[mojo] Implement pipe fusion API

mojo/edk/system/message_pipe_dispatcher.h

Index: mojo/edk/system/message_pipe_dispatcher.h
diff --git a/mojo/edk/system/message_pipe_dispatcher.h b/mojo/edk/system/message_pipe_dispatcher.h
index 7400515f93036670ec130d3fbf61b20cd31d0fb6..fff6454221d1a4d0c2d1f3af1b5eda3c7f13128c 100644
--- a/mojo/edk/system/message_pipe_dispatcher.h
+++ b/mojo/edk/system/message_pipe_dispatcher.h
@@ -39,6 +39,19 @@ class MessagePipeDispatcher : public Dispatcher {
                         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;
@@ -100,6 +113,9 @@ class MessagePipeDispatcher : public Dispatcher {
   // 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;