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

Source/modules/serviceworkers/Body.cpp

Index: Source/modules/serviceworkers/Body.cpp
diff --git a/Source/modules/serviceworkers/Body.cpp b/Source/modules/serviceworkers/Body.cpp
index cd4088d1de1a9be07a2afe53363835d8d833a604..39d6ab2ebe576086e652d0ec60e9805393ae894c 100644
--- a/Source/modules/serviceworkers/Body.cpp
+++ b/Source/modules/serviceworkers/Body.cpp
@@ -19,6 +19,13 @@ ScriptPromise Body::readAsync(ScriptState* scriptState, ResponseType type)
     if (m_bodyUsed)
         return ScriptPromise::reject(scriptState, V8ThrowException::createTypeError("Already read", scriptState->isolate()));
 
+    // executionContext may be 0 if V8::TerminateExecution() is called in the
+    // main thread.

[haraken, 2014/09/11 16:20:01]

Let's elaborate the comment a bit more.

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).

[horo, 2014/09/11 16:49:55]

Done.

+    // FIXME: Handle this case in more better way.
+    ExecutionContext* executionContext = scriptState->executionContext();
+    if (executionContext == 0)

[haraken, 2014/09/11 16:20:01]

if (!executionContext)

[horo, 2014/09/11 16:49:55]

Done.

+        return ScriptPromise();
+
     m_bodyUsed = true;
     m_responseType = type;
 
@@ -62,7 +69,7 @@ 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;
 }