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diff --git a/docshell/base/BrowsingContextGroup.h b/docshell/base/BrowsingContextGroup.h
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+++ b/docshell/base/BrowsingContextGroup.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_BrowsingContextGroup_h
+#define mozilla_dom_BrowsingContextGroup_h
+
+#include "mozilla/dom/BrowsingContext.h"
+#include "mozilla/FunctionRef.h"
+#include "nsRefPtrHashtable.h"
+#include "nsHashKeys.h"
+#include "nsTArray.h"
+#include "nsTHashtable.h"
+#include "nsWrapperCache.h"
+#include "nsXULAppAPI.h"
+
+namespace mozilla {
+class ThrottledEventQueue;
+
+namespace dom {
+
+class BrowsingContext;
+class WindowContext;
+class ContentParent;
+class DocGroup;
+
+// A BrowsingContextGroup represents the Unit of Related Browsing Contexts in
+// the standard.
+//
+// A BrowsingContext may not hold references to other BrowsingContext objects
+// which are not in the same BrowsingContextGroup.
+class BrowsingContextGroup final : public nsWrapperCache {
+ public:
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(BrowsingContextGroup)
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_NATIVE_CLASS(BrowsingContextGroup)
+
+  // Interact with the list of synced contexts. This controls the lifecycle of
+  // the BrowsingContextGroup and contexts loaded within them.
+  void Register(nsISupports* aContext);
+  void Unregister(nsISupports* aContext);
+
+  // Control which processes will be used to host documents loaded in this
+  // BrowsingContextGroup. There should only ever be one host process per remote
+  // type.
+  //
+  // A new host process will be subscribed to the BrowsingContextGroup unless it
+  // is still launching, in which case it will subscribe itself when it is done
+  // launching.
+  void EnsureHostProcess(ContentParent* aProcess);
+
+  // A removed host process will no longer be used to host documents loaded in
+  // this BrowsingContextGroup.
+  void RemoveHostProcess(ContentParent* aProcess);
+
+  // Synchronize the current BrowsingContextGroup state down to the given
+  // content process, and continue updating it.
+  //
+  // You rarely need to call this directy, as it's automatically called by
+  // |EnsureHostProcess| as needed.
+  void Subscribe(ContentParent* aProcess);
+
+  // Stop synchromizing the current BrowsingContextGroup state down to a given
+  // content process. The content process must no longer be a host process.
+  void Unsubscribe(ContentParent* aProcess);
+
+  // Look up the process which should be used to host documents with this
+  // RemoteType. This will be a non-dead process associated with this
+  // BrowsingContextGroup, if possible.
+  ContentParent* GetHostProcess(const nsACString& aRemoteType);
+
+  // When a BrowsingContext is being discarded, we may want to keep the
+  // corresponding BrowsingContextGroup alive until the other process
+  // acknowledges the BrowsingContext has been discarded. A `KeepAlive` will be
+  // added to the `BrowsingContextGroup`, delaying destruction.
+  void AddKeepAlive();
+  void RemoveKeepAlive();
+
+  // Call when we want to check if we should suspend or resume all top level
+  // contexts.
+  void UpdateToplevelsSuspendedIfNeeded();
+
+  // Get a reference to the list of toplevel contexts in this
+  // BrowsingContextGroup.
+  nsTArray<RefPtr<BrowsingContext>>& Toplevels() { return mToplevels; }
+  void GetToplevels(nsTArray<RefPtr<BrowsingContext>>& aToplevels) {
+    aToplevels.AppendElements(mToplevels);
+  }
+
+  uint64_t Id() { return mId; }
+
+  nsISupports* GetParentObject() const;
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // Get or create a BrowsingContextGroup with the given ID.
+  static already_AddRefed<BrowsingContextGroup> GetOrCreate(uint64_t aId);
+  static already_AddRefed<BrowsingContextGroup> Create();
+  static already_AddRefed<BrowsingContextGroup> Select(
+      WindowContext* aParent, BrowsingContext* aOpener);
+
+  // For each 'ContentParent', except for 'aExcludedParent',
+  // associated with this group call 'aCallback'.
+  template <typename Func>
+  void EachOtherParent(ContentParent* aExcludedParent, Func&& aCallback) {
+    MOZ_DIAGNOSTIC_ASSERT(XRE_IsParentProcess());
+    for (auto iter = mSubscribers.Iter(); !iter.Done(); iter.Next()) {
+      if (iter.Get()->GetKey() != aExcludedParent) {
+        aCallback(iter.Get()->GetKey());
+      }
+    }
+  }
+
+  // For each 'ContentParent' associated with
+  // this group call 'aCallback'.
+  template <typename Func>
+  void EachParent(Func&& aCallback) {
+    MOZ_DIAGNOSTIC_ASSERT(XRE_IsParentProcess());
+    for (auto iter = mSubscribers.Iter(); !iter.Done(); iter.Next()) {
+      aCallback(iter.Get()->GetKey());
+    }
+  }
+
+  nsresult QueuePostMessageEvent(already_AddRefed<nsIRunnable>&& aRunnable);
+
+  void FlushPostMessageEvents();
+
+  // Increase or decrease the suspension level in InputTaskManager
+  void UpdateInputTaskManagerIfNeeded(bool aIsActive);
+
+  static BrowsingContextGroup* GetChromeGroup();
+
+  void GetDocGroups(nsTArray<DocGroup*>& aDocGroups);
+
+  // Called by Document when a Document needs to be added to a DocGroup.
+  already_AddRefed<DocGroup> AddDocument(const nsACString& aKey,
+                                         Document* aDocument);
+
+  // Called by Document when a Document needs to be removed to a DocGroup.
+  void RemoveDocument(const nsACString& aKey, Document* aDocument);
+
+  auto DocGroups() const { return mDocGroups.ConstIter(); }
+
+  mozilla::ThrottledEventQueue* GetTimerEventQueue() const {
+    return mTimerEventQueue;
+  }
+
+  mozilla::ThrottledEventQueue* GetWorkerEventQueue() const {
+    return mWorkerEventQueue;
+  }
+
+  static void GetAllGroups(nsTArray<RefPtr<BrowsingContextGroup>>& aGroups);
+
+  void IncInputEventSuspensionLevel();
+  void DecInputEventSuspensionLevel();
+
+ private:
+  friend class CanonicalBrowsingContext;
+
+  explicit BrowsingContextGroup(uint64_t aId);
+  ~BrowsingContextGroup();
+
+  void MaybeDestroy();
+  void Destroy();
+
+  bool ShouldSuspendAllTopLevelContexts() const;
+
+  bool HasActiveBC();
+  void DecInputTaskManagerSuspensionLevel();
+  void IncInputTaskManagerSuspensionLevel();
+
+  uint64_t mId;
+
+  uint32_t mKeepAliveCount = 0;
+
+#ifdef DEBUG
+  bool mDestroyed = false;
+#endif
+
+  // A BrowsingContextGroup contains a series of {Browsing,Window}Context
+  // objects. They are addressed using a hashtable to avoid linear lookup when
+  // adding or removing elements from the set.
+  //
+  // FIXME: This list is only required over a counter to keep nested
+  // non-discarded contexts within discarded contexts alive. It should be
+  // removed in the future.
+  // FIXME: Consider introducing a better common base than `nsISupports`?
+  nsTHashtable<nsRefPtrHashKey<nsISupports>> mContexts;
+
+  // The set of toplevel browsing contexts in the current BrowsingContextGroup.
+  nsTArray<RefPtr<BrowsingContext>> mToplevels;
+
+  //  Whether or not all toplevels in this group should be suspended
+  bool mToplevelsSuspended = false;
+
+  // DocGroups are thread-safe, and not able to be cycle collected,
+  // but we still keep strong pointers. When all Documents are removed
+  // from DocGroup (by the BrowsingContextGroup), the DocGroup is
+  // removed from here.
+  nsRefPtrHashtable<nsCStringHashKey, DocGroup> mDocGroups;
+
+  // The content process which will host documents in this BrowsingContextGroup
+  // which need to be loaded with a given remote type.
+  //
+  // A non-launching host process must also be a subscriber, though a launching
+  // host process may not yet be subscribed, and a subscriber need not be a host
+  // process.
+  nsRefPtrHashtable<nsCStringHashKey, ContentParent> mHosts;
+
+  nsTHashtable<nsRefPtrHashKey<ContentParent>> mSubscribers;
+
+  // A queue to store postMessage events during page load, the queue will be
+  // flushed once the page is loaded
+  RefPtr<mozilla::ThrottledEventQueue> mPostMessageEventQueue;
+
+  RefPtr<mozilla::ThrottledEventQueue> mTimerEventQueue;
+  RefPtr<mozilla::ThrottledEventQueue> mWorkerEventQueue;
+
+  // A counter to keep track of the input event suspension level of this BCG
+  //
+  // We use BrowsingContextGroup to emulate process isolation in Fission, so
+  // documents within the same the same BCG will behave like they share
+  // the same input task queue.
+  uint32_t mInputEventSuspensionLevel = 0;
+  // Whether this BCG has increased the suspension level in InputTaskManager
+  bool mHasIncreasedInputTaskManagerSuspensionLevel = false;
+};
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // !defined(mozilla_dom_BrowsingContextGroup_h)