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diff --git a/dom/localstorage/LSObject.h b/dom/localstorage/LSObject.h
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+/* -*- 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 "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 ipc {
+
+class PrincipalInfo;
+
+}  // namespace ipc
+
+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 {
+  typedef mozilla::ipc::PrincipalInfo PrincipalInfo;
+
+  friend nsGlobalWindowInner;
+
+  UniquePtr<PrincipalInfo> mPrincipalInfo;
+  UniquePtr<PrincipalInfo> mStoragePrincipalInfo;
+
+  RefPtr<LSDatabase> mDatabase;
+  RefPtr<LSObserver> mObserver;
+
+  uint32_t mPrivateBrowsingId;
+  Maybe<nsID> mClientId;
+  nsCString mOrigin;
+  nsCString mOriginKey;
+  nsString mDocumentURI;
+
+  bool mInExplicitSnapshot;
+
+ public:
+  static void Initialize();
+
+  /**
+   * 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);
+
+  /**
+   * Used for requests from the parent process to the parent process; in that
+   * case we want ActorsParent to know our event-target and this is better than
+   * trying to tunnel the pointer through IPC.
+   */
+  static already_AddRefed<nsISerialEventTarget> GetSyncLoopEventTarget();
+
+  /**
+   * Helper invoked by ContentChild::OnChannelReceivedMessage when a sync IPC
+   * message is received.  This will be invoked on the IPC I/O thread and it
+   * will set the gPendingSyncMessage flag to true.  It will also force the sync
+   * loop (if it's active) to check the gPendingSyncMessage flag which will
+   * result in premature finish of the loop.
+   *
+   * This is necessary to unblock the main thread when a sync IPC message is
+   * received to avoid the potential for browser deadlock.  This should only
+   * occur in (ugly) testing scenarios where CPOWs are in use.
+   *
+   * Aborted sync loop will result in the underlying LSRequest being explicitly
+   * canceled, resulting in the parent sending an NS_ERROR_FAILURE result.
+   */
+  static void OnSyncMessageReceived();
+
+  /*
+   * Helper invoked by ContentChild::OnMessageReceived when a sync IPC message
+   * has been handled.  This will be invoked on the main thread and it will
+   * set the gPendingSyncMessage flag to false.
+   */
+  static void OnSyncMessageHandled();
+
+  void AssertIsOnOwningThread() const { NS_ASSERT_OWNINGTHREAD(LSObject); }
+
+  const nsString& DocumentURI() const { return mDocumentURI; }
+
+  LSRequestChild* StartRequest(nsIEventTarget* aMainEventTarget,
+                               const LSRequestParams& aParams,
+                               LSRequestChildCallback* aCallback);
+
+  // Storage overrides.
+  StorageType Type() const override;
+
+  bool IsForkOf(const Storage* aStorage) const override;
+
+  int64_t GetOriginQuotaUsage() const 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 EndExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                           ErrorResult& aError) override;
+
+  bool GetHasActiveSnapshot(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);
+
+  nsresult EndExplicitSnapshotInternal();
+
+  // Storage overrides.
+  void LastRelease() override;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_localstorage_LSObject_h