ReadableStream.cancel's parameter should be optional.

Source/modules/fetch/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/fetch/Body.h"
 
 #include "bindings/core/v8/ExceptionState.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
 #include "bindings/core/v8/ScriptState.h"
@@ skipping 123 matching lines @@
         visitor->trace(m_stream);
         BodyStreamBuffer::Observer::trace(visitor);
         UnderlyingSource::trace(visitor);
     }
 
 private:
     // UnderlyingSource functions.
     void pullSource() override { }
     ScriptPromise cancelSource(ScriptState* scriptState, ScriptValue reason) override
     {
-        return ScriptPromise();
+        close();
+        m_bodyStreamBuffer->unregisterObserver();
+        return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));

[tyoshino (SeeGerritForStatus), 2015/03/23 03:57:21]

which call terminates the fetch?

[yhirano, 2015/03/23 05:47:32]

You're right, and I found implementing the cancellation operation is a bit hard.
So I would fix the IDL related issue in this CL and fix the fetch termination
after that.

     }
 
     // BodyStreamBuffer::Observer functions.
     void onWrite() override
     {
         ASSERT(m_state == Streaming);
         while (RefPtr<DOMArrayBuffer> buf = m_bodyStreamBuffer->read()) {
             write(buf);
         }
     }
@@ skipping 328 matching lines @@
 
 void Body::didBlobHandleReceiveError(PassRefPtrWillBeRawPtr<DOMException> exception)
 {
     if (!m_resolver)
         return;
     m_resolver->reject(exception);
     m_resolver.clear();
 }
 
 } // namespace blink