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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 "mozilla/Maybe.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/StateWatching.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Unused.h"
+
+#include "mozilla/Logging.h"
+#include "nsISupportsImpl.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+ * Given that semantics may change and comments tend to go out of date, we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 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 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(mMirrors[i],
+                                                               &AbstractMirror<T>::NotifyDisconnected),
+                                             AbstractThread::DontAssertDispatchSuccess);
+      }
+      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(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>(aMirror, &AbstractMirror<T>::UpdateValue, mValue);;
+    }
+
+    T mValue;
+    Maybe<T> mInitialValue;
+    nsTArray<RefPtr<AbstractMirror<T>>> mMirrors;
+  };
+public:
+
+  // NB: Because mirror-initiated disconnection can race with canonical-
+  // initiated disconnection, a canonical should never be reinitialized.
+  // Forward control operations to the Impl.
+  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());
+  }
+
+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;
+    }
+
+    virtual void UpdateValue(const T& aNewValue) override
+    {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      if (mValue != aNewValue) {
+        mValue = aNewValue;
+        WatchTarget::NotifyWatchers();
+      }
+    }
+
+    virtual 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<StorensRefPtrPassByPtr<AbstractMirror<T>>>
+                                  (aCanonical, &AbstractCanonical<T>::AddMirror, this);
+      aCanonical->OwnerThread()->Dispatch(r.forget(), AbstractThread::DontAssertDispatchSuccess);
+      mCanonical = aCanonical;
+    }
+  public:
+
+    void DisconnectIfConnected()
+    {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      if (!IsConnected()) {
+        return;
+      }
+
+      MIRROR_LOG("%s [%p] Disconnecting from %p", mName, this, mCanonical.get());
+      nsCOMPtr<nsIRunnable> r = NewRunnableMethod<StorensRefPtrPassByPtr<AbstractMirror<T>>>
+                                  (mCanonical, &AbstractCanonical<T>::RemoveMirror, this);
+      mCanonical->OwnerThread()->Dispatch(r.forget(), AbstractThread::DontAssertDispatchSuccess);
+      mCanonical = nullptr;
+    }
+
+  protected:
+    ~Impl() { MOZ_DIAGNOSTIC_ASSERT(!IsConnected()); }
+
+  private:
+    T mValue;
+    RefPtr<AbstractCanonical<T>> mCanonical;
+  };
+public:
+
+  // Forward control operations to the Impl<T>.
+  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