ServiceWorker: Add null check of ExecutionContext in skipWaiting.

Source/modules/serviceworkers/ServiceWorkerGlobalScope.cpp

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 108 matching lines @@
     return m_clients;
 }
 
 void ServiceWorkerGlobalScope::close(ExceptionState& exceptionState)
 {
     exceptionState.throwDOMException(InvalidAccessError, "Not supported.");
 }
 
 ScriptPromise ServiceWorkerGlobalScope::skipWaiting(ScriptState* scriptState)
 {
+    ExecutionContext* executionContext = scriptState->executionContext();
+    if (!executionContext)
+        return ScriptPromise();

[falken, 2015/01/08 04:31:52]

I have some questions.

1. What happens if TerminateExecution is called after this, is still safe to use
this executionContext variable? +haraken@ mentioned in
https://codereview.chromium.org/535193002/ crashes could still occur in that
scenario.
2. Why is the TerminateExecution getting called in the first place? We schedule
stopping the worker 30 seconds after it's started, it shouldn't happen during
this layout test.

[xiang, 2015/01/08 08:16:13]

I'm not sure about it in this case, from my code reading, it should be OK.
Anyway I added a "FIXME" comment.
I think the unregister() will trigger worker stop. 2 of 3 tests in this file
will unregister() without waiting the worker "complete" message.

[falken, 2015/01/08 08:28:05]

I see! That looks right.

+
     RefPtrWillBeRawPtr<ScriptPromiseResolver> resolver = ScriptPromiseResolver::create(scriptState);
     ScriptPromise promise = resolver->promise();
 
-    ExecutionContext* executionContext = scriptState->executionContext();
     ServiceWorkerGlobalScopeClient::from(executionContext)->skipWaiting(new SkipWaitingCallback(resolver));
     return promise;
 }
 
 bool ServiceWorkerGlobalScope::addEventListener(const AtomicString& eventType, PassRefPtr<EventListener> listener, bool useCapture)
 {
     if (m_didEvaluateScript) {
         if (eventType == EventTypeNames::install) {
             RefPtrWillBeRawPtr<ConsoleMessage> consoleMessage = ConsoleMessage::create(JSMessageSource, WarningMessageLevel, "Event handler of 'install' event must be added on the initial evaluation of worker script.");
             addMessageToWorkerConsole(consoleMessage.release());
@@ skipping 50 matching lines @@
 void ServiceWorkerGlobalScope::logExceptionToConsole(const String& errorMessage, int scriptId, const String& sourceURL, int lineNumber, int columnNumber, PassRefPtrWillBeRawPtr<ScriptCallStack> callStack)
 {
     WorkerGlobalScope::logExceptionToConsole(errorMessage, scriptId, sourceURL, lineNumber, columnNumber, callStack);
     RefPtrWillBeRawPtr<ConsoleMessage> consoleMessage = ConsoleMessage::create(JSMessageSource, ErrorMessageLevel, errorMessage, sourceURL, lineNumber);
     consoleMessage->setScriptId(scriptId);
     consoleMessage->setCallStack(callStack);
     addMessageToWorkerConsole(consoleMessage.release());
 }
 
 } // namespace blink