Improve stack traces for async functions

test/mjsunit/harmony/debug-async-function-async-task-event.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --harmony-async-await --expose-debug-as debug --allow-natives-syntax
 
 // The test observes the callbacks that async/await makes to the inspector
-// to make accurate stack traces. The limited number of events is an
-// indirect indication that we are not doing extra Promise processing that
-// could be associated with memory leaks (v8:5380).
+// to make accurate stack traces. The pattern is based on saving a stack once
+// with enqueueRecurring and restoring it multiple times.
+
+// Additionally, the limited number of events is an indirect indication that
+// we are not doing extra Promise processing that could be associated with memory
+// leaks (v8:5380). In particular, no stacks are saved and restored for extra
+// Promise handling on throwaway Promises.
+
 // TODO(littledan): Write a test that demonstrates that the memory leak in
 // the exception case is fixed.
 
 Debug = debug.Debug;
 
 var base_id = -1;
 var exception = null;
 var expected = [
-  'enqueue #1',
+  'enqueueRecurring #1',
   'willHandle #1',
   'then #1',
-  'enqueue #2',
   'didHandle #1',
-  'willHandle #2',
+  'willHandle #1',
   'then #2',
-  'didHandle #2',
+  'cancel #1',
+  'didHandle #1',
 ];
 
 function assertLog(msg) {
   print(msg);
   assertTrue(expected.length > 0);
   assertEquals(expected.shift(), msg);
   if (!expected.length) {
     Debug.setListener(null);
   }
 }
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.AsyncTaskEvent) return;
   try {
     if (base_id < 0)
       base_id = event_data.id();
     var id = event_data.id() - base_id + 1;
-    assertTrue("Promise.resolve" == event_data.name() ||
-               "PromiseResolveThenableJob" == event_data.name());
+    assertTrue("async function" == event_data.name());
     assertLog(event_data.type() + " #" + id);
   } catch (e) {
     print(e + e.stack)
     exception = e;
   }
 }
 
 Debug.setListener(listener);
 
 var resolver;
 var p = new Promise(function(resolve, reject) {
   resolver = resolve;
 });
 
 async function main() {
   await p;
   assertLog("then #1");
   await undefined;
   assertLog("then #2");
 }
 main();
 resolver();
 
 %RunMicrotasks();
 
 assertNull(exception);