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diff --git a/xpcom/threads/StateMirroring.h b/xpcom/threads/StateMirroring.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/. */
+
+#if !defined(StateMirroring_h_)
+#  define StateMirroring_h_
+
+#  include <cstddef>
+#  include "mozilla/AbstractThread.h"
+#  include "mozilla/AlreadyAddRefed.h"
+#  include "mozilla/Assertions.h"
+#  include "mozilla/Logging.h"
+#  include "mozilla/Maybe.h"
+#  include "mozilla/RefPtr.h"
+#  include "mozilla/StateWatching.h"
+#  include "nsCOMPtr.h"
+#  include "nsIRunnable.h"
+#  include "nsISupports.h"
+#  include "nsTArray.h"
+#  include "nsThreadUtils.h"
+
+/*
+ * The state-mirroring machinery allows pieces of interesting state to be
+ * observed on multiple thread without locking. The basic strategy is to track
+ * changes in a canonical value and post updates to other threads that hold
+ * mirrors for that value.
+ *
+ * One problem with the naive implementation of such a system is that some
+ * pieces of state need to be updated atomically, and certain other operations
+ * need to wait for these atomic updates to complete before executing. The
+ * state-mirroring machinery solves this problem by requiring that its owner
+ * thread uses tail dispatch, and posting state update events (which should
+ * always be run first by TaskDispatcher implementations) to that tail
+ * dispatcher. This ensures that state changes are always atomic from the
+ * perspective of observing threads.
+ *
+ * Given that state-mirroring is an automatic background process, we try to
+ * avoid burdening the caller with worrying too much about teardown. To that
+ * end, we don't assert dispatch success for any of the notifications, and
+ * assume that any canonical or mirror owned by a thread for whom dispatch fails
+ * will soon be disconnected by its holder anyway.
+ *
+ * Given that semantics may change and comments tend to go out of date, we
+ * deliberately don't provide usage examples here. Grep around to find them.
+ */
+
+namespace mozilla {
+
+// Mirror<T> and Canonical<T> inherit WatchTarget, so we piggy-back on the
+// logging that WatchTarget already does. Given that, it makes sense to share
+// the same log module.
+#  define MIRROR_LOG(x, ...)       \
+    MOZ_ASSERT(gStateWatchingLog); \
+    MOZ_LOG(gStateWatchingLog, LogLevel::Debug, (x, ##__VA_ARGS__))
+
+template <typename T>
+class AbstractMirror;
+
+/*
+ * AbstractCanonical is a superclass from which all Canonical values must
+ * inherit. It serves as the interface of operations which may be performed (via
+ * asynchronous dispatch) by other threads, in particular by the corresponding
+ * Mirror value.
+ */
+template <typename T>
+class AbstractCanonical {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractCanonical)
+  AbstractCanonical(AbstractThread* aThread) : mOwnerThread(aThread) {}
+  virtual void AddMirror(AbstractMirror<T>* aMirror) = 0;
+  virtual void RemoveMirror(AbstractMirror<T>* aMirror) = 0;
+
+  AbstractThread* OwnerThread() const { return mOwnerThread; }
+
+ protected:
+  virtual ~AbstractCanonical() {}
+  RefPtr<AbstractThread> mOwnerThread;
+};
+
+/*
+ * AbstractMirror is a superclass from which all Mirror values must
+ * inherit. It serves as the interface of operations which may be performed (via
+ * asynchronous dispatch) by other threads, in particular by the corresponding
+ * Canonical value.
+ */
+template <typename T>
+class AbstractMirror {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractMirror)
+  AbstractMirror(AbstractThread* aThread) : mOwnerThread(aThread) {}
+  virtual void ConnectedOnCanonicalThread(AbstractCanonical<T>* aCanonical) = 0;
+  virtual void UpdateValue(const T& aNewValue) = 0;
+  virtual void NotifyDisconnected() = 0;
+
+  AbstractThread* OwnerThread() const { return mOwnerThread; }
+
+ protected:
+  virtual ~AbstractMirror() {}
+  RefPtr<AbstractThread> mOwnerThread;
+};
+
+/*
+ * Canonical<T> is a wrapper class that allows a given value to be mirrored by
+ * other threads. It maintains a list of active mirrors, and queues updates for
+ * them when the internal value changes. When changing the value, the caller
+ * needs to pass a TaskDispatcher object, which fires the updates at the
+ * appropriate time. Canonical<T> is also a WatchTarget, and may be set up to
+ * trigger other routines (on the same thread) when the canonical value changes.
+ *
+ * Canonical<T> is intended to be used as a member variable, so it doesn't
+ * actually inherit AbstractCanonical<T> (a refcounted type). Rather, it
+ * contains an inner class called |Impl| that implements most of the interesting
+ * logic.
+ */
+template <typename T>
+class Canonical {
+ public:
+  Canonical(AbstractThread* aThread, const T& aInitialValue,
+            const char* aName) {
+    mImpl = new Impl(aThread, aInitialValue, aName);
+  }
+
+  ~Canonical() {}
+
+ private:
+  class Impl : public AbstractCanonical<T>, public WatchTarget {
+   public:
+    using AbstractCanonical<T>::OwnerThread;
+
+    Impl(AbstractThread* aThread, const T& aInitialValue, const char* aName)
+        : AbstractCanonical<T>(aThread),
+          WatchTarget(aName),
+          mValue(aInitialValue) {
+      MIRROR_LOG("%s [%p] initialized", mName, this);
+      MOZ_ASSERT(aThread->SupportsTailDispatch(),
+                 "Can't get coherency without tail dispatch");
+    }
+
+    void ConnectMirror(AbstractMirror<T>* aMirror) {
+      MIRROR_LOG("%s [%p] canonical-init connecting mirror %p", mName, this,
+                 aMirror);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(OwnerThread()->RequiresTailDispatch(aMirror->OwnerThread()),
+                 "Can't get coherency without tail dispatch");
+      aMirror->ConnectedOnCanonicalThread(this);
+      AddMirror(aMirror);
+    }
+
+    void AddMirror(AbstractMirror<T>* aMirror) override {
+      MIRROR_LOG("%s [%p] adding mirror %p", mName, this, aMirror);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(!mMirrors.Contains(aMirror));
+      mMirrors.AppendElement(aMirror);
+      aMirror->OwnerThread()->DispatchStateChange(MakeNotifier(aMirror));
+    }
+
+    void RemoveMirror(AbstractMirror<T>* aMirror) override {
+      MIRROR_LOG("%s [%p] removing mirror %p", mName, this, aMirror);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(mMirrors.Contains(aMirror));
+      mMirrors.RemoveElement(aMirror);
+    }
+
+    void DisconnectAll() {
+      MIRROR_LOG("%s [%p] Disconnecting all mirrors", mName, this);
+      for (size_t i = 0; i < mMirrors.Length(); ++i) {
+        mMirrors[i]->OwnerThread()->Dispatch(
+            NewRunnableMethod("AbstractMirror::NotifyDisconnected", mMirrors[i],
+                              &AbstractMirror<T>::NotifyDisconnected));
+      }
+      mMirrors.Clear();
+    }
+
+    operator const T&() {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      return mValue;
+    }
+
+    void Set(const T& aNewValue) {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+
+      if (aNewValue == mValue) {
+        return;
+      }
+
+      // Notify same-thread watchers. The state watching machinery will make
+      // sure that notifications run at the right time.
+      NotifyWatchers();
+
+      // Check if we've already got a pending update. If so we won't schedule
+      // another one.
+      bool alreadyNotifying = mInitialValue.isSome();
+
+      // Stash the initial value if needed, then update to the new value.
+      if (mInitialValue.isNothing()) {
+        mInitialValue.emplace(mValue);
+      }
+      mValue = aNewValue;
+
+      // We wait until things have stablized before sending state updates so
+      // that we can avoid sending multiple updates, and possibly avoid sending
+      // any updates at all if the value ends up where it started.
+      if (!alreadyNotifying) {
+        AbstractThread::DispatchDirectTask(NewRunnableMethod(
+            "Canonical::Impl::DoNotify", this, &Impl::DoNotify));
+      }
+    }
+
+    Impl& operator=(const T& aNewValue) {
+      Set(aNewValue);
+      return *this;
+    }
+    Impl& operator=(const Impl& aOther) {
+      Set(aOther);
+      return *this;
+    }
+    Impl(const Impl& aOther) = delete;
+
+   protected:
+    ~Impl() { MOZ_DIAGNOSTIC_ASSERT(mMirrors.IsEmpty()); }
+
+   private:
+    void DoNotify() {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(mInitialValue.isSome());
+      bool same = mInitialValue.ref() == mValue;
+      mInitialValue.reset();
+
+      if (same) {
+        MIRROR_LOG("%s [%p] unchanged - not sending update", mName, this);
+        return;
+      }
+
+      for (size_t i = 0; i < mMirrors.Length(); ++i) {
+        mMirrors[i]->OwnerThread()->DispatchStateChange(
+            MakeNotifier(mMirrors[i]));
+      }
+    }
+
+    already_AddRefed<nsIRunnable> MakeNotifier(AbstractMirror<T>* aMirror) {
+      return NewRunnableMethod<T>("AbstractMirror::UpdateValue", aMirror,
+                                  &AbstractMirror<T>::UpdateValue, mValue);
+    }
+
+    T mValue;
+    Maybe<T> mInitialValue;
+    nsTArray<RefPtr<AbstractMirror<T>>> mMirrors;
+  };
+
+ public:
+  /*
+   * Connect this Canonical to aMirror. Note that the canonical value starts
+   * being mirrored to aMirror immediately, and requires one thread hop to
+   * aMirror's owning thread before the connection is established.
+   *
+   * Note that this could race with Mirror::DisconnectIfConnected(). It is up to
+   * the caller to provide the guarantee that disconnection happens after the
+   * connection has been established. There is no race between this and
+   * DisconnectAll().
+   */
+  void ConnectMirror(AbstractMirror<T>* aMirror) {
+    return mImpl->ConnectMirror(aMirror);
+  }
+  void DisconnectAll() { return mImpl->DisconnectAll(); }
+
+  // Access to the Impl.
+  operator Impl&() { return *mImpl; }
+  Impl* operator&() { return mImpl; }
+
+  // Access to the T.
+  const T& Ref() const { return *mImpl; }
+  operator const T&() const { return Ref(); }
+  void Set(const T& aNewValue) { mImpl->Set(aNewValue); }
+  Canonical& operator=(const T& aNewValue) {
+    Set(aNewValue);
+    return *this;
+  }
+  Canonical& operator=(const Canonical& aOther) {
+    Set(aOther);
+    return *this;
+  }
+  Canonical(const Canonical& aOther) = delete;
+
+ private:
+  RefPtr<Impl> mImpl;
+};
+
+/*
+ * Mirror<T> is a wrapper class that allows a given value to mirror that of a
+ * Canonical<T> owned by another thread. It registers itself with a
+ * Canonical<T>, and is periodically updated with new values. Mirror<T> is also
+ * a WatchTarget, and may be set up to trigger other routines (on the same
+ * thread) when the mirrored value changes.
+ *
+ * Mirror<T> is intended to be used as a member variable, so it doesn't actually
+ * inherit AbstractMirror<T> (a refcounted type). Rather, it contains an inner
+ * class called |Impl| that implements most of the interesting logic.
+ */
+template <typename T>
+class Mirror {
+ public:
+  Mirror(AbstractThread* aThread, const T& aInitialValue, const char* aName) {
+    mImpl = new Impl(aThread, aInitialValue, aName);
+  }
+
+  ~Mirror() {
+    // As a member of complex objects, a Mirror<T> may be destroyed on a
+    // different thread than its owner, or late in shutdown during CC. Given
+    // that, we require manual disconnection so that callers can put things in
+    // the right place.
+    MOZ_DIAGNOSTIC_ASSERT(!mImpl->IsConnected());
+    mImpl->AssertNoIncomingConnects();
+  }
+
+ private:
+  class Impl : public AbstractMirror<T>, public WatchTarget {
+   public:
+    using AbstractMirror<T>::OwnerThread;
+
+    Impl(AbstractThread* aThread, const T& aInitialValue, const char* aName)
+        : AbstractMirror<T>(aThread),
+          WatchTarget(aName),
+          mValue(aInitialValue) {
+      MIRROR_LOG("%s [%p] initialized", mName, this);
+      MOZ_ASSERT(aThread->SupportsTailDispatch(),
+                 "Can't get coherency without tail dispatch");
+    }
+
+    operator const T&() {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      return mValue;
+    }
+
+    void ConnectedOnCanonicalThread(AbstractCanonical<T>* aCanonical) override {
+      MOZ_ASSERT(aCanonical->OwnerThread()->IsCurrentThreadIn());
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+      ++mIncomingConnects;
+#  endif
+      OwnerThread()->DispatchStateChange(
+          NewRunnableMethod<StoreRefPtrPassByPtr<AbstractCanonical<T>>>(
+              "Mirror::Impl::SetCanonical", this, &Impl::SetCanonical,
+              aCanonical));
+    }
+
+    void SetCanonical(AbstractCanonical<T>* aCanonical) {
+      MIRROR_LOG("%s [%p] Canonical-init setting canonical %p", mName, this,
+                 aCanonical);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(!IsConnected());
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+      --mIncomingConnects;
+#  endif
+      mCanonical = aCanonical;
+    }
+
+    void UpdateValue(const T& aNewValue) override {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      if (mValue != aNewValue) {
+        mValue = aNewValue;
+        WatchTarget::NotifyWatchers();
+      }
+    }
+
+    void NotifyDisconnected() override {
+      MIRROR_LOG("%s [%p] Notifed of disconnection from %p", mName, this,
+                 mCanonical.get());
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      mCanonical = nullptr;
+    }
+
+    bool IsConnected() const { return !!mCanonical; }
+
+    void Connect(AbstractCanonical<T>* aCanonical) {
+      MIRROR_LOG("%s [%p] Connecting to %p", mName, this, aCanonical);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(!IsConnected());
+      MOZ_ASSERT(OwnerThread()->RequiresTailDispatch(aCanonical->OwnerThread()),
+                 "Can't get coherency without tail dispatch");
+
+      nsCOMPtr<nsIRunnable> r =
+          NewRunnableMethod<StoreRefPtrPassByPtr<AbstractMirror<T>>>(
+              "AbstractCanonical::AddMirror", aCanonical,
+              &AbstractCanonical<T>::AddMirror, this);
+      aCanonical->OwnerThread()->Dispatch(r.forget());
+      mCanonical = aCanonical;
+    }
+
+    void DisconnectIfConnected() {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      if (!IsConnected()) {
+        return;
+      }
+
+      MIRROR_LOG("%s [%p] Disconnecting from %p", mName, this,
+                 mCanonical.get());
+      nsCOMPtr<nsIRunnable> r =
+          NewRunnableMethod<StoreRefPtrPassByPtr<AbstractMirror<T>>>(
+              "AbstractCanonical::RemoveMirror", mCanonical,
+              &AbstractCanonical<T>::RemoveMirror, this);
+      mCanonical->OwnerThread()->Dispatch(r.forget());
+      mCanonical = nullptr;
+    }
+
+    void AssertNoIncomingConnects() {
+      MOZ_DIAGNOSTIC_ASSERT(mIncomingConnects == 0);
+    }
+
+   protected:
+    ~Impl() { MOZ_DIAGNOSTIC_ASSERT(!IsConnected()); }
+
+   private:
+    T mValue;
+    RefPtr<AbstractCanonical<T>> mCanonical;
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    std::atomic<size_t> mIncomingConnects = 0;
+#  endif
+  };
+
+ public:
+  // Forward control operations to the Impl<T>.
+  /*
+   * Connect aCanonical to this Mirror. Note that this requires one thread hop
+   * back to aCanonical's owning thread before the canonical value starts
+   * being mirrored, and another to our owning thread before the connection is
+   * established.
+   *
+   * Note that this mirror-initialized connection could race with
+   * Canonical::DisconnectAll(). It is up to the caller to provide the guarantee
+   * that disconnection happens after the connection has been established. There
+   * is no race between this and DisconnectIfConnected().
+   */
+  void Connect(AbstractCanonical<T>* aCanonical) { mImpl->Connect(aCanonical); }
+  void DisconnectIfConnected() { mImpl->DisconnectIfConnected(); }
+
+  // Access to the Impl<T>.
+  operator Impl&() { return *mImpl; }
+  Impl* operator&() { return mImpl; }
+
+  // Access to the T.
+  const T& Ref() const { return *mImpl; }
+  operator const T&() const { return Ref(); }
+
+ private:
+  RefPtr<Impl> mImpl;
+};
+
+#  undef MIRROR_LOG
+
+}  // namespace mozilla
+
+#endif