Bug 1767214: Avoid executing WorkerMainThreadRunnable::Run late in shutdown. r=dom-worker-reviewers,jesup

Differential Revision: https://phabricator.services.mozilla.com/D145577

dom/workers/WorkerRunnable.cpp

@@ -11,6 +11,7 @@
 #include "jsapi.h"
 #include "jsfriendapi.h"
 #include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/AppShutdown.h"
 #include "mozilla/Assertions.h"
 #include "mozilla/DebugOnly.h"
 #include "mozilla/ErrorResult.h"
@@ -584,6 +585,12 @@ NS_IMETHODIMP
 WorkerMainThreadRunnable::Run() {
   AssertIsOnMainThread();
 
+  // This shouldn't be necessary once we're better about making sure no workers
+  // are created during shutdown in earlier phases.
+  if (AppShutdown::IsInOrBeyond(ShutdownPhase::XPCOMShutdownThreads)) {
+    return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
+  }
+
   bool runResult = MainThreadRun();
 
   RefPtr<MainThreadStopSyncLoopRunnable> response =