diff -r 000000000000 -r 6474c204b198 xpcom/glue/nsProxyRelease.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/glue/nsProxyRelease.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,206 @@
+/* -*- Mode: C++; tab-width: 3; indent-tabs-mode: nil; c-basic-offset: 3 -*- */
+/* 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 nsProxyRelease_h__
+#define nsProxyRelease_h__
+
+#include "nsIEventTarget.h"
+#include "nsIThread.h"
+#include "nsCOMPtr.h"
+#include "nsAutoPtr.h"
+#include "MainThreadUtils.h"
+#include "mozilla/Likely.h"
+
+#ifdef XPCOM_GLUE_AVOID_NSPR
+#error NS_ProxyRelease implementation depends on NSPR.
+#endif
+
+/**
+ * Ensure that a nsCOMPtr is released on the target thread.
+ *
+ * @see NS_ProxyRelease(nsIEventTarget*, nsISupports*, bool)
+ */
+template <class T>
+inline NS_HIDDEN_(nsresult)
+NS_ProxyRelease
+    (nsIEventTarget *target, nsCOMPtr<T> &doomed, bool alwaysProxy=false)
+{
+   T* raw = nullptr;
+   doomed.swap(raw);
+   return NS_ProxyRelease(target, raw, alwaysProxy);
+}
+
+/**
+ * Ensure that a nsRefPtr is released on the target thread.
+ *
+ * @see NS_ProxyRelease(nsIEventTarget*, nsISupports*, bool)
+ */
+template <class T>
+inline NS_HIDDEN_(nsresult)
+NS_ProxyRelease
+    (nsIEventTarget *target, nsRefPtr<T> &doomed, bool alwaysProxy=false)
+{
+   T* raw = nullptr;
+   doomed.swap(raw);
+   return NS_ProxyRelease(target, raw, alwaysProxy);
+}
+
+/**
+ * Ensures that the delete of a nsISupports object occurs on the target thread.
+ *
+ * @param target
+ *        the target thread where the doomed object should be released.
+ * @param doomed
+ *        the doomed object; the object to be released on the target thread.
+ * @param alwaysProxy
+ *        normally, if NS_ProxyRelease is called on the target thread, then the
+ *        doomed object will released directly.  however, if this parameter is
+ *        true, then an event will always be posted to the target thread for
+ *        asynchronous release.
+ */
+NS_COM_GLUE nsresult
+NS_ProxyRelease
+    (nsIEventTarget *target, nsISupports *doomed, bool alwaysProxy=false);
+
+/**
+ * Class to safely handle main-thread-only pointers off the main thread.
+ *
+ * Classes like XPCWrappedJS are main-thread-only, which means that it is
+ * forbidden to call methods on instances of these classes off the main thread.
+ * For various reasons (see bug 771074), this restriction recently began to
+ * apply to AddRef/Release as well.
+ *
+ * This presents a problem for consumers that wish to hold a callback alive
+ * on non-main-thread code. A common example of this is the proxy callback
+ * pattern, where non-main-thread code holds a strong-reference to the callback
+ * object, and dispatches new Runnables (also with a strong reference) to the
+ * main thread in order to execute the callback. This involves several AddRef
+ * and Release calls on the other thread, which is (now) verboten.
+ *
+ * The basic idea of this class is to introduce a layer of indirection.
+ * nsMainThreadPtrHolder is a threadsafe reference-counted class that internally
+ * maintains one strong reference to the main-thread-only object. It must be
+ * instantiated on the main thread (so that the AddRef of the underlying object
+ * happens on the main thread), but consumers may subsequently pass references
+ * to the holder anywhere they please. These references are meant to be opaque
+ * when accessed off-main-thread (assertions enforce this).
+ *
+ * The semantics of nsRefPtr<nsMainThreadPtrHolder<T> > would be cumbersome, so
+ * we also introduce nsMainThreadPtrHandle<T>, which is conceptually identical
+ * to the above (though it includes various convenience methods). The basic
+ * pattern is as follows.
+ *
+ * // On the main thread:
+ * nsCOMPtr<nsIFooCallback> callback = ...;
+ * nsMainThreadPtrHandle<nsIFooCallback> callbackHandle =
+ *   new nsMainThreadPtrHolder<nsIFooCallback>(callback);
+ * // Pass callbackHandle to structs/classes that might be accessed on other
+ * // threads.
+ *
+ * All structs and classes that might be accessed on other threads should store
+ * an nsMainThreadPtrHandle<T> rather than an nsCOMPtr<T>.
+ */
+template<class T>
+class nsMainThreadPtrHolder MOZ_FINAL
+{
+public:
+  // We can only acquire a pointer on the main thread. We to fail fast for
+  // threading bugs, so by default we assert if our pointer is used or acquired
+  // off-main-thread. But some consumers need to use the same pointer for
+  // multiple classes, some of which are main-thread-only and some of which
+  // aren't. So we allow them to explicitly disable this strict checking.
+  nsMainThreadPtrHolder(T* ptr, bool strict = true) : mRawPtr(nullptr), mStrict(strict) {
+    // We can only AddRef our pointer on the main thread, which means that the
+    // holder must be constructed on the main thread.
+    MOZ_ASSERT(!mStrict || NS_IsMainThread());
+    NS_IF_ADDREF(mRawPtr = ptr);
+  }
+
+  // We can be released on any thread.
+  ~nsMainThreadPtrHolder() {
+    if (NS_IsMainThread()) {
+      NS_IF_RELEASE(mRawPtr);
+    } else if (mRawPtr) {
+      nsCOMPtr<nsIThread> mainThread;
+      NS_GetMainThread(getter_AddRefs(mainThread));
+      if (!mainThread) {
+        NS_WARNING("Couldn't get main thread! Leaking pointer.");
+        return;
+      }
+      NS_ProxyRelease(mainThread, mRawPtr);
+    }
+  }
+
+  T* get() {
+    // Nobody should be touching the raw pointer off-main-thread.
+    if (mStrict && MOZ_UNLIKELY(!NS_IsMainThread())) {
+      NS_ERROR("Can't dereference nsMainThreadPtrHolder off main thread");
+      MOZ_CRASH();
+    }
+    return mRawPtr;
+  }
+
+  bool operator==(const nsMainThreadPtrHolder<T>& aOther) const { return mRawPtr == aOther.mRawPtr; }
+
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(nsMainThreadPtrHolder<T>)
+
+private:
+  // Our wrapped pointer.
+  T* mRawPtr;
+
+  // Whether to strictly enforce thread invariants in this class.
+  bool mStrict;
+
+  // Copy constructor and operator= not implemented. Once constructed, the
+  // holder is immutable.
+  T& operator=(nsMainThreadPtrHolder& other);
+  nsMainThreadPtrHolder(const nsMainThreadPtrHolder& other);
+};
+
+template<class T>
+class nsMainThreadPtrHandle
+{
+  nsRefPtr<nsMainThreadPtrHolder<T> > mPtr;
+
+  public:
+  nsMainThreadPtrHandle() : mPtr(nullptr) {}
+  nsMainThreadPtrHandle(nsMainThreadPtrHolder<T> *aHolder) : mPtr(aHolder) {}
+  nsMainThreadPtrHandle(const nsMainThreadPtrHandle& aOther) : mPtr(aOther.mPtr) {}
+  nsMainThreadPtrHandle& operator=(const nsMainThreadPtrHandle& aOther) {
+    mPtr = aOther.mPtr;
+    return *this;
+  }
+
+  // These all call through to nsMainThreadPtrHolder, and thus implicitly
+  // assert that we're on the main thread. Off-main-thread consumers must treat
+  // these handles as opaque.
+  T* get()
+  {
+    if (mPtr) {
+      return mPtr.get()->get();
+    }
+    return nullptr;
+  }
+  const T* get() const
+  {
+    if (mPtr) {
+      return mPtr.get()->get();
+    }
+    return nullptr;
+  }
+
+  operator T*() { return get(); }
+  T* operator->() { return get(); }
+
+  // These are safe to call on other threads with appropriate external locking.
+  bool operator==(const nsMainThreadPtrHandle<T>& aOther) const {
+    if (!mPtr || !aOther.mPtr)
+      return mPtr == aOther.mPtr;
+    return *mPtr == *aOther.mPtr;
+  }
+  bool operator!() { return !mPtr; }
+};
+
+#endif