[mojo] Implement pipe fusion API

mojo/edk/system/message_pipe_dispatcher.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 MOJO_EDK_SYSTEM_MESSAGE_PIPE_DISPATCHER_H_
 #define MOJO_EDK_SYSTEM_MESSAGE_PIPE_DISPATCHER_H_
 
 #include <stdint.h>
 
 #include <queue>
@@ skipping 21 matching lines @@
   // state, and one can never return to a disconnected once connected.
   //
   // |pipe_id| is a unique identifier which can be used to track pipe endpoints
   // as they're passed around. |endpoint| is either 0 or 1 and again is only
   // used for tracking pipes (one side is always 0, the other is always 1.)
   MessagePipeDispatcher(NodeController* node_controller,
                         const ports::PortRef& port,
                         uint64_t pipe_id,
                         int endpoint);
 
+  // Begins a fuse operation for this pipe. Returns |true| if fusing is possible
+  // and |false| otherwise.
+  bool BeginFuse();
+
+  // Cancels a fuse operation for this pipe.
+  void CancelFuse();
+
+  // Fuses this pipe with |other|. |BeginFuse()| must have been called on both
+  // this and |other| first, and |Fuse()| itself must only be called on one of
+  // the two, passing the other as an argument. Both dispatchers are closed as a
+  // result of this operation regardless of whether it succeeds or fails.
+  bool CompleteFuse(MessagePipeDispatcher* other);

[yzshen1, 2016/03/11 17:17:20]

It seems possible to only expose a single Fuse() method, and make
BeginFuse/CancelFuse private methods. WDYT?

+
   // Dispatcher:
   Type GetType() const override;
   MojoResult Close() override;
   MojoResult Watch(MojoHandleSignals signals,
                    const Watcher::WatchCallback& callback,
                    uintptr_t context) override;
   MojoResult CancelWatch(uintptr_t context) override;
   MojoResult WriteMessage(const void* bytes,
                           uint32_t num_bytes,
                           const DispatcherInTransit* dispatchers,
@@ skipping 41 matching lines @@
 
   // These are safe to access from any thread without locking.
   NodeController* const node_controller_;
   const ports::PortRef port_;
   const uint64_t pipe_id_;
   const int endpoint_;
 
   // Guards access to all the fields below.
   mutable base::Lock signal_lock_;
 
+  // Indicates if a fuse operation has been initiated for this pipe.
+  bool is_fusing_ = false;
+
   // This is not the same is |port_transferred_|. It's only held true between
   // BeginTransit() and Complete/CancelTransit().
   bool in_transit_ = false;
 
   bool port_transferred_ = false;
   bool port_closed_ = false;
   AwakableList awakables_;
 
   DISALLOW_COPY_AND_ASSIGN(MessagePipeDispatcher);
 };
 
 }  // namespace edk
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_MESSAGE_PIPE_DISPATCHER_H_