Async/await Promise dependency graph

src/js/harmony-async-await.js

Index: src/js/harmony-async-await.js
diff --git a/src/js/harmony-async-await.js b/src/js/harmony-async-await.js
index 4109b77fb54174366cdae648b0dae7fc0c5b02af..6bf4410e3daa8bd305b36894daa419c5e7258b40 100644
--- a/src/js/harmony-async-await.js
+++ b/src/js/harmony-async-await.js
@@ -54,10 +54,11 @@ function PromiseCastResolved(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)
@@ -69,32 +70,44 @@ function AsyncFunctionAwait(generator, awaited, mark) {
   var onRejected =
       (sentError) => %_Call(AsyncFunctionThrow, generator, sentError);
 
-  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);
+  var debug_is_active = DEBUG_IS_ACTIVE;
+
+  if (!IS_UNDEFINED(outerPromise) && debug_is_active && IsPromise(awaited)) {
+    // Mark the reject handler callback to continue recursing to outerPromise
+    SET_PRIVATE(onRejected, promiseAwaitHandlerSymbol, outerPromise);
   }
 
   // Just forwarding the exception, so no debugEvent for throwawayCapability
   var throwawayCapability = NewPromiseCapability(GlobalPromise, false);
-  return PerformPromiseThen(promise, onFulfilled, onRejected,
+  PerformPromiseThen(promise, onFulfilled, onRejected,
                             throwawayCapability);

[jgruber, 2016/09/12 13:22:00]

Nit: needs formatting.

[Dan Ehrenberg, 2016/09/12 22:49:25]

Moved to one line

+
+  if (debug_is_active) {

[jgruber, 2016/09/12 13:22:00]

Should this also check for !IS_UNDEFINED(outerPromise)? Maybe this block could
be merged with the other promiseAwaitHandlerSymbol block above if it's moved
until after NewPromiseCapability and before PerformPromiseThen.

[Dan Ehrenberg, 2016/09/12 22:51:23]

Added the IS_UNDEFINED check and merged the blocks. Seems valid as the
dependency graph is not needed synchronously.

+    // Mark the dependency to outerPromise in case the throwaway Promise is
+    // found on the Promise stack
+    SET_PRIVATE(throwawayCapability.promise, promiseAwaitHandlerSymbol,
+                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);
+  return 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);
+  return outerPromise;
 }
 
 // How the parser rejects promises from async/await desugaring