Fix deadlock that can occur while handling service messages at a breakpoint.

runtime/bin/dbg_message.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "bin/dbg_connection.h"
 #include "bin/dbg_message.h"
 #include "bin/dartutils.h"
 #include "bin/lockers.h"
 #include "bin/thread.h"
 #include "bin/utils.h"
@@ skipping 1078 matching lines @@
   MonitorLocker ml(&msg_queue_lock_);
   is_running_ = false;
 
   // Request notification on isolate messages.  This allows us to
   // respond to vm service messages while at breakpoint.
   Dart_SetMessageNotifyCallback(DbgMsgQueueList::NotifyIsolate);
 
   while (true) {
     // Handle all available vm service messages, up to a resume
     // request.
-    if (Dart_HandleServiceMessages()) {
-      break;
+    while (Dart_HasServiceMessages()) {
+      // Release the message queue lock before handling service
+      // messages.  This allows notifications to come in while we are
+      // processing long requests and avoids deadlock with the PortMap
+      // lock in the vm.
+      msg_queue_lock_.Exit();
+      bool resume = Dart_HandleServiceMessages();
+      msg_queue_lock_.Enter();
+      if (resume) {
+        // Resume requested through the vm service.
+        break;
+      }
     }
 
     // Handle all available debug messages, up to a resume request.
     if (HandlePendingMessages()) {
       break;
     }
 
     // Wait for more debug or vm service messages.
     Monitor::WaitResult res = ml.Wait();
     ASSERT(res == Monitor::kNotified);
@@ skipping 300 matching lines @@
     } else {
       ASSERT(kind == kShutdown);
       RemoveIsolateMsgQueue(isolate_id);
     }
   }
   Dart_ExitScope();
 }
 
 }  // namespace bin
 }  // namespace dart