Fix races with MessagePipeReader due to the Mojo IPC channel being thread-safe.

ipc/mojo/ipc_message_pipe_reader.h

 // 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.
 
 #ifndef IPC_IPC_MESSAGE_PIPE_READER_H_
 #define IPC_IPC_MESSAGE_PIPE_READER_H_
 
 #include <vector>
 
+#include "base/atomicops.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/threading/thread_checker.h"
 #include "ipc/ipc_message.h"
 #include "third_party/mojo/src/mojo/public/c/environment/async_waiter.h"
 #include "third_party/mojo/src/mojo/public/cpp/system/core.h"
 
 namespace IPC {
 namespace internal {
 
 class AsyncHandleWaiter;
 
 // A helper class to handle bytestream directly over mojo::MessagePipe
 // in template-method pattern. MessagePipeReader manages the lifetime
 // of given MessagePipe and participates the event loop, and
 // read the stream and call the client when it is ready.
 //
 // Each client has to:
 //
 //  * Provide a subclass implemenation of a specific use of a MessagePipe
 //    and implement callbacks.
 //  * Create the subclass instance with a MessagePipeHandle.
 //    The constructor automatically start listening on the pipe.
 //
-// MessageReader has to be used in IO thread. It isn't thread-safe.
+// All functions must be called on the IO thread, except for Send(), which can
+// be called on any thread. All |Delegate| functions will be called on the IO
+// thread.
 //
 class MessagePipeReader {
  public:
   class Delegate {
    public:
     virtual void OnMessageReceived(Message& message) = 0;
     virtual void OnPipeClosed(MessagePipeReader* reader) = 0;
     virtual void OnPipeError(MessagePipeReader* reader) = 0;
   };
 
@@ skipping 11 matching lines @@
 
     void operator()(MessagePipeReader* ptr) const;
   };
 
   // Both parameters must be non-null.
   // Build a reader that reads messages from |handle| and lets |delegate| know.
   // Note that MessagePipeReader doesn't delete |delete|.
   MessagePipeReader(mojo::ScopedMessagePipeHandle handle, Delegate* delegate);
   virtual ~MessagePipeReader();
 
-  MojoHandle handle() const { return pipe_.get().value(); }
+  MojoHandle handle() const { return handle_copy_; }
 
   // Returns received bytes.
   const std::vector<char>& data_buffer() const {
     return data_buffer_;
   }
 
   // Delegate received handles ownership. The subclass should take the
   // ownership over in its OnMessageReceived(). They will leak otherwise.
   void TakeHandleBuffer(std::vector<MojoHandle>* handle_buffer) {
     handle_buffer_.swap(*handle_buffer);
@@ skipping 14 matching lines @@
 
  private:
   void OnMessageReceived();
   void OnPipeClosed();
   void OnPipeError(MojoResult error);
 
   MojoResult ReadMessageBytes();
   void PipeIsReady(MojoResult wait_result);
   void ReadAvailableMessages();
 
+  // Helper for accessing |pending_send_error_| because of the use of atomics.
+  MojoResult pending_send_error() const {
+    return base::subtle::NoBarrier_Load(&pending_send_error_);
+  }
+
   std::vector<char>  data_buffer_;
   std::vector<MojoHandle> handle_buffer_;
   mojo::ScopedMessagePipeHandle pipe_;
+  // Constant copy of the message pipe handle. For use by Send(), which can run
+  // concurrently on non-IO threads.
+  // TODO(amistry): This isn't quite right because handles can be re-used and
+  // using this can run into the ABA problem. Currently, this is highly unlikely
+  // because Mojo internally uses an increasing uint32_t as handle values, but
+  // this could change.
+  const MojoHandle handle_copy_;
   // |delegate_| and |async_waiter_| are null once the message pipe is closed.
   Delegate* delegate_;
   scoped_ptr<AsyncHandleWaiter> async_waiter_;
-  MojoResult pending_send_error_;
+  base::subtle::Atomic32 pending_send_error_;
+  base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(MessagePipeReader);
 };
 
 }  // namespace internal
 }  // namespace IPC
 
 #endif  // IPC_IPC_MESSAGE_PIPE_READER_H_