Fix NotifyPauseOnStart and NotifyPauseOnExit dead lock

runtime/vm/message_handler.cc

Index: runtime/vm/message_handler.cc
diff --git a/runtime/vm/message_handler.cc b/runtime/vm/message_handler.cc
index 6a473dd74033f4c6d019be2437de0c7a8ec29aa9..976c601a4b73b8ded6b3f5ec4f4f994bf098dcd3 100644
--- a/runtime/vm/message_handler.cc
+++ b/runtime/vm/message_handler.cc
@@ -242,7 +242,12 @@ void MessageHandler::TaskCallback() {
     // main() function.
     if (pause_on_start()) {
       if (!paused_on_start_) {
+        // Temporarily drop the lock when calling out to NotifyPauseOnStart.
+        // This avoids a dead lock that can occur when this message handler
+        // tries to post a message while a message is being posted to it.
+        monitor_.Exit();
         NotifyPauseOnStart();
+        monitor_.Enter();
         paused_on_start_ = true;
       }
       HandleMessages(false, false);
@@ -278,7 +283,12 @@ void MessageHandler::TaskCallback() {
             OS::PrintErr("Isolate %s paused before exiting. "
                        "Use the Observatory to release it.\n", name());
           }
+          // Temporarily drop the lock when calling out to NotifyPauseOnExit.
+          // This avoids a dead lock that can occur when this message handler
+          // tries to post a message while a message is being posted to it.
+          monitor_.Exit();
           NotifyPauseOnExit();

[turnidge, 2015/08/24 17:39:55]

Handle this like run_end_callback as discussed offline...

+          monitor_.Enter();
           paused_on_exit_ = true;
         }
       } else {