Mojo: Make Connector::Accept() MOJO_CHECK-fail if it sees MOJO_RESULT_BUSY.

mojo/public/cpp/bindings/lib/connector.cc

 // Copyright 2013 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 "mojo/public/cpp/bindings/lib/connector.h"
 
 #include <stddef.h>
 
 #include "mojo/public/cpp/bindings/error_handler.h"
 #include "mojo/public/cpp/environment/logging.h"
@@ skipping 79 matching lines @@
       // to track their lifetime any longer.
       message->mutable_handles()->clear();
       break;
     case MOJO_RESULT_FAILED_PRECONDITION:
       // There's no point in continuing to write to this pipe since the other
       // end is gone. Avoid writing any future messages. Hide write failures
       // from the caller since we'd like them to continue consuming any backlog
       // of incoming messages before regarding the message pipe as closed.
       drop_writes_ = true;
       break;
+    case MOJO_RESULT_BUSY:
+      // We'd get a "busy" result if one of the message's handles is:
+      //   - |message_pipe_|'s own handle;
+      //   - simultaneously being used on another thread; or
+      //   - in a "busy" state that prohibits it from being transferred (e.g.,
+      //     a data pipe handle in the middle of a two-phase read/write,
+      //     regardless of which thread that two-phase read/write is happening
+      //     on).
+      // TODO(vtl): I wonder if this should be a |MOJO_DCHECK()|. (But, until
+      // crbug.com/389666, etc. are resolved, this will make tests fail quickly
+      // rather than hanging.)
+      MOJO_CHECK(false) << "Race condition or other bug detected";
+      return false;
     default:
       // This particular write was rejected, presumably because of bad input.
       // The pipe is not necessarily in a bad state.
       return false;
   }
   return true;
 }
 
 // static
 void Connector::CallOnHandleReady(void* closure, MojoResult result) {
@@ skipping 73 matching lines @@
   // should end early.
   if (destroyed_flag_) {
     *destroyed_flag_ = true;  // Propagate flag.
   }
   if (error_handler_)
     error_handler_->OnConnectionError();
 }
 
 }  // namespace internal
 }  // namespace mojo