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diff --git a/dom/base/nsWindowMemoryReporter.h b/dom/base/nsWindowMemoryReporter.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 nsWindowMemoryReporter_h__
+#define nsWindowMemoryReporter_h__
+
+#include "nsIMemoryReporter.h"
+#include "nsIObserver.h"
+#include "nsITimer.h"
+#include "nsTHashMap.h"
+#include "nsTHashSet.h"
+#include "nsWeakReference.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/TimeStamp.h"
+
+class nsGlobalWindowInner;
+
+/**
+ * nsWindowMemoryReporter is responsible for the 'explicit/window-objects'
+ * memory reporter.
+ *
+ * We classify DOM window objects into one of three categories:
+ *
+ *   - "active" windows, which are displayed in a tab (as the top-level window
+ *     or an iframe),
+ *
+ *   - "cached" windows, which are in the fastback cache (aka the bfcache), and
+ *
+ *   - "detached" windows, which have a null docshell.  A window becomes
+ *     detached when its <iframe> or tab containing the window is destroyed --
+ *     i.e., when the window is no longer active or cached.
+ *
+ * Additionally, we classify a subset of detached windows as "ghost" windows.
+ * Although ghost windows can happen legitimately (a page can hold a reference
+ * to a cross-domain window and then close its container), the presence of
+ * ghost windows is often indicative of a memory leak.
+ *
+ * A window is a ghost if it meets the following three criteria:
+ *
+ *   1) The window is detached.
+ *
+ *   2) There exist no non-detached windows with the same base domain as
+ *      the window's principal.  (For example, the base domain of
+ *      "wiki.mozilla.co.uk" is "mozilla.co.uk".)  This criterion makes us less
+ *      likely to flag a legitimately held-alive detached window as a ghost.
+ *
+ *   3) The window has met criteria (1) and (2) above for at least
+ *      memory.ghost_window_timeout_seconds.  This criterion is in place so we
+ *      don't immediately declare a window a ghost before the GC/CC has had a
+ *      chance to run.
+ *
+ * nsWindowMemoryReporter observes window detachment and uses mDetachedWindows
+ * to remember when a window first met criteria (1) and (2).  When we generate
+ * a memory report, we use this accounting to determine which windows are
+ * ghosts.
+ *
+ *
+ * We use the following memory reporter path for active and cached windows:
+ *
+ *   explicit/window-objects/top(<top-outer-uri>, id=<top-outer-id>)/
+ *       <category>/window(<window-uri>)/...
+ *
+ * For detached and ghost windows, we use
+ *
+ *   explicit/window-objects/top(none)/<category>/window(<window-uri>)/...
+ *
+ * Where
+ *
+ * - <category> is "active", "cached", "detached", or "ghost", as described
+ *   above.
+ *
+ * - <top-outer-id> is the window id of the top outer window (i.e. the tab, or
+ *   the top level chrome window).  Exposing this ensures that each tab gets
+ *   its own sub-tree, even if multiple tabs are showing the same URI.
+ *
+ * - <top-uri> is the URI of the top window.  Excepting special windows (such
+ *   as browser.xhtml or hiddenWindow.html) it's what the address bar shows for
+ *   the tab.
+ *
+ */
+class nsWindowMemoryReporter final : public nsIMemoryReporter,
+                                     public nsIObserver,
+                                     public nsSupportsWeakReference {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIMEMORYREPORTER
+  NS_DECL_NSIOBSERVER
+
+  static void Init();
+
+#ifdef DEBUG
+  /**
+   * Unlink all known ghost windows, to enable investigating what caused them
+   * to become ghost windows in the first place.
+   */
+  static void UnlinkGhostWindows();
+#endif
+
+  static nsWindowMemoryReporter* Get();
+  void ObserveDOMWindowDetached(nsGlobalWindowInner* aWindow);
+
+  static int64_t GhostWindowsDistinguishedAmount();
+
+ private:
+  ~nsWindowMemoryReporter();
+
+  // Protect ctor, use Init() instead.
+  nsWindowMemoryReporter();
+
+  /**
+   * Get the number of seconds for which a window must satisfy ghost criteria
+   * (1) and (2) before we deem that it satisfies criterion (3).
+   */
+  uint32_t GetGhostTimeout();
+
+  void ObserveAfterMinimizeMemoryUsage();
+
+  /**
+   * Iterate over all weak window pointers in mDetachedWindows and update our
+   * accounting of which windows meet ghost criterion (2).
+   *
+   * This method also cleans up mDetachedWindows, removing entries for windows
+   * which have been destroyed or are no longer detached.
+   *
+   * If aOutGhostIDs is non-null, we populate it with the Window IDs of the
+   * ghost windows.
+   *
+   * This is called asynchronously after we observe a DOM window being detached
+   * from its docshell, and also right before we generate a memory report.
+   */
+  void CheckForGhostWindows(nsTHashSet<uint64_t>* aOutGhostIDs = nullptr);
+
+  /**
+   * Eventually do a check for ghost windows, if we haven't done one recently
+   * and we aren't already planning to do one soon.
+   */
+  void AsyncCheckForGhostWindows();
+
+  /**
+   * Kill the check timer, if it exists.
+   */
+  void KillCheckTimer();
+
+  static void CheckTimerFired(nsITimer* aTimer, void* aClosure);
+
+  /**
+   * Maps a weak reference to a detached window (nsIWeakReference) to the time
+   * when we observed that the window met ghost criterion (2) above.
+   *
+   * If the window has not yet met criterion (2) it maps to the null timestamp.
+   *
+   * (Although windows are not added to this table until they're detached, it's
+   * possible for a detached window to become non-detached, and we won't
+   * remove it from the table until CheckForGhostWindows runs.)
+   */
+  nsTHashMap<nsISupportsHashKey, mozilla::TimeStamp> mDetachedWindows;
+
+  /**
+   * Track the last time we ran CheckForGhostWindows(), to avoid running it
+   * too often after a DOM window is detached.
+   */
+  mozilla::TimeStamp mLastCheckForGhostWindows;
+
+  nsCOMPtr<nsITimer> mCheckTimer;
+
+  bool mCycleCollectorIsRunning;
+
+  bool mCheckTimerWaitingForCCEnd;
+
+  int64_t mGhostWindowCount;
+};
+
+#endif  // nsWindowMemoryReporter_h__