Introduce CompositeDataConsumerHandle.

Source/modules/fetch/CompositeDataConsumerHandle.h

+// Copyright 2015 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.
+
+#ifndef CompositeDataConsumerHandle_h
+#define CompositeDataConsumerHandle_h
+
+#include "public/platform/WebDataConsumerHandle.h"
+#include "wtf/OwnPtr.h"
+#include "wtf/PassOwnPtr.h"
+#include "wtf/RefPtr.h"
+
+namespace blink {
+
+class WebThread;
+
+// This is a utility class to construct a composite data consumer handle. It
+// ownes a web data consumer handle and delegates methods. A user can update

[hiroshige, 2015/06/03 08:45:05]

nit: s/ownes/owns/

[yhirano, 2015/06/04 05:43:02]

Done.

+// the handle by using |update| method.
+class CompositeDataConsumerHandle final : public WebDataConsumerHandle {
+public:
+    static PassOwnPtr<CompositeDataConsumerHandle> create(PassOwnPtr<WebDataConsumerHandle> handle)
+    {
+        return adoptPtr(new CompositeDataConsumerHandle(handle));
+    }
+    ~CompositeDataConsumerHandle() override;
+
+    Result read(void* data, size_t /* size */, Flags, size_t* readSize) override;
+    Result beginRead(const void** buffer, Flags, size_t* available) override;
+    Result endRead(size_t readSize) override;
+    void registerClient(Client* /* client */) override;
+    void unregisterClient() override;
+
+    // This function should be called on the thread on which this object
+    // was created.

[hiroshige, 2015/06/03 08:45:05]

Why does this thread restriction exist?
From the code, update() seems callable from any thread.

(Also, I wonder if we can remove some mutexes by limiting threads where update()
is called, but this current thread restriction doesn't remove mutexes...)

[yhirano, 2015/06/04 05:43:02]

I would like to leave a future optimization opportunity by limiting thread.
Currently I don't think of a use case that needs calling |update| on arbitrary
threads, but if it exists, loosing the restriction will be OK.

+    void update(PassOwnPtr<WebDataConsumerHandle>);
+
+    // Returns a handle that returns ShouldWait for read / beginRead / endRead
+    // operations.
+    static PassOwnPtr<WebDataConsumerHandle> createWaitingHandle();

[hiroshige, 2015/06/03 08:45:05]

It would be better to add a comment explaining why createWaitingHandle(),
createDoneHandle() and related classes are placed here (as explained in the CL
description), because there are no direct dependencies bewteen them and
CompositeDataConsumerHandle.

[yhirano, 2015/06/04 05:43:02]

Done.

+
+    // Returns a handle that returns Done for read / beginRead / endRead
+    // operations.
+    static PassOwnPtr<WebDataConsumerHandle> createDoneHandle();
+
+private:
+    explicit CompositeDataConsumerHandle(PassOwnPtr<WebDataConsumerHandle>);
+
+    class MutexHolder;
+    void updateInternal(PassOwnPtr<WebDataConsumerHandle>);
+    static void updateOnTheRightThread(CompositeDataConsumerHandle*, PassOwnPtr<WebDataConsumerHandle>, PassRefPtr<MutexHolder>);
+
+    // TODO(yhirano): Consider using atomic operations instead of mutex if
+    // possible.
+    OwnPtr<WebDataConsumerHandle> m_handle;
+    RefPtr<MutexHolder> m_mutex;
+    Client* m_client;
+    WebThread* m_clientThread;
+};
+
+} // namespace blink
+
+#endif // CompositeDataConsumerHandle_h