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

Source/modules/serviceworkers/Body.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "modules/serviceworkers/Body.h"
 
 #include "bindings/core/v8/ScriptPromiseResolver.h"
 #include "bindings/core/v8/ScriptState.h"
 #include "bindings/core/v8/V8ThrowException.h"
 #include "core/fileapi/Blob.h"
 #include "core/fileapi/FileReaderLoader.h"
 #include "core/fileapi/FileReaderLoaderClient.h"
 
 namespace blink {
 
 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;
 
     ASSERT(!m_resolver);
     m_resolver = ScriptPromiseResolver::create(scriptState);
     ScriptPromise promise = m_resolver->promise();
 
     FileReaderLoader::ReadType readType = FileReaderLoader::ReadAsText;
     RefPtr<BlobDataHandle> blobHandle = blobDataHandle();
     if (!blobHandle.get()) {
@@ skipping 23 matching lines @@
         ASSERT_NOT_REACHED();
         break;
     case ResponseAsJSON:
     case ResponseAsText:
         break;
     default:
         ASSERT_NOT_REACHED();
     }
 
     m_loader = adoptPtr(new FileReaderLoader(readType, this));
-    m_loader->start(scriptState->executionContext(), blobHandle);
+    m_loader->start(executionContext, blobHandle);
 
     return promise;
 }
 
 ScriptPromise Body::arrayBuffer(ScriptState* scriptState)
 {
     return readAsync(scriptState, ResponseAsArrayBuffer);
 }
 
 ScriptPromise Body::blob(ScriptState* scriptState)
@@ skipping 97 matching lines @@
 {
     ASSERT(m_resolver);
     if (!m_resolver->executionContext() || m_resolver->executionContext()->activeDOMObjectsAreStopped())
         return;
 
     m_resolver->resolve("");
     m_resolver.clear();
 }
 
 } // namespace blink