Bug 1409664 - P8. Make explicit shutdown of TaskQueue optional. r=gerald

Summary:
With this change TaskQueue no longer requires an explicit call to BeginShutdown(). The taskqueue will automatically shutdown once it's now longer referenced.

Depends on D1620

Tags: #secure-revision

Bug #: 1409664

Differential Revision: https://phabricator.services.mozilla.com/D1621

xpcom/threads/TaskQueue.cpp

@@ -89,8 +89,8 @@ TaskQueue::TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
 
 TaskQueue::~TaskQueue()
 {
-  MonitorAutoLock mon(mQueueMonitor);
-  MOZ_ASSERT(mIsShutdown);
+  // No one is referencing this TaskQueue anymore, meaning no tasks can be
+  // pending as all Runner hold a reference to this TaskQueue.
 }
 
 TaskDispatcher&
@@ -178,7 +178,6 @@ TaskQueue::BeginShutdown()
   if (AbstractThread* currentThread = AbstractThread::GetCurrent()) {
     currentThread->TailDispatchTasksFor(this);
   }
-
   MonitorAutoLock mon(mQueueMonitor);
   mIsShutdown = true;
   RefPtr<ShutdownPromise> p = mShutdownPromise.Ensure(__func__);

xpcom/threads/TaskQueue.h

@@ -45,6 +45,10 @@ typedef MozPromise<bool, bool, false> ShutdownPromise;
 // This arrangement lets you prioritize work by dispatching runnables directly
 // to TQ1.  You can issue many runnables for important work.  Meanwhile the TQ2
 // and TQ3 work will always execute at most one runnable and then yield.
+//
+// A TaskQueue does not require explicit shutdown, however it provides a
+// BeginShutdown() method that places TaskQueue in a shut down state and returns
+// a promise that gets resolved once all pending tasks have completed
 class TaskQueue : public AbstractThread
 {
   class EventTargetWrapper;