  LSRequestPreloadDatastoreResponse;
+  LSRequestPrepareDatastoreResponse;
+  LSRequestPrepareObserverResponse;
+};
+
+/**
+ * An asynchronous protocol for issuing requests that are used in a synchronous
+ * fashion by LocalStorage via LSObject's RequestHelper mechanism.  This differs
+ * from LSSimpleRequest which is implemented and used asynchronously.
+ *
+ * See `PBackgroundLSSharedTypes.ipdlh` for more on the request types, the
+ * response types above for their corresponding responses, and `RequestHelper`
+ * for more on the usage and lifecycle of this mechanism.
+ */
+[ManualDealloc, ChildImpl=virtual, ParentImpl=virtual]
+protocol PBackgroundLSRequest
+{
+  manager PBackground;
+
+parent:
+  // The Cancel message is used to avoid a possible dead lock caused by a CPOW
+  // sending a synchronous message from the main thread in the chrome process
+  // to the main thread in the content process at the time we are blocking
+  // the main thread in the content process to handle a request.
+  // We use the PBackground thread on the parent side to handle requests, but
+  // sometimes we need to get information from principals and that's currently
+  // only possible on the main thread. So if the main thread in the chrome
+  // process is blocked by a CPOW operation, our request must wait for the CPOW
+  // operation to complete. However the CPOW operation can't complete either
+  // because we are blocking the main thread in the content process.
+  // The dead lock is prevented by canceling our nested event loop in the
+  // content process when we receive a synchronous IPC message from the parent.
+  //
+  // Note that cancellation isn't instantaneous.  It's just an asynchronous flow
+  // that definitely doesn't involve the main thread in the parent process, so
+  // we're guaranteed to unblock the main-thread in the content process and
+  // allow the sync IPC to make progress.  When Cancel() is received by the
+  // parent, it will Send__delete__.  The child will either send Cancel or
+  // Finish, but not both.
+  async Cancel();
+
+  /**
+   * Sent by the child in response to Ready, requesting that __delete__ be sent
+   * with the result.  The child will either send Finish or Cancel, but not
+   * both.  No further message will be sent from the child after invoking one.
+   */
+  async Finish();
+
+child:
+  /**
+   * The deletion is sent with the result of the request directly in response to
+   * either Cancel or Finish.
+   */
+  async __delete__(LSRequestResponse response);
+
+  /**
+   * Sent by the parent when it has completed whatever async stuff it needs to
+   * do and is ready to send the results.  It then awaits the Finish() call to
+   * send the results.  This may seem redundant, but it's not.  If the
+   * __delete__ was sent directly, it's possible there could be a race where
+   * Cancel() would be received by the parent after it had already sent
+   * __delete__.  (Which may no longer be fatal thanks to improvements to the
+   * IPC layer, but it would still lead to warnings, etc.  And we don't
+   * expect PBackground to be highly contended nor the RemoteLazyInputStream
+   * thread.)
+   */
+  async Ready();
+};
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/localstorage/PBackgroundLSSharedTypes.ipdlh b/dom/localstorage/PBackgroundLSSharedTypes.ipdlh
new file mode 100644
index 0000000000..d71b0e6111
--- /dev/null
+++ b/dom/localstorage/PBackgroundLSSharedTypes.ipdlh
@@ -0,0 +1,96 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+include PBackgroundSharedTypes;
+include ProtocolTypes;
+
+include "mozilla/dom/localstorage/SerializationHelpers.h";
+
+using mozilla::dom::LSValue
+  from "mozilla/dom/LSValue.h";
+
+namespace mozilla {
+namespace dom {
+
+struct LSRequestCommonParams
+{
+  PrincipalInfo principalInfo;
+  PrincipalInfo storagePrincipalInfo;
+  nsCString originKey;
+};
+
+struct LSRequestPreloadDatastoreParams
+{
+  LSRequestCommonParams commonParams;
+};
+
+struct LSRequestPrepareDatastoreParams
+{
+  LSRequestCommonParams commonParams;
+  nsID? clientId;
+  PrincipalInfo? clientPrincipalInfo;
+};
+
+/**
+ * In order to validate the principal with the client, we need to provide an
+ * additional principalInfo for the client. The client is using the foreign
+ * principal, see StoragePrincipalHelper.h for details, which is different from
+ * the principalInfo. So, we need to pass the principalInfo from the client So
+ * that we can verify it with the given client Id.
+ *
+ * Note that the storagePrincipalInfo is used to access the right cookie jar
+ * according to the Storage Access. This is passed in order to access the
+ * correct local storage. Essentially, the storage principal and the client
+ * principal are using the PartitionKey in their OriginAttributes. But, the
+ * existence of the PartitionKey between them is depending on different
+ * conditions. Namely, the storage principal depends on the Storage Access but
+ * the client principal depends on whether it's in a third party.
+ */
+struct LSRequestPrepareObserverParams
+{
+  PrincipalInfo principalInfo;
+  PrincipalInfo storagePrincipalInfo;
+  nsID? clientId;
+  PrincipalInfo? clientPrincipalInfo;
+};
+
+union LSRequestParams
+{
+  LSRequestPreloadDatastoreParams;
+  LSRequestPrepareDatastoreParams;
+  LSRequestPrepareObserverParams;
+};
+
+struct LSSimpleRequestPreloadedParams
+{
+  PrincipalInfo principalInfo;
+  PrincipalInfo storagePrincipalInfo;
+};
+
+struct LSSimpleRequestGetStateParams
+{
+  PrincipalInfo principalInfo;
+  PrincipalInfo storagePrincipalInfo;
+};
+
+union LSSimpleRequestParams
+{
+  LSSimpleRequestPreloadedParams;
+  LSSimpleRequestGetStateParams;
+};
+
+/**
+ * LocalStorage key/value pair wire representations.  `value` may be void in
+ * cases where there is a value but it is not being sent for memory/bandwidth
+ * conservation purposes.  (It's not possible to have a null/undefined `value`
+ * as Storage is defined explicitly as a String store.)
+ */
+struct LSItemInfo
+{
+  nsString key;
+  LSValue value;
+};
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/localstorage/PBackgroundLSSimpleRequest.ipdl b/dom/localstorage/PBackgroundLSSimpleRequest.ipdl
new file mode 100644
index 0000000000..9a6a22d836
--- /dev/null
+++ b/dom/localstorage/PBackgroundLSSimpleRequest.ipdl
@@ -0,0 +1,59 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+include protocol PBackground;
+
+include PBackgroundLSSharedTypes;
+
+namespace mozilla {
+namespace dom {
+
+/**
+ * Response to a `LSSimpleRequestPreloadedParams` request indicating whether the
+ * origin was preloaded.
+ */
+struct LSSimpleRequestPreloadedResponse
+{
+  bool preloaded;
+};
+
+struct LSSimpleRequestGetStateResponse
+{
+  LSItemInfo[] itemInfos;
+};
+
+/**
+ * Discriminated union which can contain an error code (`nsresult`) or
+ * particular simple request response.
+ */
+union LSSimpleRequestResponse
+{
+  nsresult;
+  LSSimpleRequestPreloadedResponse;
+  LSSimpleRequestGetStateResponse;
+};
+
+/**
+ * Simple requests are async-only from both a protocol perspective and the
+ * manner in which they're used.  In comparison, PBackgroundLSRequests are
+ * async only from a protocol perspective; they are used synchronously from the
+ * main thread via LSObject's RequestHelper mechanism.  (With the caveat that
+ * nsILocalStorageManager does expose LSRequests asynchronously.)
+ *
+ * These requests use the common idiom where the arguments to the request are
+ * sent in the constructor and the result is sent in the __delete__ response.
+ * Request types are indicated by the Params variant used and those live in
+ * `PBackgroundLSSharedTypes.ipdlh`.
+ */
+[ManualDealloc, ChildImpl=virtual, ParentImpl=virtual]
+protocol PBackgroundLSSimpleRequest
+{
+  manager PBackground;
+
+child:
+  async __delete__(LSSimpleRequestResponse response);
+};
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/localstorage/PBackgroundLSSnapshot.ipdl b/dom/localstorage/PBackgroundLSSnapshot.ipdl
new file mode 100644
index 0000000000..45c744c840
--- /dev/null
+++ b/dom/localstorage/PBackgroundLSSnapshot.ipdl
@@ -0,0 +1,153 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+include protocol PBackground;
+include protocol PBackgroundLSDatabase;
+
+include PBackgroundLSSharedTypes;
+
+include "mozilla/dom/localstorage/SerializationHelpers.h";
+
+using mozilla::dom::LSValue
+  from "mozilla/dom/LSValue.h";
+
+namespace mozilla {
+namespace dom {
+
+struct LSSetItemInfo
+{
+  nsString key;
+  LSValue value;
+};
+
+struct LSRemoveItemInfo
+{
+  nsString key;
+};
+
+struct LSClearInfo
+{
+};
+
+/**
+ * Union of LocalStorage mutation types.
+ */
+union LSWriteInfo
+{
+  LSSetItemInfo;
+  LSRemoveItemInfo;
+  LSClearInfo;
+};
+
+struct LSSetItemAndNotifyInfo
+{
+  nsString key;
+  LSValue oldValue;
+  LSValue value;
+};
+
+struct LSRemoveItemAndNotifyInfo
+{
+  nsString key;
+  LSValue oldValue;
+};
+
+/**
+ * Union of LocalStorage mutation types.
+ */
+union LSWriteAndNotifyInfo
+{
+  LSSetItemAndNotifyInfo;
+  LSRemoveItemAndNotifyInfo;
+  LSClearInfo;
+};
+
+[ManualDealloc, ChildImpl=virtual, ParentImpl=virtual]
+sync protocol PBackgroundLSSnapshot
+{
+  manager PBackgroundLSDatabase;
+
+parent:
+  async DeleteMe();
+
+  async AsyncCheckpoint(LSWriteInfo[] writeInfos);
+
+  async AsyncCheckpointAndNotify(LSWriteAndNotifyInfo[] writeAndNotifyInfos);
+
+  // A synchronous checkpoint. This should only be used by the snapshotting code
+  // to checkpoint an explicit snapshot.
+  sync SyncCheckpoint(LSWriteInfo[] writeInfos);
+
+  // A synchronous checkpoint and notify. This should only be used by the
+  // snapshotting code to checkpoint and notify an explicit snapshot.
+  sync SyncCheckpointAndNotify(LSWriteAndNotifyInfo[] writeAndNotifyInfos);
+
+  async AsyncFinish();
+
+  // A synchronous finish. This should only be used by the snapshotting code to
+  // end an explicit snapshot.
+  sync SyncFinish();
+
+  async Loaded();
+
+  /**
+   * Invoked on demand to load an item that didn't fit into the initial
+   * snapshot prefill and also some additional key/value pairs to lower down
+   * the need to use this synchronous message again.
+   *
+   * This needs to be synchronous because LocalStorage's semantics are
+   * synchronous.  Note that the Snapshot in the PBackground parent already
+   * has the answers to this request immediately available without needing to
+   * consult any other threads or perform any I/O.
+   */
+  sync LoadValueAndMoreItems(nsString key)
+    returns (LSValue value, LSItemInfo[] itemInfos);
+
+  /**
+   * Invoked on demand to load all keys in in their canonical order if they
+   * didn't fit into the initial snapshot prefill.
+   *
+   * This needs to be synchronous because LocalStorage's semantics are
+   * synchronous.  Note that the Snapshot in the PBackground parent already
+   * has the answers to this request immediately available without needing to
+   * consult any other threads or perform any I/O.
+   */
+  sync LoadKeys()
+    returns (nsString[] keys);
+
+  /**
+   * This needs to be synchronous because LocalStorage's semantics are
+   * synchronous.  Note that the Snapshot in the PBackground parent typically
+   * doesn't need to consult any other threads or perform any I/O to handle
+   * this request.  However, it has to call a quota manager method that can
+   * potentially do I/O directly on the PBackground thread.  It can only happen
+   * rarely in a storage pressure (low storage space) situation.  Specifically,
+   * after we get a list of origin directories for eviction, we will delete
+   * them directly on the PBackground thread.  This doesn't cause any
+   * performance problems, but avoiding I/O completely might need to be done as
+   * a futher optimization.
+   */
+  sync IncreasePeakUsage(int64_t minSize)
+    returns (int64_t size);
+
+child:
+  /**
+   * Compels the child LSSnapshot to Checkpoint() and Finish(), effectively
+   * compelling the snapshot to flush any issued mutations and close itself.
+   * The child LSSnapshot does that either immediately if it's just waiting
+   * to be reused or when it gets into a stable state.
+   *
+   * This message is expected to be sent in the following two cases only:
+   *   1. The state of the underlying Datastore starts to differ from the state
+   *      captured at the time of snapshot creation.
+   *   2. The last private browsing context exits.  And in that case we expect
+   *      all private browsing globals to already have been destroyed.
+   */
+  async MarkDirty();
+
+  async __delete__();
+};
+
+} // namespace dom
+} // namespace mozilla
diff --git a/dom/localstorage/ReportInternalError.cpp b/dom/localstorage/ReportInternalError.cpp
new file mode 100644
index 0000000000..88fa4fd6e2
--- /dev/null
+++ b/dom/localstorage/ReportInternalError.cpp
@@ -0,0 +1,31 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ReportInternalError.h"
+
+#include <cinttypes>
+#include "nsContentUtils.h"
+#include "nsPrintfCString.h"
+#include "nsString.h"
+
+namespace mozilla::dom::localstorage {
+
+void ReportInternalError(const char* aFile, uint32_t aLine, const char* aStr) {
+  // Get leaf of file path
+  for (const char* p = aFile; *p; ++p) {
+    if (*p == '/' && *(p + 1)) {
+      aFile = p + 1;
+    }
+  }
+
+  nsContentUtils::LogSimpleConsoleError(
+      NS_ConvertUTF8toUTF16(
+          nsPrintfCString("LocalStorage %s: %s:%" PRIu32, aStr, aFile, aLine)),
+      "localstorage"_ns, false,
+      true /* Internal errors are chrome context only*/);
+}
+
+}  // namespace mozilla::dom::localstorage
diff --git a/dom/localstorage/ReportInternalError.h b/dom/localstorage/ReportInternalError.h
new file mode 100644
index 0000000000..9cfd1cd72a
--- /dev/null
+++ b/dom/localstorage/ReportInternalError.h
@@ -0,0 +1,29 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_ReportInternalError_h
+#define mozilla_dom_localstorage_ReportInternalError_h
+
+#include <cstdint>
+#include "mozilla/Attributes.h"
+#include "nsDebug.h"
+
+#define LS_WARNING(...)                                                 \
+  do {                                                                  \
+    nsPrintfCString s(__VA_ARGS__);                                     \
+    mozilla::dom::localstorage::ReportInternalError(__FILE__, __LINE__, \
+                                                    s.get());           \
+    NS_WARNING(s.get());                                                \
+  } while (0)
+
+namespace mozilla::dom::localstorage {
+
+MOZ_COLD void ReportInternalError(const char* aFile, uint32_t aLine,
+                                  const char* aStr);
+
+}  // namespace mozilla::dom::localstorage
+
+#endif  // mozilla_dom_localstorage_ReportInternalError_h
diff --git a/dom/localstorage/SerializationHelpers.h b/dom/localstorage/SerializationHelpers.h
new file mode 100644
index 0000000000..f9b499671c
--- /dev/null
+++ b/dom/localstorage/SerializationHelpers.h
@@ -0,0 +1,62 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_SerializationHelpers_h
+#define mozilla_dom_localstorage_SerializationHelpers_h
+
+#include <string>
+#include "chrome/common/ipc_message_utils.h"
+#include "ipc/EnumSerializer.h"
+#include "mozilla/dom/LSSnapshot.h"
+#include "mozilla/dom/LSValue.h"
+
+namespace IPC {
+
+template <>
+struct ParamTraits<mozilla::dom::LSSnapshot::LoadState>
+    : public ContiguousEnumSerializer<
+          mozilla::dom::LSSnapshot::LoadState,
+          mozilla::dom::LSSnapshot::LoadState::Initial,
+          mozilla::dom::LSSnapshot::LoadState::EndGuard> {};
+
+template <>
+struct ParamTraits<mozilla::dom::LSValue::CompressionType>
+    : public ContiguousEnumSerializer<
+          mozilla::dom::LSValue::CompressionType,
+          mozilla::dom::LSValue::CompressionType::UNCOMPRESSED,
+          mozilla::dom::LSValue::CompressionType::NUM_TYPES> {};
+
+static_assert(
+    0u == static_cast<uint8_t>(mozilla::dom::LSValue::ConversionType::NONE));
+template <>
+struct ParamTraits<mozilla::dom::LSValue::ConversionType>
+    : public ContiguousEnumSerializer<
+          mozilla::dom::LSValue::ConversionType,
+          mozilla::dom::LSValue::ConversionType::NONE,
+          mozilla::dom::LSValue::ConversionType::NUM_TYPES> {};
+
+template <>
+struct ParamTraits<mozilla::dom::LSValue> {
+  typedef mozilla::dom::LSValue paramType;
+
+  static void Write(MessageWriter* aWriter, const paramType& aParam) {
+    WriteParam(aWriter, aParam.mBuffer);
+    WriteParam(aWriter, aParam.mUTF16Length);
+    WriteParam(aWriter, aParam.mConversionType);
+    WriteParam(aWriter, aParam.mCompressionType);
+  }
+
+  static bool Read(MessageReader* aReader, paramType* aResult) {
+    return ReadParam(aReader, &aResult->mBuffer) &&
+           ReadParam(aReader, &aResult->mUTF16Length) &&
+           ReadParam(aReader, &aResult->mConversionType) &&
+           ReadParam(aReader, &aResult->mCompressionType);
+  }
+};
+
+}  // namespace IPC
+
+#endif  // mozilla_dom_localstorage_SerializationHelpers_h
diff --git a/dom/localstorage/SnappyUtils.cpp b/dom/localstorage/SnappyUtils.cpp
new file mode 100644
index 0000000000..cf47421bc5
--- /dev/null
+++ b/dom/localstorage/SnappyUtils.cpp
@@ -0,0 +1,80 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "SnappyUtils.h"
+
+#include <stddef.h>
+#include "mozilla/Assertions.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/fallible.h"
+#include "nsDebug.h"
+#include "nsString.h"
+#include "snappy/snappy.h"
+
+namespace mozilla::dom {
+
+static_assert(SNAPPY_VERSION == 0x010109);
+
+bool SnappyCompress(const nsACString& aSource, nsACString& aDest) {
+  MOZ_ASSERT(!aSource.IsVoid());
+
+  size_t uncompressedLength = aSource.Length();
+
+  if (uncompressedLength <= 16) {
+    aDest.SetIsVoid(true);
+    return true;
+  }
+
+  size_t compressedLength = snappy::MaxCompressedLength(uncompressedLength);
+
+  if (NS_WARN_IF(!aDest.SetLength(compressedLength, fallible))) {
+    return false;
+  }
+
+  snappy::RawCompress(aSource.BeginReading(), uncompressedLength,
+                      aDest.BeginWriting(), &compressedLength);
+
+  if (compressedLength >= uncompressedLength) {
+    aDest.SetIsVoid(true);
+    return true;
+  }
+
+  if (NS_WARN_IF(!aDest.SetLength(compressedLength, fallible))) {
+    return false;
+  }
+
+  return true;
+}
+
+bool SnappyUncompress(const nsACString& aSource, nsACString& aDest) {
+  MOZ_ASSERT(!aSource.IsVoid());
+
+  const char* compressed = aSource.BeginReading();
+
+  auto compressedLength = static_cast<size_t>(aSource.Length());
+
+  size_t uncompressedLength = 0u;
+  if (!snappy::GetUncompressedLength(compressed, compressedLength,
+                                     &uncompressedLength)) {
+    return false;
+  }
+
+  CheckedUint32 checkedLength(uncompressedLength);
+  if (!checkedLength.isValid()) {
+    return false;
+  }
+
+  aDest.SetLength(checkedLength.value());
+
+  if (!snappy::RawUncompress(compressed, compressedLength,
+                             aDest.BeginWriting())) {
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/localstorage/SnappyUtils.h b/dom/localstorage/SnappyUtils.h
new file mode 100644
index 0000000000..45c09a9465
--- /dev/null
+++ b/dom/localstorage/SnappyUtils.h
@@ -0,0 +1,20 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_SnappyUtils_h
+#define mozilla_dom_localstorage_SnappyUtils_h
+
+#include "nsStringFwd.h"
+
+namespace mozilla::dom {
+
+bool SnappyCompress(const nsACString& aSource, nsACString& aDest);
+
+bool SnappyUncompress(const nsACString& aSource, nsACString& aDest);
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_localstorage_SnappyUtils_h
diff --git a/dom/localstorage/moz.build b/dom/localstorage/moz.build
new file mode 100644
index 0000000000..a7f5e0b359
--- /dev/null
+++ b/dom/localstorage/moz.build
@@ -0,0 +1,75 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+with Files("**"):
+    BUG_COMPONENT = ("Core", "Storage: localStorage & sessionStorage")
+
+MOCHITEST_MANIFESTS += ["test/mochitest.ini"]
+
+BROWSER_CHROME_MANIFESTS += ["test/browser.ini"]
+
+XPCSHELL_TESTS_MANIFESTS += ["test/unit/xpcshell.ini"]
+
+TEST_HARNESS_FILES.xpcshell.dom.localstorage.test.unit += [
+    "test/unit/databaseShadowing-shared.js",
+]
+
+TEST_DIRS += ["test/gtest"]
+
+XPIDL_SOURCES += [
+    "nsILocalStorageManager.idl",
+]
+
+XPIDL_MODULE = "dom_localstorage"
+
+EXPORTS.mozilla.dom.localstorage += [
+    "ActorsParent.h",
+    "SerializationHelpers.h",
+]
+
+EXPORTS.mozilla.dom += [
+    "LocalStorageCommon.h",
+    "LocalStorageManager2.h",
+    "LSObject.h",
+    "LSObserver.h",
+    "LSSnapshot.h",
+    "LSValue.h",
+    "LSWriteOptimizer.h",
+    "LSWriteOptimizerImpl.h",
+    "SnappyUtils.h",
+]
+
+UNIFIED_SOURCES += [
+    "ActorsChild.cpp",
+    "ActorsParent.cpp",
+    "LocalStorageCommon.cpp",
+    "LocalStorageManager2.cpp",
+    "LSDatabase.cpp",
+    "LSObject.cpp",
+    "LSObserver.cpp",
+    "LSSnapshot.cpp",
+    "LSValue.cpp",
+    "LSWriteOptimizer.cpp",
+    "ReportInternalError.cpp",
+    "SnappyUtils.cpp",
+]
+
+IPDL_SOURCES += [
+    "PBackgroundLSDatabase.ipdl",
+    "PBackgroundLSObserver.ipdl",
+    "PBackgroundLSRequest.ipdl",
+    "PBackgroundLSSharedTypes.ipdlh",
+    "PBackgroundLSSimpleRequest.ipdl",
+    "PBackgroundLSSnapshot.ipdl",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
+
+LOCAL_INCLUDES += [
+    "/dom/file/ipc",
+]
diff --git a/dom/localstorage/nsILocalStorageManager.idl b/dom/localstorage/nsILocalStorageManager.idl
new file mode 100644
index 0000000000..0e8db22482
--- /dev/null
+++ b/dom/localstorage/nsILocalStorageManager.idl
@@ -0,0 +1,38 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsIPrincipal;
+
+/**
+ * Methods specific to LocalStorage, see nsIDOMStorageManager for methods shared
+ * with SessionStorage.  Methods may migrate there as SessionStorage is
+ * overhauled.
+ */
+[scriptable, builtinclass, uuid(d4f534da-2744-4db3-8774-8b187c64ade9)]
+interface nsILocalStorageManager : nsISupports
+{
+  readonly attribute boolean nextGenLocalStorageEnabled;
+
+  /**
+   * Trigger preload of LocalStorage for the given principal.  For use by
+   * ContentParent::AboutToLoadHttpFtpDocumentForChild to maximize the
+   * amount of time we have to load the data off disk before the page might
+   * attempt to touch LocalStorage.
+   *
+   * This method will not create a QuotaManager-managed directory on disk if
+   * one does not already exist for the principal.
+   */
+  [implicit_jscontext]
+  Promise preload(in nsIPrincipal aPrincipal);
+
+  [implicit_jscontext]
+  Promise isPreloaded(in nsIPrincipal aPrincipal);
+
+  [implicit_jscontext]
+  Promise getState(in nsIPrincipal aPrincipal);
+};
diff --git a/dom/localstorage/test/browser.ini b/dom/localstorage/test/browser.ini
new file mode 100644
index 0000000000..8119342ecb
--- /dev/null
+++ b/dom/localstorage/test/browser.ini
@@ -0,0 +1,6 @@
+[DEFAULT]
+skip-if = (buildapp != "browser")
+support-files =
+  page_private_ls.html
+
+[browser_private_ls.js]
diff --git a/dom/localstorage/test/browser_private_ls.js b/dom/localstorage/test/browser_private_ls.js
new file mode 100644
index 0000000000..5b375c90e1
--- /dev/null
+++ b/dom/localstorage/test/browser_private_ls.js
@@ -0,0 +1,44 @@
+/**
+ * This test is mainly to verify that datastores are cleared when the last
+ * private browsing context exited.
+ */
+
+async function lsCheckFunc() {
+  let storage = content.localStorage;
+
+  if (storage.length) {
+    return false;
+  }
+
+  // Store non-ASCII value to verify bug 1552428.
+  storage.setItem("foo", "úžasné");
+
+  return true;
+}
+
+function checkTabWindowLS(tab) {
+  return SpecialPowers.spawn(tab.linkedBrowser, [], lsCheckFunc);
+}
+
+add_task(async function () {
+  const pageUrl =
+    "http://example.com/browser/dom/localstorage/test/page_private_ls.html";
+
+  for (let i = 0; i < 2; i++) {
+    let privateWin = await BrowserTestUtils.openNewBrowserWindow({
+      private: true,
+    });
+
+    let privateTab = await BrowserTestUtils.openNewForegroundTab(
+      privateWin.gBrowser,
+      pageUrl
+    );
+
+    ok(
+      await checkTabWindowLS(privateTab),
+      "LS works correctly in a private-browsing page."
+    );
+
+    await BrowserTestUtils.closeWindow(privateWin);
+  }
+});
diff --git a/dom/localstorage/test/gtest/TestLocalStorage.cpp b/dom/localstorage/test/gtest/TestLocalStorage.cpp
new file mode 100644
index 0000000000..6c5621f248
--- /dev/null
+++ b/dom/localstorage/test/gtest/TestLocalStorage.cpp
@@ -0,0 +1,140 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "gtest/gtest.h"
+#include "mozilla/NullPrincipal.h"
+#include "mozilla/dom/LocalStorageCommon.h"
+#include "mozilla/dom/StorageUtils.h"
+#include "mozilla/ipc/BackgroundUtils.h"
+#include "mozilla/ipc/PBackgroundSharedTypes.h"
+#include "nsCOMPtr.h"
+#include "nsContentUtils.h"
+#include "nsIURI.h"
+#include "nsNetUtil.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+using namespace mozilla::dom::StorageUtils;
+using namespace mozilla::ipc;
+
+namespace {
+
+struct OriginKeyTest {
+  const char* mSpec;
+  const char* mOriginKey;
+  const char* mQuotaKey;
+};
+
+already_AddRefed<nsIPrincipal> GetContentPrincipal(const char* aSpec) {
+  nsCOMPtr<nsIURI> uri;
+  nsresult rv = NS_NewURI(getter_AddRefs(uri), nsDependentCString(aSpec));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return nullptr;
+  }
+
+  OriginAttributes attrs;
+
+  nsCOMPtr<nsIPrincipal> principal =
+      BasePrincipal::CreateContentPrincipal(uri, attrs);
+
+  return principal.forget();
+}
+
+void CheckGeneratedOriginKey(nsIPrincipal* aPrincipal, const char* aOriginKey,
+                             const char* aQuotaKey) {
+  nsCString originAttrSuffix;
+  nsCString originKey;
+  nsCString quotaKey;
+
+  aPrincipal->OriginAttributesRef().CreateSuffix(originAttrSuffix);
+
+  nsresult rv = aPrincipal->GetStorageOriginKey(originKey);
+  if (aOriginKey) {
+    ASSERT_EQ(rv, NS_OK) << "GetStorageOriginKey should not fail";
+    EXPECT_TRUE(originKey == nsDependentCString(aOriginKey));
+  } else {
+    ASSERT_NE(rv, NS_OK) << "GetStorageOriginKey should fail";
+  }
+
+  rv = aPrincipal->GetLocalStorageQuotaKey(quotaKey);
+  if (aQuotaKey) {
+    ASSERT_EQ(rv, NS_OK) << "GetLocalStorageQuotaKey should not fail";
+    EXPECT_TRUE(quotaKey == nsDependentCString(aQuotaKey));
+  } else {
+    ASSERT_NE(rv, NS_OK) << "GetLocalStorageQuotaKey should fail";
+  }
+
+  PrincipalInfo principalInfo;
+  rv = PrincipalToPrincipalInfo(aPrincipal, &principalInfo);
+  ASSERT_EQ(rv, NS_OK) << "PrincipalToPrincipalInfo should not fail";
+
+  const auto res = GenerateOriginKey2(principalInfo);
+  if (aOriginKey) {
+    ASSERT_TRUE(res.isOk())
+    << "GenerateOriginKey2 should not fail";
+    EXPECT_TRUE(res.inspect().second == nsDependentCString(aOriginKey));
+  } else {
+    ASSERT_TRUE(res.isErr())
+    << "GenerateOriginKey2 should fail";
+  }
+}
+
+}  // namespace
+
+TEST(LocalStorage, OriginKey)
+{
+  // Check the system principal.
+  nsCOMPtr<nsIScriptSecurityManager> secMan =
+      nsContentUtils::GetSecurityManager();
+  ASSERT_TRUE(secMan)
+  << "GetSecurityManager() should not fail";
+
+  nsCOMPtr<nsIPrincipal> principal;
+  secMan->GetSystemPrincipal(getter_AddRefs(principal));
+  ASSERT_TRUE(principal)
+  << "GetSystemPrincipal() should not fail";
+
+  CheckGeneratedOriginKey(principal, nullptr, nullptr);
+
+  // Check the null principal.
+  principal = NullPrincipal::CreateWithoutOriginAttributes();
+  ASSERT_TRUE(principal)
+  << "CreateWithoutOriginAttributes() should not fail";
+
+  CheckGeneratedOriginKey(principal, nullptr, nullptr);
+
+  // Check content principals.
+  static const OriginKeyTest tests[] = {
+      {"http://localhost", "tsohlacol.:http:80", ":tsohlacol."},
+      {"http://www.mozilla.org", "gro.allizom.www.:http:80", ":gro.allizom."},
+      {"https://www.mozilla.org", "gro.allizom.www.:https:443",
+       ":gro.allizom."},
+      {"http://www.mozilla.org:32400", "gro.allizom.www.:http:32400",
+       ":gro.allizom."},
+      {"file:///Users/Joe/Sites/", "/setiS/eoJ/sresU/.:file",
+       ":/setiS/eoJ/sresU/."},
+      {"file:///Users/Joe/Sites/#foo", "/setiS/eoJ/sresU/.:file",
+       ":/setiS/eoJ/sresU/."},
+      {"file:///Users/Joe/Sites/?foo", "/setiS/eoJ/sresU/.:file",
+       ":/setiS/eoJ/sresU/."},
+      {"file:///Users/Joe/Sites", "/eoJ/sresU/.:file", ":/eoJ/sresU/."},
+      {"file:///Users/Joe/Sites#foo", "/eoJ/sresU/.:file", ":/eoJ/sresU/."},
+      {"file:///Users/Joe/Sites?foo", "/eoJ/sresU/.:file", ":/eoJ/sresU/."},
+      {"moz-extension://53711a8f-65ed-e742-9671-1f02e267c0bc/"
+       "_generated_background_page.html",
+       "cb0c762e20f1-1769-247e-de56-f8a11735.:moz-extension",
+       ":cb0c762e20f1-1769-247e-de56-f8a11735."},
+      {"http://[::1]:8/test.html", "1::.:http:8", ":1::."},
+  };
+
+  for (const auto& test : tests) {
+    principal = GetContentPrincipal(test.mSpec);
+    ASSERT_TRUE(principal)
+    << "GetContentPrincipal() should not fail";
+
+    CheckGeneratedOriginKey(principal, test.mOriginKey, test.mQuotaKey);
+  }
+}
diff --git a/dom/localstorage/test/gtest/moz.build b/dom/localstorage/test/gtest/moz.build
new file mode 100644
index 0000000000..f9d3c7bb0b
--- /dev/null
+++ b/dom/localstorage/test/gtest/moz.build
@@ -0,0 +1,17 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, you can obtain one at http://mozilla.org/MPL/2.0/.
+
+UNIFIED_SOURCES = [
+    "TestLocalStorage.cpp",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul-gtest"
+
+LOCAL_INCLUDES += [
+    "/dom/localstorage",
+]
diff --git a/dom/localstorage/test/helpers.js b/dom/localstorage/test/helpers.js
new file mode 100644
index 0000000000..f9dbdd21dc
--- /dev/null
+++ b/dom/localstorage/test/helpers.js
@@ -0,0 +1,78 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+// testSteps is expected to be defined by the test using this file.
+/* global testSteps:false */
+
+function executeSoon(aFun) {
+  SpecialPowers.Services.tm.dispatchToMainThread({
+    run() {
+      aFun();
+    },
+  });
+}
+
+function clearAllDatabases() {
+  let qms = SpecialPowers.Services.qms;
+  let principal = SpecialPowers.wrap(document).nodePrincipal;
+  let request = qms.clearStoragesForPrincipal(principal);
+  return request;
+}
+
+if (!window.runTest) {
+  window.runTest = async function () {
+    SimpleTest.waitForExplicitFinish();
+
+    info("Pushing preferences");
+
+    await SpecialPowers.pushPrefEnv({
+      set: [
+        ["dom.storage.testing", true],
+        ["dom.quotaManager.testing", true],
+      ],
+    });
+
+    info("Clearing old databases");
+
+    await requestFinished(clearAllDatabases());
+
+    SimpleTest.registerCleanupFunction(async function () {
+      await requestFinished(clearAllDatabases());
+    });
+  };
+}
+
+function returnToEventLoop() {
+  return new Promise(function (resolve) {
+    executeSoon(resolve);
+  });
+}
+
+function getLocalStorage() {
+  return localStorage;
+}
+
+class RequestError extends Error {
+  constructor(resultCode, resultName) {
+    super(`Request failed (code: ${resultCode}, name: ${resultName})`);
+    this.name = "RequestError";
+    this.resultCode = resultCode;
+    this.resultName = resultName;
+  }
+}
+
+async function requestFinished(request) {
+  await new Promise(function (resolve) {
+    request.callback = SpecialPowers.wrapCallback(function () {
+      resolve();
+    });
+  });
+
+  if (request.resultCode !== SpecialPowers.Cr.NS_OK) {
+    throw new RequestError(request.resultCode, request.resultName);
+  }
+
+  return request.result;
+}
diff --git a/dom/localstorage/test/mochitest.ini b/dom/localstorage/test/mochitest.ini
new file mode 100644
index 0000000000..500bdd0a1c
--- /dev/null
+++ b/dom/localstorage/test/mochitest.ini
@@ -0,0 +1,10 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+[DEFAULT]
+support-files =
+  helpers.js
+  unit/test_largeItems.js
+
+[test_largeItems.html]
diff --git a/dom/localstorage/test/page_private_ls.html b/dom/localstorage/test/page_private_ls.html
new file mode 100644
index 0000000000..795a814981
--- /dev/null
+++ b/dom/localstorage/test/page_private_ls.html
@@ -0,0 +1,9 @@
+<!doctype html>
+<html>
+<head>
+  <meta charset="utf-8">
+</head>
+<body>
+  All the interesting stuff happens in ContentTask.spawn() calls.
+</body>
+</html>
diff --git a/dom/localstorage/test/test_largeItems.html b/dom/localstorage/test/test_largeItems.html
new file mode 100644
index 0000000000..92316085fc
--- /dev/null
+++ b/dom/localstorage/test/test_largeItems.html
@@ -0,0 +1,19 @@
+<!--
+  Any copyright is dedicated to the Public Domain.
+  http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<html>
+<head>
+  <title>Large Items Test</title>
+
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+
+  <script type="text/javascript" src="unit/test_largeItems.js"></script>
+  <script type="text/javascript" src="helpers.js"></script>
+
+</head>
+
+<body onload="runTest();"></body>
+
+</html>
diff --git a/dom/localstorage/test/unit/archive_profile.zip b/dom/localstorage/test/unit/archive_profile.zip
new file mode 100644
index 0000000000..71b2d1e5f9
--- /dev/null
+++ b/dom/localstorage/test/unit/archive_profile.zip
diff --git a/dom/localstorage/test/unit/corruptedDatabase_missingUsageFile_profile.zip b/dom/localstorage/test/unit/corruptedDatabase_missingUsageFile_profile.zip
new file mode 100644
index 0000000000..8cfa6e3d43
--- /dev/null
+++ b/dom/localstorage/test/unit/corruptedDatabase_missingUsageFile_profile.zip
diff --git a/dom/localstorage/test/unit/corruptedDatabase_profile.zip b/dom/localstorage/test/unit/corruptedDatabase_profile.zip
new file mode 100644
index 0000000000..2f60db2a45
--- /dev/null
+++ b/dom/localstorage/test/unit/corruptedDatabase_profile.zip
diff --git a/dom/localstorage/test/unit/databaseShadowing-shared.js b/dom/localstorage/test/unit/databaseShadowing-shared.js
new file mode 100644
index 0000000000..ffee8579cb
--- /dev/null
+++ b/dom/localstorage/test/unit/databaseShadowing-shared.js
@@ -0,0 +1,130 @@
+/* import-globals-from head.js */
+
+const principalInfos = [
+  { url: "http://example.com", attrs: {} },
+
+  { url: "http://origin.test", attrs: {} },
+
+  { url: "http://prefix.test", attrs: {} },
+  { url: "http://prefix.test", attrs: { userContextId: 10 } },
+
+  { url: "http://pattern.test", attrs: { userContextId: 15 } },
+  { url: "http://pattern.test:8080", attrs: { userContextId: 15 } },
+  { url: "https://pattern.test", attrs: { userContextId: 15 } },
+];
+
+const surrogate = String.fromCharCode(0xdc00);
+const replacement = String.fromCharCode(0xfffd);
+const beginning = "beginning";
+const ending = "ending";
+const complexValue = beginning + surrogate + surrogate + ending;
+const corruptedValue = beginning + replacement + replacement + ending;
+
+function enableNextGenLocalStorage() {
+  info("Setting pref");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+}
+
+function disableNextGenLocalStorage() {
+  info("Setting pref");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    true
+  );
+}
+
+function storeData() {
+  for (let i = 0; i < principalInfos.length; i++) {
+    let principalInfo = principalInfos[i];
+    let principal = getPrincipal(principalInfo.url, principalInfo.attrs);
+
+    info("Getting storage");
+
+    let storage = getLocalStorage(principal);
+
+    info("Adding data");
+
+    storage.setItem("key0", "value0");
+    storage.clear();
+    storage.setItem("key1", "value1");
+    storage.removeItem("key1");
+    storage.setItem("key2", "value2");
+    storage.setItem("complexKey", complexValue);
+
+    info("Closing storage");
+
+    storage.close();
+  }
+}
+
+function exportShadowDatabase(name) {
+  info("Verifying shadow database");
+
+  let profileDir = Services.dirsvc.get("ProfD", Ci.nsIFile);
+  let shadowDatabase = profileDir.clone();
+  shadowDatabase.append("webappsstore.sqlite");
+
+  let exists = shadowDatabase.exists();
+  ok(exists, "Shadow database does exist");
+
+  info("Copying shadow database");
+
+  let currentDir = Services.dirsvc.get("CurWorkD", Ci.nsIFile);
+  shadowDatabase.copyTo(currentDir, name);
+}
+
+function importShadowDatabase(name) {
+  info("Verifying shadow database");
+
+  let currentDir = Services.dirsvc.get("CurWorkD", Ci.nsIFile);
+  let shadowDatabase = currentDir.clone();
+  shadowDatabase.append(name);
+
+  let exists = shadowDatabase.exists();
+  if (!exists) {
+    return false;
+  }
+
+  info("Copying shadow database");
+
+  let profileDir = Services.dirsvc.get("ProfD", Ci.nsIFile);
+  shadowDatabase.copyTo(profileDir, "webappsstore.sqlite");
+
+  return true;
+}
+
+function verifyData(clearedOrigins, migrated = false) {
+  for (let i = 0; i < principalInfos.length; i++) {
+    let principalInfo = principalInfos[i];
+    let principal = getPrincipal(principalInfo.url, principalInfo.attrs);
+
+    info("Getting storage");
+
+    let storage = getLocalStorage(principal);
+
+    info("Verifying data");
+
+    if (clearedOrigins.includes(i)) {
+      ok(storage.getItem("key2") == null, "Correct value");
+      ok(storage.getItem("complexKey") == null, "Correct value");
+    } else {
+      ok(storage.getItem("key0") == null, "Correct value");
+      ok(storage.getItem("key1") == null, "Correct value");
+      is(storage.getItem("key2"), "value2", "Correct value");
+      is(
+        storage.getItem("complexKey"),
+        migrated ? corruptedValue : complexValue,
+        "Correct value"
+      );
+    }
+
+    info("Closing storage");
+
+    storage.close();
+  }
+}
diff --git a/dom/localstorage/test/unit/groupMismatch_profile.zip b/dom/localstorage/test/unit/groupMismatch_profile.zip
new file mode 100644
index 0000000000..182b013de0
--- /dev/null
+++ b/dom/localstorage/test/unit/groupMismatch_profile.zip
diff --git a/dom/localstorage/test/unit/head.js b/dom/localstorage/test/unit/head.js
new file mode 100644
index 0000000000..8ddc42ac4f
--- /dev/null
+++ b/dom/localstorage/test/unit/head.js
@@ -0,0 +1,332 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+// Tests are expected to define testSteps.
+/* globals testSteps */
+
+const NS_ERROR_DOM_QUOTA_EXCEEDED_ERR = 22;
+
+function is(a, b, msg) {
+  Assert.equal(a, b, msg);
+}
+
+function ok(cond, msg) {
+  Assert.ok(!!cond, msg);
+}
+
+add_setup(function () {
+  do_get_profile();
+
+  enableTesting();
+
+  Cu.importGlobalProperties(["crypto"]);
+
+  registerCleanupFunction(resetTesting);
+});
+
+function returnToEventLoop() {
+  return new Promise(function (resolve) {
+    executeSoon(resolve);
+  });
+}
+
+function enableTesting() {
+  Services.prefs.setBoolPref("dom.simpleDB.enabled", true);
+  Services.prefs.setBoolPref("dom.storage.testing", true);
+
+  // xpcshell globals don't have associated clients in the Clients API sense, so
+  // we need to disable client validation so that the unit tests are allowed to
+  // use LocalStorage.
+  Services.prefs.setBoolPref("dom.storage.client_validation", false);
+
+  Services.prefs.setBoolPref("dom.quotaManager.testing", true);
+}
+
+function resetTesting() {
+  Services.prefs.clearUserPref("dom.quotaManager.testing");
+  Services.prefs.clearUserPref("dom.storage.client_validation");
+  Services.prefs.clearUserPref("dom.storage.testing");
+  Services.prefs.clearUserPref("dom.simpleDB.enabled");
+}
+
+function setGlobalLimit(globalLimit) {
+  Services.prefs.setIntPref(
+    "dom.quotaManager.temporaryStorage.fixedLimit",
+    globalLimit
+  );
+}
+
+function resetGlobalLimit() {
+  Services.prefs.clearUserPref("dom.quotaManager.temporaryStorage.fixedLimit");
+}
+
+function setOriginLimit(originLimit) {
+  Services.prefs.setIntPref("dom.storage.default_quota", originLimit);
+}
+
+function resetOriginLimit() {
+  Services.prefs.clearUserPref("dom.storage.default_quota");
+}
+
+function setTimeout(callback, timeout) {
+  let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+
+  timer.initWithCallback(
+    {
+      notify() {
+        callback();
+      },
+    },
+    timeout,
+    Ci.nsITimer.TYPE_ONE_SHOT
+  );
+
+  return timer;
+}
+
+function initStorage() {
+  return Services.qms.init();
+}
+
+function initTemporaryStorage() {
+  return Services.qms.initTemporaryStorage();
+}
+
+function initPersistentOrigin(principal) {
+  return Services.qms.initializePersistentOrigin(principal);
+}
+
+function initTemporaryOrigin(persistence, principal) {
+  return Services.qms.initializeTemporaryOrigin(persistence, principal);
+}
+
+function getOriginUsage(principal, fromMemory = false) {
+  let request = Services.qms.getUsageForPrincipal(
+    principal,
+    function () {},
+    fromMemory
+  );
+
+  return request;
+}
+
+function clear() {
+  let request = Services.qms.clear();
+
+  return request;
+}
+
+function clearOriginsByPattern(pattern) {
+  let request = Services.qms.clearStoragesForOriginAttributesPattern(pattern);
+
+  return request;
+}
+
+function clearOriginsByPrefix(principal, persistence) {
+  let request = Services.qms.clearStoragesForPrincipal(
+    principal,
+    persistence,
+    null,
+    true
+  );
+
+  return request;
+}
+
+function clearOrigin(principal, persistence) {
+  let request = Services.qms.clearStoragesForPrincipal(principal, persistence);
+
+  return request;
+}
+
+function reset() {
+  let request = Services.qms.reset();
+
+  return request;
+}
+
+function resetOrigin(principal) {
+  let request = Services.qms.resetStoragesForPrincipal(
+    principal,
+    "default",
+    "ls"
+  );
+
+  return request;
+}
+
+function installPackage(packageName) {
+  let currentDir = Services.dirsvc.get("CurWorkD", Ci.nsIFile);
+
+  let packageFile = currentDir.clone();
+  packageFile.append(packageName + ".zip");
+
+  let zipReader = Cc["@mozilla.org/libjar/zip-reader;1"].createInstance(
+    Ci.nsIZipReader
+  );
+  zipReader.open(packageFile);
+
+  let entryNames = [];
+  let entries = zipReader.findEntries(null);
+  while (entries.hasMore()) {
+    let entry = entries.getNext();
+    entryNames.push(entry);
+  }
+  entryNames.sort();
+
+  for (let entryName of entryNames) {
+    let zipentry = zipReader.getEntry(entryName);
+
+    let file = getRelativeFile(entryName);
+
+    if (zipentry.isDirectory) {
+      file.create(Ci.nsIFile.DIRECTORY_TYPE, parseInt("0755", 8));
+    } else {
+      let istream = zipReader.getInputStream(entryName);
+
+      var ostream = Cc[
+        "@mozilla.org/network/file-output-stream;1"
+      ].createInstance(Ci.nsIFileOutputStream);
+      ostream.init(file, -1, parseInt("0644", 8), 0);
+
+      let bostream = Cc[
+        "@mozilla.org/network/buffered-output-stream;1"
+      ].createInstance(Ci.nsIBufferedOutputStream);
+      bostream.init(ostream, 32768);
+
+      bostream.writeFrom(istream, istream.available());
+
+      istream.close();
+      bostream.close();
+    }
+  }
+
+  zipReader.close();
+}
+
+function getProfileDir() {
+  return Services.dirsvc.get("ProfD", Ci.nsIFile);
+}
+
+// Given a "/"-delimited path relative to the profile directory,
+// return an nsIFile representing the path.  This does not test
+// for the existence of the file or parent directories.
+// It is safe even on Windows where the directory separator is not "/",
+// but make sure you're not passing in a "\"-delimited path.
+function getRelativeFile(relativePath) {
+  let profileDir = getProfileDir();
+
+  let file = profileDir.clone();
+  relativePath.split("/").forEach(function (component) {
+    file.append(component);
+  });
+
+  return file;
+}
+
+function repeatChar(count, ch) {
+  if (count == 0) {
+    return "";
+  }
+
+  let result = ch;
+  let count2 = count / 2;
+
+  // Double the input until it is long enough.
+  while (result.length <= count2) {
+    result += result;
+  }
+
+  // Use substring to hit the precise length target without using extra memory.
+  return result + result.substring(0, count - result.length);
+}
+
+function getPrincipal(url, attrs) {
+  let uri = Services.io.newURI(url);
+  if (!attrs) {
+    attrs = {};
+  }
+  return Services.scriptSecurityManager.createContentPrincipal(uri, attrs);
+}
+
+function getCurrentPrincipal() {
+  return Cc["@mozilla.org/systemprincipal;1"].createInstance(Ci.nsIPrincipal);
+}
+
+function getDefaultPrincipal() {
+  return getPrincipal("http://example.com");
+}
+
+function getSimpleDatabase(principal, persistence) {
+  let connection = Cc["@mozilla.org/dom/sdb-connection;1"].createInstance(
+    Ci.nsISDBConnection
+  );
+
+  if (!principal) {
+    principal = getDefaultPrincipal();
+  }
+
+  connection.init(principal, persistence);
+
+  return connection;
+}
+
+function getLocalStorage(principal) {
+  if (!principal) {
+    principal = getDefaultPrincipal();
+  }
+
+  return Services.domStorageManager.createStorage(
+    null,
+    principal,
+    principal,
+    ""
+  );
+}
+
+class RequestError extends Error {
+  constructor(resultCode, resultName) {
+    super(`Request failed (code: ${resultCode}, name: ${resultName})`);
+    this.name = "RequestError";
+    this.resultCode = resultCode;
+    this.resultName = resultName;
+  }
+}
+
+async function requestFinished(request) {
+  await new Promise(function (resolve) {
+    request.callback = function () {
+      resolve();
+    };
+  });
+
+  if (request.resultCode !== Cr.NS_OK) {
+    throw new RequestError(request.resultCode, request.resultName);
+  }
+
+  return request.result;
+}
+
+function loadSubscript(path) {
+  let file = do_get_file(path, false);
+  let uri = Services.io.newFileURI(file);
+  Services.scriptloader.loadSubScript(uri.spec);
+}
+
+async function readUsageFromUsageFile(usageFile) {
+  let file = await File.createFromNsIFile(usageFile);
+
+  let buffer = await new Promise(resolve => {
+    let reader = new FileReader();
+    reader.onloadend = () => resolve(reader.result);
+    reader.readAsArrayBuffer(file);
+  });
+
+  // Manually getting the lower 32-bits because of the lack of support for
+  // 64-bit values currently from DataView/JS (other than the BigInt support
+  // that's currently behind a flag).
+  let view = new DataView(buffer, 8, 4);
+  return view.getUint32();
+}
diff --git a/dom/localstorage/test/unit/make_migration_emptyValue.js b/dom/localstorage/test/unit/make_migration_emptyValue.js
new file mode 100644
index 0000000000..12484ad7f3
--- /dev/null
+++ b/dom/localstorage/test/unit/make_migration_emptyValue.js
@@ -0,0 +1,23 @@
+/*
+  Any copyright is dedicated to the Public Domain.
+  http://creativecommons.org/publicdomain/zero/1.0/
+*/
+
+async function testSteps() {
+  const data = {
+    key: "foo",
+    value: "",
+  };
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref("dom.storage.next_gen", false);
+
+  info("Getting storage");
+
+  const storage = getLocalStorage();
+
+  info("Adding data");
+
+  storage.setItem(data.key, data.value);
+}
diff --git a/dom/localstorage/test/unit/migration_emptyValue_profile.zip b/dom/localstorage/test/unit/migration_emptyValue_profile.zip
new file mode 100644
index 0000000000..b829beae77
--- /dev/null
+++ b/dom/localstorage/test/unit/migration_emptyValue_profile.zip
diff --git a/dom/localstorage/test/unit/migration_profile.zip b/dom/localstorage/test/unit/migration_profile.zip
new file mode 100644
index 0000000000..19dc3d4805
--- /dev/null
+++ b/dom/localstorage/test/unit/migration_profile.zip
diff --git a/dom/localstorage/test/unit/schema3upgrade_profile.zip b/dom/localstorage/test/unit/schema3upgrade_profile.zip
new file mode 100644
index 0000000000..1ee9bfbf2e
--- /dev/null
+++ b/dom/localstorage/test/unit/schema3upgrade_profile.zip
diff --git a/dom/localstorage/test/unit/schema4upgrade_profile.zip b/dom/localstorage/test/unit/schema4upgrade_profile.zip
new file mode 100644
index 0000000000..ae8ba09606
--- /dev/null
+++ b/dom/localstorage/test/unit/schema4upgrade_profile.zip
diff --git a/dom/localstorage/test/unit/stringLength2_profile.zip b/dom/localstorage/test/unit/stringLength2_profile.zip
new file mode 100644
index 0000000000..de4d0fc3aa
--- /dev/null
+++ b/dom/localstorage/test/unit/stringLength2_profile.zip
diff --git a/dom/localstorage/test/unit/stringLength_profile.zip b/dom/localstorage/test/unit/stringLength_profile.zip
new file mode 100644
index 0000000000..6cac890860
--- /dev/null
+++ b/dom/localstorage/test/unit/stringLength_profile.zip
diff --git a/dom/localstorage/test/unit/test_archive.js b/dom/localstorage/test/unit/test_archive.js
new file mode 100644
index 0000000000..51af78db1b
--- /dev/null
+++ b/dom/localstorage/test/unit/test_archive.js
@@ -0,0 +1,78 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const lsArchiveFile = "storage/ls-archive.sqlite";
+
+  const principalInfo = {
+    url: "http://example.com",
+    attrs: {},
+  };
+
+  function checkStorage() {
+    let principal = getPrincipal(principalInfo.url, principalInfo.attrs);
+    let storage = getLocalStorage(principal);
+    try {
+      storage.open();
+      ok(true, "Did not throw");
+    } catch (ex) {
+      ok(false, "Should not have thrown");
+    }
+  }
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref("dom.storage.next_gen", true);
+
+  info("Sub test case 1 - Archive file is a directory.");
+
+  info("Clearing");
+
+  let request = clear();
+  await requestFinished(request);
+
+  let archiveFile = getRelativeFile(lsArchiveFile);
+
+  archiveFile.create(Ci.nsIFile.DIRECTORY_TYPE, parseInt("0755", 8));
+
+  checkStorage();
+
+  info("Sub test case 2 - Corrupted archive file.");
+
+  info("Clearing");
+
+  request = clear();
+  await requestFinished(request);
+
+  let ostream = Cc["@mozilla.org/network/file-output-stream;1"].createInstance(
+    Ci.nsIFileOutputStream
+  );
+  ostream.init(archiveFile, -1, parseInt("0644", 8), 0);
+  ostream.write("foobar", 6);
+  ostream.close();
+
+  checkStorage();
+
+  info("Sub test case 3 - Nonupdateable archive file.");
+
+  info("Clearing");
+
+  request = clear();
+  await requestFinished(request);
+
+  info("Installing package");
+
+  // The profile contains storage.sqlite and storage/ls-archive.sqlite
+  // storage/ls-archive.sqlite was taken from FF 54 to force an upgrade.
+  // There's just one record in the webappsstore2 table. The record was
+  // modified by renaming the origin attribute userContextId to userContextKey.
+  // This triggers an error during the upgrade.
+  installPackage("archive_profile");
+
+  let fileSize = archiveFile.fileSize;
+  ok(fileSize > 0, "archive file size is greater than zero");
+
+  checkStorage();
+});
diff --git a/dom/localstorage/test/unit/test_clientValidation.js b/dom/localstorage/test/unit/test_clientValidation.js
new file mode 100644
index 0000000000..26dc2bfdf7
--- /dev/null
+++ b/dom/localstorage/test/unit/test_clientValidation.js
@@ -0,0 +1,32 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/**
+ * Because this is an xpcshell global, it does not have an associated client id.
+ * We turn on client validation for LocalStorage and ensure that we don't have
+ * access to LocalStorage.
+ */
+add_task(async function testSteps() {
+  const principal = getPrincipal("http://example.com");
+
+  info("Setting prefs");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+  Services.prefs.setBoolPref("dom.storage.client_validation", true);
+
+  info("Getting storage");
+
+  try {
+    getLocalStorage(principal);
+    ok(false, "Should have thrown");
+  } catch (ex) {
+    ok(true, "Did throw");
+    is(ex.name, "NS_ERROR_FAILURE", "Threw right Exception");
+    is(ex.result, Cr.NS_ERROR_FAILURE, "Threw with right result");
+  }
+});
diff --git a/dom/localstorage/test/unit/test_corruptedDatabase.js b/dom/localstorage/test/unit/test_corruptedDatabase.js
new file mode 100644
index 0000000000..da73bb92f6
--- /dev/null
+++ b/dom/localstorage/test/unit/test_corruptedDatabase.js
@@ -0,0 +1,73 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+async function doTest(profile) {
+  info("Testing profile " + profile);
+
+  info("Clearing");
+
+  let request = clear();
+  await requestFinished(request);
+
+  info("Installing package");
+
+  installPackage(profile);
+
+  const principal = getPrincipal("http://example.org");
+
+  let storage = getLocalStorage(principal);
+
+  let length = storage.length;
+
+  ok(length === 0, "Correct length");
+
+  info("Resetting origin");
+
+  request = resetOrigin(principal);
+  await requestFinished(request);
+
+  info("Getting usage");
+
+  request = getOriginUsage(principal);
+  await requestFinished(request);
+
+  is(request.result.usage, 0, "Correct usage");
+}
+
+add_task(async function testSteps() {
+  info("Setting pref");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+
+  // XXX This should be refactored into separate sub test cases.
+
+  const profiles = [
+    // This profile contains one localStorage, all localStorage related files, a
+    // script for localStorage creation and the storage database:
+    // - storage/default/http+++example.org/ls
+    // - storage/ls-archive.sqlite
+    // - create_db.js
+    // - storage.sqlite
+    // - webappsstore.sqlite
+    // The file create_db.js in the package was run locally, specifically it was
+    // temporarily added to xpcshell.ini and then executed:
+    //   mach xpcshell-test --interactive dom/localstorage/test/unit/create_db.js
+    // Note: to make it become the profile in the test, additional manual steps
+    // are needed.
+    // 1. Manually change first 6 chars in data.sqlite to "foobar".
+    // 2. Remove the folder "storage/temporary".
+    "corruptedDatabase_profile",
+    // This profile is the same as corruptedDatabase_profile, except that the usage
+    // file (storage/default/http+++example.org/ls/usage) is missing.
+    "corruptedDatabase_missingUsageFile_profile",
+  ];
+
+  for (const profile of profiles) {
+    await doTest(profile);
+  }
+});
diff --git a/dom/localstorage/test/unit/test_databaseShadowing1.js b/dom/localstorage/test/unit/test_databaseShadowing1.js
new file mode 100644
index 0000000000..8582f434fc
--- /dev/null
+++ b/dom/localstorage/test/unit/test_databaseShadowing1.js
@@ -0,0 +1,23 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/* import-globals-from databaseShadowing-shared.js */
+loadSubscript("databaseShadowing-shared.js");
+
+add_task(async function testSteps() {
+  enableNextGenLocalStorage();
+
+  storeData();
+
+  verifyData([]);
+
+  // Wait for all database connections to close.
+  let request = reset();
+  await requestFinished(request);
+
+  exportShadowDatabase("shadowdb.sqlite");
+
+  // The shadow database is now prepared for test_databaseShadowing2.js
+});
diff --git a/dom/localstorage/test/unit/test_databaseShadowing2.js b/dom/localstorage/test/unit/test_databaseShadowing2.js
new file mode 100644
index 0000000000..f7b0ddb1a2
--- /dev/null
+++ b/dom/localstorage/test/unit/test_databaseShadowing2.js
@@ -0,0 +1,17 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/* import-globals-from databaseShadowing-shared.js */
+loadSubscript("databaseShadowing-shared.js");
+
+add_task(async function testSteps() {
+  // The shadow database was prepared in test_databaseShadowing1.js
+
+  disableNextGenLocalStorage();
+
+  ok(importShadowDatabase("shadowdb.sqlite"), "Import succeeded");
+
+  verifyData([], /* migrated */ true);
+});
diff --git a/dom/localstorage/test/unit/test_databaseShadowing_clearOrigin1.js b/dom/localstorage/test/unit/test_databaseShadowing_clearOrigin1.js
new file mode 100644
index 0000000000..d88fde52e5
--- /dev/null
+++ b/dom/localstorage/test/unit/test_databaseShadowing_clearOrigin1.js
@@ -0,0 +1,30 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/* import-globals-from databaseShadowing-shared.js */
+loadSubscript("databaseShadowing-shared.js");
+
+add_task(async function testSteps() {
+  enableNextGenLocalStorage();
+
+  storeData();
+
+  verifyData([]);
+
+  let principal = getPrincipal("http://origin.test", {});
+  let request = clearOrigin(principal, "default");
+  await requestFinished(request);
+
+  verifyData([1]);
+
+  // Wait for all database connections to close.
+  request = reset();
+  await requestFinished(request);
+
+  exportShadowDatabase("shadowdb-clearedOrigin.sqlite");
+
+  // The shadow database is now prepared for
+  // test_databaseShadowing_clearOrigin2.js
+});
diff --git a/dom/localstorage/test/unit/test_databaseShadowing_clearOrigin2.js b/dom/localstorage/test/unit/test_databaseShadowing_clearOrigin2.js
new file mode 100644
index 0000000000..83d792b496
--- /dev/null
+++ b/dom/localstorage/test/unit/test_databaseShadowing_clearOrigin2.js
@@ -0,0 +1,17 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/* import-globals-from databaseShadowing-shared.js */
+loadSubscript("databaseShadowing-shared.js");
+
+add_task(async function testSteps() {
+  // The shadow database was prepared in test_databaseShadowing_clearOrigin1.js
+
+  disableNextGenLocalStorage();
+
+  ok(importShadowDatabase("shadowdb-clearedOrigin.sqlite"), "Import succeeded");
+
+  verifyData([1], /* migrated */ true);
+});
diff --git a/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPattern1.js b/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPattern1.js
new file mode 100644
index 0000000000..70367bbeff
--- /dev/null
+++ b/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPattern1.js
@@ -0,0 +1,29 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/* import-globals-from databaseShadowing-shared.js */
+loadSubscript("databaseShadowing-shared.js");
+
+add_task(async function testSteps() {
+  enableNextGenLocalStorage();
+
+  storeData();
+
+  verifyData([]);
+
+  let request = clearOriginsByPattern(JSON.stringify({ userContextId: 15 }));
+  await requestFinished(request);
+
+  verifyData([4, 5, 6]);
+
+  // Wait for all database connections to close.
+  request = reset();
+  await requestFinished(request);
+
+  exportShadowDatabase("shadowdb-clearedOriginsByPattern.sqlite");
+
+  // The shadow database is now prepared for
+  // test_databaseShadowing_clearOriginsByPattern2.js
+});
diff --git a/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPattern2.js b/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPattern2.js
new file mode 100644
index 0000000000..6c4d794d04
--- /dev/null
+++ b/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPattern2.js
@@ -0,0 +1,21 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/* import-globals-from databaseShadowing-shared.js */
+loadSubscript("databaseShadowing-shared.js");
+
+add_task(async function testSteps() {
+  // The shadow database was prepared in
+  // test_databaseShadowing_clearOriginsByPattern1.js
+
+  disableNextGenLocalStorage();
+
+  ok(
+    importShadowDatabase("shadowdb-clearedOriginsByPattern.sqlite"),
+    "Import succeeded"
+  );
+
+  verifyData([4, 5, 6], /* migrated */ true);
+});
diff --git a/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPrefix1.js b/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPrefix1.js
new file mode 100644
index 0000000000..2b605e953f
--- /dev/null
+++ b/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPrefix1.js
@@ -0,0 +1,28 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/* import-globals-from databaseShadowing-shared.js */
+loadSubscript("databaseShadowing-shared.js");
+
+add_task(async function testSteps() {
+  enableNextGenLocalStorage();
+
+  storeData();
+
+  verifyData([]);
+
+  let principal = getPrincipal("http://prefix.test", {});
+  let request = clearOriginsByPrefix(principal, "default");
+  await requestFinished(request);
+
+  // Wait for all database connections to close.
+  request = reset();
+  await requestFinished(request);
+
+  exportShadowDatabase("shadowdb-clearedOriginsByPrefix.sqlite");
+
+  // The shadow database is now prepared for
+  // test_databaseShadowing_clearOriginsByPrefix2.js
+});
diff --git a/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPrefix2.js b/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPrefix2.js
new file mode 100644
index 0000000000..892a470723
--- /dev/null
+++ b/dom/localstorage/test/unit/test_databaseShadowing_clearOriginsByPrefix2.js
@@ -0,0 +1,21 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/* import-globals-from databaseShadowing-shared.js */
+loadSubscript("databaseShadowing-shared.js");
+
+add_task(async function testSteps() {
+  // The shadow database was prepared in
+  // test_databaseShadowing_clearOriginsByPrefix1.js
+
+  disableNextGenLocalStorage();
+
+  ok(
+    importShadowDatabase("shadowdb-clearedOriginsByPrefix.sqlite"),
+    "Import succeeded"
+  );
+
+  verifyData([2, 3], /* migrated */ true);
+});
diff --git a/dom/localstorage/test/unit/test_eviction.js b/dom/localstorage/test/unit/test_eviction.js
new file mode 100644
index 0000000000..d8c10b771f
--- /dev/null
+++ b/dom/localstorage/test/unit/test_eviction.js
@@ -0,0 +1,91 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const globalLimitKB = 5 * 1024;
+
+  const data = {};
+  data.sizeKB = 1 * 1024;
+  data.key = "A";
+  data.value = repeatChar(data.sizeKB * 1024 - data.key.length, ".");
+  data.urlCount = globalLimitKB / data.sizeKB;
+
+  function getSpec(index) {
+    return "http://example" + index + ".com";
+  }
+
+  info("Setting prefs");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+  Services.prefs.setBoolPref("dom.storage.snapshot_reusing", false);
+
+  info("Setting limits");
+
+  setGlobalLimit(globalLimitKB);
+
+  let request = clear();
+  await requestFinished(request);
+
+  info("Getting storages");
+
+  let storages = [];
+  for (let i = 0; i < data.urlCount; i++) {
+    let storage = getLocalStorage(getPrincipal(getSpec(i)));
+    storages.push(storage);
+  }
+
+  info("Filling up entire default storage");
+
+  for (let i = 0; i < data.urlCount; i++) {
+    storages[i].setItem(data.key, data.value);
+    await returnToEventLoop();
+  }
+
+  info("Verifying no more data can be written");
+
+  for (let i = 0; i < data.urlCount; i++) {
+    try {
+      storages[i].setItem("B", "");
+      ok(false, "Should have thrown");
+    } catch (ex) {
+      ok(true, "Did throw");
+      ok(DOMException.isInstance(ex), "Threw DOMException");
+      is(ex.name, "QuotaExceededError", "Threw right DOMException");
+      is(ex.code, NS_ERROR_DOM_QUOTA_EXCEEDED_ERR, "Threw with right code");
+    }
+  }
+
+  info("Closing first origin");
+
+  storages[0].close();
+
+  let principal = getPrincipal("http://example0.com");
+
+  request = resetOrigin(principal);
+  await requestFinished(request);
+
+  info("Getting usage for first origin");
+
+  request = getOriginUsage(principal);
+  await requestFinished(request);
+
+  is(request.result.usage, data.sizeKB * 1024, "Correct usage");
+
+  info("Verifying more data data can be written");
+
+  for (let i = 1; i < data.urlCount; i++) {
+    storages[i].setItem("B", "");
+  }
+
+  info("Getting usage for first origin");
+
+  request = getOriginUsage(principal);
+  await requestFinished(request);
+
+  is(request.result.usage, 0, "Zero usage");
+});
diff --git a/dom/localstorage/test/unit/test_flushing.js b/dom/localstorage/test/unit/test_flushing.js
new file mode 100644
index 0000000000..17f015d5ef
--- /dev/null
+++ b/dom/localstorage/test/unit/test_flushing.js
@@ -0,0 +1,72 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+/* eslint-disable mozilla/no-arbitrary-setTimeout */
+
+/**
+ * This test is mainly to verify that the flush operation detaches the shadow
+ * database in the event of early return due to error. See bug 1559029.
+ */
+
+add_task(async function testSteps() {
+  const principal1 = getPrincipal("http://example1.com");
+
+  const usageFile1 = getRelativeFile(
+    "storage/default/http+++example1.com/ls/usage"
+  );
+
+  const principal2 = getPrincipal("http://example2.com");
+
+  const data = {
+    key: "foo",
+    value: "bar",
+  };
+
+  const flushSleepTimeSec = 6;
+
+  info("Setting prefs");
+
+  Services.prefs.setBoolPref("dom.storage.next_gen", true);
+
+  info("Getting storage 1");
+
+  let storage1 = getLocalStorage(principal1);
+
+  info("Adding item");
+
+  storage1.setItem(data.key, data.value);
+
+  info("Creating usage as a directory");
+
+  // This will cause a failure during the flush for first principal.
+  usageFile1.create(Ci.nsIFile.DIRECTORY_TYPE, parseInt("0755", 8));
+
+  info("Getting storage 2");
+
+  let storage2 = getLocalStorage(principal2);
+
+  info("Adding item");
+
+  storage2.setItem(data.key, data.value);
+
+  // The flush for second principal shouldn't be affected by failed flush for
+  // first principal.
+
+  info(
+    "Sleeping for " +
+      flushSleepTimeSec +
+      " seconds to let all flushes " +
+      "finish"
+  );
+
+  await new Promise(function (resolve) {
+    setTimeout(resolve, flushSleepTimeSec * 1000);
+  });
+
+  info("Resetting");
+
+  // Wait for all database connections to close.
+  let request = reset();
+  await requestFinished(request);
+});
diff --git a/dom/localstorage/test/unit/test_groupLimit.js b/dom/localstorage/test/unit/test_groupLimit.js
new file mode 100644
index 0000000000..92ff07b7ab
--- /dev/null
+++ b/dom/localstorage/test/unit/test_groupLimit.js
@@ -0,0 +1,85 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const groupLimitKB = 10 * 1024;
+
+  const globalLimitKB = groupLimitKB * 5;
+
+  const originLimit = 10 * 1024;
+
+  const urls = [
+    "http://example.com",
+    "http://test1.example.com",
+    "https://test2.example.com",
+    "http://test3.example.com:8080",
+  ];
+
+  const data = {};
+  data.sizeKB = 5 * 1024;
+  data.key = "A";
+  data.value = repeatChar(data.sizeKB * 1024 - data.key.length, ".");
+  data.urlCount = groupLimitKB / data.sizeKB;
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+  Services.prefs.setBoolPref("dom.storage.snapshot_reusing", false);
+
+  info("Setting limits");
+
+  setGlobalLimit(globalLimitKB);
+
+  let request = clear();
+  await requestFinished(request);
+
+  setOriginLimit(originLimit);
+
+  info("Getting storages");
+
+  let storages = [];
+  for (let i = 0; i < urls.length; i++) {
+    let storage = getLocalStorage(getPrincipal(urls[i]));
+    storages.push(storage);
+  }
+
+  info("Filling up the whole group");
+
+  for (let i = 0; i < data.urlCount; i++) {
+    storages[i].setItem(data.key, data.value);
+    await returnToEventLoop();
+  }
+
+  info("Verifying no more data can be written");
+
+  for (let i = 0; i < urls.length; i++) {
+    try {
+      storages[i].setItem("B", "");
+      ok(false, "Should have thrown");
+    } catch (ex) {
+      ok(true, "Did throw");
+      ok(DOMException.isInstance(ex), "Threw DOMException");
+      is(ex.name, "QuotaExceededError", "Threw right DOMException");
+      is(ex.code, NS_ERROR_DOM_QUOTA_EXCEEDED_ERR, "Threw with right code");
+    }
+  }
+
+  info("Clearing first origin");
+
+  storages[0].clear();
+
+  // Let the internal snapshot finish (usage is not descreased until all
+  // snapshots finish)..
+  await returnToEventLoop();
+
+  info("Verifying more data can be written");
+
+  for (let i = 0; i < urls.length; i++) {
+    storages[i].setItem("B", "");
+  }
+});
diff --git a/dom/localstorage/test/unit/test_groupMismatch.js b/dom/localstorage/test/unit/test_groupMismatch.js
new file mode 100644
index 0000000000..46533aa2f7
--- /dev/null
+++ b/dom/localstorage/test/unit/test_groupMismatch.js
@@ -0,0 +1,45 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/**
+ * This test is mainly to verify that metadata files with old group information
+ * get updated, so writing to local storage won't cause a crash because of null
+ * quota object. See bug 1516333.
+ */
+
+add_task(async function testSteps() {
+  const principal = getPrincipal("https://foo.bar.mozilla-iot.org");
+
+  info("Clearing");
+
+  let request = clear();
+  await requestFinished(request);
+
+  info("Installing package");
+
+  // The profile contains one initialized origin directory, a script for origin
+  // initialization and the storage database:
+  // - storage/default/https+++foo.bar.mozilla-iot.org
+  // - create_db.js
+  // - storage.sqlite
+  // The file create_db.js in the package was run locally, specifically it was
+  // temporarily added to xpcshell.ini and then executed:
+  //   mach xpcshell-test --interactive dom/localstorage/test/unit/create_db.js
+  // Note: to make it become the profile in the test, additional manual steps
+  // are needed.
+  // 1. Manually change the group in .metadata and .metadata-v2 from
+  //    "bar.mozilla-iot.org" to "mozilla-iot.org".
+  // 2. Remove the folder "storage/temporary".
+  // 3. Remove the file "storage/ls-archive.sqlite".
+  installPackage("groupMismatch_profile");
+
+  info("Getting storage");
+
+  let storage = getLocalStorage(principal);
+
+  info("Adding item");
+
+  storage.setItem("foo", "bar");
+});
diff --git a/dom/localstorage/test/unit/test_largeItems.js b/dom/localstorage/test/unit/test_largeItems.js
new file mode 100644
index 0000000000..3ea6bd21b4
--- /dev/null
+++ b/dom/localstorage/test/unit/test_largeItems.js
@@ -0,0 +1,88 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/**
+ * Test repeatedly setting values that are just under the LocalStorage quota
+ * limit without yielding control flow in order to verify that the write
+ * optimizer is present / works. If there was no write optimizer present, the
+ * IPC message size limit would be exceeded, resulting in a crash.
+ */
+
+add_task(async function testSteps() {
+  const globalLimitKB = 5 * 1024;
+
+  // 18 and more iterations would produce an IPC message with size greater than
+  // 256 MB if write optimizer was not present. This number was determined
+  // experimentally by running the test with disabled write optimizer.
+  const numberOfIterations = 18;
+
+  const randomStringBlockSize = 65536;
+
+  // We need to use a random string because LS internally tries to compress
+  // values.
+  function getRandomString(size) {
+    let crypto = this.window ? this.window.crypto : this.crypto;
+    let decoder = new TextDecoder("ISO-8859-2");
+
+    function getRandomStringBlock(array) {
+      crypto.getRandomValues(array);
+      return decoder.decode(array);
+    }
+
+    let string = "";
+
+    let quotient = size / randomStringBlockSize;
+    if (quotient) {
+      let array = new Uint8Array(randomStringBlockSize);
+      for (let i = 1; i <= quotient; i++) {
+        string += getRandomStringBlock(array);
+      }
+    }
+
+    let remainder = size % randomStringBlockSize;
+    if (remainder) {
+      let array = new Uint8Array(remainder);
+      string += getRandomStringBlock(array);
+    }
+
+    return string;
+  }
+
+  const data = {};
+  data.key = "foo";
+  data.value = getRandomString(
+    globalLimitKB * 1024 -
+      data.key.length -
+      numberOfIterations.toString().length
+  );
+
+  info("Setting pref");
+
+  // By disabling snapshot reusing, we guarantee that the snapshot will be
+  // checkpointed once control returns to the event loop.
+  if (this.window) {
+    await SpecialPowers.pushPrefEnv({
+      set: [["dom.storage.snapshot_reusing", false]],
+    });
+  } else {
+    Services.prefs.setBoolPref("dom.storage.snapshot_reusing", false);
+  }
+
+  info("Getting storage");
+
+  let storage = getLocalStorage();
+
+  info("Adding/updating item");
+
+  for (var i = 0; i < numberOfIterations; i++) {
+    storage.setItem(data.key, data.value + i);
+  }
+
+  info("Returning to event loop");
+
+  await returnToEventLoop();
+
+  ok(!storage.hasSnapshot, "Snapshot successfully finished");
+});
diff --git a/dom/localstorage/test/unit/test_lsng_enabled.js b/dom/localstorage/test/unit/test_lsng_enabled.js
new file mode 100644
index 0000000000..d978aaa901
--- /dev/null
+++ b/dom/localstorage/test/unit/test_lsng_enabled.js
@@ -0,0 +1,13 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/**
+ * This test is mainly to verify that LSNG is not accidentally disabled which
+ * can lead to a data loss in a combination with disabled shadow writes.
+ */
+
+add_task(async function testSteps() {
+  ok(Services.domStorageManager.nextGenLocalStorageEnabled, "LSNG enabled");
+});
diff --git a/dom/localstorage/test/unit/test_migration.js b/dom/localstorage/test/unit/test_migration.js
new file mode 100644
index 0000000000..1249eb076f
--- /dev/null
+++ b/dom/localstorage/test/unit/test_migration.js
@@ -0,0 +1,127 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const principalInfos = [
+    { url: "http://localhost", attrs: {} },
+    { url: "http://www.mozilla.org", attrs: {} },
+    { url: "http://example.com", attrs: {} },
+    { url: "http://example.org", attrs: { userContextId: 5 } },
+
+    { url: "http://origin.test", attrs: {} },
+
+    { url: "http://prefix.test", attrs: {} },
+    { url: "http://prefix.test", attrs: { userContextId: 10 } },
+
+    { url: "http://pattern.test", attrs: { userContextId: 15 } },
+    { url: "http://pattern.test:8080", attrs: { userContextId: 15 } },
+    { url: "https://pattern.test", attrs: { userContextId: 15 } },
+  ];
+
+  const data = {
+    key: "foo",
+    value: "bar",
+  };
+
+  function verifyData(clearedOrigins) {
+    info("Getting storages");
+
+    let storages = [];
+    for (let i = 0; i < principalInfos.length; i++) {
+      let principalInfo = principalInfos[i];
+      let principal = getPrincipal(principalInfo.url, principalInfo.attrs);
+      let storage = getLocalStorage(principal);
+      storages.push(storage);
+    }
+
+    info("Verifying data");
+
+    for (let i = 0; i < storages.length; i++) {
+      let value = storages[i].getItem(data.key + i);
+      if (clearedOrigins.includes(i)) {
+        is(value, null, "Correct value");
+      } else {
+        is(value, data.value + i, "Correct value");
+      }
+    }
+  }
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+
+  info("Stage 1 - Testing archived data migration");
+
+  info("Clearing");
+
+  let request = clear();
+  await requestFinished(request);
+
+  info("Installing package");
+
+  // The profile contains storage.sqlite and webappsstore.sqlite. The file
+  // create_db.js in the package was run locally, specifically it was
+  // temporarily added to xpcshell.ini and then executed:
+  // mach xpcshell-test --interactive dom/localstorage/test/unit/create_db.js
+  installPackage("migration_profile");
+
+  verifyData([]);
+
+  info("Stage 2 - Testing archived data clearing");
+
+  for (let type of ["origin", "prefix", "pattern"]) {
+    info("Clearing");
+
+    request = clear();
+    await requestFinished(request);
+
+    info("Installing package");
+
+    // See the comment for the first installPackage() call.
+    installPackage("migration_profile");
+
+    let clearedOrigins = [];
+
+    switch (type) {
+      case "origin": {
+        let principal = getPrincipal("http://origin.test", {});
+        request = clearOrigin(principal, "default");
+        await requestFinished(request);
+
+        clearedOrigins.push(4);
+
+        break;
+      }
+
+      case "prefix": {
+        let principal = getPrincipal("http://prefix.test", {});
+        request = clearOriginsByPrefix(principal, "default");
+        await requestFinished(request);
+
+        clearedOrigins.push(5, 6);
+
+        break;
+      }
+
+      case "pattern": {
+        request = clearOriginsByPattern(JSON.stringify({ userContextId: 15 }));
+        await requestFinished(request);
+
+        clearedOrigins.push(7, 8, 9);
+
+        break;
+      }
+
+      default: {
+        throw new Error("Unknown type: " + type);
+      }
+    }
+
+    verifyData(clearedOrigins);
+  }
+});
diff --git a/dom/localstorage/test/unit/test_migration_emptyValue.js b/dom/localstorage/test/unit/test_migration_emptyValue.js
new file mode 100644
index 0000000000..dd09c82e88
--- /dev/null
+++ b/dom/localstorage/test/unit/test_migration_emptyValue.js
@@ -0,0 +1,37 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const data = {
+    key: "foo",
+    value: "",
+  };
+
+  info("Clearing");
+
+  let request = clear();
+  await requestFinished(request);
+
+  info("Installing package");
+
+  // The profile contains storage.sqlite and webappsstore.sqlite.
+  // The archive migration_emptyValue_profile.zip was created by running
+  // make_migration_emptyValue.js locally, specifically the special test was
+  // temporarily activated in xpcshell.ini and then it was run as:
+  // mach test --interactive dom/localstorage/test/unit/make_migration_emptyValue.js
+  // Before packaging, additional manual steps are needed:
+  // 1. Folder "cache2" is removed.
+  // 2. Folder "crashes" is removed.
+  // 3. File "mozinfo.json" is removed.
+  installPackage("migration_emptyValue_profile");
+
+  info("Getting storage");
+
+  const storage = getLocalStorage();
+
+  info("Verifying data");
+
+  is(storage.getItem(data.key), data.value, "Correct value");
+});
diff --git a/dom/localstorage/test/unit/test_old_lsng_pref.js b/dom/localstorage/test/unit/test_old_lsng_pref.js
new file mode 100644
index 0000000000..d502ee8779
--- /dev/null
+++ b/dom/localstorage/test/unit/test_old_lsng_pref.js
@@ -0,0 +1,17 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/**
+ * This test is mainly to verify that the old pref for switching LS
+ * implementations has no effect anymore.
+ */
+
+add_task(async function testSteps() {
+  info("Setting pref");
+
+  Services.prefs.setBoolPref("dom.storage.next_gen", false);
+
+  ok(Services.domStorageManager.nextGenLocalStorageEnabled, "LSNG enabled");
+});
diff --git a/dom/localstorage/test/unit/test_orderingAfterRemoveAdd.js b/dom/localstorage/test/unit/test_orderingAfterRemoveAdd.js
new file mode 100644
index 0000000000..88a2e45d2a
--- /dev/null
+++ b/dom/localstorage/test/unit/test_orderingAfterRemoveAdd.js
@@ -0,0 +1,70 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const url = "http://example.com";
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref("dom.storage.snapshot_reusing", false);
+
+  const items = [
+    { key: "key01", value: "value01" },
+    { key: "key02", value: "value02" },
+    { key: "key03", value: "value03" },
+    { key: "key04", value: "value04" },
+    { key: "key05", value: "value05" },
+  ];
+
+  info("Getting storage");
+
+  let storage = getLocalStorage(getPrincipal(url));
+
+  // 1st snapshot
+
+  info("Adding data");
+
+  for (let item of items) {
+    storage.setItem(item.key, item.value);
+  }
+
+  info("Returning to event loop");
+
+  await returnToEventLoop();
+
+  // 2nd snapshot
+
+  // Remove first two items, add some new items and add the two items back.
+
+  storage.removeItem("key01");
+  storage.removeItem("key02");
+
+  storage.setItem("key06", "value06");
+  storage.setItem("key07", "value07");
+  storage.setItem("key08", "value08");
+
+  storage.setItem("key01", "value01");
+  storage.setItem("key02", "value02");
+
+  info("Saving key order");
+
+  let savedKeys = Object.keys(storage);
+
+  info("Returning to event loop");
+
+  await returnToEventLoop();
+
+  // 3rd snapshot
+
+  info("Verifying key order");
+
+  let keys = Object.keys(storage);
+
+  is(keys.length, savedKeys.length);
+
+  for (let i = 0; i < keys.length; i++) {
+    is(keys[i], savedKeys[i], "Correct key");
+  }
+});
diff --git a/dom/localstorage/test/unit/test_originInit.js b/dom/localstorage/test/unit/test_originInit.js
new file mode 100644
index 0000000000..48afdf971b
--- /dev/null
+++ b/dom/localstorage/test/unit/test_originInit.js
@@ -0,0 +1,372 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const storageDirName = "storage";
+  const persistenceTypeDefaultDirName = "default";
+  const persistenceTypePersistentDirName = "permanent";
+
+  const principal = getPrincipal("http://example.com");
+
+  const originDirName = "http+++example.com";
+
+  const clientLSDirName = "ls";
+
+  const dataFile = getRelativeFile(
+    `${storageDirName}/${persistenceTypeDefaultDirName}/${originDirName}/` +
+      `${clientLSDirName}/data.sqlite`
+  );
+
+  const usageJournalFile = getRelativeFile(
+    `${storageDirName}/${persistenceTypeDefaultDirName}/${originDirName}/` +
+      `${clientLSDirName}/usage-journal`
+  );
+
+  const usageFile = getRelativeFile(
+    `${storageDirName}/${persistenceTypeDefaultDirName}/${originDirName}/` +
+      `${clientLSDirName}/usage`
+  );
+
+  const persistentLSDir = getRelativeFile(
+    `${storageDirName}/${persistenceTypePersistentDirName}/${originDirName}/` +
+      `${clientLSDirName}`
+  );
+
+  const data = {};
+  data.key = "key1";
+  data.value = "value1";
+  data.usage = data.key.length + data.value.length;
+
+  const usageFileCookie = 0x420a420a;
+
+  async function createTestOrigin() {
+    let storage = getLocalStorage(principal);
+
+    storage.setItem(data.key, data.value);
+
+    let request = reset();
+    await requestFinished(request);
+  }
+
+  async function createPersistentTestOrigin() {
+    let database = getSimpleDatabase(principal, "persistent");
+
+    let request = database.open("data");
+    await requestFinished(request);
+
+    request = reset();
+    await requestFinished(request);
+  }
+
+  function removeFile(file) {
+    file.remove(false);
+  }
+
+  function createEmptyFile(file) {
+    file.create(Ci.nsIFile.NORMAL_FILE_TYPE, 0o0644);
+  }
+
+  function createEmptyDirectory(dir) {
+    dir.create(Ci.nsIFile.DIRECTORY_TYPE, 0o0755);
+  }
+
+  function getBinaryOutputStream(file) {
+    var ostream = Cc[
+      "@mozilla.org/network/file-output-stream;1"
+    ].createInstance(Ci.nsIFileOutputStream);
+    ostream.init(file, -1, parseInt("0644", 8), 0);
+
+    let bstream = Cc["@mozilla.org/binaryoutputstream;1"].createInstance(
+      Ci.nsIBinaryOutputStream
+    );
+    bstream.setOutputStream(ostream);
+
+    return bstream;
+  }
+
+  async function initTestOrigin() {
+    let request = initStorage();
+    await requestFinished(request);
+
+    request = initTemporaryStorage();
+    await requestFinished(request);
+
+    request = initTemporaryOrigin("default", principal);
+    await requestFinished(request);
+  }
+
+  async function initPersistentTestOrigin() {
+    let request = initStorage();
+    await requestFinished(request);
+
+    request = initPersistentOrigin(principal);
+    await requestFinished(request);
+  }
+
+  async function checkFiles(wantData, wantUsage) {
+    let exists = dataFile.exists();
+    if (wantData) {
+      ok(exists, "Data file does exist");
+    } else {
+      ok(!exists, "Data file doesn't exist");
+    }
+
+    exists = usageJournalFile.exists();
+    ok(!exists, "Usage journal file doesn't exist");
+
+    exists = usageFile.exists();
+    if (wantUsage) {
+      ok(exists, "Usage file does exist");
+    } else {
+      ok(!exists, "Usage file doesn't exist");
+      return;
+    }
+
+    let usage = await readUsageFromUsageFile(usageFile);
+    ok(usage == data.usage, "Correct usage");
+  }
+
+  async function clearTestOrigin() {
+    let request = clearOrigin(principal, "default");
+    await requestFinished(request);
+  }
+
+  async function clearPersistentTestOrigin() {
+    let request = clearOrigin(principal, "persistent");
+    await requestFinished(request);
+  }
+
+  info("Setting prefs");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+
+  info(
+    "Stage 1 - " +
+      "data.sqlite file doesn't exist, " +
+      "usage-journal file doesn't exist, " +
+      "any usage file exists"
+  );
+
+  await createTestOrigin();
+
+  removeFile(dataFile);
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ false, /* wantUsage */ false);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 2 - " +
+      "data.sqlite file doesn't exist, " +
+      "any usage-journal file exists, " +
+      "any usage file exists"
+  );
+
+  await createTestOrigin();
+
+  removeFile(dataFile);
+  createEmptyFile(usageJournalFile);
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ false, /* wantUsage */ false);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 3 - " +
+      "valid data.sqlite file exists, " +
+      "usage-journal file doesn't exist, " +
+      "usage file doesn't exist"
+  );
+
+  await createTestOrigin();
+
+  removeFile(usageFile);
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ true, /* wantUsage */ true);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 4 - " +
+      "valid data.sqlite file exists, " +
+      "usage-journal file doesn't exist, " +
+      "invalid (wrong file size) usage file exists"
+  );
+
+  await createTestOrigin();
+
+  removeFile(usageFile);
+  createEmptyFile(usageFile);
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ true, /* wantUsage */ true);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 5 - " +
+      "valid data.sqlite file exists, " +
+      "usage-journal file doesn't exist, " +
+      "invalid (wrong cookie) usage file exists"
+  );
+
+  await createTestOrigin();
+
+  let stream = getBinaryOutputStream(usageFile);
+  stream.write32(usageFileCookie - 1);
+  stream.write64(data.usage);
+  stream.close();
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ true, /* wantUsage */ true);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 6 - " +
+      "valid data.sqlite file exists, " +
+      "usage-journal file doesn't exist, " +
+      "valid usage file exists"
+  );
+
+  await createTestOrigin();
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ true, /* wantUsage */ true);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 7 - " +
+      "valid data.sqlite file exists, " +
+      "any usage-journal exists, " +
+      "usage file doesn't exist"
+  );
+
+  await createTestOrigin();
+
+  createEmptyFile(usageJournalFile);
+  removeFile(usageFile);
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ true, /* wantUsage */ true);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 8 - " +
+      "valid data.sqlite file exists, " +
+      "any usage-journal exists, " +
+      "invalid (wrong file size) usage file exists"
+  );
+
+  await createTestOrigin();
+
+  createEmptyFile(usageJournalFile);
+  removeFile(usageFile);
+  createEmptyFile(usageFile);
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ true, /* wantUsage */ true);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 9 - " +
+      "valid data.sqlite file exists, " +
+      "any usage-journal exists, " +
+      "invalid (wrong cookie) usage file exists"
+  );
+
+  await createTestOrigin();
+
+  createEmptyFile(usageJournalFile);
+  stream = getBinaryOutputStream(usageFile);
+  stream.write32(usageFileCookie - 1);
+  stream.write64(data.usage);
+  stream.close();
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ true, /* wantUsage */ true);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 10 - " +
+      "valid data.sqlite file exists, " +
+      "any usage-journal exists, " +
+      "invalid (wrong usage) usage file exists"
+  );
+
+  await createTestOrigin();
+
+  createEmptyFile(usageJournalFile);
+  stream = getBinaryOutputStream(usageFile);
+  stream.write32(usageFileCookie);
+  stream.write64(data.usage - 1);
+  stream.close();
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ true, /* wantUsage */ true);
+
+  await clearTestOrigin();
+
+  info(
+    "Stage 11 - " +
+      "valid data.sqlite file exists, " +
+      "any usage-journal exists, " +
+      "valid usage file exists"
+  );
+
+  await createTestOrigin();
+
+  createEmptyFile(usageJournalFile);
+
+  await initTestOrigin();
+
+  await checkFiles(/* wantData */ true, /* wantUsage */ true);
+
+  await clearTestOrigin();
+
+  // Verify that InitializeOrigin doesn't fail when a
+  // storage/permanent/${origin}/ls exists.
+  info(
+    "Stage 12 - Testing initialization of ls directory placed in permanent " +
+      "origin directory"
+  );
+
+  await createPersistentTestOrigin();
+
+  createEmptyDirectory(persistentLSDir);
+
+  try {
+    await initPersistentTestOrigin();
+
+    ok(true, "Should not have thrown");
+  } catch (ex) {
+    ok(false, "Should not have thrown");
+  }
+
+  let exists = persistentLSDir.exists();
+  ok(exists, "ls directory in permanent origin directory does exist");
+
+  await clearPersistentTestOrigin();
+});
diff --git a/dom/localstorage/test/unit/test_preloading.js b/dom/localstorage/test/unit/test_preloading.js
new file mode 100644
index 0000000000..c3b7c808eb
--- /dev/null
+++ b/dom/localstorage/test/unit/test_preloading.js
@@ -0,0 +1,87 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const principals = [
+    getPrincipal("http://example.com", {}),
+    getPrincipal("http://example.com", { privateBrowsingId: 1 }),
+  ];
+
+  async function isPreloaded(principal) {
+    return Services.domStorageManager.isPreloaded(principal);
+  }
+
+  info("Setting prefs");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+  Services.prefs.setBoolPref("dom.storage.snapshot_reusing", false);
+
+  for (const principal of principals) {
+    info("Getting storage");
+
+    let storage = getLocalStorage(principal);
+
+    ok(
+      !(await isPreloaded(principal)),
+      "Data is not preloaded after getting storage"
+    );
+
+    info("Opening storage");
+
+    storage.open();
+
+    ok(await isPreloaded(principal), "Data is preloaded after opening storage");
+
+    info("Closing storage");
+
+    storage.close();
+
+    if (principal.privateBrowsingId > 0) {
+      ok(
+        await isPreloaded(principal),
+        "Data is still preloaded after closing storage"
+      );
+
+      info("Closing private session");
+
+      Services.obs.notifyObservers(null, "last-pb-context-exited");
+
+      ok(
+        !(await isPreloaded(principal)),
+        "Data is not preloaded anymore after closing private session"
+      );
+    } else {
+      ok(
+        !(await isPreloaded(principal)),
+        "Data is not preloaded anymore after closing storage"
+      );
+    }
+
+    info("Opening storage again");
+
+    storage.open();
+
+    ok(
+      await isPreloaded(principal),
+      "Data is preloaded after opening storage again"
+    );
+
+    info("Clearing origin");
+
+    let request = clearOrigin(
+      principal,
+      principal.privateBrowsingId > 0 ? "private" : "default"
+    );
+    await requestFinished(request);
+
+    ok(
+      !(await isPreloaded(principal)),
+      "Data is not preloaded after clearing origin"
+    );
+  }
+});
diff --git a/dom/localstorage/test/unit/test_schema3upgrade.js b/dom/localstorage/test/unit/test_schema3upgrade.js
new file mode 100644
index 0000000000..4b851642ea
--- /dev/null
+++ b/dom/localstorage/test/unit/test_schema3upgrade.js
@@ -0,0 +1,39 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const url = "http://example.com";
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref("dom.storage.next_gen", true);
+
+  info("Clearing");
+
+  let request = clear();
+  await requestFinished(request);
+
+  info("Installing package");
+
+  // The profile contains one initialized origin directory with local
+  // storage data, local storage archive, a script for origin initialization,
+  // the storage database and the web apps store database:
+  // - storage/default/http+++example.com
+  // - storage/ls-archive.sqlite
+  // - create_db.js
+  // - storage.sqlite
+  // - webappsstore.sqlite
+  // The file create_db.js in the package was run locally (with a build with
+  // local storage archive version 1 and database schema version 2),
+  // specifically it was temporarily added to xpcshell.ini and then executed:
+  //   mach xpcshell-test --interactive dom/localstorage/test/unit/create_db.js
+  // Note: to make it become the profile in the test, additional manual steps
+  // are needed.
+  // 1. Remove the folder "storage/temporary".
+  installPackage("schema3upgrade_profile");
+
+  let storage = getLocalStorage(getPrincipal(url));
+  storage.open();
+});
diff --git a/dom/localstorage/test/unit/test_schema4upgrade.js b/dom/localstorage/test/unit/test_schema4upgrade.js
new file mode 100644
index 0000000000..a6c308af35
--- /dev/null
+++ b/dom/localstorage/test/unit/test_schema4upgrade.js
@@ -0,0 +1,39 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const url = "http://example.com";
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref("dom.storage.next_gen", true);
+
+  info("Clearing");
+
+  let request = clear();
+  await requestFinished(request);
+
+  info("Installing package");
+
+  // The profile contains one initialized origin directory with local
+  // storage data, local storage archive, a script for origin initialization,
+  // the storage database and the web apps store database:
+  // - storage/default/http+++example.com
+  // - storage/ls-archive.sqlite
+  // - storage.sqlite
+  // - test_create_db.js
+  // - webappsstore.sqlite
+  //
+  // The file test_create_db.js in the package was run locally by
+  // adding it temporarily to xpcshell.ini and then executed with
+  //   mach xpcshell-test --headless dom/localstorage/test/unit/test_create_db.js
+  // Note: to make it become the profile in the test, additional manual steps
+  // are needed.
+  // 1. Remove the folder "storage/temporary".
+  installPackage("schema4upgrade_profile");
+
+  let storage = getLocalStorage(getPrincipal(url));
+  storage.open();
+});
diff --git a/dom/localstorage/test/unit/test_snapshotting.js b/dom/localstorage/test/unit/test_snapshotting.js
new file mode 100644
index 0000000000..4b639395f7
--- /dev/null
+++ b/dom/localstorage/test/unit/test_snapshotting.js
@@ -0,0 +1,330 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const url = "http://example.com";
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref("dom.storage.snapshot_reusing", false);
+
+  const items = [
+    { key: "key01", value: "value01" },
+    { key: "key02", value: "value02" },
+    { key: "key03", value: "value03" },
+    { key: "key04", value: "value04" },
+    { key: "key05", value: "value05" },
+    { key: "key06", value: "value06" },
+    { key: "key07", value: "value07" },
+    { key: "key08", value: "value08" },
+    { key: "key09", value: "value09" },
+    { key: "key10", value: "value10" },
+  ];
+
+  let sizeOfOneKey;
+  let sizeOfOneValue;
+  let sizeOfOneItem;
+  let sizeOfKeys = 0;
+  let sizeOfItems = 0;
+
+  for (let i = 0; i < items.length; i++) {
+    let item = items[i];
+    let sizeOfKey = item.key.length;
+    let sizeOfValue = item.value.length;
+    let sizeOfItem = sizeOfKey + sizeOfValue;
+    if (i == 0) {
+      sizeOfOneKey = sizeOfKey;
+      sizeOfOneValue = sizeOfValue;
+      sizeOfOneItem = sizeOfItem;
+    }
+    sizeOfKeys += sizeOfKey;
+    sizeOfItems += sizeOfItem;
+  }
+
+  info("Size of one key is " + sizeOfOneKey);
+  info("Size of one value is " + sizeOfOneValue);
+  info("Size of one item is " + sizeOfOneItem);
+  info("Size of keys is " + sizeOfKeys);
+  info("Size of items is " + sizeOfItems);
+
+  const prefillValues = [
+    // Zero prefill (prefill disabled)
+    0,
+    // Less than one key length prefill
+    sizeOfOneKey - 1,
+    // Greater than one key length and less than one item length prefill
+    sizeOfOneKey + 1,
+    // Precisely one item length prefill
+    sizeOfOneItem,
+    // Precisely two times one item length prefill
+    2 * sizeOfOneItem,
+    // Precisely three times one item length prefill
+    3 * sizeOfOneItem,
+    // Precisely four times one item length prefill
+    4 * sizeOfOneItem,
+    // Precisely size of keys prefill
+    sizeOfKeys,
+    // Less than size of keys plus one value length prefill
+    sizeOfKeys + sizeOfOneValue - 1,
+    // Precisely size of keys plus one value length prefill
+    sizeOfKeys + sizeOfOneValue,
+    // Greater than size of keys plus one value length and less than size of
+    // keys plus two times one value length prefill
+    sizeOfKeys + sizeOfOneValue + 1,
+    // Precisely size of keys plus two times one value length prefill
+    sizeOfKeys + 2 * sizeOfOneValue,
+    // Precisely size of keys plus three times one value length prefill
+    sizeOfKeys + 3 * sizeOfOneValue,
+    // Precisely size of keys plus four times one value length prefill
+    sizeOfKeys + 4 * sizeOfOneValue,
+    // Precisely size of keys plus five times one value length prefill
+    sizeOfKeys + 5 * sizeOfOneValue,
+    // Precisely size of keys plus six times one value length prefill
+    sizeOfKeys + 6 * sizeOfOneValue,
+    // Precisely size of keys plus seven times one value length prefill
+    sizeOfKeys + 7 * sizeOfOneValue,
+    // Precisely size of keys plus eight times one value length prefill
+    sizeOfKeys + 8 * sizeOfOneValue,
+    // Precisely size of keys plus nine times one value length prefill
+    sizeOfKeys + 9 * sizeOfOneValue,
+    // Precisely size of items prefill
+    sizeOfItems,
+    // Unlimited prefill
+    -1,
+  ];
+
+  for (let prefillValue of prefillValues) {
+    info("Setting prefill value to " + prefillValue);
+
+    Services.prefs.setIntPref("dom.storage.snapshot_prefill", prefillValue);
+
+    const gradualPrefillValues = [
+      // Zero gradual prefill
+      0,
+      // Less than one key length gradual prefill
+      sizeOfOneKey - 1,
+      // Greater than one key length and less than one item length gradual
+      // prefill
+      sizeOfOneKey + 1,
+      // Precisely one item length gradual prefill
+      sizeOfOneItem,
+      // Precisely two times one item length gradual prefill
+      2 * sizeOfOneItem,
+      // Precisely three times one item length gradual prefill
+      3 * sizeOfOneItem,
+      // Precisely four times one item length gradual prefill
+      4 * sizeOfOneItem,
+      // Precisely five times one item length gradual prefill
+      5 * sizeOfOneItem,
+      // Precisely six times one item length gradual prefill
+      6 * sizeOfOneItem,
+      // Precisely seven times one item length gradual prefill
+      7 * sizeOfOneItem,
+      // Precisely eight times one item length gradual prefill
+      8 * sizeOfOneItem,
+      // Precisely nine times one item length gradual prefill
+      9 * sizeOfOneItem,
+      // Precisely size of items prefill
+      sizeOfItems,
+      // Unlimited gradual prefill
+      -1,
+    ];
+
+    for (let gradualPrefillValue of gradualPrefillValues) {
+      info("Setting gradual prefill value to " + gradualPrefillValue);
+
+      Services.prefs.setIntPref(
+        "dom.storage.snapshot_gradual_prefill",
+        gradualPrefillValue
+      );
+
+      info("Getting storage");
+
+      let storage = getLocalStorage(getPrincipal(url));
+
+      // 1st snapshot
+
+      info("Adding data");
+
+      for (let item of items) {
+        storage.setItem(item.key, item.value);
+      }
+
+      info("Saving key order");
+
+      // This forces GetKeys to be called internally.
+      let savedKeys = Object.keys(storage);
+
+      // GetKey should match GetKeys
+      for (let i = 0; i < savedKeys.length; i++) {
+        is(storage.key(i), savedKeys[i], "Correct key");
+      }
+
+      info("Returning to event loop");
+
+      // Returning to event loop forces the internal snapshot to finish.
+      await returnToEventLoop();
+
+      // 2nd snapshot
+
+      info("Verifying length");
+
+      is(storage.length, items.length, "Correct length");
+
+      info("Verifying key order");
+
+      let keys = Object.keys(storage);
+
+      is(keys.length, savedKeys.length);
+
+      for (let i = 0; i < keys.length; i++) {
+        is(keys[i], savedKeys[i], "Correct key");
+      }
+
+      info("Verifying values");
+
+      for (let item of items) {
+        is(storage.getItem(item.key), item.value, "Correct value");
+      }
+
+      info("Returning to event loop");
+
+      await returnToEventLoop();
+
+      // 3rd snapshot
+
+      // Force key2 to load.
+      storage.getItem("key02");
+
+      // Fill out write infos a bit.
+      storage.removeItem("key05");
+      storage.setItem("key05", "value05");
+      storage.removeItem("key05");
+      storage.setItem("key11", "value11");
+      storage.setItem("key05", "value05");
+
+      items.push({ key: "key11", value: "value11" });
+
+      info("Verifying length");
+
+      is(storage.length, items.length, "Correct length");
+
+      // This forces to get all keys from the parent and then apply write infos
+      // on already cached values.
+      savedKeys = Object.keys(storage);
+
+      info("Verifying values");
+
+      for (let item of items) {
+        is(storage.getItem(item.key), item.value, "Correct value");
+      }
+
+      storage.removeItem("key11");
+
+      items.pop();
+
+      info("Returning to event loop");
+
+      await returnToEventLoop();
+
+      // 4th snapshot
+
+      // Force loading of all items.
+      info("Verifying length");
+
+      is(storage.length, items.length, "Correct length");
+
+      info("Verifying values");
+
+      for (let item of items) {
+        is(storage.getItem(item.key), item.value, "Correct value");
+      }
+
+      is(storage.getItem("key11"), null, "Correct value");
+
+      info("Returning to event loop");
+
+      await returnToEventLoop();
+
+      // 5th snapshot
+
+      // Force loading of all keys.
+      info("Saving key order");
+
+      savedKeys = Object.keys(storage);
+
+      // Force loading of all items.
+      info("Verifying length");
+
+      is(storage.length, items.length, "Correct length");
+
+      info("Verifying values");
+
+      for (let item of items) {
+        is(storage.getItem(item.key), item.value, "Correct value");
+      }
+
+      is(storage.getItem("key11"), null, "Correct value");
+
+      info("Returning to event loop");
+
+      await returnToEventLoop();
+
+      // 6th snapshot
+      info("Verifying unknown item");
+
+      is(storage.getItem("key11"), null, "Correct value");
+
+      info("Verifying unknown item again");
+
+      is(storage.getItem("key11"), null, "Correct value");
+
+      info("Returning to event loop");
+
+      await returnToEventLoop();
+
+      // 7th snapshot
+
+      // Save actual key order.
+      info("Saving key order");
+
+      savedKeys = Object.keys(storage);
+
+      await returnToEventLoop();
+
+      // 8th snapshot
+
+      // Force loading of all items, but in reverse order.
+      info("Getting values");
+
+      for (let i = items.length - 1; i >= 0; i--) {
+        let item = items[i];
+        storage.getItem(item.key);
+      }
+
+      info("Verifying key order");
+
+      keys = Object.keys(storage);
+
+      is(keys.length, savedKeys.length);
+
+      for (let i = 0; i < keys.length; i++) {
+        is(keys[i], savedKeys[i], "Correct key");
+      }
+
+      await returnToEventLoop();
+
+      // 9th snapshot
+
+      info("Clearing");
+
+      storage.clear();
+
+      info("Returning to event loop");
+
+      await returnToEventLoop();
+    }
+  }
+});
diff --git a/dom/localstorage/test/unit/test_stringLength.js b/dom/localstorage/test/unit/test_stringLength.js
new file mode 100644
index 0000000000..52ee59f7d6
--- /dev/null
+++ b/dom/localstorage/test/unit/test_stringLength.js
@@ -0,0 +1,74 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const principal = getPrincipal("http://example.org");
+
+  const data = {};
+  data.key = "foobar";
+  data.secondKey = "foobaz";
+  data.value = {
+    length: 25637,
+  };
+  data.usage = data.key.length + data.value.length;
+
+  async function checkUsage(expectedUsage) {
+    info("Checking usage");
+
+    // This forces any pending changes to be flushed to disk.  It also forces
+    // data to be reloaded from disk at next localStorage API call.
+    request = resetOrigin(principal);
+    await requestFinished(request);
+
+    request = getOriginUsage(principal);
+    await requestFinished(request);
+
+    is(request.result.usage, expectedUsage, "Correct usage");
+  }
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+
+  info("Stage 1 - Checking usage after profile installation");
+
+  info("Clearing");
+
+  let request = clear();
+  await requestFinished(request);
+
+  info("Installing package");
+
+  // The profile contains storage.sqlite and webappsstore.sqlite.
+  installPackage("stringLength_profile");
+
+  await checkUsage(0);
+
+  info("Stage 2 - Checking usage after archived data migration");
+
+  info("Opening database");
+
+  let storage = getLocalStorage(principal);
+  storage.open();
+
+  await checkUsage(data.usage);
+
+  info("Stage 3 - Checking usage after copying the value");
+
+  info("Adding a second copy of the value");
+
+  let value = storage.getItem(data.key);
+  storage.setItem(data.secondKey, value);
+
+  await checkUsage(2 * data.usage);
+
+  info("Stage 4 - Checking length of the copied value");
+
+  value = storage.getItem(data.secondKey);
+  ok(value.length === data.value.length, "Correct string length");
+});
diff --git a/dom/localstorage/test/unit/test_stringLength2.js b/dom/localstorage/test/unit/test_stringLength2.js
new file mode 100644
index 0000000000..f1a1a902cf
--- /dev/null
+++ b/dom/localstorage/test/unit/test_stringLength2.js
@@ -0,0 +1,79 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/**
+ * This test is mainly to verify that string length is correctly computed for
+ * database values containing NULs. See bug 1541681.
+ */
+
+add_task(async function testSteps() {
+  const principal = getPrincipal("http://example.org");
+
+  const data = {};
+  data.key = "foobar";
+  data.secondKey = "foobaz";
+  data.value = {
+    length: 19253,
+  };
+  data.usage = data.key.length + data.value.length;
+
+  async function checkUsage(expectedUsage) {
+    info("Checking usage");
+
+    // This forces any pending changes to be flushed to disk.  It also forces
+    // data to be reloaded from disk at next localStorage API call.
+    request = resetOrigin(principal);
+    await requestFinished(request);
+
+    request = getOriginUsage(principal);
+    await requestFinished(request);
+
+    is(request.result.usage, expectedUsage, "Correct usage");
+  }
+
+  info("Setting pref");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+
+  info("Stage 1 - Checking usage after profile installation");
+
+  info("Clearing");
+
+  let request = clear();
+  await requestFinished(request);
+
+  info("Installing package");
+
+  // The profile contains storage.sqlite and webappsstore.sqlite.
+  installPackage("stringLength2_profile");
+
+  await checkUsage(0);
+
+  info("Stage 2 - Checking usage after archived data migration");
+
+  info("Opening database");
+
+  let storage = getLocalStorage(principal);
+  storage.open();
+
+  await checkUsage(data.usage);
+
+  info("Stage 3 - Checking usage after copying the value");
+
+  info("Adding a second copy of the value");
+
+  let value = storage.getItem(data.key);
+  storage.setItem(data.secondKey, value);
+
+  await checkUsage(2 * data.usage);
+
+  info("Stage 4 - Checking length of the copied value");
+
+  value = storage.getItem(data.secondKey);
+  ok(value.length === data.value.length, "Correct string length");
+});
diff --git a/dom/localstorage/test/unit/test_unicodeCharacters.js b/dom/localstorage/test/unit/test_unicodeCharacters.js
new file mode 100644
index 0000000000..3ea8923e0a
--- /dev/null
+++ b/dom/localstorage/test/unit/test_unicodeCharacters.js
@@ -0,0 +1,202 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+const interpretChar = (chars, index) => {
+  return chars.charCodeAt(index).toString(16).padStart(4, "0");
+};
+
+const hexEncode = str => {
+  let result = "";
+  const len = str.length;
+  for (let i = 0; i < len; ++i) {
+    result += interpretChar(str, i);
+  }
+  return result;
+};
+
+const collectCorrupted = (expected, actual) => {
+  const len = Math.min(expected.length, actual.length);
+  let notEquals = [];
+  for (let i = 0; i < len; ++i) {
+    if (expected[i] !== actual[i]) {
+      notEquals.push([hexEncode(expected[i]), hexEncode(actual[i])]);
+    }
+  }
+  return notEquals;
+};
+
+const sanitizeOutputWithSurrogates = (testValue, prefix = "") => {
+  let utf8What = prefix;
+  for (let i = 0; i < testValue.length; ++i) {
+    const valueChar = testValue.charCodeAt(i);
+    const isPlanar = 0xd800 <= valueChar && valueChar <= 0xdfff;
+    utf8What += isPlanar ? "\\u" + interpretChar(testValue, i) : testValue[i];
+  }
+  return utf8What;
+};
+
+const getEncodingSample = () => {
+  const expectedSample =
+    "3681207208613504e0a5028800b945551988c60050008027ebc2808c00d38e806e03d8210ac906722b85499be9d00000";
+
+  let result = "";
+  const len = expectedSample.length;
+  for (let i = 0; i < len; i += 4) {
+    result += String.fromCharCode(parseInt(expectedSample.slice(i, i + 4), 16));
+  }
+  return result;
+};
+
+const getSeparatedBasePlane = () => {
+  let result = "";
+  for (let i = 0xffff; i >= 0; --i) {
+    result += String.fromCharCode(i) + "\n";
+  }
+  return result;
+};
+
+const getJoinedBasePlane = () => {
+  let result = "";
+  for (let i = 0; i <= 0xffff; ++i) {
+    result += String.fromCharCode(i);
+  }
+  return result;
+};
+
+const getSurrogateCombinations = () => {
+  const upperLead = String.fromCharCode(0xdbff);
+  const lowerTrail = String.fromCharCode(0xdc00);
+
+  const regularSlot = ["w", "abcdefghijklmnopqrst", "aaaaaaaaaaaaaaaaaaaa", ""];
+  const surrogateSlot = [lowerTrail, upperLead];
+
+  let samples = [];
+  for (const leadSnippet of regularSlot) {
+    for (const firstSlot of surrogateSlot) {
+      for (const trailSnippet of regularSlot) {
+        for (const secondSlot of surrogateSlot) {
+          samples.push(leadSnippet + firstSlot + secondSlot + trailSnippet);
+        }
+        samples.push(leadSnippet + firstSlot + trailSnippet);
+      }
+    }
+  }
+
+  return samples;
+};
+
+const fetchFrom = async (itemKey, sample, meanwhile) => {
+  const principal = getPrincipal("http://example.com/", {});
+
+  let request = clearOrigin(principal);
+  await requestFinished(request);
+
+  const storage = getLocalStorage(principal);
+
+  await storage.setItem(itemKey, sample);
+
+  await meanwhile(principal);
+
+  return storage.getItem(itemKey);
+};
+
+/**
+ * Value fetched from existing snapshot based on
+ * existing in-memory datastore in the parent process
+ * without any communication between content/parent
+ */
+const fetchFromExistingSnapshotExistingDatastore = async (itemKey, sample) => {
+  return fetchFrom(itemKey, sample, async () => {});
+};
+
+/**
+ * Value fetched from newly created snapshot based on
+ * existing in-memory datastore in the parent process
+ */
+const fetchFromNewSnapshotExistingDatastore = async (itemKey, sample) => {
+  return fetchFrom(itemKey, sample, async () => {
+    await returnToEventLoop();
+  });
+};
+
+/**
+ * Value fetched from newly created snapshot based on newly created
+ * in-memory datastore based on database in the parent process
+ */
+const fetchFromNewSnapshotNewDatastore = async (itemKey, sample) => {
+  return fetchFrom(itemKey, sample, async principal => {
+    let request = resetOrigin(principal);
+    await requestFinished(request);
+  });
+};
+
+add_task(async function testSteps() {
+  /* This test is based on bug 1681300 */
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+  Services.prefs.setBoolPref("dom.storage.snapshot_reusing", false);
+
+  const reportWhat = (testKey, testValue) => {
+    if (testKey.length + testValue.length > 82) {
+      return testKey;
+    }
+    return sanitizeOutputWithSurrogates(testValue, /* prefix */ testKey + ":");
+  };
+
+  const testFetchMode = async (testType, storeAndLookup) => {
+    const testPairs = [
+      { testEmptyValue: [""] },
+      { testSampleKey: [getEncodingSample()] },
+      { testSeparatedKey: [getSeparatedBasePlane()] },
+      { testJoinedKey: [getJoinedBasePlane()] },
+      { testCombinations: getSurrogateCombinations() },
+    ];
+
+    for (const testPair of testPairs) {
+      for (const [testKey, expectedValues] of Object.entries(testPair)) {
+        for (const expected of expectedValues) {
+          const actual = await storeAndLookup(testKey, expected);
+          const testInfo = reportWhat(testKey, expected);
+          is(
+            null != actual,
+            true,
+            testType + ": Value not null for " + testInfo
+          );
+          is(
+            expected.length,
+            actual.length,
+            testType + ": Returned size for " + testInfo
+          );
+
+          const notEquals = collectCorrupted(expected, actual);
+          for (let i = 0; i < notEquals.length; ++i) {
+            is(
+              notEquals[i][0],
+              notEquals[i][1],
+              testType + ": Unequal character at " + i + " for " + testInfo
+            );
+          }
+        }
+      }
+    }
+  };
+
+  await testFetchMode(
+    "ExistingSnapshotExistingDatastore",
+    fetchFromExistingSnapshotExistingDatastore
+  );
+
+  await testFetchMode(
+    "NewSnapshotExistingDatastore",
+    fetchFromNewSnapshotExistingDatastore
+  );
+
+  await testFetchMode(
+    "NewSnapshotNewDatastore",
+    fetchFromNewSnapshotNewDatastore
+  );
+});
diff --git a/dom/localstorage/test/unit/test_uri_encoding_edge_cases.js b/dom/localstorage/test/unit/test_uri_encoding_edge_cases.js
new file mode 100644
index 0000000000..0da4e8584d
--- /dev/null
+++ b/dom/localstorage/test/unit/test_uri_encoding_edge_cases.js
@@ -0,0 +1,69 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/**
+ * This test verifies that group and origin strings for URIs with special
+ * characters are consistent between calling
+ * EnsureQuotaForOringin/EnsureOriginIsInitailized and GetQuotaObject in
+ * PrepareDatastoreOp, so writing to local storage won't cause a crash because
+ * of a null quota object. See bug 1516333.
+ */
+
+add_task(async function testSteps() {
+  /**
+   * The edge cases are specified in this array of origins. Each edge case must
+   * contain two properties uri and path (origin directory path relative to the
+   * profile directory).
+   */
+  const origins = [
+    {
+      uri: "file:///test'.html",
+      path: "storage/default/file++++test'.html",
+    },
+    {
+      uri: "file:///test>.html",
+      path: "storage/default/file++++test%3E.html",
+    },
+  ];
+
+  info("Setting prefs");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+
+  for (let origin of origins) {
+    const principal = getPrincipal(origin.uri);
+
+    let originDir = getRelativeFile(origin.path);
+
+    info("Checking the origin directory existence");
+
+    ok(
+      !originDir.exists(),
+      `The origin directory ${origin.path} should not exists`
+    );
+
+    info("Getting storage");
+
+    let storage = getLocalStorage(principal);
+
+    info("Adding item");
+
+    storage.setItem("foo", "bar");
+
+    info("Resetting origin");
+
+    // This forces any pending changes to be flushed to disk (including origin
+    // directory creation).
+    let request = resetOrigin(principal);
+    await requestFinished(request);
+
+    info("Checking the origin directory existence");
+
+    ok(originDir.exists(), `The origin directory ${origin.path} should exist`);
+  }
+});
diff --git a/dom/localstorage/test/unit/test_usage.js b/dom/localstorage/test/unit/test_usage.js
new file mode 100644
index 0000000000..552a45e4a6
--- /dev/null
+++ b/dom/localstorage/test/unit/test_usage.js
@@ -0,0 +1,69 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const data = {};
+  data.key = "key1";
+  data.value = "value1";
+  data.usage = data.key.length + data.value.length;
+
+  const principal = getPrincipal("http://example.com");
+
+  info("Setting prefs");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+
+  info("Stage 1 - Testing usage after adding item");
+
+  info("Getting storage");
+
+  let storage = getLocalStorage(principal);
+
+  info("Adding item");
+
+  storage.setItem(data.key, data.value);
+
+  info("Resetting origin");
+
+  let request = resetOrigin(principal);
+  await requestFinished(request);
+
+  info("Getting usage");
+
+  request = getOriginUsage(principal);
+  await requestFinished(request);
+
+  is(request.result.usage, data.usage, "Correct usage");
+
+  info("Resetting");
+
+  request = reset();
+  await requestFinished(request);
+
+  info("Stage 2 - Testing usage after removing item");
+
+  info("Getting storage");
+
+  storage = getLocalStorage(principal);
+
+  info("Removing item");
+
+  storage.removeItem(data.key);
+
+  info("Resetting origin");
+
+  request = resetOrigin(principal);
+  await requestFinished(request);
+
+  info("Getting usage");
+
+  request = getOriginUsage(principal);
+  await requestFinished(request);
+
+  is(request.result.usage, 0, "Correct usage");
+});
diff --git a/dom/localstorage/test/unit/test_usageAfterMigration.js b/dom/localstorage/test/unit/test_usageAfterMigration.js
new file mode 100644
index 0000000000..a0bd5efd5b
--- /dev/null
+++ b/dom/localstorage/test/unit/test_usageAfterMigration.js
@@ -0,0 +1,164 @@
+/**
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+add_task(async function testSteps() {
+  const principal = getPrincipal("http://example.com");
+
+  const dataFile = getRelativeFile(
+    "storage/default/http+++example.com/ls/data.sqlite"
+  );
+
+  const usageJournalFile = getRelativeFile(
+    "storage/default/http+++example.com/ls/usage-journal"
+  );
+
+  const usageFile = getRelativeFile(
+    "storage/default/http+++example.com/ls/usage"
+  );
+
+  const data = {};
+  data.key = "foo";
+  data.value = "bar";
+  data.usage = data.key.length + data.value.length;
+
+  async function createStorageForMigration(createUsageDir) {
+    info("Clearing");
+
+    let request = clear();
+    await requestFinished(request);
+
+    info("Installing package");
+
+    // The profile contains storage.sqlite and webappsstore.sqlite. The file
+    // create_db.js in the package was run locally, specifically it was
+    // temporarily added to xpcshell.ini and then executed:
+    // mach xpcshell-test --interactive dom/localstorage/test/unit/create_db.js
+    installPackage("usageAfterMigration_profile");
+
+    if (createUsageDir) {
+      // Origin must be initialized before the usage dir is created.
+
+      info("Initializing storage");
+
+      request = initStorage();
+      await requestFinished(request);
+
+      info("Initializing temporary storage");
+
+      request = initTemporaryStorage();
+      await requestFinished(request);
+
+      info("Initializing origin");
+
+      request = initTemporaryOrigin("default", principal);
+      await requestFinished(request);
+
+      info("Creating usage as a directory");
+
+      // This will cause a failure during migration.
+      usageFile.create(Ci.nsIFile.DIRECTORY_TYPE, parseInt("0755", 8));
+    }
+  }
+
+  function verifyData() {
+    ok(dataFile.exists(), "Data file does exist");
+  }
+
+  async function verifyUsage(success) {
+    info("Verifying usage in memory");
+
+    let request = getOriginUsage(principal, /* fromMemory */ true);
+    await requestFinished(request);
+
+    if (success) {
+      is(request.result.usage, data.usage, "Correct usage");
+    } else {
+      is(request.result.usage, 0, "Zero usage");
+    }
+
+    info("Verifying usage on disk");
+
+    if (success) {
+      ok(!usageJournalFile.exists(), "Usage journal file doesn't exist");
+      ok(usageFile.exists(), "Usage file does exist");
+      let usage = await readUsageFromUsageFile(usageFile);
+      is(usage, data.usage, "Correct usage");
+    } else {
+      ok(usageJournalFile.exists(), "Usage journal file does exist");
+      ok(usageFile.exists(), "Usage file does exist");
+    }
+  }
+
+  info("Setting prefs");
+
+  Services.prefs.setBoolPref(
+    "dom.storage.enable_unsupported_legacy_implementation",
+    false
+  );
+
+  info("Stage 1 - Testing usage after successful data migration");
+
+  await createStorageForMigration(/* createUsageDir */ false);
+
+  info("Getting storage");
+
+  let storage = getLocalStorage(principal);
+
+  info("Opening");
+
+  storage.open();
+
+  verifyData();
+
+  await verifyUsage(/* success */ true);
+
+  info("Stage 2 - Testing usage after unsuccessful data migration");
+
+  await createStorageForMigration(/* createUsageDir */ true);
+
+  info("Getting storage");
+
+  storage = getLocalStorage(principal);
+
+  info("Opening");
+
+  try {
+    storage.open();
+    ok(false, "Should have thrown");
+  } catch (ex) {
+    ok(true, "Did throw");
+  }
+
+  verifyData();
+
+  await verifyUsage(/* success */ false);
+
+  info("Stage 3 - Testing usage after unsuccessful/successful data migration");
+
+  await createStorageForMigration(/* createUsageDir */ true);
+
+  info("Getting storage");
+
+  storage = getLocalStorage(principal);
+
+  info("Opening");
+
+  try {
+    storage.open();
+    ok(false, "Should have thrown");
+  } catch (ex) {
+    ok(true, "Did throw");
+  }
+
+  usageFile.remove(true);
+
+  info("Opening");
+
+  storage.open();
+
+  verifyData();
+
+  await verifyUsage(/* success */ true);
+});
diff --git a/dom/localstorage/test/unit/usageAfterMigration_profile.zip b/dom/localstorage/test/unit/usageAfterMigration_profile.zip
new file mode 100644
index 0000000000..30a73292c3
--- /dev/null
+++ b/dom/localstorage/test/unit/usageAfterMigration_profile.zip
diff --git a/dom/localstorage/test/unit/xpcshell.ini b/dom/localstorage/test/unit/xpcshell.ini
new file mode 100644
index 0000000000..9b86f34349
--- /dev/null
+++ b/dom/localstorage/test/unit/xpcshell.ini
@@ -0,0 +1,73 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+[DEFAULT]
+head = head.js
+tags = condprof
+support-files =
+  archive_profile.zip
+  corruptedDatabase_profile.zip
+  corruptedDatabase_missingUsageFile_profile.zip
+  groupMismatch_profile.zip
+  migration_profile.zip
+  schema3upgrade_profile.zip
+  schema4upgrade_profile.zip
+  stringLength2_profile.zip
+  stringLength_profile.zip
+  usageAfterMigration_profile.zip
+
+[make_migration_emptyValue.js]
+skip-if = true # Only used for recreating migration_emptyValue_profile.zip
+[test_archive.js]
+[test_clientValidation.js]
+[test_corruptedDatabase.js]
+[test_databaseShadowing1.js]
+prefs =
+  dom.storage.shadow_writes=true
+run-sequentially = test_databaseShadowing2.js depends on a file produced by this test
+[test_databaseShadowing2.js]
+run-sequentially = this test depends on a file produced by test_databaseShadowing1.js
+[test_databaseShadowing_clearOrigin1.js]
+prefs =
+  dom.storage.shadow_writes=true
+run-sequentially = test_databaseShadowing_clearOrigin2.js depends on a file produced by this test
+[test_databaseShadowing_clearOrigin2.js]
+run-sequentially = this test depends on a file produced by test_databaseShadowing_clearOrigin1.js
+[test_databaseShadowing_clearOriginsByPattern1.js]
+prefs =
+  dom.storage.shadow_writes=true
+run-sequentially = test_databaseShadowing_clearOriginsByPattern2.js depends on a file produced by this test
+[test_databaseShadowing_clearOriginsByPattern2.js]
+run-sequentially = this test depends on a file produced by test_databaseShadowing_clearOriginsByPattern1.js
+[test_databaseShadowing_clearOriginsByPrefix1.js]
+prefs =
+  dom.storage.shadow_writes=true
+run-sequentially = test_databaseShadowing_clearOriginsByPrefix2.js depends on a file produced by this test
+[test_databaseShadowing_clearOriginsByPrefix2.js]
+run-sequentially = this test depends on a file produced by test_databaseShadowing_clearOriginsByPrefix1.js
+[test_eviction.js]
+[test_flushing.js]
+[test_groupLimit.js]
+[test_groupMismatch.js]
+[test_largeItems.js]
+[test_lsng_enabled.js]
+[test_migration.js]
+[test_migration_emptyValue.js]
+support-files =
+  migration_emptyValue_profile.zip
+[test_old_lsng_pref.js]
+[test_orderingAfterRemoveAdd.js]
+[test_originInit.js]
+[test_preloading.js]
+[test_schema3upgrade.js]
+[test_schema4upgrade.js]
+[test_snapshotting.js]
+skip-if = tsan # Unreasonably slow, bug 1612707
+requesttimeoutfactor = 4
+[test_stringLength.js]
+[test_stringLength2.js]
+[test_unicodeCharacters.js]
+[test_uri_encoding_edge_cases.js]
+[test_usage.js]
+[test_usageAfterMigration.js]