Fix a bug PauseOnExit notification.

When we notified before, we could end up entering an isolate when it
was busy handling another message.  By moving the notification up and
clearing the "task" later, we ensure that we own the isolate for the
duration of the notification.

BUG=
[email protected]

Review URL: https://codereview.chromium.org//1324543002 .

Author: turnidge

runtime/vm/message_handler.cc

@@ -238,7 +238,6 @@ void MessageHandler::TaskCallback() {
   ASSERT(Isolate::Current() == NULL);
   bool ok = true;
   bool run_end_callback = false;
-  bool notify_paused_on_exit = false;
   {
     MonitorLocker ml(&monitor_);
     // Initialize the message handler by running its start function,
@@ -255,9 +254,11 @@ void MessageHandler::TaskCallback() {
         NotifyPauseOnStart();
         monitor_.Enter();
       }
+      // More messages may have come in while we released monitor_.
       HandleMessages(false, false);
       if (pause_on_start()) {
         // Still paused.
+        ASSERT(oob_queue_->IsEmpty());
         task_ = NULL;  // No task in queue.
         return;
       } else {
@@ -280,36 +281,50 @@ void MessageHandler::TaskCallback() {
     if (ok) {
       ok = HandleMessages(true, true);
     }
-    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());
+                         "Use the Observatory to release it.\n", name());
           }
-          notify_paused_on_exit = true;
+          // 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.
           paused_on_exit_ = true;
           paused_timestamp_ = OS::GetCurrentTimeMillis();
+          monitor_.Exit();
+          NotifyPauseOnExit();
+          monitor_.Enter();
         }
-      } else {
-        if (FLAG_trace_isolates) {
+        // More messages may have come in while we released monitor_.
+        HandleMessages(false, false);
+        if (pause_on_exit()) {
+          // Still paused.
+          ASSERT(oob_queue_->IsEmpty());
+          task_ = NULL;  // No task in queue.
+          return;
+        } else {
+          paused_on_exit_ = false;
+          paused_timestamp_ = -1;
+        }
+      }
+      if (FLAG_trace_isolates) {
         OS::Print("[-] Stopping message handler (%s):\n"
                   "\thandler:    %s\n",
                   (ok ? "no live ports" : "error"),
                   name());
-        }
-        pool_ = NULL;
-        run_end_callback = true;
-        paused_on_exit_ = false;
-        paused_timestamp_ = -1;
       }
+      pool_ = NULL;
+      run_end_callback = true;
     }
-  }
-  // At this point we no longer hold the message handler lock.
-  if (notify_paused_on_exit) {
-    NotifyPauseOnExit();
+
+    // Clear the task_ last.  We don't want any other tasks to start up
+    // until we are done with all messages and pause notifications.
+    ASSERT(oob_queue_->IsEmpty());
+    ASSERT(!ok || queue_->IsEmpty());
+    task_ = NULL;
   }
   if (run_end_callback && end_callback_ != NULL) {
     end_callback_(callback_data_);