Bug 1151656 - Implement the ability to assert tail dispatch. r=mattwoodrow

In order to make sure that the MDSM properly dispatches everything via tail
dispatch, we want verification that is more robust than simple inspection.

dom/media/AbstractThread.cpp

@@ -20,6 +20,7 @@ template<>
 nsresult
 AbstractThreadImpl<nsIThread>::Dispatch(already_AddRefed<nsIRunnable> aRunnable)
 {
+  MediaTaskQueue::AssertInTailDispatchIfNeeded();
   nsCOMPtr<nsIRunnable> r = aRunnable;
   return mTarget->Dispatch(r, NS_DISPATCH_NORMAL);
 }

dom/media/MediaTaskQueue.cpp

@@ -20,13 +20,15 @@ MediaTaskQueue::InitStatics()
   }
 }
 
-MediaTaskQueue::MediaTaskQueue(TemporaryRef<SharedThreadPool> aPool)
+MediaTaskQueue::MediaTaskQueue(TemporaryRef<SharedThreadPool> aPool,
+                               bool aRequireTailDispatch)
   : mPool(aPool)
   , mQueueMonitor("MediaTaskQueue::Queue")
   , mTailDispatcher(nullptr)
   , mIsRunning(false)
   , mIsShutdown(false)
   , mIsFlushing(false)
+  , mRequireTailDispatch(aRequireTailDispatch)
 {
   MOZ_COUNT_CTOR(MediaTaskQueue);
 }
@@ -41,6 +43,7 @@ MediaTaskQueue::~MediaTaskQueue()
 nsresult
 MediaTaskQueue::Dispatch(TemporaryRef<nsIRunnable> aRunnable)
 {
+  AssertInTailDispatchIfNeeded(); // Do this before acquiring the monitor.
   MonitorAutoLock mon(mQueueMonitor);
   return DispatchLocked(aRunnable, AbortIfFlushing);
 }
@@ -56,6 +59,7 @@ MediaTaskQueue::TailDispatcher()
 nsresult
 MediaTaskQueue::ForceDispatch(TemporaryRef<nsIRunnable> aRunnable)
 {
+  AssertInTailDispatchIfNeeded(); // Do this before acquiring the monitor.
   MonitorAutoLock mon(mQueueMonitor);
   return DispatchLocked(aRunnable, Forced);
 }
@@ -178,6 +182,7 @@ FlushableMediaTaskQueue::Flush()
 nsresult
 FlushableMediaTaskQueue::FlushAndDispatch(TemporaryRef<nsIRunnable> aRunnable)
 {
+  AssertInTailDispatchIfNeeded(); // Do this before acquiring the monitor.
   MonitorAutoLock mon(mQueueMonitor);
   AutoSetFlushing autoFlush(this);
   FlushLocked();

dom/media/MediaTaskQueue.h

@@ -38,7 +38,7 @@ public:
   // if the caller is not running in a MediaTaskQueue.
   static MediaTaskQueue* GetCurrentQueue() { return sCurrentQueueTLS.get(); }
 
-  explicit MediaTaskQueue(TemporaryRef<SharedThreadPool> aPool);
+  explicit MediaTaskQueue(TemporaryRef<SharedThreadPool> aPool, bool aRequireTailDispatch = false);
 
   nsresult Dispatch(TemporaryRef<nsIRunnable> aRunnable);
 
@@ -48,6 +48,31 @@ public:
   // May only be called when running within the task queue it is invoked up.
   TaskDispatcher& TailDispatcher();
 
+  // Returns true if this task queue requires all dispatches performed by its
+  // tasks to go through the tail dispatcher.
+  bool RequiresTailDispatch() { return mRequireTailDispatch; }
+
+#ifdef DEBUG
+  static void AssertInTailDispatchIfNeeded()
+  {
+    // See if we're currently running in a task queue that asserts tail
+    // dispatch.
+    MediaTaskQueue* currentQueue = MediaTaskQueue::GetCurrentQueue();
+    if (!currentQueue || !currentQueue->RequiresTailDispatch()) {
+      return;
+    }
+
+    // This is a bit tricky. The only moment when we're running in a task queue
+    // but don't have mTailDispatcher set is precisely the moment that we're
+    // doing tail dispatch (i.e. when AutoTaskGuard's destructor has already
+    // run and AutoTaskDispatcher's destructor is currently running).
+    MOZ_ASSERT(!currentQueue->mTailDispatcher,
+               "Not allowed to dispatch tasks directly from this task queue - use TailDispatcher()");
+  }
+#else
+  static void AssertInTailDispatchIfNeeded() {}
+#endif
+
   // For AbstractThread.
   nsresult Dispatch(already_AddRefed<nsIRunnable> aRunnable) override
   {
@@ -157,6 +182,10 @@ protected:
   // True if we're flushing; we reject new tasks if we're flushing.
   bool mIsFlushing;
 
+  // True if we want to require that every task dispatched from tasks running in
+  // this queue go through our queue's tail dispatcher.
+  bool mRequireTailDispatch;
+
   class Runner : public nsRunnable {
   public:
     explicit Runner(MediaTaskQueue* aQueue)