Bug 1741118 - Allow the MainAsCurrent TaskQueueWrapper to be deleted on itself. r=ng

This is prohibited for regular TaskQueueWrappers, as Delete must join (i.e.
block) the thread after initiating shutdown. But MainAsCurrent is special as
main thread doesn't go away just because MainAsCurrent goes away. It is also
gtest-only, and thus tightly coupled with MediaPipelineTest behavior.

Differential Revision: https://phabricator.services.mozilla.com/D132221

media/webrtc/signaling/gtest/mediapipeline_unittest.cpp

@@ -54,12 +54,23 @@ class MainAsCurrent : public TaskQueueWrapper {
   MainAsCurrent()
       : TaskQueueWrapper(MakeRefPtr<TaskQueue>(
             do_AddRef(GetMainThreadEventTarget()), "MainAsCurrentTaskQueue")),
-        mSetter(this) {}
-  void Delete() override {
-    MOZ_RELEASE_ASSERT(!NS_IsMainThread(),
-                       "Releasing a main-thread-TaskQueue on main might hang");
-    TaskQueueWrapper::Delete();
+        mSetter(this) {
+    MOZ_RELEASE_ASSERT(NS_IsMainThread());
   }
+
+  void Delete() override {
+    MOZ_RELEASE_ASSERT(NS_IsMainThread());
+    // We release the MainAsCurrent TaskQueue wrapper off-main since it sits on
+    // top of main. It blocks the current thread during shutdown through
+    // TaskQueue::AwaitIdle, which will hang unless it's already idle.
+    // NS_DISPATCH_SYNC will still process other main thread tasks, allowing the
+    // task queue to process events and shut down.
+    NS_DispatchBackgroundTask(
+        NS_NewRunnableFunction("MainAsCurrent off-main deleter",
+                               [this] { TaskQueueWrapper::Delete(); }),
+        NS_DISPATCH_SYNC);
+  }
+
   ~MainAsCurrent() = default;
 
  private:
@@ -468,14 +479,6 @@ class MediaPipelineTest : public ::testing::Test {
   ~MediaPipelineTest() {
     p1_.Shutdown();
     p2_.Shutdown();
-    // We release the MainAsCurrent TaskQueue wrapper off-main since it sits on
-    // top of main. It blocks the current thread during shutdown through
-    // TaskQueue::AwaitIdle, which will hang unless it's already idle.
-    NS_DispatchBackgroundTask(
-        NS_NewRunnableFunction(
-            "MainAsCurrent off-main deleter",
-            [tq = std::move(main_task_queue_)]() mutable { tq = nullptr; }),
-        NS_DISPATCH_SYNC);
   }
 
   static void SetUpTestCase() {
@@ -584,7 +587,7 @@ class MediaPipelineTest : public ::testing::Test {
   }
 
  protected:
-  // main_task_queue has this type to make sure it goes through Delete() when
+  // main_task_queue_ has this type to make sure it goes through Delete() when
   // we're destroyed.
   UniquePtr<TaskQueueWrapper> main_task_queue_;
   const RefPtr<SharedWebrtcState> shared_state_;