Normative: guard against reentrancy in AsyncGenerator.prototype.return

Author: bakkot

spec.html

@@ -48461,7 +48461,7 @@ <h1>
               1. Let _result_ be Completion(_generatorBody_()).
             1. Assert: If we return here, the async generator either threw an exception or performed either an implicit or explicit return.
             1. Remove _acGenContext_ from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
-            1. Set _acGenerator_.[[AsyncGeneratorState]] to ~completed~.
+            1. Set _acGenerator_.[[AsyncGeneratorState]] to ~awaiting-return~.
             1. If _result_ is a normal completion, set _result_ to NormalCompletion(*undefined*).
             1. If _result_ is a return completion, set _result_ to NormalCompletion(_result_.[[Value]]).
             1. Perform AsyncGeneratorCompleteStep(_acGenerator_, _result_, *true*).
@@ -48626,28 +48626,28 @@ <h1>
         <dl class="header">
         </dl>
         <emu-alg>
+          1. Assert: _generator_.[[AsyncGeneratorState]] is ~awaiting-return~.
           1. Let _queue_ be _generator_.[[AsyncGeneratorQueue]].
           1. Assert: _queue_ is not empty.
           1. Let _next_ be the first element of _queue_.
           1. Let _completion_ be Completion(_next_.[[Completion]]).
           1. Assert: _completion_ is a return completion.
           1. Let _promiseCompletion_ be Completion(PromiseResolve(%Promise%, _completion_.[[Value]])).
           1. If _promiseCompletion_ is an abrupt completion, then
-            1. Set _generator_.[[AsyncGeneratorState]] to ~completed~.
             1. Perform AsyncGeneratorCompleteStep(_generator_, _promiseCompletion_, *true*).
             1. Perform AsyncGeneratorDrainQueue(_generator_).
             1. Return ~unused~.
           1. Assert: _promiseCompletion_ is a normal completion.
           1. Let _promise_ be _promiseCompletion_.[[Value]].
           1. Let _fulfilledClosure_ be a new Abstract Closure with parameters (_value_) that captures _generator_ and performs the following steps when called:
-            1. Set _generator_.[[AsyncGeneratorState]] to ~completed~.
+            1. Assert: _generator_.[[AsyncGeneratorState]] is ~awaiting-return~.
             1. Let _result_ be NormalCompletion(_value_).
             1. Perform AsyncGeneratorCompleteStep(_generator_, _result_, *true*).
             1. Perform AsyncGeneratorDrainQueue(_generator_).
             1. Return *undefined*.
           1. Let _onFulfilled_ be CreateBuiltinFunction(_fulfilledClosure_, 1, *""*, « »).
           1. Let _rejectedClosure_ be a new Abstract Closure with parameters (_reason_) that captures _generator_ and performs the following steps when called:
-            1. Set _generator_.[[AsyncGeneratorState]] to ~completed~.
+            1. Assert: _generator_.[[AsyncGeneratorState]] is ~awaiting-return~.
             1. Let _result_ be ThrowCompletion(_reason_).
             1. Perform AsyncGeneratorCompleteStep(_generator_, _result_, *true*).
             1. Perform AsyncGeneratorDrainQueue(_generator_).
@@ -48669,22 +48669,25 @@ <h1>
           <dd>It drains the generator's AsyncGeneratorQueue until it encounters an AsyncGeneratorRequest which holds a return completion.</dd>
         </dl>
         <emu-alg>
-          1. Assert: _generator_.[[AsyncGeneratorState]] is ~completed~.
+          1. Assert: _generator_.[[AsyncGeneratorState]] is ~awaiting-return~.
           1. Let _queue_ be _generator_.[[AsyncGeneratorQueue]].
-          1. If _queue_ is empty, return ~unused~.
+          1. If _queue_ is empty, then
+            1. Set _generator_.[[AsyncGeneratorState]] to ~completed~.
+            1. Return ~unused~.
           1. Let _done_ be *false*.
           1. Repeat, while _done_ is *false*,
             1. Let _next_ be the first element of _queue_.
             1. Let _completion_ be Completion(_next_.[[Completion]]).
             1. If _completion_ is a return completion, then
-              1. Set _generator_.[[AsyncGeneratorState]] to ~awaiting-return~.
               1. Perform AsyncGeneratorAwaitReturn(_generator_).
               1. Set _done_ to *true*.
             1. Else,
               1. If _completion_ is a normal completion, then
                 1. Set _completion_ to NormalCompletion(*undefined*).
               1. Perform AsyncGeneratorCompleteStep(_generator_, _completion_, *true*).
-              1. If _queue_ is empty, set _done_ to *true*.
+              1. If _queue_ is empty, then
+                1. Set _generator_.[[AsyncGeneratorState]] to ~completed~.
+                1. Set _done_ to *true*.
           1. Return ~unused~.
         </emu-alg>
       </emu-clause>