Adding upstream version 115.7.0esr.upstream/115.7.0esr

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

15 files changed, 3167 insertions, 0 deletions

diff --git a/dom/fs/api/FileSystemDirectoryHandle.cpp b/dom/fs/api/FileSystemDirectoryHandle.cpp
new file mode 100644
index 0000000000..3529f29e78
--- /dev/null
+++ b/dom/fs/api/FileSystemDirectoryHandle.cpp
@@ -0,0 +1,176 @@
+/* -*- 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 "FileSystemDirectoryHandle.h"
+
+#include "FileSystemDirectoryIteratorFactory.h"
+#include "fs/FileSystemRequestHandler.h"
+#include "js/StructuredClone.h"
+#include "js/TypeDecls.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/FileSystemDirectoryHandleBinding.h"
+#include "mozilla/dom/FileSystemHandleBinding.h"
+#include "mozilla/dom/FileSystemLog.h"
+#include "mozilla/dom/FileSystemManager.h"
+#include "mozilla/dom/PFileSystemManager.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/StorageManager.h"
+#include "nsJSUtils.h"
+
+namespace mozilla::dom {
+
+FileSystemDirectoryHandle::FileSystemDirectoryHandle(
+    nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+    const fs::FileSystemEntryMetadata& aMetadata,
+    fs::FileSystemRequestHandler* aRequestHandler)
+    : FileSystemHandle(aGlobal, aManager, aMetadata, aRequestHandler) {}
+
+FileSystemDirectoryHandle::FileSystemDirectoryHandle(
+    nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+    const fs::FileSystemEntryMetadata& aMetadata)
+    : FileSystemDirectoryHandle(aGlobal, aManager, aMetadata,
+                                new fs::FileSystemRequestHandler()) {}
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(FileSystemDirectoryHandle,
+                                               FileSystemHandle)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(FileSystemDirectoryHandle, FileSystemHandle)
+
+// WebIDL Boilerplate
+
+JSObject* FileSystemDirectoryHandle::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return FileSystemDirectoryHandle_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// WebIDL Interface
+
+FileSystemHandleKind FileSystemDirectoryHandle::Kind() const {
+  return FileSystemHandleKind::Directory;
+}
+
+void FileSystemDirectoryHandle::InitAsyncIteratorData(
+    IteratorData& aData, iterator_t::IteratorType aType, ErrorResult& aError) {
+  aData.mImpl =
+      fs::FileSystemDirectoryIteratorFactory::Create(mMetadata, aType);
+}
+
+already_AddRefed<Promise> FileSystemDirectoryHandle::GetNextIterationResult(
+    FileSystemDirectoryHandle::iterator_t* aIterator, ErrorResult& aError) {
+  LOG_VERBOSE(("GetNextIterationResult"));
+  return aIterator->Data().mImpl->Next(mGlobal, mManager, aError);
+}
+
+already_AddRefed<Promise> FileSystemDirectoryHandle::GetFileHandle(
+    const nsAString& aName, const FileSystemGetFileOptions& aOptions,
+    ErrorResult& aError) {
+  MOZ_ASSERT(!mMetadata.entryId().IsEmpty());
+
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  fs::Name name(aName);
+  fs::FileSystemChildMetadata metadata(mMetadata.entryId(), name);
+  mRequestHandler->GetFileHandle(mManager, metadata, aOptions.mCreate, promise,
+                                 aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  return promise.forget();
+}
+
+already_AddRefed<Promise> FileSystemDirectoryHandle::GetDirectoryHandle(
+    const nsAString& aName, const FileSystemGetDirectoryOptions& aOptions,
+    ErrorResult& aError) {
+  MOZ_ASSERT(!mMetadata.entryId().IsEmpty());
+
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  fs::Name name(aName);
+  fs::FileSystemChildMetadata metadata(mMetadata.entryId(), name);
+  mRequestHandler->GetDirectoryHandle(mManager, metadata, aOptions.mCreate,
+                                      promise, aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  return promise.forget();
+}
+
+already_AddRefed<Promise> FileSystemDirectoryHandle::RemoveEntry(
+    const nsAString& aName, const FileSystemRemoveOptions& aOptions,
+    ErrorResult& aError) {
+  MOZ_ASSERT(!mMetadata.entryId().IsEmpty());
+
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  fs::Name name(aName);
+  fs::FileSystemChildMetadata metadata(mMetadata.entryId(), name);
+
+  mRequestHandler->RemoveEntry(mManager, metadata, aOptions.mRecursive, promise,
+                               aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  return promise.forget();
+}
+
+already_AddRefed<Promise> FileSystemDirectoryHandle::Resolve(
+    FileSystemHandle& aPossibleDescendant, ErrorResult& aError) {
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  LOG_VERBOSE(("Resolve"));
+
+  fs::FileSystemEntryPair pair(mMetadata.entryId(),
+                               aPossibleDescendant.GetId());
+  mRequestHandler->Resolve(mManager, pair, promise, aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  return promise.forget();
+}
+
+// [Serializable] implementation
+
+// static
+already_AddRefed<FileSystemDirectoryHandle>
+FileSystemDirectoryHandle::ReadStructuredClone(
+    JSContext* aCx, nsIGlobalObject* aGlobal,
+    JSStructuredCloneReader* aReader) {
+  uint32_t kind = static_cast<uint32_t>(FileSystemHandleKind::EndGuard_);
+
+  if (!JS_ReadBytes(aReader, reinterpret_cast<void*>(&kind),
+                    sizeof(uint32_t))) {
+    return nullptr;
+  }
+
+  if (kind != static_cast<uint32_t>(FileSystemHandleKind::Directory)) {
+    return nullptr;
+  }
+
+  RefPtr<FileSystemDirectoryHandle> result =
+      FileSystemHandle::ConstructDirectoryHandle(aCx, aGlobal, aReader);
+  if (!result) {
+    return nullptr;
+  }
+
+  return result.forget();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/fs/api/FileSystemDirectoryHandle.h b/dom/fs/api/FileSystemDirectoryHandle.h
new file mode 100644
index 0000000000..36ac7455c5
--- /dev/null
+++ b/dom/fs/api/FileSystemDirectoryHandle.h
@@ -0,0 +1,86 @@
+/* -*- 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_FS_FILESYSTEMDIRECTORYHANDLE_H_
+#define DOM_FS_FILESYSTEMDIRECTORYHANDLE_H_
+
+#include "mozilla/dom/FileSystemDirectoryIterator.h"
+#include "mozilla/dom/FileSystemHandle.h"
+#include "mozilla/dom/IterableIterator.h"
+
+namespace mozilla {
+
+class ErrorResult;
+
+namespace dom {
+
+struct FileSystemGetFileOptions;
+struct FileSystemGetDirectoryOptions;
+struct FileSystemRemoveOptions;
+
+class FileSystemDirectoryHandle final : public FileSystemHandle {
+ public:
+  using iterator_t = AsyncIterableIterator<FileSystemDirectoryHandle>;
+
+  FileSystemDirectoryHandle(nsIGlobalObject* aGlobal,
+                            RefPtr<FileSystemManager>& aManager,
+                            const fs::FileSystemEntryMetadata& aMetadata,
+                            fs::FileSystemRequestHandler* aRequestHandler);
+
+  FileSystemDirectoryHandle(nsIGlobalObject* aGlobal,
+                            RefPtr<FileSystemManager>& aManager,
+                            const fs::FileSystemEntryMetadata& aMetadata);
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(FileSystemDirectoryHandle,
+                                           FileSystemHandle)
+
+  // WebIDL Boilerplate
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // WebIDL Interface
+  FileSystemHandleKind Kind() const override;
+
+  struct IteratorData {
+    UniquePtr<FileSystemDirectoryIterator::Impl> mImpl;
+  };
+
+  void InitAsyncIteratorData(IteratorData& aData,
+                             iterator_t::IteratorType aType,
+                             ErrorResult& aError);
+
+  [[nodiscard]] already_AddRefed<Promise> GetNextIterationResult(
+      iterator_t* aIterator, ErrorResult& aError);
+
+  already_AddRefed<Promise> GetFileHandle(
+      const nsAString& aName, const FileSystemGetFileOptions& aOptions,
+      ErrorResult& aError);
+
+  already_AddRefed<Promise> GetDirectoryHandle(
+      const nsAString& aName, const FileSystemGetDirectoryOptions& aOptions,
+      ErrorResult& aError);
+
+  already_AddRefed<Promise> RemoveEntry(const nsAString& aName,
+                                        const FileSystemRemoveOptions& aOptions,
+                                        ErrorResult& aError);
+
+  already_AddRefed<Promise> Resolve(FileSystemHandle& aPossibleDescendant,
+                                    ErrorResult& aError);
+
+  // [Serializable]
+  static already_AddRefed<FileSystemDirectoryHandle> ReadStructuredClone(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      JSStructuredCloneReader* aReader);
+
+ private:
+  ~FileSystemDirectoryHandle() = default;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // DOM_FS_FILESYSTEMDIRECTORYHANDLE_H_
diff --git a/dom/fs/api/FileSystemDirectoryIterator.cpp b/dom/fs/api/FileSystemDirectoryIterator.cpp
new file mode 100644
index 0000000000..45e7511e4a
--- /dev/null
+++ b/dom/fs/api/FileSystemDirectoryIterator.cpp
@@ -0,0 +1,53 @@
+/* -*- 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 "FileSystemDirectoryIterator.h"
+
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/FileSystemDirectoryIteratorBinding.h"
+#include "mozilla/dom/FileSystemManager.h"
+#include "mozilla/dom/Promise.h"
+
+namespace mozilla::dom {
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(FileSystemDirectoryIterator)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+NS_IMPL_CYCLE_COLLECTING_ADDREF(FileSystemDirectoryIterator);
+NS_IMPL_CYCLE_COLLECTING_RELEASE(FileSystemDirectoryIterator);
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(FileSystemDirectoryIterator, mGlobal);
+
+FileSystemDirectoryIterator::FileSystemDirectoryIterator(
+    nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+    UniquePtr<Impl> aImpl)
+    : mGlobal(aGlobal), mManager(aManager), mImpl(std::move(aImpl)) {}
+
+// WebIDL Boilerplate
+
+nsIGlobalObject* FileSystemDirectoryIterator::GetParentObject() const {
+  return mGlobal;
+}
+
+JSObject* FileSystemDirectoryIterator::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return FileSystemDirectoryIterator_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// WebIDL Interface
+
+already_AddRefed<Promise> FileSystemDirectoryIterator::Next(
+    ErrorResult& aError) {
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  MOZ_ASSERT(mImpl);
+  return mImpl->Next(mGlobal, mManager, aError);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/fs/api/FileSystemDirectoryIterator.h b/dom/fs/api/FileSystemDirectoryIterator.h
new file mode 100644
index 0000000000..d761ae6d2b
--- /dev/null
+++ b/dom/fs/api/FileSystemDirectoryIterator.h
@@ -0,0 +1,70 @@
+/* -*- 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_FS_FILESYSTEMDIRECTORYITERATOR_H_
+#define DOM_FS_FILESYSTEMDIRECTORYITERATOR_H_
+
+#include "mozilla/dom/IterableIterator.h"
+#include "nsCOMPtr.h"
+#include "nsISupports.h"
+#include "nsWrapperCache.h"
+
+class nsIGlobalObject;
+
+namespace mozilla {
+
+class ErrorResult;
+
+namespace dom {
+
+class FileSystemManager;
+class IterableIteratorBase;
+class Promise;
+
+// XXX This class isn't used to support iteration anymore. `Impl` should be
+// extracted elsewhere and `FileSystemDirectoryIterator` should be removed
+// completely
+class FileSystemDirectoryIterator : public nsISupports, public nsWrapperCache {
+ public:
+  class Impl {
+   public:
+    virtual already_AddRefed<Promise> Next(nsIGlobalObject* aGlobal,
+                                           RefPtr<FileSystemManager>& aManager,
+                                           ErrorResult& aError) = 0;
+    virtual ~Impl() = default;
+  };
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(FileSystemDirectoryIterator)
+
+  explicit FileSystemDirectoryIterator(nsIGlobalObject* aGlobal,
+                                       RefPtr<FileSystemManager>& aManager,
+                                       UniquePtr<Impl> aImpl);
+
+  // WebIDL Boilerplate
+  nsIGlobalObject* GetParentObject() const;
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // WebIDL Interface
+  already_AddRefed<Promise> Next(ErrorResult& aError);
+
+ protected:
+  virtual ~FileSystemDirectoryIterator() = default;
+
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  RefPtr<FileSystemManager> mManager;
+
+ private:
+  UniquePtr<Impl> mImpl;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // DOM_FS_FILESYSTEMDIRECTORYITERATOR_H_
diff --git a/dom/fs/api/FileSystemFileHandle.cpp b/dom/fs/api/FileSystemFileHandle.cpp
new file mode 100644
index 0000000000..4d8306857f
--- /dev/null
+++ b/dom/fs/api/FileSystemFileHandle.cpp
@@ -0,0 +1,122 @@
+/* -*- 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 "FileSystemFileHandle.h"
+
+#include "fs/FileSystemRequestHandler.h"
+#include "js/StructuredClone.h"
+#include "js/TypeDecls.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/FileSystemFileHandleBinding.h"
+#include "mozilla/dom/FileSystemHandleBinding.h"
+#include "mozilla/dom/FileSystemManager.h"
+#include "mozilla/dom/PFileSystemManager.h"
+#include "mozilla/dom/Promise.h"
+
+namespace mozilla::dom {
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(FileSystemFileHandle,
+                                               FileSystemHandle)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(FileSystemFileHandle, FileSystemHandle)
+
+FileSystemFileHandle::FileSystemFileHandle(
+    nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+    const fs::FileSystemEntryMetadata& aMetadata,
+    fs::FileSystemRequestHandler* aRequestHandler)
+    : FileSystemHandle(aGlobal, aManager, aMetadata, aRequestHandler) {}
+
+FileSystemFileHandle::FileSystemFileHandle(
+    nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+    const fs::FileSystemEntryMetadata& aMetadata)
+    : FileSystemFileHandle(aGlobal, aManager, aMetadata,
+                           new fs::FileSystemRequestHandler()) {}
+
+// WebIDL Boilerplate
+
+JSObject* FileSystemFileHandle::WrapObject(JSContext* aCx,
+                                           JS::Handle<JSObject*> aGivenProto) {
+  return FileSystemFileHandle_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// WebIDL Interface
+
+FileSystemHandleKind FileSystemFileHandle::Kind() const {
+  return FileSystemHandleKind::File;
+}
+
+already_AddRefed<Promise> FileSystemFileHandle::GetFile(ErrorResult& aError) {
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  mRequestHandler->GetFile(mManager, mMetadata, promise, aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  return promise.forget();
+}
+
+already_AddRefed<Promise> FileSystemFileHandle::CreateWritable(
+    const FileSystemCreateWritableOptions& aOptions, ErrorResult& aError) {
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  mRequestHandler->GetWritable(mManager, mMetadata, aOptions.mKeepExistingData,
+                               promise, aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  return promise.forget();
+}
+
+already_AddRefed<Promise> FileSystemFileHandle::CreateSyncAccessHandle(
+    ErrorResult& aError) {
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  mRequestHandler->GetAccessHandle(mManager, mMetadata, promise, aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  return promise.forget();
+}
+
+// [Serializable] implementation
+
+// static
+already_AddRefed<FileSystemFileHandle>
+FileSystemFileHandle::ReadStructuredClone(JSContext* aCx,
+                                          nsIGlobalObject* aGlobal,
+                                          JSStructuredCloneReader* aReader) {
+  uint32_t kind = static_cast<uint32_t>(FileSystemHandleKind::EndGuard_);
+
+  if (!JS_ReadBytes(aReader, reinterpret_cast<void*>(&kind),
+                    sizeof(uint32_t))) {
+    return nullptr;
+  }
+
+  if (kind != static_cast<uint32_t>(FileSystemHandleKind::File)) {
+    return nullptr;
+  }
+
+  RefPtr<FileSystemFileHandle> result =
+      FileSystemHandle::ConstructFileHandle(aCx, aGlobal, aReader);
+  if (!result) {
+    return nullptr;
+  }
+
+  return result.forget();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/fs/api/FileSystemFileHandle.h b/dom/fs/api/FileSystemFileHandle.h
new file mode 100644
index 0000000000..c0606c85a6
--- /dev/null
+++ b/dom/fs/api/FileSystemFileHandle.h
@@ -0,0 +1,61 @@
+/* -*- 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_FS_FILESYSTEMFILEHANDLE_H_
+#define DOM_FS_FILESYSTEMFILEHANDLE_H_
+
+#include "mozilla/dom/FileSystemHandle.h"
+
+namespace mozilla {
+
+class ErrorResult;
+
+namespace dom {
+
+struct FileSystemCreateWritableOptions;
+
+class FileSystemFileHandle final : public FileSystemHandle {
+ public:
+  FileSystemFileHandle(nsIGlobalObject* aGlobal,
+                       RefPtr<FileSystemManager>& aManager,
+                       const fs::FileSystemEntryMetadata& aMetadata,
+                       fs::FileSystemRequestHandler* aRequestHandler);
+
+  FileSystemFileHandle(nsIGlobalObject* aGlobal,
+                       RefPtr<FileSystemManager>& aManager,
+                       const fs::FileSystemEntryMetadata& aMetadata);
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(FileSystemFileHandle,
+                                           FileSystemHandle)
+
+  // WebIDL Boilerplate
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // WebIDL interface
+  FileSystemHandleKind Kind() const override;
+
+  already_AddRefed<Promise> GetFile(ErrorResult& aError);
+
+  already_AddRefed<Promise> CreateWritable(
+      const FileSystemCreateWritableOptions& aOptions, ErrorResult& aError);
+
+  already_AddRefed<Promise> CreateSyncAccessHandle(ErrorResult& aError);
+
+  // [Serializable]
+  static already_AddRefed<FileSystemFileHandle> ReadStructuredClone(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      JSStructuredCloneReader* aReader);
+
+ private:
+  ~FileSystemFileHandle() = default;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // DOM_FS_FILESYSTEMFILEHANDLE_H_
diff --git a/dom/fs/api/FileSystemHandle.cpp b/dom/fs/api/FileSystemHandle.cpp
new file mode 100644
index 0000000000..beca83d0bf
--- /dev/null
+++ b/dom/fs/api/FileSystemHandle.cpp
@@ -0,0 +1,319 @@
+/* -*- 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 "FileSystemHandle.h"
+
+#include "FileSystemDirectoryHandle.h"
+#include "FileSystemFileHandle.h"
+#include "fs/FileSystemRequestHandler.h"
+#include "js/StructuredClone.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/FileSystemHandleBinding.h"
+#include "mozilla/dom/FileSystemLog.h"
+#include "mozilla/dom/FileSystemManager.h"
+#include "mozilla/dom/Promise-inl.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/StorageManager.h"
+#include "mozilla/dom/StructuredCloneHolder.h"
+#include "mozilla/dom/quota/QuotaCommon.h"
+#include "mozilla/ipc/PBackgroundSharedTypes.h"
+#include "nsJSPrincipals.h"
+#include "nsString.h"
+#include "prio.h"
+#include "private/pprio.h"
+#include "xpcpublic.h"
+
+namespace mozilla::dom {
+
+namespace {
+
+bool ConstructHandleMetadata(JSContext* aCx, nsIGlobalObject* aGlobal,
+                             JSStructuredCloneReader* aReader,
+                             const bool aDirectory,
+                             fs::FileSystemEntryMetadata& aMetadata) {
+  using namespace mozilla::dom::fs;
+
+  EntryId entryId;
+  if (!entryId.SetLength(32u, fallible)) {
+    return false;
+  }
+
+  if (!JS_ReadBytes(aReader, static_cast<void*>(entryId.BeginWriting()), 32u)) {
+    return false;
+  }
+
+  Name name;
+  if (!StructuredCloneHolder::ReadString(aReader, name)) {
+    return false;
+  }
+
+  mozilla::ipc::PrincipalInfo storageKey;
+  if (!nsJSPrincipals::ReadPrincipalInfo(aReader, storageKey)) {
+    return false;
+  }
+
+  QM_TRY_UNWRAP(auto hasEqualStorageKey,
+                aGlobal->HasEqualStorageKey(storageKey), false);
+
+  if (!hasEqualStorageKey) {
+    LOG(("Blocking deserialization of %s due to cross-origin",
+         NS_ConvertUTF16toUTF8(name).get()));
+    return false;
+  }
+
+  LOG_VERBOSE(("Deserializing %s", NS_ConvertUTF16toUTF8(name).get()));
+
+  aMetadata = fs::FileSystemEntryMetadata(entryId, name, aDirectory);
+  return true;
+}
+
+}  // namespace
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(FileSystemHandle)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(FileSystemHandle)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(FileSystemHandle)
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(FileSystemHandle)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(FileSystemHandle)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mGlobal)
+  // Don't unlink mManager!
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(FileSystemHandle)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mGlobal)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mManager)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+FileSystemHandle::FileSystemHandle(
+    nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+    const fs::FileSystemEntryMetadata& aMetadata,
+    fs::FileSystemRequestHandler* aRequestHandler)
+    : mGlobal(aGlobal),
+      mManager(aManager),
+      mMetadata(aMetadata),
+      mRequestHandler(aRequestHandler) {
+  MOZ_ASSERT(!mMetadata.entryId().IsEmpty());
+}
+
+// WebIDL Boilerplate
+
+nsIGlobalObject* FileSystemHandle::GetParentObject() const { return mGlobal; }
+
+JSObject* FileSystemHandle::WrapObject(JSContext* aCx,
+                                       JS::Handle<JSObject*> aGivenProto) {
+  return FileSystemHandle_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// WebIDL Interface
+
+void FileSystemHandle::GetName(nsAString& aResult) {
+  aResult = mMetadata.entryName();
+}
+
+already_AddRefed<Promise> FileSystemHandle::IsSameEntry(
+    FileSystemHandle& aOther, ErrorResult& aError) const {
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  // Handles the case of "dir = createdir foo; removeEntry(foo); file =
+  // createfile foo; issameentry(dir, file)"
+  const bool result = mMetadata.entryId().Equals(aOther.mMetadata.entryId()) &&
+                      Kind() == aOther.Kind();
+  promise->MaybeResolve(result);
+
+  return promise.forget();
+}
+
+already_AddRefed<Promise> FileSystemHandle::Move(const nsAString& aName,
+                                                 ErrorResult& aError) {
+  LOG(("Move %s to %s", NS_ConvertUTF16toUTF8(mMetadata.entryName()).get(),
+       NS_ConvertUTF16toUTF8(aName).get()));
+
+  fs::EntryId parent;  // empty means same directory
+  return Move(parent, aName, aError);
+}
+
+already_AddRefed<Promise> FileSystemHandle::Move(
+    FileSystemDirectoryHandle& aParent, ErrorResult& aError) {
+  LOG(("Move %s to %s/%s", NS_ConvertUTF16toUTF8(mMetadata.entryName()).get(),
+       NS_ConvertUTF16toUTF8(aParent.mMetadata.entryName()).get(),
+       NS_ConvertUTF16toUTF8(mMetadata.entryName()).get()));
+  return Move(aParent, mMetadata.entryName(), aError);
+}
+
+already_AddRefed<Promise> FileSystemHandle::Move(
+    FileSystemDirectoryHandle& aParent, const nsAString& aName,
+    ErrorResult& aError) {
+  LOG(("Move %s to %s/%s", NS_ConvertUTF16toUTF8(mMetadata.entryName()).get(),
+       NS_ConvertUTF16toUTF8(aParent.mMetadata.entryName()).get(),
+       NS_ConvertUTF16toUTF8(aName).get()));
+  return Move(aParent.mMetadata.entryId(), aName, aError);
+}
+
+already_AddRefed<Promise> FileSystemHandle::Move(const fs::EntryId& aParentId,
+                                                 const nsAString& aName,
+                                                 ErrorResult& aError) {
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  fs::Name name(aName);
+  if (!aParentId.IsEmpty()) {
+    fs::FileSystemChildMetadata newMetadata;
+    newMetadata.parentId() = aParentId;
+    newMetadata.childName() = aName;
+    mRequestHandler->MoveEntry(mManager, this, mMetadata, newMetadata, promise,
+                               aError);
+  } else {
+    mRequestHandler->RenameEntry(mManager, this, mMetadata, name, promise,
+                                 aError);
+  }
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  // Other handles to this will be broken, and the spec is ok with this, but we
+  // need to update our EntryId and name
+  promise->AddCallbacksWithCycleCollectedArgs(
+      [name](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+             FileSystemHandle* aHandle) {
+        // XXX Fix entryId!
+        LOG(("Changing FileSystemHandle name from %s to %s",
+             NS_ConvertUTF16toUTF8(aHandle->mMetadata.entryName()).get(),
+             NS_ConvertUTF16toUTF8(name).get()));
+        aHandle->mMetadata.entryName() = name;
+      },
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         FileSystemHandle* aHandle) {
+        LOG(("reject of move for %s",
+             NS_ConvertUTF16toUTF8(aHandle->mMetadata.entryName()).get()));
+      },
+      RefPtr(this));
+
+  return promise.forget();
+}
+
+// [Serializable] implementation
+
+// static
+already_AddRefed<FileSystemHandle> FileSystemHandle::ReadStructuredClone(
+    JSContext* aCx, nsIGlobalObject* aGlobal,
+    JSStructuredCloneReader* aReader) {
+  LOG_VERBOSE(("Reading File/DirectoryHandle"));
+
+  uint32_t kind = static_cast<uint32_t>(FileSystemHandleKind::EndGuard_);
+
+  if (!JS_ReadBytes(aReader, reinterpret_cast<void*>(&kind),
+                    sizeof(uint32_t))) {
+    return nullptr;
+  }
+
+  if (kind == static_cast<uint32_t>(FileSystemHandleKind::Directory)) {
+    RefPtr<FileSystemHandle> result =
+        FileSystemHandle::ConstructDirectoryHandle(aCx, aGlobal, aReader);
+    return result.forget();
+  }
+
+  if (kind == static_cast<uint32_t>(FileSystemHandleKind::File)) {
+    RefPtr<FileSystemHandle> result =
+        FileSystemHandle::ConstructFileHandle(aCx, aGlobal, aReader);
+    return result.forget();
+  }
+
+  return nullptr;
+}
+
+bool FileSystemHandle::WriteStructuredClone(
+    JSContext* aCx, JSStructuredCloneWriter* aWriter) const {
+  LOG_VERBOSE(("Writing File/DirectoryHandle"));
+  MOZ_ASSERT(mMetadata.entryId().Length() == 32);
+
+  auto kind = static_cast<uint32_t>(Kind());
+  if (NS_WARN_IF(!JS_WriteBytes(aWriter, static_cast<void*>(&kind),
+                                sizeof(uint32_t)))) {
+    return false;
+  }
+
+  if (NS_WARN_IF(!JS_WriteBytes(
+          aWriter, static_cast<const void*>(mMetadata.entryId().get()),
+          mMetadata.entryId().Length()))) {
+    return false;
+  }
+
+  if (!StructuredCloneHolder::WriteString(aWriter, mMetadata.entryName())) {
+    return false;
+  }
+
+  // Needed to make sure the destination nsIGlobalObject is from the same
+  // origin/principal
+  QM_TRY_INSPECT(const auto& storageKey, mGlobal->GetStorageKey(), false);
+
+  return nsJSPrincipals::WritePrincipalInfo(aWriter, storageKey);
+}
+
+// static
+already_AddRefed<FileSystemFileHandle> FileSystemHandle::ConstructFileHandle(
+    JSContext* aCx, nsIGlobalObject* aGlobal,
+    JSStructuredCloneReader* aReader) {
+  LOG(("Reading FileHandle"));
+
+  fs::FileSystemEntryMetadata metadata;
+  if (!ConstructHandleMetadata(aCx, aGlobal, aReader, /* aDirectory */ false,
+                               metadata)) {
+    return nullptr;
+  }
+
+  RefPtr<StorageManager> storageManager = aGlobal->GetStorageManager();
+  if (!storageManager) {
+    return nullptr;
+  }
+
+  // Note that the actor may not exist or may not be connected yet.
+  RefPtr<FileSystemManager> fileSystemManager =
+      storageManager->GetFileSystemManager();
+
+  RefPtr<FileSystemFileHandle> fsHandle =
+      new FileSystemFileHandle(aGlobal, fileSystemManager, metadata);
+
+  return fsHandle.forget();
+}
+
+// static
+already_AddRefed<FileSystemDirectoryHandle>
+FileSystemHandle::ConstructDirectoryHandle(JSContext* aCx,
+                                           nsIGlobalObject* aGlobal,
+                                           JSStructuredCloneReader* aReader) {
+  LOG(("Reading DirectoryHandle"));
+
+  fs::FileSystemEntryMetadata metadata;
+  if (!ConstructHandleMetadata(aCx, aGlobal, aReader, /* aDirectory */ true,
+                               metadata)) {
+    return nullptr;
+  }
+
+  RefPtr<StorageManager> storageManager = aGlobal->GetStorageManager();
+  if (!storageManager) {
+    return nullptr;
+  }
+
+  // Note that the actor may not exist or may not be connected yet.
+  RefPtr<FileSystemManager> fileSystemManager =
+      storageManager->GetFileSystemManager();
+
+  RefPtr<FileSystemDirectoryHandle> fsHandle =
+      new FileSystemDirectoryHandle(aGlobal, fileSystemManager, metadata);
+
+  return fsHandle.forget();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/fs/api/FileSystemHandle.h b/dom/fs/api/FileSystemHandle.h
new file mode 100644
index 0000000000..76ad66f5ea
--- /dev/null
+++ b/dom/fs/api/FileSystemHandle.h
@@ -0,0 +1,107 @@
+/* -*- 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_FS_FILESYSTEMHANDLE_H_
+#define DOM_FS_FILESYSTEMHANDLE_H_
+
+#include "mozilla/dom/PFileSystemManager.h"
+#include "nsCOMPtr.h"
+#include "nsISupports.h"
+#include "nsWrapperCache.h"
+
+class nsIGlobalObject;
+
+namespace mozilla {
+
+class ErrorResult;
+
+namespace dom {
+
+class FileSystemDirectoryHandle;
+class FileSystemFileHandle;
+enum class FileSystemHandleKind : uint8_t;
+class FileSystemManager;
+class FileSystemManagerChild;
+class Promise;
+
+namespace fs {
+class FileSystemRequestHandler;
+}  // namespace fs
+
+class FileSystemHandle : public nsISupports, public nsWrapperCache {
+ public:
+  FileSystemHandle(nsIGlobalObject* aGlobal,
+                   RefPtr<FileSystemManager>& aManager,
+                   const fs::FileSystemEntryMetadata& aMetadata,
+                   fs::FileSystemRequestHandler* aRequestHandler);
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(FileSystemHandle)
+
+  const fs::EntryId& GetId() const { return mMetadata.entryId(); }
+
+  // WebIDL Boilerplate
+  nsIGlobalObject* GetParentObject() const;
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // WebIDL Interface
+  virtual FileSystemHandleKind Kind() const = 0;
+
+  void GetName(nsAString& aResult);
+
+  already_AddRefed<Promise> IsSameEntry(FileSystemHandle& aOther,
+                                        ErrorResult& aError) const;
+
+  // [Serializable] implementation
+  static already_AddRefed<FileSystemHandle> ReadStructuredClone(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      JSStructuredCloneReader* aReader);
+
+  virtual bool WriteStructuredClone(JSContext* aCx,
+                                    JSStructuredCloneWriter* aWriter) const;
+
+  already_AddRefed<Promise> Move(const nsAString& aName, ErrorResult& aError);
+
+  already_AddRefed<Promise> Move(FileSystemDirectoryHandle& aParent,
+                                 ErrorResult& aError);
+
+  already_AddRefed<Promise> Move(FileSystemDirectoryHandle& aParent,
+                                 const nsAString& aName, ErrorResult& aError);
+
+  already_AddRefed<Promise> Move(const fs::EntryId& aParentId,
+                                 const nsAString& aName, ErrorResult& aError);
+
+  void UpdateMetadata(const fs::FileSystemEntryMetadata& aMetadata) {
+    mMetadata = aMetadata;
+  }
+
+ protected:
+  virtual ~FileSystemHandle() = default;
+
+  static already_AddRefed<FileSystemFileHandle> ConstructFileHandle(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      JSStructuredCloneReader* aReader);
+
+  static already_AddRefed<FileSystemDirectoryHandle> ConstructDirectoryHandle(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      JSStructuredCloneReader* aReader);
+
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  RefPtr<FileSystemManager> mManager;
+
+  // move() can change names/directories
+  fs::FileSystemEntryMetadata mMetadata;
+
+  const UniquePtr<fs::FileSystemRequestHandler> mRequestHandler;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // DOM_FS_FILESYSTEMHANDLE_H_
diff --git a/dom/fs/api/FileSystemManager.cpp b/dom/fs/api/FileSystemManager.cpp
new file mode 100644
index 0000000000..099739f8a1
--- /dev/null
+++ b/dom/fs/api/FileSystemManager.cpp
@@ -0,0 +1,153 @@
+/* -*- 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 "mozilla/dom/FileSystemManager.h"
+
+#include "FileSystemBackgroundRequestHandler.h"
+#include "fs/FileSystemRequestHandler.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/FileSystemManagerChild.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/StorageManager.h"
+#include "mozilla/dom/fs/ManagedMozPromiseRequestHolder.h"
+#include "mozilla/dom/quota/QuotaCommon.h"
+#include "mozilla/dom/quota/ResultExtensions.h"
+
+namespace mozilla::dom {
+
+FileSystemManager::FileSystemManager(
+    nsIGlobalObject* aGlobal, RefPtr<StorageManager> aStorageManager,
+    RefPtr<FileSystemBackgroundRequestHandler> aBackgroundRequestHandler)
+    : mGlobal(aGlobal),
+      mStorageManager(std::move(aStorageManager)),
+      mBackgroundRequestHandler(std::move(aBackgroundRequestHandler)),
+      mRequestHandler(new fs::FileSystemRequestHandler()) {}
+
+FileSystemManager::FileSystemManager(nsIGlobalObject* aGlobal,
+                                     RefPtr<StorageManager> aStorageManager)
+    : FileSystemManager(aGlobal, std::move(aStorageManager),
+                        MakeRefPtr<FileSystemBackgroundRequestHandler>()) {}
+
+FileSystemManager::~FileSystemManager() { MOZ_ASSERT(mShutdown); }
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(FileSystemManager)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+NS_IMPL_CYCLE_COLLECTING_ADDREF(FileSystemManager);
+NS_IMPL_CYCLE_COLLECTING_RELEASE(FileSystemManager);
+NS_IMPL_CYCLE_COLLECTION(FileSystemManager, mGlobal, mStorageManager);
+
+void FileSystemManager::Shutdown() {
+  mShutdown.Flip();
+
+  auto shutdownAndDisconnect = [self = RefPtr(this)]() {
+    self->mBackgroundRequestHandler->Shutdown();
+
+    for (RefPtr<PromiseRequestHolder<BoolPromise>> holder :
+         self->mPromiseRequestHolders.ForwardRange()) {
+      holder->DisconnectIfExists();
+    }
+  };
+
+  if (NS_IsMainThread()) {
+    if (mBackgroundRequestHandler->FileSystemManagerChildStrongRef()) {
+      mBackgroundRequestHandler->FileSystemManagerChildStrongRef()
+          ->CloseAllWritables(
+              [shutdownAndDisconnect = std::move(shutdownAndDisconnect)]() {
+                shutdownAndDisconnect();
+              });
+    } else {
+      shutdownAndDisconnect();
+    }
+  } else {
+    if (mBackgroundRequestHandler->FileSystemManagerChildStrongRef()) {
+      // FileSystemAccessHandles and FileSystemWritableFileStreams prevent
+      // shutdown until they are full closed, so at this point, they all should
+      // be closed.
+      MOZ_ASSERT(mBackgroundRequestHandler->FileSystemManagerChildStrongRef()
+                     ->AllSyncAccessHandlesClosed());
+      MOZ_ASSERT(mBackgroundRequestHandler->FileSystemManagerChildStrongRef()
+                     ->AllWritableFileStreamsClosed());
+    }
+
+    shutdownAndDisconnect();
+  }
+}
+
+const RefPtr<FileSystemManagerChild>& FileSystemManager::ActorStrongRef()
+    const {
+  return mBackgroundRequestHandler->FileSystemManagerChildStrongRef();
+}
+
+void FileSystemManager::RegisterPromiseRequestHolder(
+    PromiseRequestHolder<BoolPromise>* aHolder) {
+  mPromiseRequestHolders.AppendElement(aHolder);
+}
+
+void FileSystemManager::UnregisterPromiseRequestHolder(
+    PromiseRequestHolder<BoolPromise>* aHolder) {
+  mPromiseRequestHolders.RemoveElement(aHolder);
+}
+
+void FileSystemManager::BeginRequest(
+    std::function<void(const RefPtr<FileSystemManagerChild>&)>&& aSuccess,
+    std::function<void(nsresult)>&& aFailure) {
+  MOZ_ASSERT(!mShutdown);
+
+  MOZ_ASSERT(mGlobal);
+
+  // Check if we're allowed to use storage
+  if (mGlobal->GetStorageAccess() < StorageAccess::eSessionScoped) {
+    aFailure(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  if (mBackgroundRequestHandler->FileSystemManagerChildStrongRef()) {
+    aSuccess(mBackgroundRequestHandler->FileSystemManagerChildStrongRef());
+    return;
+  }
+
+  QM_TRY_INSPECT(const auto& principalInfo, mGlobal->GetStorageKey(), QM_VOID,
+                 [&aFailure](nsresult rv) { aFailure(rv); });
+
+  auto holder = MakeRefPtr<PromiseRequestHolder<BoolPromise>>(this);
+
+  mBackgroundRequestHandler->CreateFileSystemManagerChild(principalInfo)
+      ->Then(GetCurrentSerialEventTarget(), __func__,
+             [self = RefPtr<FileSystemManager>(this), holder,
+              success = std::move(aSuccess), failure = std::move(aFailure)](
+                 const BoolPromise::ResolveOrRejectValue& aValue) {
+               holder->Complete();
+
+               if (aValue.IsResolve()) {
+                 success(self->mBackgroundRequestHandler
+                             ->FileSystemManagerChildStrongRef());
+               } else {
+                 failure(aValue.RejectValue());
+               }
+             })
+      ->Track(*holder);
+}
+
+already_AddRefed<Promise> FileSystemManager::GetDirectory(ErrorResult& aError) {
+  MOZ_ASSERT(mGlobal);
+
+  RefPtr<Promise> promise = Promise::Create(mGlobal, aError);
+  if (NS_WARN_IF(aError.Failed())) {
+    return nullptr;
+  }
+
+  MOZ_ASSERT(promise);
+
+  mRequestHandler->GetRootHandle(this, promise, aError);
+  if (NS_WARN_IF(aError.Failed())) {
+    return nullptr;
+  }
+
+  return promise.forget();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/fs/api/FileSystemManager.h b/dom/fs/api/FileSystemManager.h
new file mode 100644
index 0000000000..b910eacea2
--- /dev/null
+++ b/dom/fs/api/FileSystemManager.h
@@ -0,0 +1,102 @@
+/* -*- 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_FS_CHILD_FILESYSTEMMANAGER_H_
+#define DOM_FS_CHILD_FILESYSTEMMANAGER_H_
+
+#include <functional>
+
+#include "mozilla/MozPromise.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/dom/FlippedOnce.h"
+#include "mozilla/dom/quota/ForwardDecls.h"
+#include "nsCOMPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupports.h"
+#include "nsTObserverArray.h"
+
+class nsIGlobalObject;
+
+namespace mozilla {
+
+class ErrorResult;
+
+namespace dom {
+
+class FileSystemManagerChild;
+class FileSystemBackgroundRequestHandler;
+class StorageManager;
+
+namespace fs {
+class FileSystemRequestHandler;
+template <typename Manager, typename PromiseType>
+class ManagedMozPromiseRequestHolder;
+}  // namespace fs
+
+// `FileSystemManager` is supposed to be held by `StorageManager` and thus
+// there should always be only one `FileSystemManager` per `nsIGlobalObject`.
+// `FileSystemManager` is responsible for creating and eventually caching
+// `FileSystemManagerChild` which is required for communication with the parent
+// process. `FileSystemHandle` is also expected to hold `FileSystemManager`,
+// but it should never clear the strong reference during cycle collection's
+// unlink phase to keep the actor alive. `FileSystemSyncAccessHandle` and
+// `FileSystemWritableFileStream` are also expected to hold `FileSystemManager`,
+// and they shouldn't clear the strong reference during cycle collection's
+// unlink phase as well even though they have their own actor. Those actors
+// are managed by the top level actor, so if the top level actor is destroyed,
+// the whole chain of managed actors would be destroyed as well.
+class FileSystemManager : public nsISupports {
+ public:
+  template <typename PromiseType>
+  using PromiseRequestHolder =
+      fs::ManagedMozPromiseRequestHolder<FileSystemManager, PromiseType>;
+
+  FileSystemManager(
+      nsIGlobalObject* aGlobal, RefPtr<StorageManager> aStorageManager,
+      RefPtr<FileSystemBackgroundRequestHandler> aBackgroundRequestHandler);
+
+  FileSystemManager(nsIGlobalObject* aGlobal,
+                    RefPtr<StorageManager> aStorageManager);
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(FileSystemManager)
+
+  bool IsShutdown() const { return mShutdown; }
+
+  void Shutdown();
+
+  const RefPtr<FileSystemManagerChild>& ActorStrongRef() const;
+
+  void RegisterPromiseRequestHolder(PromiseRequestHolder<BoolPromise>* aHolder);
+
+  void UnregisterPromiseRequestHolder(
+      PromiseRequestHolder<BoolPromise>* aHolder);
+
+  void BeginRequest(
+      std::function<void(const RefPtr<FileSystemManagerChild>&)>&& aSuccess,
+      std::function<void(nsresult)>&& aFailure);
+
+  already_AddRefed<Promise> GetDirectory(ErrorResult& aError);
+
+ private:
+  virtual ~FileSystemManager();
+
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  RefPtr<StorageManager> mStorageManager;
+
+  const RefPtr<FileSystemBackgroundRequestHandler> mBackgroundRequestHandler;
+  const UniquePtr<fs::FileSystemRequestHandler> mRequestHandler;
+
+  nsTObserverArray<PromiseRequestHolder<BoolPromise>*> mPromiseRequestHolders;
+
+  FlippedOnce<false> mShutdown;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // DOM_FS_CHILD_FILESYSTEMMANAGER_H_
diff --git a/dom/fs/api/FileSystemSyncAccessHandle.cpp b/dom/fs/api/FileSystemSyncAccessHandle.cpp
new file mode 100644
index 0000000000..c04f8e11a5
--- /dev/null
+++ b/dom/fs/api/FileSystemSyncAccessHandle.cpp
@@ -0,0 +1,650 @@
+/* -*- 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 "FileSystemSyncAccessHandle.h"
+
+#include "fs/FileSystemAsyncCopy.h"
+#include "fs/FileSystemRequestHandler.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/FixedBufferOutputStream.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/TaskQueue.h"
+#include "mozilla/dom/FileSystemAccessHandleChild.h"
+#include "mozilla/dom/FileSystemAccessHandleControlChild.h"
+#include "mozilla/dom/FileSystemHandleBinding.h"
+#include "mozilla/dom/FileSystemLog.h"
+#include "mozilla/dom/FileSystemManager.h"
+#include "mozilla/dom/FileSystemManagerChild.h"
+#include "mozilla/dom/FileSystemSyncAccessHandleBinding.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/UnionTypes.h"
+#include "mozilla/dom/WorkerCommon.h"
+#include "mozilla/dom/WorkerPrivate.h"
+#include "mozilla/dom/WorkerRef.h"
+#include "mozilla/dom/fs/IPCRejectReporter.h"
+#include "mozilla/dom/fs/TargetPtrHolder.h"
+#include "mozilla/dom/quota/QuotaCommon.h"
+#include "mozilla/dom/quota/ResultExtensions.h"
+#include "mozilla/ipc/RandomAccessStreamUtils.h"
+#include "nsNetCID.h"
+#include "nsStringStream.h"
+
+namespace mozilla::dom {
+
+namespace {
+
+using SizePromise = Int64Promise;
+const auto CreateAndRejectSizePromise = CreateAndRejectInt64Promise;
+
+}  // namespace
+
+FileSystemSyncAccessHandle::FileSystemSyncAccessHandle(
+    nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+    mozilla::ipc::RandomAccessStreamParams&& aStreamParams,
+    RefPtr<FileSystemAccessHandleChild> aActor,
+    RefPtr<FileSystemAccessHandleControlChild> aControlActor,
+    RefPtr<TaskQueue> aIOTaskQueue,
+    const fs::FileSystemEntryMetadata& aMetadata)
+    : mGlobal(aGlobal),
+      mManager(aManager),
+      mActor(std::move(aActor)),
+      mControlActor(std::move(aControlActor)),
+      mIOTaskQueue(std::move(aIOTaskQueue)),
+      mStreamParams(std::move(aStreamParams)),
+      mMetadata(aMetadata),
+      mState(State::Initial) {
+  LOG(("Created SyncAccessHandle %p", this));
+
+  // Connect with the actor directly in the constructor. This way the actor
+  // can call `FileSystemSyncAccessHandle::ClearActor` when we call
+  // `PFileSystemAccessHandleChild::Send__delete__` even when
+  // FileSystemSyncAccessHandle::Create fails, in which case the not yet
+  // fully constructed FileSystemSyncAccessHandle is being destroyed.
+  mActor->SetAccessHandle(this);
+
+  mControlActor->SetAccessHandle(this);
+}
+
+FileSystemSyncAccessHandle::~FileSystemSyncAccessHandle() {
+  MOZ_ASSERT(!mActor);
+  MOZ_ASSERT(IsClosed());
+}
+
+// static
+Result<RefPtr<FileSystemSyncAccessHandle>, nsresult>
+FileSystemSyncAccessHandle::Create(
+    nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+    mozilla::ipc::RandomAccessStreamParams&& aStreamParams,
+    mozilla::ipc::ManagedEndpoint<PFileSystemAccessHandleChild>&&
+        aAccessHandleChildEndpoint,
+    mozilla::ipc::Endpoint<PFileSystemAccessHandleControlChild>&&
+        aAccessHandleControlChildEndpoint,
+    const fs::FileSystemEntryMetadata& aMetadata) {
+  WorkerPrivate* const workerPrivate = GetCurrentThreadWorkerPrivate();
+  MOZ_ASSERT(workerPrivate);
+
+  auto accessHandleChild = MakeRefPtr<FileSystemAccessHandleChild>();
+
+  QM_TRY(MOZ_TO_RESULT(
+      aManager->ActorStrongRef()->BindPFileSystemAccessHandleEndpoint(
+          std::move(aAccessHandleChildEndpoint), accessHandleChild)));
+
+  auto accessHandleControlChild =
+      MakeRefPtr<FileSystemAccessHandleControlChild>();
+
+  aAccessHandleControlChildEndpoint.Bind(accessHandleControlChild,
+                                         workerPrivate->ControlEventTarget());
+
+  QM_TRY_UNWRAP(auto streamTransportService,
+                MOZ_TO_RESULT_GET_TYPED(nsCOMPtr<nsIEventTarget>,
+                                        MOZ_SELECT_OVERLOAD(do_GetService),
+                                        NS_STREAMTRANSPORTSERVICE_CONTRACTID));
+
+  RefPtr<TaskQueue> ioTaskQueue = TaskQueue::Create(
+      streamTransportService.forget(), "FileSystemSyncAccessHandle");
+  QM_TRY(MOZ_TO_RESULT(ioTaskQueue));
+
+  RefPtr<FileSystemSyncAccessHandle> result = new FileSystemSyncAccessHandle(
+      aGlobal, aManager, std::move(aStreamParams), std::move(accessHandleChild),
+      std::move(accessHandleControlChild), std::move(ioTaskQueue), aMetadata);
+
+  auto autoClose = MakeScopeExit([result] {
+    MOZ_ASSERT(result->mState == State::Initial);
+    result->mState = State::Closed;
+    result->mActor->SendClose();
+  });
+
+  workerPrivate->AssertIsOnWorkerThread();
+
+  RefPtr<StrongWorkerRef> workerRef = StrongWorkerRef::Create(
+      workerPrivate, "FileSystemSyncAccessHandle", [result]() {
+        if (result->IsOpen()) {
+          // We don't need to use the result, we just need to begin the closing
+          // process.
+          Unused << result->BeginClose();
+        }
+      });
+  QM_TRY(MOZ_TO_RESULT(workerRef));
+
+  autoClose.release();
+
+  result->mWorkerRef = std::move(workerRef);
+  result->mState = State::Open;
+
+  return result;
+}
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(FileSystemSyncAccessHandle)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(FileSystemSyncAccessHandle)
+NS_IMPL_CYCLE_COLLECTING_RELEASE_WITH_LAST_RELEASE(FileSystemSyncAccessHandle,
+                                                   LastRelease())
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(FileSystemSyncAccessHandle)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(FileSystemSyncAccessHandle)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mGlobal)
+  // Don't unlink mManager!
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+  if (tmp->IsOpen()) {
+    // We don't need to use the result, we just need to begin the closing
+    // process.
+    Unused << tmp->BeginClose();
+  }
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(FileSystemSyncAccessHandle)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mGlobal)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mManager)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+void FileSystemSyncAccessHandle::LastRelease() {
+  // We can't call `FileSystemSyncAccessHandle::Close` here because it may need
+  // to keep FileSystemSyncAccessHandle object alive which isn't possible when
+  // the object is about to be deleted. There are other mechanisms which ensure
+  // that the object is correctly closed before destruction. For example the
+  // object unlinking and the worker shutdown (we get notified about it via the
+  // callback passed to `StrongWorkerRef`) are used to close the object if it
+  // hasn't been closed yet.
+
+  if (mActor) {
+    PFileSystemAccessHandleChild::Send__delete__(mActor);
+
+    // `PFileSystemAccessHandleChild::Send__delete__` is supposed to call
+    // `FileSystemAccessHandleChild::ActorDestroy` which in turn calls
+    // `FileSystemSyncAccessHandle::ClearActor`, so `mActor` should be be null
+    // at this point.
+    MOZ_ASSERT(!mActor);
+  }
+
+  if (mControlActor) {
+    mControlActor->Close();
+
+    // `FileSystemAccessHandleControlChild::Close` is supposed to call
+    // `FileSystemAccessHandleControlChild::ActorDestroy` which in turn calls
+    // `FileSystemSyncAccessHandle::ClearControlActor`, so `mControlActor`
+    // should be be null at this point.
+    MOZ_ASSERT(!mControlActor);
+  }
+}
+
+void FileSystemSyncAccessHandle::ClearActor() {
+  MOZ_ASSERT(mActor);
+
+  mActor = nullptr;
+}
+
+void FileSystemSyncAccessHandle::ClearControlActor() {
+  // `mControlActor` is initialized in the constructor and this method is
+  // supposed to be called only once.
+  MOZ_ASSERT(mControlActor);
+
+  mControlActor = nullptr;
+}
+
+bool FileSystemSyncAccessHandle::IsOpen() const {
+  MOZ_ASSERT(mState != State::Initial);
+
+  return mState == State::Open;
+}
+
+bool FileSystemSyncAccessHandle::IsClosing() const {
+  MOZ_ASSERT(mState != State::Initial);
+
+  return mState == State::Closing;
+}
+
+bool FileSystemSyncAccessHandle::IsClosed() const {
+  MOZ_ASSERT(mState != State::Initial);
+
+  return mState == State::Closed;
+}
+
+RefPtr<BoolPromise> FileSystemSyncAccessHandle::BeginClose() {
+  MOZ_ASSERT(IsOpen());
+
+  mState = State::Closing;
+
+  InvokeAsync(mIOTaskQueue, __func__,
+              [selfHolder = fs::TargetPtrHolder(this)]() {
+                if (selfHolder->mStream) {
+                  LOG(("%p: Closing", selfHolder->mStream.get()));
+
+                  selfHolder->mStream->OutputStream()->Close();
+                  selfHolder->mStream = nullptr;
+                } else {
+                  LOG(("Closing (no stream)"));
+
+                  // If the stream was not deserialized, `mStreamParams` still
+                  // contains a pre-opened file descriptor which needs to be
+                  // closed here by moving `mStreamParams` to a local variable
+                  // (the file descriptor will be closed for real when
+                  // `streamParams` goes out of scope).
+
+                  mozilla::ipc::RandomAccessStreamParams streamParams(
+                      std::move(selfHolder->mStreamParams));
+                }
+
+                return BoolPromise::CreateAndResolve(true, __func__);
+              })
+      ->Then(mWorkerRef->Private()->ControlEventTarget(), __func__,
+             [self = RefPtr(this)](const BoolPromise::ResolveOrRejectValue&) {
+               return self->mIOTaskQueue->BeginShutdown();
+             })
+      ->Then(
+          mWorkerRef->Private()->ControlEventTarget(), __func__,
+          [self = RefPtr(this)](const ShutdownPromise::ResolveOrRejectValue&) {
+            if (self->mControlActor) {
+              RefPtr<BoolPromise::Private> promise =
+                  new BoolPromise::Private(__func__);
+
+              self->mControlActor->SendClose(
+                  [promise](void_t&&) { promise->Resolve(true, __func__); },
+                  [promise](const mozilla::ipc::ResponseRejectReason& aReason) {
+                    fs::IPCRejectReporter(aReason);
+
+                    promise->Reject(NS_ERROR_FAILURE, __func__);
+                  });
+
+              return RefPtr<BoolPromise>(promise);
+            }
+
+            return BoolPromise::CreateAndResolve(true, __func__);
+          })
+      ->Then(mWorkerRef->Private()->ControlEventTarget(), __func__,
+             [self = RefPtr(this)](const BoolPromise::ResolveOrRejectValue&) {
+               self->mWorkerRef = nullptr;
+
+               self->mState = State::Closed;
+
+               self->mClosePromiseHolder.ResolveIfExists(true, __func__);
+             });
+
+  return OnClose();
+}
+
+RefPtr<BoolPromise> FileSystemSyncAccessHandle::OnClose() {
+  MOZ_ASSERT(mState == State::Closing);
+
+  return mClosePromiseHolder.Ensure(__func__);
+}
+
+// WebIDL Boilerplate
+
+nsIGlobalObject* FileSystemSyncAccessHandle::GetParentObject() const {
+  return mGlobal;
+}
+
+JSObject* FileSystemSyncAccessHandle::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return FileSystemSyncAccessHandle_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// WebIDL Interface
+
+uint64_t FileSystemSyncAccessHandle::Read(
+    const MaybeSharedArrayBufferViewOrMaybeSharedArrayBuffer& aBuffer,
+    const FileSystemReadWriteOptions& aOptions, ErrorResult& aRv) {
+  return ReadOrWrite(aBuffer, aOptions, /* aRead */ true, aRv);
+}
+
+uint64_t FileSystemSyncAccessHandle::Write(
+    const MaybeSharedArrayBufferViewOrMaybeSharedArrayBuffer& aBuffer,
+    const FileSystemReadWriteOptions& aOptions, ErrorResult& aRv) {
+  return ReadOrWrite(aBuffer, aOptions, /* aRead */ false, aRv);
+}
+
+void FileSystemSyncAccessHandle::Truncate(uint64_t aSize, ErrorResult& aError) {
+  if (!IsOpen()) {
+    aError.ThrowInvalidStateError("SyncAccessHandle is closed");
+    return;
+  }
+
+  MOZ_ASSERT(mWorkerRef);
+
+  AutoSyncLoopHolder syncLoop(mWorkerRef->Private(), Canceling);
+
+  nsCOMPtr<nsISerialEventTarget> syncLoopTarget =
+      syncLoop.GetSerialEventTarget();
+  QM_TRY(MOZ_TO_RESULT(syncLoopTarget), [&aError](nsresult) {
+    aError.ThrowInvalidStateError("Worker is shutting down");
+  });
+
+  InvokeAsync(
+      mIOTaskQueue, __func__,
+      [selfHolder = fs::TargetPtrHolder(this), aSize]() {
+        QM_TRY(MOZ_TO_RESULT(selfHolder->EnsureStream()),
+               CreateAndRejectBoolPromise);
+
+        LOG(("%p: Truncate to %" PRIu64, selfHolder->mStream.get(), aSize));
+        int64_t offset = 0;
+        QM_TRY(MOZ_TO_RESULT(selfHolder->mStream->Tell(&offset)),
+               CreateAndRejectBoolPromise);
+        QM_TRY(MOZ_TO_RESULT(selfHolder->mStream->Seek(
+                   nsISeekableStream::NS_SEEK_SET, aSize)),
+               CreateAndRejectBoolPromise);
+
+        QM_TRY(MOZ_TO_RESULT(selfHolder->mStream->SetEOF()),
+               CreateAndRejectBoolPromise);
+        // restore cursor position (clamp to end of file)
+        QM_TRY(MOZ_TO_RESULT(selfHolder->mStream->Seek(
+                   nsISeekableStream::NS_SEEK_SET,
+                   std::min((uint64_t)offset, aSize))),
+               CreateAndRejectBoolPromise);
+
+        return BoolPromise::CreateAndResolve(true, __func__);
+      })
+      ->Then(syncLoopTarget, __func__,
+             [this, &syncLoopTarget](
+                 const BoolPromise::ResolveOrRejectValue& aValue) {
+               MOZ_ASSERT(mWorkerRef);
+
+               mWorkerRef->Private()->AssertIsOnWorkerThread();
+
+               mWorkerRef->Private()->StopSyncLoop(
+                   syncLoopTarget,
+                   aValue.IsResolve() ? NS_OK : aValue.RejectValue());
+             });
+
+  QM_TRY(MOZ_TO_RESULT(syncLoop.Run()),
+         [&aError](const nsresult rv) { aError.Throw(rv); });
+}
+
+uint64_t FileSystemSyncAccessHandle::GetSize(ErrorResult& aError) {
+  if (!IsOpen()) {
+    aError.ThrowInvalidStateError("SyncAccessHandle is closed");
+    return 0;
+  }
+
+  MOZ_ASSERT(mWorkerRef);
+
+  AutoSyncLoopHolder syncLoop(mWorkerRef->Private(), Canceling);
+
+  nsCOMPtr<nsISerialEventTarget> syncLoopTarget =
+      syncLoop.GetSerialEventTarget();
+  QM_TRY(MOZ_TO_RESULT(syncLoopTarget), [&aError](nsresult) {
+    aError.ThrowInvalidStateError("Worker is shutting down");
+    return 0;
+  });
+
+  // XXX Could we somehow pass the size to `StopSyncLoop` and then get it via
+  // `QM_TRY_INSPECT(const auto& size, syncLoop.Run)` ?
+  // Could we use Result<UniquePtr<...>, nsresult> for that ?
+  int64_t size;
+
+  InvokeAsync(mIOTaskQueue, __func__,
+              [selfHolder = fs::TargetPtrHolder(this)]() {
+                QM_TRY(MOZ_TO_RESULT(selfHolder->EnsureStream()),
+                       CreateAndRejectSizePromise);
+
+                nsCOMPtr<nsIFileMetadata> fileMetadata =
+                    do_QueryInterface(selfHolder->mStream);
+                MOZ_ASSERT(fileMetadata);
+
+                QM_TRY_INSPECT(
+                    const auto& size,
+                    MOZ_TO_RESULT_INVOKE_MEMBER(fileMetadata, GetSize),
+                    CreateAndRejectSizePromise);
+
+                LOG(("%p: GetSize %" PRIu64, selfHolder->mStream.get(), size));
+
+                return SizePromise::CreateAndResolve(size, __func__);
+              })
+      ->Then(syncLoopTarget, __func__,
+             [this, &syncLoopTarget,
+              &size](const Int64Promise::ResolveOrRejectValue& aValue) {
+               MOZ_ASSERT(mWorkerRef);
+
+               mWorkerRef->Private()->AssertIsOnWorkerThread();
+
+               if (aValue.IsResolve()) {
+                 size = aValue.ResolveValue();
+
+                 mWorkerRef->Private()->StopSyncLoop(syncLoopTarget, NS_OK);
+               } else {
+                 mWorkerRef->Private()->StopSyncLoop(syncLoopTarget,
+                                                     aValue.RejectValue());
+               }
+             });
+
+  QM_TRY(MOZ_TO_RESULT(syncLoop.Run()), [&aError](const nsresult rv) {
+    aError.Throw(rv);
+    return 0;
+  });
+
+  return size;
+}
+
+void FileSystemSyncAccessHandle::Flush(ErrorResult& aError) {
+  if (!IsOpen()) {
+    aError.ThrowInvalidStateError("SyncAccessHandle is closed");
+    return;
+  }
+
+  MOZ_ASSERT(mWorkerRef);
+
+  AutoSyncLoopHolder syncLoop(mWorkerRef->Private(), Canceling);
+
+  nsCOMPtr<nsISerialEventTarget> syncLoopTarget =
+      syncLoop.GetSerialEventTarget();
+  QM_TRY(MOZ_TO_RESULT(syncLoopTarget), [&aError](nsresult) {
+    aError.ThrowInvalidStateError("Worker is shutting down");
+  });
+
+  InvokeAsync(mIOTaskQueue, __func__,
+              [selfHolder = fs::TargetPtrHolder(this)]() {
+                QM_TRY(MOZ_TO_RESULT(selfHolder->EnsureStream()),
+                       CreateAndRejectBoolPromise);
+
+                LOG(("%p: Flush", selfHolder->mStream.get()));
+
+                QM_TRY(
+                    MOZ_TO_RESULT(selfHolder->mStream->OutputStream()->Flush()),
+                    CreateAndRejectBoolPromise);
+
+                return BoolPromise::CreateAndResolve(true, __func__);
+              })
+      ->Then(syncLoopTarget, __func__,
+             [this, &syncLoopTarget](
+                 const BoolPromise::ResolveOrRejectValue& aValue) {
+               MOZ_ASSERT(mWorkerRef);
+
+               mWorkerRef->Private()->AssertIsOnWorkerThread();
+
+               mWorkerRef->Private()->StopSyncLoop(
+                   syncLoopTarget,
+                   aValue.IsResolve() ? NS_OK : aValue.RejectValue());
+             });
+
+  QM_TRY(MOZ_TO_RESULT(syncLoop.Run()),
+         [&aError](const nsresult rv) { aError.Throw(rv); });
+}
+
+void FileSystemSyncAccessHandle::Close() {
+  if (!(IsOpen() || IsClosing())) {
+    return;
+  }
+
+  MOZ_ASSERT(mWorkerRef);
+
+  // Normally mWorkerRef can be used directly for stopping the sync loop, but
+  // the async close is special because mWorkerRef is cleared as part of the
+  // operation. That's why we need to use this extra strong ref to the
+  // `StrongWorkerRef`.
+  RefPtr<StrongWorkerRef> workerRef = mWorkerRef;
+
+  AutoSyncLoopHolder syncLoop(workerRef->Private(), Killing);
+
+  nsCOMPtr<nsISerialEventTarget> syncLoopTarget =
+      syncLoop.GetSerialEventTarget();
+  MOZ_ASSERT(syncLoopTarget);
+
+  InvokeAsync(syncLoopTarget, __func__, [self = RefPtr(this)]() {
+    if (self->IsOpen()) {
+      return self->BeginClose();
+    }
+    return self->OnClose();
+  })->Then(syncLoopTarget, __func__, [&workerRef, &syncLoopTarget]() {
+    MOZ_ASSERT(workerRef);
+
+    workerRef->Private()->AssertIsOnWorkerThread();
+
+    workerRef->Private()->StopSyncLoop(syncLoopTarget, NS_OK);
+  });
+
+  MOZ_ALWAYS_SUCCEEDS(syncLoop.Run());
+}
+
+uint64_t FileSystemSyncAccessHandle::ReadOrWrite(
+    const MaybeSharedArrayBufferViewOrMaybeSharedArrayBuffer& aBuffer,
+    const FileSystemReadWriteOptions& aOptions, const bool aRead,
+    ErrorResult& aRv) {
+  if (!IsOpen()) {
+    aRv.ThrowInvalidStateError("SyncAccessHandle is closed");
+    return 0;
+  }
+
+  MOZ_ASSERT(mWorkerRef);
+
+  auto throwAndReturn = [&aRv](const nsresult rv) {
+    aRv.Throw(rv);
+    return 0;
+  };
+
+  const auto dataSpan = [&aBuffer]() {
+    if (aBuffer.IsArrayBuffer()) {
+      const ArrayBuffer& buffer = aBuffer.GetAsArrayBuffer();
+      buffer.ComputeState();
+      return Span{buffer.Data(), buffer.Length()};
+    }
+    MOZ_ASSERT(aBuffer.IsArrayBufferView());
+    const ArrayBufferView& buffer = aBuffer.GetAsArrayBufferView();
+    buffer.ComputeState();
+    return Span{buffer.Data(), buffer.Length()};
+  }();
+
+  CheckedInt<int64_t> offset = 0;
+  if (aOptions.mAt.WasPassed()) {
+    // Handle seek before read ('at')
+    offset = CheckedInt<int64_t>(aOptions.mAt.Value());
+    QM_TRY(MOZ_TO_RESULT(offset.isValid()), throwAndReturn);
+  }
+  AutoSyncLoopHolder syncLoop(mWorkerRef->Private(), Canceling);
+
+  nsCOMPtr<nsISerialEventTarget> syncLoopTarget =
+      syncLoop.GetSerialEventTarget();
+  QM_TRY(MOZ_TO_RESULT(syncLoopTarget), [&aRv](nsresult) {
+    aRv.ThrowInvalidStateError("Worker is shutting down");
+    return 0;
+  });
+
+  uint64_t totalCount = 0;
+
+  InvokeAsync(
+      mIOTaskQueue, __func__,
+      [selfHolder = fs::TargetPtrHolder(this), dataSpan,
+       use_offset = aOptions.mAt.WasPassed(), offset, aRead, &totalCount]() {
+        QM_TRY(MOZ_TO_RESULT(selfHolder->EnsureStream()),
+               CreateAndRejectBoolPromise);
+
+        if (use_offset) {
+          LOG_VERBOSE(("%p: Seeking to %" PRIu64, selfHolder->mStream.get(),
+                       offset.value()));
+
+          QM_TRY(MOZ_TO_RESULT(selfHolder->mStream->Seek(
+                     nsISeekableStream::NS_SEEK_SET, offset.value())),
+                 CreateAndRejectBoolPromise);
+        }
+
+        nsCOMPtr<nsIInputStream> inputStream;
+        nsCOMPtr<nsIOutputStream> outputStream;
+
+        if (aRead) {
+          LOG_VERBOSE(("%p: Reading %zu bytes", selfHolder->mStream.get(),
+                       dataSpan.Length()));
+
+          inputStream = selfHolder->mStream->InputStream();
+
+          outputStream =
+              FixedBufferOutputStream::Create(AsWritableChars(dataSpan));
+        } else {
+          LOG_VERBOSE(("%p: Writing %zu bytes", selfHolder->mStream.get(),
+                       dataSpan.Length()));
+
+          QM_TRY(MOZ_TO_RESULT(NS_NewByteInputStream(
+                     getter_AddRefs(inputStream), AsChars(dataSpan),
+                     NS_ASSIGNMENT_DEPEND)),
+                 CreateAndRejectBoolPromise);
+
+          outputStream = selfHolder->mStream->OutputStream();
+        }
+
+        auto promiseHolder = MakeUnique<MozPromiseHolder<BoolPromise>>();
+        RefPtr<BoolPromise> promise = promiseHolder->Ensure(__func__);
+
+        QM_TRY(MOZ_TO_RESULT(fs::AsyncCopy(
+                   inputStream, outputStream, GetCurrentSerialEventTarget(),
+                   aRead ? NS_ASYNCCOPY_VIA_WRITESEGMENTS
+                         : NS_ASYNCCOPY_VIA_READSEGMENTS,
+                   /* aCloseSource */ !aRead, /* aCloseSink */ aRead,
+                   [&totalCount](uint32_t count) { totalCount += count; },
+                   [promiseHolder = std::move(promiseHolder)](nsresult rv) {
+                     promiseHolder->ResolveIfExists(true, __func__);
+                   })),
+               CreateAndRejectBoolPromise);
+
+        return promise;
+      })
+      ->Then(syncLoopTarget, __func__,
+             [this, &syncLoopTarget](
+                 const BoolPromise::ResolveOrRejectValue& aValue) {
+               MOZ_ASSERT(mWorkerRef);
+
+               mWorkerRef->Private()->AssertIsOnWorkerThread();
+
+               mWorkerRef->Private()->StopSyncLoop(syncLoopTarget, NS_OK);
+             });
+
+  MOZ_ALWAYS_SUCCEEDS(syncLoop.Run());
+
+  return totalCount;
+}
+
+nsresult FileSystemSyncAccessHandle::EnsureStream() {
+  if (!mStream) {
+    QM_TRY_UNWRAP(mStream, DeserializeRandomAccessStream(mStreamParams),
+                  NS_ERROR_FAILURE);
+
+    mozilla::ipc::RandomAccessStreamParams streamParams(
+        std::move(mStreamParams));
+  }
+
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/fs/api/FileSystemSyncAccessHandle.h b/dom/fs/api/FileSystemSyncAccessHandle.h
new file mode 100644
index 0000000000..49f470b60b
--- /dev/null
+++ b/dom/fs/api/FileSystemSyncAccessHandle.h
@@ -0,0 +1,133 @@
+/* -*- 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_FS_FILESYSTEMSYNCACCESSHANDLE_H_
+#define DOM_FS_FILESYSTEMSYNCACCESSHANDLE_H_
+
+#include "mozilla/dom/PFileSystemManager.h"
+#include "mozilla/dom/quota/ForwardDecls.h"
+#include "nsCOMPtr.h"
+#include "nsISupports.h"
+#include "nsWrapperCache.h"
+
+class nsIGlobalObject;
+
+namespace mozilla {
+
+class ErrorResult;
+class TaskQueue;
+
+namespace dom {
+
+class FileSystemAccessHandleChild;
+class FileSystemAccessHandleControlChild;
+struct FileSystemReadWriteOptions;
+class FileSystemManager;
+class MaybeSharedArrayBufferViewOrMaybeSharedArrayBuffer;
+class Promise;
+class StrongWorkerRef;
+
+class FileSystemSyncAccessHandle final : public nsISupports,
+                                         public nsWrapperCache {
+ public:
+  enum struct State : uint8_t { Initial = 0, Open, Closing, Closed };
+
+  static Result<RefPtr<FileSystemSyncAccessHandle>, nsresult> Create(
+      nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+      mozilla::ipc::RandomAccessStreamParams&& aStreamParams,
+      mozilla::ipc::ManagedEndpoint<PFileSystemAccessHandleChild>&&
+          aAccessHandleChildEndpoint,
+      mozilla::ipc::Endpoint<PFileSystemAccessHandleControlChild>&&
+          aAccessHandleControlChildEndpoint,
+      const fs::FileSystemEntryMetadata& aMetadata);
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(FileSystemSyncAccessHandle)
+
+  void LastRelease();
+
+  void ClearActor();
+
+  void ClearControlActor();
+
+  bool IsOpen() const;
+
+  bool IsClosing() const;
+
+  bool IsClosed() const;
+
+  [[nodiscard]] RefPtr<BoolPromise> BeginClose();
+
+  [[nodiscard]] RefPtr<BoolPromise> OnClose();
+
+  // WebIDL Boilerplate
+  nsIGlobalObject* GetParentObject() const;
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // WebIDL Interface
+  uint64_t Read(
+      const MaybeSharedArrayBufferViewOrMaybeSharedArrayBuffer& aBuffer,
+      const FileSystemReadWriteOptions& aOptions, ErrorResult& aRv);
+
+  uint64_t Write(
+      const MaybeSharedArrayBufferViewOrMaybeSharedArrayBuffer& aBuffer,
+      const FileSystemReadWriteOptions& aOptions, ErrorResult& aRv);
+
+  void Truncate(uint64_t aSize, ErrorResult& aError);
+
+  uint64_t GetSize(ErrorResult& aError);
+
+  void Flush(ErrorResult& aError);
+
+  void Close();
+
+ private:
+  FileSystemSyncAccessHandle(
+      nsIGlobalObject* aGlobal, RefPtr<FileSystemManager>& aManager,
+      mozilla::ipc::RandomAccessStreamParams&& aStreamParams,
+      RefPtr<FileSystemAccessHandleChild> aActor,
+      RefPtr<FileSystemAccessHandleControlChild> aControlActor,
+      RefPtr<TaskQueue> aIOTaskQueue,
+      const fs::FileSystemEntryMetadata& aMetadata);
+
+  virtual ~FileSystemSyncAccessHandle();
+
+  uint64_t ReadOrWrite(
+      const MaybeSharedArrayBufferViewOrMaybeSharedArrayBuffer& aBuffer,
+      const FileSystemReadWriteOptions& aOptions, const bool aRead,
+      ErrorResult& aRv);
+
+  nsresult EnsureStream();
+
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  RefPtr<FileSystemManager> mManager;
+
+  RefPtr<FileSystemAccessHandleChild> mActor;
+
+  RefPtr<FileSystemAccessHandleControlChild> mControlActor;
+
+  RefPtr<TaskQueue> mIOTaskQueue;
+
+  nsCOMPtr<nsIRandomAccessStream> mStream;
+
+  RefPtr<StrongWorkerRef> mWorkerRef;
+
+  MozPromiseHolder<BoolPromise> mClosePromiseHolder;
+
+  mozilla::ipc::RandomAccessStreamParams mStreamParams;
+
+  const fs::FileSystemEntryMetadata mMetadata;
+
+  State mState;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // DOM_FS_FILESYSTEMSYNCACCESSHANDLE_H_
diff --git a/dom/fs/api/FileSystemWritableFileStream.cpp b/dom/fs/api/FileSystemWritableFileStream.cpp
new file mode 100644
index 0000000000..b19a0c9fe9
--- /dev/null
+++ b/dom/fs/api/FileSystemWritableFileStream.cpp
@@ -0,0 +1,951 @@
+/* -*- 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 "FileSystemWritableFileStream.h"
+
+#include "fs/FileSystemAsyncCopy.h"
+#include "fs/FileSystemShutdownBlocker.h"
+#include "fs/FileSystemThreadSafeStreamOwner.h"
+#include "mozilla/Buffer.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/InputStreamLengthHelper.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/SpinEventLoopUntil.h"
+#include "mozilla/TaskQueue.h"
+#include "mozilla/dom/Blob.h"
+#include "mozilla/dom/FileSystemHandle.h"
+#include "mozilla/dom/FileSystemLog.h"
+#include "mozilla/dom/FileSystemManager.h"
+#include "mozilla/dom/FileSystemWritableFileStreamBinding.h"
+#include "mozilla/dom/FileSystemWritableFileStreamChild.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/PromiseNativeHandler.h"
+#include "mozilla/dom/WorkerCommon.h"
+#include "mozilla/dom/WorkerPrivate.h"
+#include "mozilla/dom/WorkerRef.h"
+#include "mozilla/dom/WritableStreamDefaultController.h"
+#include "mozilla/dom/fs/TargetPtrHolder.h"
+#include "mozilla/dom/quota/QuotaCommon.h"
+#include "mozilla/dom/quota/ResultExtensions.h"
+#include "mozilla/ipc/RandomAccessStreamUtils.h"
+#include "nsAsyncStreamCopier.h"
+#include "nsIInputStream.h"
+#include "nsIRequestObserver.h"
+#include "nsISupportsImpl.h"
+#include "nsNetCID.h"
+#include "nsNetUtil.h"
+#include "nsStreamUtils.h"
+#include "nsStringStream.h"
+
+namespace mozilla::dom {
+
+namespace {
+
+constexpr bool IsFileNotFoundError(const nsresult aRv) {
+  return NS_ERROR_DOM_FILE_NOT_FOUND_ERR == aRv ||
+         NS_ERROR_FILE_NOT_FOUND == aRv;
+}
+
+class WritableFileStreamUnderlyingSinkAlgorithms final
+    : public UnderlyingSinkAlgorithmsWrapper {
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(
+      WritableFileStreamUnderlyingSinkAlgorithms, UnderlyingSinkAlgorithmsBase)
+
+  explicit WritableFileStreamUnderlyingSinkAlgorithms(
+      FileSystemWritableFileStream& aStream)
+      : mStream(&aStream) {}
+
+  already_AddRefed<Promise> WriteCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      WritableStreamDefaultController& aController, ErrorResult& aRv) override;
+
+  already_AddRefed<Promise> CloseCallbackImpl(JSContext* aCx,
+                                              ErrorResult& aRv) override;
+
+  already_AddRefed<Promise> AbortCallbackImpl(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override;
+
+  void ReleaseObjects() override;
+
+ private:
+  ~WritableFileStreamUnderlyingSinkAlgorithms() = default;
+
+  RefPtr<FileSystemWritableFileStream> mStream;
+};
+
+}  // namespace
+
+class FileSystemWritableFileStream::Command {
+ public:
+  explicit Command(RefPtr<FileSystemWritableFileStream> aWritableFileStream)
+      : mWritableFileStream(std::move(aWritableFileStream)) {
+    MOZ_ASSERT(mWritableFileStream);
+  }
+
+  NS_INLINE_DECL_REFCOUNTING(FileSystemWritableFileStream::Command)
+
+ private:
+  ~Command() { mWritableFileStream->NoteFinishedCommand(); }
+
+  RefPtr<FileSystemWritableFileStream> mWritableFileStream;
+};
+
+class FileSystemWritableFileStream::CloseHandler {
+  enum struct State : uint8_t { Initial = 0, Open, Closing, Closed };
+
+ public:
+  CloseHandler()
+      : mShutdownBlocker(fs::FileSystemShutdownBlocker::CreateForWritable()),
+        mClosePromiseHolder(),
+        mState(State::Initial) {}
+
+  NS_INLINE_DECL_REFCOUNTING(FileSystemWritableFileStream::CloseHandler)
+
+  /**
+   * @brief Are we not yet closing?
+   */
+  bool IsOpen() const { return State::Open == mState; }
+
+  /**
+   * @brief Are we already fully closed?
+   */
+  bool IsClosed() const { return State::Closed == mState; }
+
+  /**
+   * @brief Transition from open to closing state
+   *
+   * @return true if the state was open and became closing after the call
+   * @return false in all the other cases the previous state is preserved
+   */
+  bool TestAndSetClosing() {
+    const bool isOpen = State::Open == mState;
+
+    if (isOpen) {
+      mState = State::Closing;
+    }
+
+    return isOpen;
+  }
+
+  RefPtr<BoolPromise> GetClosePromise() const {
+    MOZ_ASSERT(State::Open != mState,
+               "Please call TestAndSetClosing before GetClosePromise");
+
+    if (State::Closing == mState) {
+      return mClosePromiseHolder.Ensure(__func__);
+    }
+
+    // Instant resolve for initial state due to early shutdown or closed state
+    return BoolPromise::CreateAndResolve(true, __func__);
+  }
+
+  /**
+   * @brief Transition from initial to open state. In initial state
+   *
+   */
+  void Open() {
+    MOZ_ASSERT(State::Initial == mState);
+    mShutdownBlocker->Block();
+
+    mState = State::Open;
+  }
+
+  /**
+   * @brief Transition to closed state and resolve all pending promises.
+   *
+   */
+  void Close() {
+    mShutdownBlocker->Unblock();
+    mState = State::Closed;
+    mClosePromiseHolder.ResolveIfExists(true, __func__);
+  }
+
+ protected:
+  virtual ~CloseHandler() = default;
+
+ private:
+  RefPtr<fs::FileSystemShutdownBlocker> mShutdownBlocker;
+
+  mutable MozPromiseHolder<BoolPromise> mClosePromiseHolder;
+
+  State mState;
+};
+
+FileSystemWritableFileStream::FileSystemWritableFileStream(
+    const nsCOMPtr<nsIGlobalObject>& aGlobal,
+    RefPtr<FileSystemManager>& aManager,
+    mozilla::ipc::RandomAccessStreamParams&& aStreamParams,
+    RefPtr<FileSystemWritableFileStreamChild> aActor,
+    already_AddRefed<TaskQueue> aTaskQueue,
+    const fs::FileSystemEntryMetadata& aMetadata)
+    : WritableStream(aGlobal, HoldDropJSObjectsCaller::Explicit),
+      mManager(aManager),
+      mActor(std::move(aActor)),
+      mTaskQueue(aTaskQueue),
+      mWorkerRef(),
+      mStreamParams(std::move(aStreamParams)),
+      mMetadata(std::move(aMetadata)),
+      mCloseHandler(MakeAndAddRef<CloseHandler>()),
+      mCommandActive(false) {
+  LOG(("Created WritableFileStream %p", this));
+
+  // Connect with the actor directly in the constructor. This way the actor
+  // can call `FileSystemWritableFileStream::ClearActor` when we call
+  // `PFileSystemWritableFileStreamChild::Send__delete__` even when
+  // FileSystemWritableFileStream::Create fails, in which case the not yet
+  // fully constructed FileSystemWritableFileStream is being destroyed.
+  mActor->SetStream(this);
+
+  mozilla::HoldJSObjects(this);
+}
+
+FileSystemWritableFileStream::~FileSystemWritableFileStream() {
+  MOZ_ASSERT(!mCommandActive);
+  MOZ_ASSERT(IsClosed());
+
+  mozilla::DropJSObjects(this);
+}
+
+// https://fs.spec.whatwg.org/#create-a-new-filesystemwritablefilestream
+// * This is fallible because of OOM handling of JSAPI. See bug 1762233.
+// XXX(krosylight): _BOUNDARY because SetUpNative here can't run script because
+// StartCallback here is no-op. Can we let the static check automatically detect
+// this situation?
+/* static */
+MOZ_CAN_RUN_SCRIPT_BOUNDARY
+Result<RefPtr<FileSystemWritableFileStream>, nsresult>
+FileSystemWritableFileStream::Create(
+    const nsCOMPtr<nsIGlobalObject>& aGlobal,
+    RefPtr<FileSystemManager>& aManager,
+    mozilla::ipc::RandomAccessStreamParams&& aStreamParams,
+    RefPtr<FileSystemWritableFileStreamChild> aActor,
+    const fs::FileSystemEntryMetadata& aMetadata) {
+  MOZ_ASSERT(aGlobal);
+
+  QM_TRY_UNWRAP(auto streamTransportService,
+                MOZ_TO_RESULT_GET_TYPED(nsCOMPtr<nsIEventTarget>,
+                                        MOZ_SELECT_OVERLOAD(do_GetService),
+                                        NS_STREAMTRANSPORTSERVICE_CONTRACTID));
+
+  RefPtr<TaskQueue> taskQueue =
+      TaskQueue::Create(streamTransportService.forget(), "WritableStreamQueue");
+  MOZ_ASSERT(taskQueue);
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(aGlobal)) {
+    return Err(NS_ERROR_FAILURE);
+  }
+  JSContext* cx = jsapi.cx();
+
+  // Step 1. Let stream be a new FileSystemWritableFileStream in realm.
+  // Step 2. Set stream.[[file]] to file. (Covered by the constructor)
+  RefPtr<FileSystemWritableFileStream> stream =
+      new FileSystemWritableFileStream(
+          aGlobal, aManager, std::move(aStreamParams), std::move(aActor),
+          taskQueue.forget(), aMetadata);
+
+  auto autoClose = MakeScopeExit([stream] {
+    stream->mCloseHandler->Close();
+    stream->mActor->SendClose();
+  });
+
+  QM_TRY_UNWRAP(
+      RefPtr<StrongWorkerRef> workerRef,
+      ([stream]() -> Result<RefPtr<StrongWorkerRef>, nsresult> {
+        WorkerPrivate* const workerPrivate = GetCurrentThreadWorkerPrivate();
+        if (!workerPrivate) {
+          return RefPtr<StrongWorkerRef>();
+        }
+
+        RefPtr<StrongWorkerRef> workerRef = StrongWorkerRef::Create(
+            workerPrivate, "FileSystemWritableFileStream::Create", [stream]() {
+              if (stream->IsOpen()) {
+                // We don't need the promise, we just
+                // begin the closing process.
+                Unused << stream->BeginClose();
+              }
+            });
+        QM_TRY(MOZ_TO_RESULT(workerRef));
+
+        return workerRef;
+      }()));
+
+  // Step 3 - 5
+  auto algorithms =
+      MakeRefPtr<WritableFileStreamUnderlyingSinkAlgorithms>(*stream);
+
+  // Step 8: Set up stream with writeAlgorithm set to writeAlgorithm,
+  // closeAlgorithm set to closeAlgorithm, abortAlgorithm set to
+  // abortAlgorithm, highWaterMark set to highWaterMark, and
+  // sizeAlgorithm set to sizeAlgorithm.
+  IgnoredErrorResult rv;
+  stream->SetUpNative(cx, *algorithms,
+                      // Step 6. Let highWaterMark be 1.
+                      Some(1),
+                      // Step 7. Let sizeAlgorithm be an algorithm
+                      // that returns 1. (nullptr returns 1, See
+                      // WritableStream::Constructor for details)
+                      nullptr, rv);
+  if (rv.Failed()) {
+    return Err(rv.StealNSResult());
+  }
+
+  autoClose.release();
+
+  stream->mWorkerRef = std::move(workerRef);
+  stream->mCloseHandler->Open();
+
+  // Step 9: Return stream.
+  return stream;
+}
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(FileSystemWritableFileStream,
+                                               WritableStream)
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(FileSystemWritableFileStream)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(FileSystemWritableFileStream,
+                                                WritableStream)
+  // Per the comment for the FileSystemManager class, don't unlink mManager!
+  if (tmp->IsOpen()) {
+    Unused << tmp->BeginClose();
+  }
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(FileSystemWritableFileStream,
+                                                  WritableStream)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mManager)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+void FileSystemWritableFileStream::LastRelease() {
+  // We can't call `FileSystemWritableFileStream::Close` here because it may
+  // need to keep FileSystemWritableFileStream object alive which isn't possible
+  // when the object is about to be deleted. There are other mechanisms which
+  // ensure that the object is correctly closed before destruction. For example
+  // the object unlinking and the worker shutdown (we get notified about it via
+  // the callback passed to `StrongWorkerRef`) are used to close the object if
+  // it hasn't been closed yet.
+
+  if (mActor) {
+    PFileSystemWritableFileStreamChild::Send__delete__(mActor);
+    MOZ_ASSERT(!mActor);
+  }
+}
+
+RefPtr<FileSystemWritableFileStream::Command>
+FileSystemWritableFileStream::CreateCommand() {
+  MOZ_ASSERT(!mCommandActive);
+
+  mCommandActive = true;
+
+  return MakeRefPtr<Command>(this);
+}
+
+bool FileSystemWritableFileStream::IsCommandActive() const {
+  return mCommandActive;
+}
+
+void FileSystemWritableFileStream::ClearActor() {
+  MOZ_ASSERT(mActor);
+
+  mActor = nullptr;
+}
+
+bool FileSystemWritableFileStream::IsOpen() const {
+  return mCloseHandler->IsOpen();
+}
+
+bool FileSystemWritableFileStream::IsClosed() const {
+  return mCloseHandler->IsClosed();
+}
+
+RefPtr<BoolPromise> FileSystemWritableFileStream::BeginClose() {
+  using ClosePromise = PFileSystemWritableFileStreamChild::ClosePromise;
+  if (mCloseHandler->TestAndSetClosing()) {
+    Finish()
+        ->Then(mTaskQueue, __func__,
+               [selfHolder = fs::TargetPtrHolder(this)]() mutable {
+                 if (selfHolder->mStreamOwner) {
+                   selfHolder->mStreamOwner->Close();
+                 } else {
+                   // If the stream was not deserialized, `mStreamParams` still
+                   // contains a pre-opened file descriptor which needs to be
+                   // closed here by moving `mStreamParams` to a local variable
+                   // (the file descriptor will be closed for real when
+                   // `streamParams` goes out of scope).
+
+                   mozilla::ipc::RandomAccessStreamParams streamParams(
+                       std::move(selfHolder->mStreamParams));
+                 }
+
+                 return BoolPromise::CreateAndResolve(true, __func__);
+               })
+        ->Then(GetCurrentSerialEventTarget(), __func__,
+               [self = RefPtr(this)](const BoolPromise::ResolveOrRejectValue&) {
+                 return self->mTaskQueue->BeginShutdown();
+               })
+        ->Then(GetCurrentSerialEventTarget(), __func__,
+               [self = RefPtr(this)](
+                   const ShutdownPromise::ResolveOrRejectValue& /* aValue */) {
+                 if (!self->mActor) {
+                   return ClosePromise::CreateAndResolve(void_t(), __func__);
+                 }
+
+                 return self->mActor->SendClose();
+               })
+        ->Then(GetCurrentSerialEventTarget(), __func__,
+               [self = RefPtr(this)](
+                   const ClosePromise::ResolveOrRejectValue& aValue) {
+                 self->mWorkerRef = nullptr;
+                 self->mCloseHandler->Close();
+
+                 QM_TRY(OkIf(aValue.IsResolve()), QM_VOID);
+               });
+  }
+
+  return mCloseHandler->GetClosePromise();
+}
+
+already_AddRefed<Promise> FileSystemWritableFileStream::Write(
+    JSContext* aCx, JS::Handle<JS::Value> aChunk, ErrorResult& aError) {
+  // https://fs.spec.whatwg.org/#create-a-new-filesystemwritablefilestream
+  // Step 3. Let writeAlgorithm be an algorithm which takes a chunk argument
+  // and returns the result of running the write a chunk algorithm with stream
+  // and chunk.
+
+  // https://fs.spec.whatwg.org/#write-a-chunk
+  // Step 1. Let input be the result of converting chunk to a
+  // FileSystemWriteChunkType.
+
+  aError.MightThrowJSException();
+
+  ArrayBufferViewOrArrayBufferOrBlobOrUTF8StringOrWriteParams data;
+  if (!data.Init(aCx, aChunk)) {
+    aError.StealExceptionFromJSContext(aCx);
+    return nullptr;
+  }
+
+  // Step 2. Let p be a new promise.
+  RefPtr<Promise> promise = Promise::Create(GetParentObject(), aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  if (!IsOpen()) {
+    promise->MaybeRejectWithTypeError("WritableFileStream closed");
+    return promise.forget();
+  }
+
+  // Step 3.3. Let command be input.type if input is a WriteParams, ...
+  if (data.IsWriteParams()) {
+    const WriteParams& params = data.GetAsWriteParams();
+    switch (params.mType) {
+      // Step 3.4. If command is "write":
+      case WriteCommandType::Write: {
+        if (!params.mData.WasPassed()) {
+          promise->MaybeRejectWithSyntaxError("write() requires data");
+          return promise.forget();
+        }
+
+        // Step 3.4.2. If data is undefined, reject p with a TypeError and
+        // abort.
+        if (params.mData.Value().IsNull()) {
+          promise->MaybeRejectWithTypeError("write() of null data");
+          return promise.forget();
+        }
+
+        Maybe<uint64_t> position;
+
+        if (params.mPosition.WasPassed()) {
+          if (params.mPosition.Value().IsNull()) {
+            promise->MaybeRejectWithTypeError("write() with null position");
+            return promise.forget();
+          }
+
+          position = Some(params.mPosition.Value().Value());
+        }
+
+        Write(params.mData.Value().Value(), position, promise);
+        return promise.forget();
+      }
+
+      // Step 3.5. Otherwise, if command is "seek":
+      case WriteCommandType::Seek:
+        if (!params.mPosition.WasPassed()) {
+          promise->MaybeRejectWithSyntaxError("seek() requires a position");
+          return promise.forget();
+        }
+
+        // Step 3.5.1. If chunk.position is undefined, reject p with a
+        // TypeError and abort.
+        if (params.mPosition.Value().IsNull()) {
+          promise->MaybeRejectWithTypeError("seek() with null position");
+          return promise.forget();
+        }
+
+        Seek(params.mPosition.Value().Value(), promise);
+        return promise.forget();
+
+      // Step 3.6. Otherwise, if command is "truncate":
+      case WriteCommandType::Truncate:
+        if (!params.mSize.WasPassed()) {
+          promise->MaybeRejectWithSyntaxError("truncate() requires a size");
+          return promise.forget();
+        }
+
+        // Step 3.6.1. If chunk.size is undefined, reject p with a TypeError
+        // and abort.
+        if (params.mSize.Value().IsNull()) {
+          promise->MaybeRejectWithTypeError("truncate() with null size");
+          return promise.forget();
+        }
+
+        Truncate(params.mSize.Value().Value(), promise);
+        return promise.forget();
+
+      default:
+        MOZ_CRASH("Bad WriteParams value!");
+    }
+  }
+
+  // Step 3.3. ... and "write" otherwise.
+  // Step 3.4. If command is "write":
+  Write(data, Nothing(), promise);
+  return promise.forget();
+}
+
+// WebIDL Boilerplate
+
+JSObject* FileSystemWritableFileStream::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return FileSystemWritableFileStream_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// WebIDL Interface
+
+already_AddRefed<Promise> FileSystemWritableFileStream::Write(
+    const ArrayBufferViewOrArrayBufferOrBlobOrUTF8StringOrWriteParams& aData,
+    ErrorResult& aError) {
+  // https://fs.spec.whatwg.org/#dom-filesystemwritablefilestream-write
+  // Step 1. Let writer be the result of getting a writer for this.
+  RefPtr<WritableStreamDefaultWriter> writer = GetWriter(aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  // Step 2. Let result be the result of writing a chunk to writer given data.
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(GetParentObject())) {
+    aError.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+
+  JSContext* cx = jsapi.cx();
+
+  JS::Rooted<JSObject*> global(cx, JS::CurrentGlobalOrNull(cx));
+
+  JS::Rooted<JS::Value> val(cx);
+  if (!aData.ToJSVal(cx, global, &val)) {
+    aError.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+
+  RefPtr<Promise> promise = writer->Write(cx, val, aError);
+
+  // Step 3. Release writer.
+  writer->ReleaseLock(cx);
+
+  // Step 4. Return result.
+  return promise.forget();
+}
+
+already_AddRefed<Promise> FileSystemWritableFileStream::Seek(
+    uint64_t aPosition, ErrorResult& aError) {
+  // https://fs.spec.whatwg.org/#dom-filesystemwritablefilestream-seek
+  // Step 1. Let writer be the result of getting a writer for this.
+  RefPtr<WritableStreamDefaultWriter> writer = GetWriter(aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  // Step 2. Let result be the result of writing a chunk to writer given
+  // «[ "type" → "seek", "position" → position ]».
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(GetParentObject())) {
+    aError.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+
+  JSContext* cx = jsapi.cx();
+
+  RootedDictionary<WriteParams> writeParams(cx);
+  writeParams.mType = WriteCommandType::Seek;
+  writeParams.mPosition.Construct(aPosition);
+
+  JS::Rooted<JS::Value> val(cx);
+  if (!ToJSValue(cx, writeParams, &val)) {
+    aError.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+
+  RefPtr<Promise> promise = writer->Write(cx, val, aError);
+
+  // Step 3. Release writer.
+  writer->ReleaseLock(cx);
+
+  // Step 4. Return result.
+  return promise.forget();
+}
+
+already_AddRefed<Promise> FileSystemWritableFileStream::Truncate(
+    uint64_t aSize, ErrorResult& aError) {
+  // https://fs.spec.whatwg.org/#dom-filesystemwritablefilestream-truncate
+  // Step 1. Let writer be the result of getting a writer for this.
+  RefPtr<WritableStreamDefaultWriter> writer = GetWriter(aError);
+  if (aError.Failed()) {
+    return nullptr;
+  }
+
+  // Step 2. Let result be the result of writing a chunk to writer given
+  // «[ "type" → "truncate", "size" → size ]».
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(GetParentObject())) {
+    aError.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+
+  JSContext* cx = jsapi.cx();
+
+  RootedDictionary<WriteParams> writeParams(cx);
+  writeParams.mType = WriteCommandType::Truncate;
+  writeParams.mSize.Construct(aSize);
+
+  JS::Rooted<JS::Value> val(cx);
+  if (!ToJSValue(cx, writeParams, &val)) {
+    aError.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+
+  RefPtr<Promise> promise = writer->Write(cx, val, aError);
+
+  // Step 3. Release writer.
+  writer->ReleaseLock(cx);
+
+  // Step 4. Return result.
+  return promise.forget();
+}
+
+template <typename T>
+void FileSystemWritableFileStream::Write(const T& aData,
+                                         const Maybe<uint64_t> aPosition,
+                                         const RefPtr<Promise>& aPromise) {
+  MOZ_ASSERT(IsOpen());
+
+  auto rejectAndReturn = [&aPromise](const nsresult rv) {
+    if (IsFileNotFoundError(rv)) {
+      aPromise->MaybeRejectWithNotFoundError("File not found");
+      return;
+    }
+    aPromise->MaybeReject(rv);
+  };
+
+  nsCOMPtr<nsIInputStream> inputStream;
+
+  // https://fs.spec.whatwg.org/#write-a-chunk
+  // Step 3.4.6 If data is a BufferSource, let dataBytes be a copy of data.
+  if (aData.IsArrayBuffer() || aData.IsArrayBufferView()) {
+    const auto dataSpan = [&aData]() -> mozilla::Span<uint8_t> {
+      if (aData.IsArrayBuffer()) {
+        const ArrayBuffer& buffer = aData.GetAsArrayBuffer();
+        buffer.ComputeState();
+        return Span{buffer.Data(), buffer.Length()};
+      }
+
+      const ArrayBufferView& buffer = aData.GetAsArrayBufferView();
+      buffer.ComputeState();
+      return Span{buffer.Data(), buffer.Length()};
+    }();
+
+    // Here we copy
+
+    QM_TRY(MOZ_TO_RESULT(NS_NewByteInputStream(getter_AddRefs(inputStream),
+                                               AsChars(dataSpan),
+                                               NS_ASSIGNMENT_COPY)),
+           rejectAndReturn);
+
+    WriteImpl(std::move(inputStream), aPosition, aPromise);
+    return;
+  }
+
+  // Step 3.4.7 Otherwise, if data is a Blob ...
+  if (aData.IsBlob()) {
+    Blob& blob = aData.GetAsBlob();
+
+    ErrorResult error;
+    blob.CreateInputStream(getter_AddRefs(inputStream), error);
+    QM_TRY((MOZ_TO_RESULT(!error.Failed()).mapErr([&error](const nsresult rv) {
+             return error.StealNSResult();
+           })),
+           rejectAndReturn);
+
+    WriteImpl(std::move(inputStream), aPosition, aPromise);
+    return;
+  }
+
+  // Step 3.4.8 Otherwise ...
+  MOZ_ASSERT(aData.IsUTF8String());
+
+  // Here we copy
+  nsCString dataString;
+  if (!dataString.Assign(aData.GetAsUTF8String(), mozilla::fallible)) {
+    rejectAndReturn(NS_ERROR_OUT_OF_MEMORY);
+    return;
+  }
+
+  // Input stream takes ownership
+  QM_TRY(MOZ_TO_RESULT(NS_NewCStringInputStream(getter_AddRefs(inputStream),
+                                                std::move(dataString))),
+         rejectAndReturn);
+
+  WriteImpl(std::move(inputStream), aPosition, aPromise);
+}
+
+void FileSystemWritableFileStream::WriteImpl(
+    nsCOMPtr<nsIInputStream> aInputStream, const Maybe<uint64_t> aPosition,
+    const RefPtr<Promise>& aPromise) {
+  auto command = CreateCommand();
+
+  InvokeAsync(
+      mTaskQueue, __func__,
+      [selfHolder = fs::TargetPtrHolder(this),
+       inputStream = std::move(aInputStream), aPosition]() {
+        QM_TRY(MOZ_TO_RESULT(selfHolder->EnsureStream()),
+               CreateAndRejectInt64Promise);
+
+        if (aPosition.isSome()) {
+          LOG(("%p: Seeking to %" PRIu64, selfHolder->mStreamOwner.get(),
+               aPosition.value()));
+
+          QM_TRY(
+              MOZ_TO_RESULT(selfHolder->mStreamOwner->Seek(aPosition.value())),
+              CreateAndRejectInt64Promise);
+        }
+
+        nsCOMPtr<nsIOutputStream> streamSink =
+            selfHolder->mStreamOwner->OutputStream();
+
+        auto written = std::make_shared<int64_t>(0);
+        auto writingProgress = [written](uint32_t aDelta) {
+          *written += static_cast<int64_t>(aDelta);
+        };
+
+        auto promiseHolder = MakeUnique<MozPromiseHolder<Int64Promise>>();
+        RefPtr<Int64Promise> promise = promiseHolder->Ensure(__func__);
+
+        auto writingCompletion =
+            [written,
+             promiseHolder = std::move(promiseHolder)](nsresult aStatus) {
+              if (NS_SUCCEEDED(aStatus)) {
+                promiseHolder->ResolveIfExists(*written, __func__);
+                return;
+              }
+
+              promiseHolder->RejectIfExists(aStatus, __func__);
+            };
+
+        QM_TRY(MOZ_TO_RESULT(fs::AsyncCopy(
+                   inputStream, streamSink, selfHolder->mTaskQueue,
+                   nsAsyncCopyMode::NS_ASYNCCOPY_VIA_READSEGMENTS,
+                   /* aCloseSource */ true, /* aCloseSink */ false,
+                   std::move(writingProgress), std::move(writingCompletion))),
+               CreateAndRejectInt64Promise);
+
+        return promise;
+      })
+      ->Then(
+          GetCurrentSerialEventTarget(), __func__,
+          [command,
+           aPromise](const Int64Promise::ResolveOrRejectValue& aValue) {
+            if (aValue.IsResolve()) {
+              aPromise->MaybeResolve(aValue.ResolveValue());
+              return;
+            }
+
+            if (IsFileNotFoundError(aValue.RejectValue())) {
+              aPromise->MaybeRejectWithNotFoundError("File not found");
+            } else if (aValue.RejectValue() == NS_ERROR_FILE_NO_DEVICE_SPACE) {
+              aPromise->MaybeRejectWithQuotaExceededError("Quota exceeded");
+            } else {
+              aPromise->MaybeReject(aValue.RejectValue());
+            }
+
+            aPromise->MaybeReject(aValue.RejectValue());
+          });
+}
+
+void FileSystemWritableFileStream::Seek(uint64_t aPosition,
+                                        const RefPtr<Promise>& aPromise) {
+  MOZ_ASSERT(IsOpen());
+
+  LOG_VERBOSE(("%p: Seeking to %" PRIu64, mStreamOwner.get(), aPosition));
+
+  auto command = CreateCommand();
+
+  InvokeAsync(mTaskQueue, __func__,
+              [selfHolder = fs::TargetPtrHolder(this), aPosition]() mutable {
+                QM_TRY(MOZ_TO_RESULT(selfHolder->EnsureStream()),
+                       CreateAndRejectBoolPromise);
+
+                QM_TRY(MOZ_TO_RESULT(selfHolder->mStreamOwner->Seek(aPosition)),
+                       CreateAndRejectBoolPromise);
+
+                return BoolPromise::CreateAndResolve(true, __func__);
+              })
+      ->Then(
+          GetCurrentSerialEventTarget(), __func__,
+          [command, aPromise](const BoolPromise::ResolveOrRejectValue& aValue) {
+            if (aValue.IsReject()) {
+              auto rv = aValue.RejectValue();
+              if (IsFileNotFoundError(rv)) {
+                aPromise->MaybeRejectWithNotFoundError("File not found");
+                return;
+              }
+              aPromise->MaybeReject(rv);
+              return;
+            }
+            MOZ_ASSERT(aValue.IsResolve());
+            aPromise->MaybeResolveWithUndefined();
+          });
+}
+
+void FileSystemWritableFileStream::Truncate(uint64_t aSize,
+                                            const RefPtr<Promise>& aPromise) {
+  MOZ_ASSERT(IsOpen());
+
+  auto command = CreateCommand();
+
+  InvokeAsync(mTaskQueue, __func__,
+              [selfHolder = fs::TargetPtrHolder(this), aSize]() mutable {
+                QM_TRY(MOZ_TO_RESULT(selfHolder->EnsureStream()),
+                       CreateAndRejectBoolPromise);
+
+                QM_TRY(MOZ_TO_RESULT(selfHolder->mStreamOwner->Truncate(aSize)),
+                       CreateAndRejectBoolPromise);
+
+                return BoolPromise::CreateAndResolve(true, __func__);
+              })
+      ->Then(
+          GetCurrentSerialEventTarget(), __func__,
+          [command, aPromise](const BoolPromise::ResolveOrRejectValue& aValue) {
+            if (aValue.IsReject()) {
+              aPromise->MaybeReject(aValue.RejectValue());
+              return;
+            }
+
+            aPromise->MaybeResolveWithUndefined();
+          });
+}
+
+nsresult FileSystemWritableFileStream::EnsureStream() {
+  if (!mStreamOwner) {
+    QM_TRY_UNWRAP(MovingNotNull<nsCOMPtr<nsIRandomAccessStream>> stream,
+                  DeserializeRandomAccessStream(mStreamParams),
+                  NS_ERROR_FAILURE);
+
+    mozilla::ipc::RandomAccessStreamParams streamParams(
+        std::move(mStreamParams));
+
+    mStreamOwner = MakeRefPtr<fs::FileSystemThreadSafeStreamOwner>(
+        this, std::move(stream));
+  }
+
+  return NS_OK;
+}
+
+void FileSystemWritableFileStream::NoteFinishedCommand() {
+  MOZ_ASSERT(mCommandActive);
+
+  mCommandActive = false;
+
+  mFinishPromiseHolder.ResolveIfExists(true, __func__);
+}
+
+RefPtr<BoolPromise> FileSystemWritableFileStream::Finish() {
+  if (!mCommandActive) {
+    return BoolPromise::CreateAndResolve(true, __func__);
+  }
+
+  return mFinishPromiseHolder.Ensure(__func__);
+}
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(
+    WritableFileStreamUnderlyingSinkAlgorithms, UnderlyingSinkAlgorithmsBase)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(WritableFileStreamUnderlyingSinkAlgorithms,
+                                   UnderlyingSinkAlgorithmsBase, mStream)
+
+// Step 3 of
+// https://fs.spec.whatwg.org/#create-a-new-filesystemwritablefilestream
+already_AddRefed<Promise>
+WritableFileStreamUnderlyingSinkAlgorithms::WriteCallback(
+    JSContext* aCx, JS::Handle<JS::Value> aChunk,
+    WritableStreamDefaultController& aController, ErrorResult& aRv) {
+  return mStream->Write(aCx, aChunk, aRv);
+}
+
+// Step 4 of
+// https://fs.spec.whatwg.org/#create-a-new-filesystemwritablefilestream
+already_AddRefed<Promise>
+WritableFileStreamUnderlyingSinkAlgorithms::CloseCallbackImpl(
+    JSContext* aCx, ErrorResult& aRv) {
+  RefPtr<Promise> promise = Promise::Create(mStream->GetParentObject(), aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  if (!mStream->IsOpen()) {
+    promise->MaybeRejectWithTypeError("WritableFileStream closed");
+    return promise.forget();
+  }
+
+  mStream->BeginClose()->Then(
+      GetCurrentSerialEventTarget(), __func__,
+      [promise](const BoolPromise::ResolveOrRejectValue& aValue) {
+        // Step 2.3. Return a promise resolved with undefined.
+        if (aValue.IsResolve()) {
+          promise->MaybeResolveWithUndefined();
+          return;
+        }
+        promise->MaybeRejectWithAbortError(
+            "Internal error closing file stream");
+      });
+
+  return promise.forget();
+}
+
+// Step 5 of
+// https://fs.spec.whatwg.org/#create-a-new-filesystemwritablefilestream
+already_AddRefed<Promise>
+WritableFileStreamUnderlyingSinkAlgorithms::AbortCallbackImpl(
+    JSContext* aCx, const Optional<JS::Handle<JS::Value>>& /* aReason */,
+    ErrorResult& aRv) {
+  // https://streams.spec.whatwg.org/#writablestream-set-up
+  // Step 3. Let abortAlgorithmWrapper be an algorithm that runs these steps:
+  // Step 3.3. Return a promise resolved with undefined.
+
+  return CloseCallbackImpl(aCx, aRv);
+}
+
+void WritableFileStreamUnderlyingSinkAlgorithms::ReleaseObjects() {
+  // XXX We shouldn't be calling close here. We should just release the lock.
+  // However, calling close here is not a big issue for now because we don't
+  // write to a temporary file which would atomically replace the real file
+  // during close.
+  if (mStream->IsOpen()) {
+    Unused << mStream->BeginClose();
+  }
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/fs/api/FileSystemWritableFileStream.h b/dom/fs/api/FileSystemWritableFileStream.h
new file mode 100644
index 0000000000..ac8b8b03f3
--- /dev/null
+++ b/dom/fs/api/FileSystemWritableFileStream.h
@@ -0,0 +1,146 @@
+/* -*- 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_FS_FILESYSTEMWRITABLEFILESTREAM_H_
+#define DOM_FS_FILESYSTEMWRITABLEFILESTREAM_H_
+
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/dom/FlippedOnce.h"
+#include "mozilla/dom/PFileSystemManager.h"
+#include "mozilla/dom/WritableStream.h"
+#include "mozilla/dom/quota/ForwardDecls.h"
+
+class nsIGlobalObject;
+class nsIRandomAccessStream;
+
+namespace mozilla {
+
+template <typename T>
+class Buffer;
+class ErrorResult;
+class TaskQueue;
+
+namespace ipc {
+class RandomAccessStreamParams;
+}
+
+namespace dom {
+
+class ArrayBufferViewOrArrayBufferOrBlobOrUTF8StringOrWriteParams;
+class Blob;
+class FileSystemManager;
+class FileSystemWritableFileStreamChild;
+class OwningArrayBufferViewOrArrayBufferOrBlobOrUSVString;
+class Promise;
+class StrongWorkerRef;
+
+namespace fs {
+class FileSystemThreadSafeStreamOwner;
+}
+
+class FileSystemWritableFileStream final : public WritableStream {
+ public:
+  static Result<RefPtr<FileSystemWritableFileStream>, nsresult> Create(
+      const nsCOMPtr<nsIGlobalObject>& aGlobal,
+      RefPtr<FileSystemManager>& aManager,
+      mozilla::ipc::RandomAccessStreamParams&& aStreamParams,
+      RefPtr<FileSystemWritableFileStreamChild> aActor,
+      const fs::FileSystemEntryMetadata& aMetadata);
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(FileSystemWritableFileStream,
+                                           WritableStream)
+
+  void LastRelease() override;
+
+  void ClearActor();
+
+  class Command;
+  RefPtr<Command> CreateCommand();
+
+  bool IsCommandActive() const;
+
+  bool IsOpen() const;
+
+  bool IsClosed() const;
+
+  [[nodiscard]] RefPtr<BoolPromise> BeginClose();
+
+  already_AddRefed<Promise> Write(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                                  ErrorResult& aError);
+
+  // WebIDL Boilerplate
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // WebIDL Interface
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Write(
+      const ArrayBufferViewOrArrayBufferOrBlobOrUTF8StringOrWriteParams& aData,
+      ErrorResult& aError);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Seek(uint64_t aPosition,
+                                                    ErrorResult& aError);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Truncate(uint64_t aSize,
+                                                        ErrorResult& aError);
+
+ private:
+  class CloseHandler;
+
+  FileSystemWritableFileStream(
+      const nsCOMPtr<nsIGlobalObject>& aGlobal,
+      RefPtr<FileSystemManager>& aManager,
+      mozilla::ipc::RandomAccessStreamParams&& aStreamParams,
+      RefPtr<FileSystemWritableFileStreamChild> aActor,
+      already_AddRefed<TaskQueue> aTaskQueue,
+      const fs::FileSystemEntryMetadata& aMetadata);
+
+  virtual ~FileSystemWritableFileStream();
+
+  template <typename T>
+  void Write(const T& aData, const Maybe<uint64_t> aPosition,
+             const RefPtr<Promise>& aPromise);
+
+  void WriteImpl(nsCOMPtr<nsIInputStream> aInputStream,
+                 const Maybe<uint64_t> aPosition,
+                 const RefPtr<Promise>& aPromise);
+
+  void Seek(uint64_t aPosition, const RefPtr<Promise>& aPromise);
+
+  void Truncate(uint64_t aSize, const RefPtr<Promise>& aPromise);
+
+  nsresult EnsureStream();
+
+  void NoteFinishedCommand();
+
+  [[nodiscard]] RefPtr<BoolPromise> Finish();
+
+  RefPtr<FileSystemManager> mManager;
+
+  RefPtr<FileSystemWritableFileStreamChild> mActor;
+
+  RefPtr<TaskQueue> mTaskQueue;
+
+  RefPtr<fs::FileSystemThreadSafeStreamOwner> mStreamOwner;
+
+  RefPtr<StrongWorkerRef> mWorkerRef;
+
+  mozilla::ipc::RandomAccessStreamParams mStreamParams;
+
+  fs::FileSystemEntryMetadata mMetadata;
+
+  RefPtr<CloseHandler> mCloseHandler;
+
+  MozPromiseHolder<BoolPromise> mFinishPromiseHolder;
+
+  bool mCommandActive;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // DOM_FS_FILESYSTEMWRITABLEFILESTREAM_H_
diff --git a/dom/fs/api/moz.build b/dom/fs/api/moz.build
new file mode 100644
index 0000000000..1b9a64996d
--- /dev/null
+++ b/dom/fs/api/moz.build
@@ -0,0 +1,38 @@
+# -*- 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/.
+
+EXPORTS.mozilla.dom += [
+    "FileSystemDirectoryHandle.h",
+    "FileSystemDirectoryIterator.h",
+    "FileSystemFileHandle.h",
+    "FileSystemHandle.h",
+    "FileSystemManager.h",
+    "FileSystemSyncAccessHandle.h",
+    "FileSystemWritableFileStream.h",
+]
+
+UNIFIED_SOURCES += [
+    "FileSystemDirectoryHandle.cpp",
+    "FileSystemDirectoryIterator.cpp",
+    "FileSystemFileHandle.cpp",
+    "FileSystemHandle.cpp",
+    "FileSystemManager.cpp",
+    "FileSystemSyncAccessHandle.cpp",
+    "FileSystemWritableFileStream.cpp",
+]
+
+LOCAL_INCLUDES += [
+    "/dom/fs/child",
+    "/dom/fs/include",
+    "/netwerk/base",
+]
+
+FINAL_LIBRARY = "xul"
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+# Add libFuzzer configuration directives
+include("/tools/fuzzing/libfuzzer-config.mozbuild")