[ServiceWorker] Add NULL check of ExecutionContext in FetchBodyStream::readAsync

scriptState->executionContext() returns NULL in the following situation.

[Main thread]
- WorkerThread::stop()
 - m_workerGlobalScope->script()->scheduleExecutionTermination();
  - v8::V8::TerminateExecution(m_isolate);

[Worker thread]
- FetchBodyStream::readAsync()
 - ScriptPromiseResolver::create()
  ---> v8::Promise::Resolver::New()
     - v8::internal::Execution::Call
      - v8::internal::Invoke
       - JS
        - v8::internal::__RT_impl_Runtime_StackGuard
         - v8::internal::Isolate::TerminateExecution()
            Sets TerminationException.
 - scriptState->executionContext()
  - toExecutionContext()
   - V8WorkerGlobalScope::findInstanceInPrototypeChain()
    - V8PerIsolateData::findInstanceInPrototypeChain()
     - V8PerIsolateData::findInstanceInPrototypeChain()
      - v8::Object::FindInstanceInPrototypeChain()
       - ON_BAILOUT(isolate,
                    "v8::Object::FindInstanceInPrototypeChain()",
                    return Local<v8::Object>());
        - IsExecutionTerminatingCheck()
          Returns true.

BUG=409755
TEST=run_webkit_tests http/tests/serviceworker/request.html

Review URL: https://codereview.chromium.org/535193002

git-svn-id: svn://svn.chromium.org/blink/trunk@181872 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/modules/serviceworkers/Body.cpp

@@ -19,6 +19,16 @@ ScriptPromise Body::readAsync(ScriptState* scriptState, ResponseType type)
     if (m_bodyUsed)
         return ScriptPromise::reject(scriptState, V8ThrowException::createTypeError("Already read", scriptState->isolate()));
 
+    // When the main thread sends a V8::TerminateExecution() signal to a worker
+    // thread, any V8 API on the worker thread starts returning an empty
+    // handle. This can happen in Body::readAsync. To avoid the situation, we
+    // first check the ExecutionContext and return immediately if it's already
+    // gone (which means that the V8::TerminateExecution() signal has been sent
+    // to this worker thread).
+    ExecutionContext* executionContext = scriptState->executionContext();
+    if (!executionContext)
+        return ScriptPromise();
+
     m_bodyUsed = true;
     m_responseType = type;
 
@@ -62,7 +72,7 @@ ScriptPromise Body::readAsync(ScriptState* scriptState, ResponseType type)
     }
 
     m_loader = adoptPtr(new FileReaderLoader(readType, this));
-    m_loader->start(scriptState->executionContext(), blobHandle);
+    m_loader->start(executionContext, blobHandle);
 
     return promise;
 }