Bug 857895 - Only enforce tail dispatch if both source and dest support it. r=bholley

dom/media/AbstractThread.cpp

@@ -51,7 +51,7 @@ public:
   {
     nsCOMPtr<nsIRunnable> r = aRunnable;
     AbstractThread* currentThread;
-    if (aReason != TailDispatch && (currentThread = GetCurrent()) && currentThread->RequiresTailDispatch()) {
+    if (aReason != TailDispatch && (currentThread = GetCurrent()) && RequiresTailDispatch(currentThread)) {
       currentThread->TailDispatcher().AddTask(this, r.forget(), aFailureHandling);
       return;
     }
@@ -101,6 +101,14 @@ private:
   Maybe<AutoTaskDispatcher> mTailDispatcher;
 };
 
+bool
+AbstractThread::RequiresTailDispatch(AbstractThread* aThread) const
+{
+  // We require tail dispatch if both the source and destination
+  // threads support it.
+  return SupportsTailDispatch() && aThread->SupportsTailDispatch();
+}
+
 AbstractThread*
 AbstractThread::MainThread()
 {

dom/media/AbstractThread.h

@@ -40,7 +40,7 @@ public:
   // if the caller is not running in an AbstractThread.
   static AbstractThread* GetCurrent() { return sCurrentThreadTLS.get(); }
 
-  AbstractThread(bool aRequireTailDispatch) : mRequireTailDispatch(aRequireTailDispatch) {}
+  AbstractThread(bool aSupportsTailDispatch) : mSupportsTailDispatch(aSupportsTailDispatch) {}
 
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractThread);
 
@@ -63,9 +63,12 @@ public:
   // threads which support it.
   virtual TaskDispatcher& TailDispatcher() = 0;
 
-  // Returns true if this task queue requires all dispatches performed by its
-  // tasks to go through the tail dispatcher.
-  bool RequiresTailDispatch() const { return mRequireTailDispatch; }
+  // Returns true if this supports the tail dispatcher.
+  bool SupportsTailDispatch() const { return mSupportsTailDispatch; }
+
+  // Returns true if this thread requires all dispatches originating from
+  // aThread go through the tail dispatcher.
+  bool RequiresTailDispatch(AbstractThread* aThread) const;
 
   virtual MediaTaskQueue* AsTaskQueue() { MOZ_CRASH("Not a task queue!"); }
   virtual nsIThread* AsXPCOMThread() { MOZ_CRASH("Not an XPCOM thread!"); }
@@ -82,7 +85,7 @@ protected:
 
   // True if we want to require that every task dispatched from tasks running in
   // this queue go through our queue's tail dispatcher.
-  const bool mRequireTailDispatch;
+  const bool mSupportsTailDispatch;
 };
 
 } // namespace mozilla

dom/media/MediaTaskQueue.cpp

@@ -45,7 +45,7 @@ MediaTaskQueue::DispatchLocked(already_AddRefed<nsIRunnable> aRunnable,
 {
   nsCOMPtr<nsIRunnable> r = aRunnable;
   AbstractThread* currentThread;
-  if (aReason != TailDispatch && (currentThread = GetCurrent()) && currentThread->RequiresTailDispatch()) {
+  if (aReason != TailDispatch && (currentThread = GetCurrent()) && RequiresTailDispatch(currentThread)) {
     currentThread->TailDispatcher().AddTask(this, r.forget(), aFailureHandling);
     return NS_OK;
   }

dom/media/StateMirroring.h

@@ -130,7 +130,7 @@ private:
       : AbstractCanonical<T>(aThread), WatchTarget(aName), mValue(aInitialValue)
     {
       MIRROR_LOG("%s [%p] initialized", mName, this);
-      MOZ_ASSERT(aThread->RequiresTailDispatch(), "Can't get coherency without tail dispatch");
+      MOZ_ASSERT(aThread->SupportsTailDispatch(), "Can't get coherency without tail dispatch");
     }
 
     void AddMirror(AbstractMirror<T>* aMirror) override
@@ -303,6 +303,7 @@ private:
       : 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&()
@@ -334,6 +335,7 @@ private:
       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 = NS_NewRunnableMethodWithArg<StorensRefPtrPassByPtr<AbstractMirror<T>>>
                                   (aCanonical, &AbstractCanonical<T>::AddMirror, this);