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diff --git a/dom/storage/SessionStorageManager.h b/dom/storage/SessionStorageManager.h
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+++ b/dom/storage/SessionStorageManager.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_SessionStorageManager_h
+#define mozilla_dom_SessionStorageManager_h
+
+#include "StorageObserver.h"
+
+#include "mozilla/dom/FlippedOnce.h"
+#include "nsIDOMStorageManager.h"
+#include "nsClassHashtable.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsHashKeys.h"
+
+#include "mozilla/ipc/PBackgroundChild.h"
+#include "mozilla/ipc/PBackgroundParent.h"
+
+class nsIPrincipal;
+class nsITimer;
+
+namespace mozilla {
+class OriginAttributesPattern;
+
+namespace dom {
+
+class SSCacheCopy;
+
+bool RecvShutdownBackgroundSessionStorageManagers();
+void RecvPropagateBackgroundSessionStorageManager(uint64_t aCurrentTopContextId,
+                                                  uint64_t aTargetTopContextId);
+bool RecvRemoveBackgroundSessionStorageManager(uint64_t aTopContextId);
+
+bool RecvGetSessionStorageData(
+    uint64_t aTopContextId, uint32_t aSizeLimit, bool aCancelSessionStoreTimer,
+    ::mozilla::ipc::PBackgroundParent::GetSessionStorageManagerDataResolver&&
+        aResolver);
+
+bool RecvLoadSessionStorageData(
+    uint64_t aTopContextId,
+    nsTArray<mozilla::dom::SSCacheCopy>&& aCacheCopyList);
+
+bool RecvClearStoragesForOrigin(const nsACString& aOriginAttrs,
+                                const nsACString& aOriginKey);
+
+class BrowsingContext;
+class ContentParent;
+class SSSetItemInfo;
+class SSWriteInfo;
+class SessionStorageCache;
+class SessionStorageCacheChild;
+class SessionStorageManagerChild;
+class SessionStorageManagerParent;
+class SessionStorageObserver;
+struct OriginRecord;
+
+// sessionStorage is a data store that's unique to each tab (i.e. top-level
+// browsing context) and origin. Before the advent of Fission all the data
+// for a given tab could be stored in a single content process; now each
+// site-specific process stores only its portion of the data. As content
+// processes terminate, their sessionStorage data needs to be saved in the
+// parent process, in case the same origin appears again in the tab (e.g.
+// by navigating an iframe element). Therefore SessionStorageManager
+// objects exist in both the parent and content processes.
+//
+// Whenever a write operation for SessionStorage executes, the content process
+// sends the changes to the parent process at the next stable state. Whenever a
+// content process navigates to an origin for the first time in a given tab, the
+// parent process sends it the saved data. SessionStorageCache has a flag
+// (mLoadedOrCloned) to ensure that it's only be loaded or cloned once.
+//
+// Note: the current implementation is expected to be replaced by a new
+// implementation using LSNG.
+class SessionStorageManagerBase {
+ public:
+  SessionStorageManagerBase() = default;
+
+ protected:
+  ~SessionStorageManagerBase() = default;
+
+  struct OriginRecord {
+    OriginRecord() = default;
+    OriginRecord(OriginRecord&&) = default;
+    OriginRecord& operator=(OriginRecord&&) = default;
+    ~OriginRecord();
+
+    RefPtr<SessionStorageCache> mCache;
+
+    // A flag to ensure that cache is only loaded once.
+    FlippedOnce<false> mLoaded;
+  };
+
+  void ClearStoragesInternal(const OriginAttributesPattern& aPattern,
+                             const nsACString& aOriginScope);
+
+  void ClearStoragesForOriginInternal(const nsACString& aOriginAttrs,
+                                      const nsACString& aOriginKey);
+
+  OriginRecord* GetOriginRecord(const nsACString& aOriginAttrs,
+                                const nsACString& aOriginKey,
+                                bool aMakeIfNeeded,
+                                SessionStorageCache* aCloneFrom);
+
+  using OriginKeyHashTable = nsClassHashtable<nsCStringHashKey, OriginRecord>;
+  nsClassHashtable<nsCStringHashKey, OriginKeyHashTable> mOATable;
+};
+
+class SessionStorageManager final : public SessionStorageManagerBase,
+                                    public nsIDOMSessionStorageManager,
+                                    public StorageObserverSink {
+ public:
+  explicit SessionStorageManager(RefPtr<BrowsingContext> aBrowsingContext);
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_NSIDOMSTORAGEMANAGER
+  NS_DECL_NSIDOMSESSIONSTORAGEMANAGER
+
+  NS_DECL_CYCLE_COLLECTION_CLASS(SessionStorageManager)
+
+  bool CanLoadData();
+
+  void SetActor(SessionStorageManagerChild* aActor);
+
+  bool ActorExists() const;
+
+  void ClearActor();
+
+  nsresult EnsureManager();
+
+  nsresult LoadData(nsIPrincipal& aPrincipal, SessionStorageCache& aCache);
+
+  void CheckpointData(nsIPrincipal& aPrincipal, SessionStorageCache& aCache);
+
+  nsresult ClearStoragesForOrigin(const nsACString& aOriginAttrs,
+                                  const nsACString& aOriginKey);
+
+ private:
+  ~SessionStorageManager();
+
+  // StorageObserverSink, handler to various chrome clearing notification
+  nsresult Observe(const char* aTopic,
+                   const nsAString& aOriginAttributesPattern,
+                   const nsACString& aOriginScope) override;
+
+  nsresult GetSessionStorageCacheHelper(nsIPrincipal* aPrincipal,
+                                        bool aMakeIfNeeded,
+                                        SessionStorageCache* aCloneFrom,
+                                        RefPtr<SessionStorageCache>* aRetVal);
+
+  nsresult GetSessionStorageCacheHelper(const nsACString& aOriginAttrs,
+                                        const nsACString& aOriginKey,
+                                        bool aMakeIfNeeded,
+                                        SessionStorageCache* aCloneFrom,
+                                        RefPtr<SessionStorageCache>* aRetVal);
+
+  void ClearStorages(const OriginAttributesPattern& aPattern,
+                     const nsACString& aOriginScope);
+
+  SessionStorageCacheChild* EnsureCache(nsIPrincipal& aPrincipal,
+                                        const nsACString& aOriginKey,
+                                        SessionStorageCache& aCache);
+
+  void CheckpointDataInternal(nsIPrincipal& aPrincipal,
+                              const nsACString& aOriginKey,
+                              SessionStorageCache& aCache);
+
+  RefPtr<SessionStorageObserver> mObserver;
+
+  RefPtr<BrowsingContext> mBrowsingContext;
+
+  SessionStorageManagerChild* mActor;
+};
+
+/**
+ * A specialized SessionStorageManager class that lives on the parent process
+ * background thread. It is a shadow copy of SessionStorageManager and it's used
+ * to preserve SessionStorageCaches for the other SessionStorageManagers.
+ */
+class BackgroundSessionStorageManager final : public SessionStorageManagerBase {
+ public:
+  // Parent process getter function.
+  static BackgroundSessionStorageManager* GetOrCreate(uint64_t aTopContextId);
+
+  NS_INLINE_DECL_REFCOUNTING(BackgroundSessionStorageManager);
+
+  // Only called by CanonicalBrowsingContext::ReplaceBy on the parent process.
+  static void PropagateManager(uint64_t aCurrentTopContextId,
+                               uint64_t aTargetTopContextId);
+
+  // Only called by CanonicalBrowsingContext::CanonicalDiscard on parent
+  // process.
+  static void RemoveManager(uint64_t aTopContextId);
+
+  static void LoadData(
+      uint64_t aTopContextId,
+      const nsTArray<mozilla::dom::SSCacheCopy>& aCacheCopyList);
+
+  using DataPromise =
+      ::mozilla::ipc::PBackgroundChild::GetSessionStorageManagerDataPromise;
+  static RefPtr<DataPromise> GetData(BrowsingContext* aContext,
+                                     uint32_t aSizeLimit,
+                                     bool aClearSessionStoreTimer = false);
+
+  void GetData(uint32_t aSizeLimit, nsTArray<SSCacheCopy>& aCacheCopyList);
+
+  void CopyDataToContentProcess(const nsACString& aOriginAttrs,
+                                const nsACString& aOriginKey,
+                                nsTArray<SSSetItemInfo>& aData);
+
+  void UpdateData(const nsACString& aOriginAttrs, const nsACString& aOriginKey,
+                  const nsTArray<SSWriteInfo>& aWriteInfos);
+
+  void UpdateData(const nsACString& aOriginAttrs, const nsACString& aOriginKey,
+                  const nsTArray<SSSetItemInfo>& aData);
+
+  void ClearStorages(const OriginAttributesPattern& aPattern,
+                     const nsACString& aOriginScope);
+
+  void ClearStoragesForOrigin(const nsACString& aOriginAttrs,
+                              const nsACString& aOriginKey);
+
+  void SetCurrentBrowsingContextId(uint64_t aBrowsingContextId);
+
+  void MaybeDispatchSessionStoreUpdate();
+
+  void CancelSessionStoreUpdate();
+
+  void AddParticipatingActor(SessionStorageManagerParent* aActor);
+
+  void RemoveParticipatingActor(SessionStorageManagerParent* aActor);
+
+ private:
+  // Only be called by GetOrCreate() on the parent process.
+  explicit BackgroundSessionStorageManager(uint64_t aBrowsingContextId);
+
+  ~BackgroundSessionStorageManager();
+
+  // Sets a timer for notifying main thread that the cache has been
+  // updated. May do nothing if we're coalescing notifications.
+  void MaybeScheduleSessionStoreUpdate();
+
+  void DispatchSessionStoreUpdate();
+
+  // The most current browsing context using this manager
+  uint64_t mCurrentBrowsingContextId;
+
+  // Callback for notifying main thread of calls to `UpdateData`.
+  //
+  // A timer that is held whenever this manager has dirty state that
+  // has not yet been reflected to the main thread. The timer is used
+  // to delay notifying the main thread to ask for changes, thereby
+  // coalescing/throttling changes. (Note that SessionStorage, like
+  // LocalStorage, treats attempts to set a value to its current value
+  // as a no-op.)
+  //
+
+  // The timer is initialized with a fixed delay as soon as the state
+  // becomes dirty; additional mutations to our state will not reset
+  // the timer because then we might never flush to the main
+  // thread. The timer is cleared only when a new set of data is sent
+  // to the main thread and therefore this manager no longer has any
+  // dirty state. This means that there is a period of time after the
+  // nsITimer fires where this value is non-null but there is no
+  // scheduled timer while we wait for the main thread to request the
+  // new state. Callers of GetData can also optionally cancel the
+  // current timer to reduce the amounts of notifications.
+  //
+  // When this manager is moved to a new top-level browsing context id
+  // via a PropagateBackgroundSessionStorageManager message, the
+  // behavior of the timer doesn't change because the main thread knows
+  // about the renaming and is initiating it (and any in-flight
+  // GetSessionStorageManagerData requests will be unaffected because
+  // they use async-returns so the response is inherently matched up via
+  // the issued promise).
+  nsCOMPtr<nsITimer> mSessionStoreCallbackTimer;
+
+  nsTArray<RefPtr<SessionStorageManagerParent>> mParticipatingActors;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_SessionStorageManager_h