Fix NotifyPauseOnStart and NotifyPauseOnExit dead lock

runtime/vm/message_handler.cc

 // Copyright (c) 2011, 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 "vm/message_handler.h"
 
 #include "vm/dart.h"
 #include "vm/lockers.h"
 #include "vm/port.h"
 #include "vm/thread_interrupter.h"
@@ skipping 224 matching lines @@
   ASSERT(Isolate::Current() == NULL);
   bool ok = true;
   bool run_end_callback = false;
   {
     MonitorLocker ml(&monitor_);
     // Initialize the message handler by running its start function,
     // if we have one.  For an isolate, this will run the isolate's
     // 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);
       if (pause_on_start()) {
         // Still paused.
         task_ = NULL;  // No task in queue.
         return;
       } else {
         paused_on_start_ = false;
       }
@@ skipping 15 matching lines @@
     }
     task_ = NULL;  // No task in queue.
 
     if (!ok || !HasLivePorts()) {
       if (pause_on_exit()) {
         if (!paused_on_exit_) {
           if (FLAG_trace_service_pause_events) {
             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 {
         if (FLAG_trace_isolates) {
         OS::Print("[-] Stopping message handler (%s):\n"
                   "\thandler:    %s\n",
                   (ok ? "no live ports" : "error"),
                   name());
         }
         pool_ = NULL;
@@ skipping 79 matching lines @@
 
 
 void MessageHandler::AcquireQueues(AcquiredQueues* acquired_queues) {
   ASSERT(acquired_queues != NULL);
   // No double dipping.
   ASSERT(acquired_queues->handler_ == NULL);
   acquired_queues->Reset(this);
 }
 
 }  // namespace dart