Bug 130118 P2 Make TaskQueue implement nsIEventTarget. r=bholley

xpcom/threads/TaskQueue.cpp

@@ -6,10 +6,62 @@
 
 #include "mozilla/TaskQueue.h"
 
+#include "nsIEventTarget.h"
 #include "nsThreadUtils.h"
 
 namespace mozilla {
 
+class TaskQueue::EventTargetWrapper final : public nsIEventTarget
+{
+  RefPtr<TaskQueue> mTaskQueue;
+
+  ~EventTargetWrapper()
+  {
+  }
+
+public:
+  explicit EventTargetWrapper(TaskQueue* aTaskQueue)
+    : mTaskQueue(aTaskQueue)
+  {
+    MOZ_ASSERT(mTaskQueue);
+  }
+
+  NS_IMETHOD
+  DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags) override
+  {
+    nsCOMPtr<nsIRunnable> ref = aEvent;
+    return Dispatch(ref.forget(), aFlags);
+  }
+
+  NS_IMETHOD
+  Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags) override
+  {
+    nsCOMPtr<nsIRunnable> runnable = aEvent;
+    MonitorAutoLock mon(mTaskQueue->mQueueMonitor);
+    return mTaskQueue->DispatchLocked(/* passed by ref */runnable,
+                                      AbortIfFlushing,
+                                      DontAssertDispatchSuccess,
+                                      NormalDispatch);
+  }
+
+  NS_IMETHOD
+  DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t aFlags) override
+  {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  NS_IMETHOD
+  IsOnCurrentThread(bool* aResult) override
+  {
+    *aResult = mTaskQueue->IsCurrentThreadIn();
+    return NS_OK;
+  }
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+};
+
+NS_IMPL_ISUPPORTS(TaskQueue::EventTargetWrapper, nsIEventTarget)
+
 TaskQueue::TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
                      bool aRequireTailDispatch)
   : AbstractThread(aRequireTailDispatch)
@@ -143,6 +195,13 @@ TaskQueue::IsCurrentThreadIn()
   return in;
 }
 
+already_AddRefed<nsIEventTarget>
+TaskQueue::WrapAsEventTarget()
+{
+  nsCOMPtr<nsIEventTarget> ref = new EventTargetWrapper(this);
+  return ref.forget();
+}
+
 nsresult
 TaskQueue::Runner::Run()
 {

xpcom/threads/TaskQueue.h

@@ -29,7 +29,26 @@ typedef MozPromise<bool, bool, false> ShutdownPromise;
 // they're received, and are guaranteed to not be executed concurrently.
 // They may be executed on different threads, and a memory barrier is used
 // to make this threadsafe for objects that aren't already threadsafe.
-class TaskQueue : public AbstractThread {
+//
+// Note, since a TaskQueue can also be converted to an nsIEventTarget using
+// WrapAsEventTarget() its possible to construct a hierarchy of TaskQueues.
+// Consider these three TaskQueues:
+//
+//  TQ1 dispatches to the main thread
+//  TQ2 dispatches to TQ1
+//  TQ3 dispatches to TQ1
+//
+// This ensures there is only ever a single runnable from the entire chain on
+// the main thread.  It also ensures that TQ2 and TQ3 only have a single runnable
+// in TQ1 at any time.
+//
+// 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.
+class TaskQueue : public AbstractThread
+{
+  class EventTargetWrapper;
+
 public:
   explicit TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
                      bool aSupportsTailDispatch = false);
@@ -76,6 +95,10 @@ public:
   // the task queue.
   bool IsCurrentThreadIn() override;
 
+  // Create a new nsIEventTarget wrapper object that dispatches to this
+  // TaskQueue.
+  already_AddRefed<nsIEventTarget> WrapAsEventTarget();
+
 protected:
   virtual ~TaskQueue();