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diff --git a/docshell/shistory/ChildSHistory.h b/docshell/shistory/ChildSHistory.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/. */
+
+/**
+ * ChildSHistory represents a view of session history from a child process. It
+ * exposes getters for some cached history state, and mutators which are
+ * implemented by communicating with the actual history storage.
+ *
+ * NOTE: Currently session history is in transition, meaning that we're still
+ * using the legacy nsSHistory class internally. The API exposed from this class
+ * should be only the API which we expect to expose when this transition is
+ * complete, and special cases will need to call through the LegacySHistory()
+ * getters.
+ */
+
+#ifndef mozilla_dom_ChildSHistory_h
+#define mozilla_dom_ChildSHistory_h
+
+#include "nsCOMPtr.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "nsWrapperCache.h"
+#include "nsThreadUtils.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/LinkedList.h"
+#include "nsID.h"
+
+class nsISHEntry;
+class nsISHistory;
+
+namespace mozilla {
+namespace dom {
+
+class BrowsingContext;
+
+class ChildSHistory : public nsISupports, public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ChildSHistory)
+  nsISupports* GetParentObject() const;
+  JSObject* WrapObject(JSContext* cx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  explicit ChildSHistory(BrowsingContext* aBrowsingContext);
+
+  void SetBrowsingContext(BrowsingContext* aBrowsingContext);
+
+  // Create or destroy the session history implementation in the child process.
+  // This can be removed once session history is stored exclusively in the
+  // parent process.
+  void SetIsInProcess(bool aIsInProcess);
+  bool IsInProcess() { return !!mHistory; }
+
+  int32_t Count();
+  int32_t Index();
+
+  /**
+   * Reload the current entry in the session history.
+   */
+  void Reload(uint32_t aReloadFlags, ErrorResult& aRv);
+
+  /**
+   * The CanGo and Go methods are called with an offset from the current index.
+   * Positive numbers go forward in history, while negative numbers go
+   * backwards.
+   */
+  bool CanGo(int32_t aOffset);
+  void Go(int32_t aOffset, bool aRequireUserInteraction, ErrorResult& aRv);
+  void AsyncGo(int32_t aOffset, bool aRequireUserInteraction,
+               CallerType aCallerType, ErrorResult& aRv);
+
+  // aIndex is the new index, and aOffset is the offset between new and current.
+  void GotoIndex(int32_t aIndex, int32_t aOffset, bool aRequireUserInteraction,
+                 ErrorResult& aRv);
+
+  void RemovePendingHistoryNavigations();
+
+  /**
+   * Evicts all content viewers within the current process.
+   */
+  void EvictLocalContentViewers();
+
+  // GetLegacySHistory and LegacySHistory have been deprecated. Don't
+  // use these, but instead handle the interaction with nsISHistory in
+  // the parent process.
+  nsISHistory* GetLegacySHistory(ErrorResult& aError);
+  nsISHistory* LegacySHistory();
+
+  void SetIndexAndLength(uint32_t aIndex, uint32_t aLength,
+                         const nsID& aChangeId);
+  nsID AddPendingHistoryChange();
+  nsID AddPendingHistoryChange(int32_t aIndexDelta, int32_t aLengthDelta);
+
+  // AsyncHistoryLength is for testing.
+  void SetAsyncHistoryLength(bool aEnable, ErrorResult& aRv);
+  bool AsyncHistoryLength() { return mAsyncHistoryLength; }
+
+ private:
+  virtual ~ChildSHistory() = default;
+
+  class PendingAsyncHistoryNavigation
+      : public Runnable,
+        public mozilla::LinkedListElement<PendingAsyncHistoryNavigation> {
+   public:
+    PendingAsyncHistoryNavigation(ChildSHistory* aHistory, int32_t aOffset,
+                                  bool aRequireUserInteraction)
+        : Runnable("PendingAsyncHistoryNavigation"),
+          mHistory(aHistory),
+          mRequireUserInteraction(aRequireUserInteraction),
+          mOffset(aOffset) {}
+
+    NS_IMETHOD Run() override {
+      if (isInList()) {
+        remove();
+        mHistory->Go(mOffset, mRequireUserInteraction, IgnoreErrors());
+      }
+      return NS_OK;
+    }
+
+   private:
+    RefPtr<ChildSHistory> mHistory;
+    bool mRequireUserInteraction;
+    int32_t mOffset;
+  };
+
+  RefPtr<BrowsingContext> mBrowsingContext;
+  nsCOMPtr<nsISHistory> mHistory;
+  // Can be removed once history-in-parent is the only way
+  mozilla::LinkedList<PendingAsyncHistoryNavigation> mPendingNavigations;
+  int32_t mIndex = -1;
+  int32_t mLength = 0;
+
+  struct PendingSHistoryChange {
+    nsID mChangeID;
+    int32_t mIndexDelta;
+    int32_t mLengthDelta;
+  };
+  AutoTArray<PendingSHistoryChange, 2> mPendingSHistoryChanges;
+
+  bool mAsyncHistoryLength = false;
+
+  // Needs to start 1 above default epoch in parent
+  uint64_t mHistoryEpoch = 1;
+  bool mPendingEpoch = false;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif /* mozilla_dom_ChildSHistory_h */