Reland of Fix async/await memory leak

src/js/harmony-async-await.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.
 
 (function(global, utils, extrasUtils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
@@ skipping 48 matching lines @@
 //   yield AsyncFunctionAwait{Caught,Uncaught}(.generator, awaited)
 // The 'awaited' parameter is the value; the generator stands in
 // for the asyncContext, and mark is metadata for debugging
 function AsyncFunctionAwait(generator, awaited, mark) {
   // Promise.resolve(awaited).then(
   //     value => AsyncFunctionNext(value),
   //     error => AsyncFunctionThrow(error)
   // );
   var promise = PromiseCastResolved(awaited);
 
-  var onFulfilled =
-      (sentValue) => %_Call(AsyncFunctionNext, generator, sentValue);
-  var onRejected =
-      (sentError) => %_Call(AsyncFunctionThrow, generator, sentError);
+  var onFulfilled = sentValue => {
+    %_Call(AsyncFunctionNext, generator, sentValue);
+    // The resulting Promise is a throwaway, so it doesn't matter what it
+    // resolves to. What is important is that we don't end up keeping the
+    // whole chain of intermediate Promises alive by returning the value
+    // of AsyncFunctionNext, as that would create a memory leak.
+    return;
+  };
+  var onRejected = sentError => {
+    %_Call(AsyncFunctionThrow, generator, sentError);
+    // Similarly, returning the huge Promise here would cause a long
+    // resolution chain to find what the exception to throw is, and
+    // create a similar memory leak, and it does not matter what
+    // sort of rejection this intermediate Promise becomes.
+    return;
+  }
 
   if (mark && DEBUG_IS_ACTIVE && IsPromise(awaited)) {
     // Mark the reject handler callback such that it does not influence
     // catch prediction.
     SET_PRIVATE(onRejected, promiseAwaitHandlerSymbol, true);
   }
 
   // Just forwarding the exception, so no debugEvent for throwawayCapability
   var throwawayCapability = NewPromiseCapability(GlobalPromise, false);
 
@@ skipping 26 matching lines @@
   return RejectPromise(promise, reason, false);
 }
 
 %InstallToContext([
   "async_function_await_caught", AsyncFunctionAwaitCaught,
   "async_function_await_uncaught", AsyncFunctionAwaitUncaught,
   "reject_promise_no_debug_event", RejectPromiseNoDebugEvent,
 ]);
 
 })