Implements JS bindings for mojo shared buffer.

third_party/WebKit/Source/core/mojo/MojoSharedBuffer.cpp

+// Copyright 2017 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 "core/mojo/MojoSharedBuffer.h"
+
+#include "core/dom/DOMArrayBuffer.h"
+#include "mojo/public/cpp/system/core.h"
+
+namespace blink {
+
+MojoSharedBuffer::MojoSharedBuffer(void* data, unsigned numBytes)
+    : m_buffer(DOMArrayBuffer::create(data, numBytes)) {
+  // TODO(alokp): We need to create an ArrayBuffer that does not own data.

[alokp, 2017/02/27 19:50:45]

jbroman@: I could not find a way to create an array buffer that does not own the
underlying data. We need an External v8::ArrayBuffer here:
https://cs.chromium.org/chromium/src/v8/include/v8.h?dr&l=4059

Is there a way to create such an array buffer in blink?

[jbroman, 2017/02/27 20:14:44]

blink::DOMArrayBufferBase doesn't presently support further externalizing it. I
believe it currently assumes that all array buffers are in-process, and
allocated by PartitionAlloc (in the "array buffer" partition).

While you could in principle create a v8 array buffer backed by one (there's an
overload of New for that), that would be problematic because at present, Blink
assumes that all externalized buffers were externalized by it. Thus
V8ArrayBuffer::toImpl (or V8SharedArrayBuffer::toImpl) would have a CHECK
failure.

If you want to be able to expose a Mojo-allocated buffer in this way, you'll
have to either teach Blink to accept that there are externalized V8 buffers that
aren't DOMArrayBufferBase, or more likely, teach DOMArrayBufferBase that it
could have a buffer that comes from Mojo as opposed to from PartitionAlloc.

[alokp, 2017/02/27 20:30:34]

I think the latter - teaching DOMArrayBufferBase about external buffers - might
be easier. I will look into adding this support.

[alokp, 2017/02/28 00:22:15]

A quick patch to support externalized ArrayBuffer:
https://codereview.chromium.org/2719883004/

+}
+
+void MojoSharedBuffer::unmap() {
+  MojoUnmapBuffer(m_buffer->data());
+  // TODO(alokp): Neuter m_buffer.

[alokp, 2017/02/27 19:50:45]

jbroman@: Is Neutering the correct thing to do here?

[jbroman, 2017/02/27 20:14:44]

Yes, probably.

+}
+
+DEFINE_TRACE(MojoSharedBuffer) {
+  visitor->trace(m_buffer);
+}
+
+}  // namespace blink