Never lock the Pepper proxy lock on the IO thread

ppapi/proxy/resource_message_filter.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 PPAPI_PROXY_RESOURCE_MESSAGE_FILTER_H_
+#define PPAPI_PROXY_RESOURCE_MESSAGE_FILTER_H_
+
+#include "base/memory/ref_counted.h"
+#include "ppapi/proxy/ppapi_proxy_export.h"
+
+namespace IPC {
+class Message;
+}
+
+namespace ppapi {
+namespace proxy {
+class ResourceMessageReplyParams;
+
+// A ResourceMessageFilter lives on the IO thread and handles messages for a
+// particular resource type. This is necessary in some cases where we want to
+// reduce latency by doing some work on the IO thread rather than having to
+// PostTask to the main Pepper thread.
+//
+// Note: In some cases we can rely on a reply being associated with a
+// particular TrackedCallback, in which case we can dispatch directly to the
+// TrackedCallback's thread. See ReplyThreadRegistrar. That should be the first
+// choice for avoiding an unecessary jump to the main-thread.
+//
+// ResourceMessageFilter is for cases where there is not a one-to-one
+// relationship between a reply message and a TrackedCallback. For example, for
+// UDP Socket resources, the browser pushes data to the plugin even when the
+// plugin does not have a pending callback. We can't use the
+// ReplyThreadRegistrar, because data may arrive when there's not yet a
+// TrackedCallback to tell us what thread to use. So instead, we define a
+// UDPSocketFilter which accepts and queues UDP data on the IO thread.
+class PPAPI_PROXY_EXPORT ResourceMessageFilter
+    : public base::RefCountedThreadSafe<ResourceMessageFilter> {
+ public:
+  virtual bool OnResourceReplyReceived(
+      const ResourceMessageReplyParams& reply_params,
+      const IPC::Message& nested_msg) = 0;
+
+ protected:
+  friend class base::RefCountedThreadSafe<ResourceMessageFilter>;
+  virtual ~ResourceMessageFilter() {}
+};
+
+}  // namespace proxy
+}  // namespace ppapi
+
+#endif  // PPAPI_PROXY_RESOURCE_MESSAGE_FILTER_H_