Async/await Promise dependency graph

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 15 matching lines @@
   AsyncFunctionThrow = from.AsyncFunctionThrow;
   IsPromise = from.IsPromise;
   GlobalPromise = from.GlobalPromise;
   NewPromiseCapability = from.NewPromiseCapability;
   PromiseCreate = from.PromiseCreate;
   PerformPromiseThen = from.PerformPromiseThen;
   RejectPromise = from.RejectPromise;
   ResolvePromise = from.ResolvePromise;
 });
 
-var promiseAwaitHandlerSymbol = utils.ImportNow("promise_await_handler_symbol");
+var promiseHandledBySymbol =
+    utils.ImportNow("promise_handled_by_symbol");
+var promiseForwardingHandlerSymbol =
+    utils.ImportNow("promise_forwarding_handler_symbol");
 var promiseHandledHintSymbol =
     utils.ImportNow("promise_handled_hint_symbol");
 var promiseHasHandlerSymbol =
     utils.ImportNow("promise_has_handler_symbol");
 
 // -------------------------------------------------------------------
 
 function PromiseCastResolved(value) {
   if (IsPromise(value)) {
     return value;
   } else {
     var promise = PromiseCreate();
     ResolvePromise(promise, value);
     return promise;
   }
 }
 
 // ES#abstract-ops-async-function-await
 // AsyncFunctionAwait ( value )
 // Shared logic for the core of await. The parser desugars
 //   await awaited
 // into
-//   yield AsyncFunctionAwait{Caught,Uncaught}(.generator, awaited)
+//   yield AsyncFunctionAwait{Caught,Uncaught}(.generator, awaited, .promise)
 // The 'awaited' parameter is the value; the generator stands in
-// for the asyncContext, and mark is metadata for debugging
-function AsyncFunctionAwait(generator, awaited, mark) {
+// for the asyncContext, and .promise is the larger promise under
+// construction by the enclosing async function.
+function AsyncFunctionAwait(generator, awaited, outerPromise) {
   // Promise.resolve(awaited).then(
   //     value => AsyncFunctionNext(value),
   //     error => AsyncFunctionThrow(error)
   // );
   var promise = PromiseCastResolved(awaited);
 
   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);
 
   // The Promise will be thrown away and not handled, but it shouldn't trigger
   // unhandled reject events as its work is done
   SET_PRIVATE(throwawayCapability.promise, promiseHasHandlerSymbol, true);
 
-  return PerformPromiseThen(promise, onFulfilled, onRejected,
-                            throwawayCapability);
+  PerformPromiseThen(promise, onFulfilled, onRejected, throwawayCapability);
+
+  if (DEBUG_IS_ACTIVE && !IS_UNDEFINED(outerPromise)) {
+    if (IsPromise(awaited)) {
+      // Mark the reject handler callback to be a forwarding edge, rather
+      // than a meaningful catch handler
+      SET_PRIVATE(onRejected, promiseForwardingHandlerSymbol, true);
+    }
+
+    // Mark the dependency to outerPromise in case the throwaway Promise is
+    // found on the Promise stack
+    SET_PRIVATE(throwawayCapability.promise, promiseHandledBySymbol,
+                outerPromise);
+  }
 }
 
 // Called by the parser from the desugaring of 'await' when catch
 // prediction indicates no locally surrounding catch block
-function AsyncFunctionAwaitUncaught(generator, awaited) {
-  // TODO(littledan): Install a dependency edge from awaited to outerPromise
-  return AsyncFunctionAwait(generator, awaited, true);
+function AsyncFunctionAwaitUncaught(generator, awaited, outerPromise) {
+  AsyncFunctionAwait(generator, awaited, outerPromise);
 }
 
 // Called by the parser from the desugaring of 'await' when catch
 // prediction indicates that there is a locally surrounding catch block
-function AsyncFunctionAwaitCaught(generator, awaited) {
+function AsyncFunctionAwaitCaught(generator, awaited, outerPromise) {
   if (DEBUG_IS_ACTIVE && IsPromise(awaited)) {
     SET_PRIVATE(awaited, promiseHandledHintSymbol, true);
   }
-  return AsyncFunctionAwait(generator, awaited, false);
+  // Pass undefined for the outer Promise to not waste time setting up
+  // or following the dependency chain when this Promise is already marked
+  // as handled
+  AsyncFunctionAwait(generator, awaited, UNDEFINED);
 }
 
 // How the parser rejects promises from async/await desugaring
 function RejectPromiseNoDebugEvent(promise, reason) {
   return RejectPromise(promise, reason, false);
 }
 
 %InstallToContext([
   "async_function_await_caught", AsyncFunctionAwaitCaught,
   "async_function_await_uncaught", AsyncFunctionAwaitUncaught,
   "reject_promise_no_debug_event", RejectPromiseNoDebugEvent,
 ]);
 
 })