Bug 1584396 - Do not assert about other thread dispatches at worker shutdown. r=dom-workers-and-storage-reviewers,ttung

Differential Revision: https://phabricator.services.mozilla.com/D78836

dom/workers/WorkerThread.cpp

@@ -127,10 +127,12 @@ void WorkerThread::SetWorker(const WorkerThreadFriendKey& /* aKey */,
 
       MOZ_ASSERT(mWorkerPrivate);
       MOZ_ASSERT(!mAcceptingNonWorkerRunnables);
-      MOZ_ASSERT(!mOtherThreadsDispatchingViaEventTarget,
-                 "XPCOM Dispatch hapenning at the same time our thread is "
-                 "being unset! This should not be possible!");
-
+      // mOtherThreadsDispatchingViaEventTarget can still be non-zero here
+      // because WorkerThread::Dispatch isn't atomic so a thread initiating
+      // dispatch can have dispatched a runnable at this thread allowing us to
+      // begin shutdown before that thread gets a chance to decrement
+      // mOtherThreadsDispatchingViaEventTarget back to 0.  So we need to wait
+      // for that.
       while (mOtherThreadsDispatchingViaEventTarget) {
         mWorkerPrivateCondVar.Wait();
       }