Fix a race condition during worker thread initialization

Source/core/workers/WorkerThread.cpp

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 196 matching lines @@
     V8CacheOptions v8CacheOptions = startupData->m_v8CacheOptions;
 
     {
         MutexLocker lock(m_threadStateMutex);
 
         // The worker was terminated before the thread had a chance to run.
         if (m_terminated) {
             // Notify the proxy that the WorkerGlobalScope has been disposed of.
             // This can free this thread object, hence it must not be touched afterwards.
             m_workerReportingProxy.workerThreadTerminated();
+            // Notify the main thread that it is safe to deallocate our resources.
+            m_terminationEvent->signal();
             return;
         }
 
         m_microtaskRunner = adoptPtr(new WorkerMicrotaskRunner(this));
         initializeBackingThread();
         backingThread().addTaskObserver(m_microtaskRunner.get());
 
         m_isolate = initializeIsolate();
         m_workerGlobalScope = createWorkerGlobalScope(startupData);
         m_workerGlobalScope->scriptLoaded(sourceCode.length(), cachedMetaData.get() ? cachedMetaData->size() : 0);
@@ skipping 89 matching lines @@
     m_terminated = true;
 
     // Signal the thread to notify that the thread's stopping.
     if (m_shutdownEvent)
         m_shutdownEvent->signal();
 
     // If the thread has already initiated shut down, just return.
     if (m_shutdown)
         return;
 
-    // If the worker thread was never initialized, complete the termination immediately.
-    if (!m_workerGlobalScope) {
-        m_terminationEvent->signal();
+    // If the worker thread was never initialized, don't start another
+    // shutdown, but still wait for the thread to signal when termination has
+    // completed.
+    if (!m_workerGlobalScope)
         return;
-    }
 
     // Ensure that tasks are being handled by thread event loop. If script execution weren't forbidden, a while(1) loop in JS could keep the thread alive forever.
     m_workerGlobalScope->script()->willScheduleExecutionTermination();
     terminateV8Execution();
 
     InspectorInstrumentation::didKillAllExecutionContextTasks(m_workerGlobalScope.get());
     m_debuggerMessageQueue.kill();
     backingThread().postTask(FROM_HERE, new Task(threadSafeBind(&WorkerThread::shutdown, AllowCrossThreadAccess(this))));
 }
 
@@ skipping 142 matching lines @@
     InspectorInstrumentation::didLeaveNestedRunLoop(m_workerGlobalScope.get());
 }
 
 void WorkerThread::setWorkerInspectorController(WorkerInspectorController* workerInspectorController)
 {
     MutexLocker locker(m_workerInspectorControllerMutex);
     m_workerInspectorController = workerInspectorController;
 }
 
 } // namespace blink