m.[[CycleRoot]] can be null in GatherAvailableAncestors

[allstarschh]

// main.js
import {} from from "./tla.js";
import {} from from "./throw.js';

// tla.js
await 1;

// throw.js
throw 1;

Evaluate(main.js)

1. InnerModuleEvaluation(main.js)
   1. InnerModuleEvaluation(tla.js)
      1. module(tla.js).[[AsyncEvaluationOrder]] += 1
      2. add module(main.js) to module(tla.js).[[AsyncParentModules]]
   2. InnerModuleEvalation(throw.js)
      1. module(throw.js).ExecuteModule() // throws
      2. returns Abrupt completion
2. Exit InnerModuleEvaluation(main.js) with abrupt completion

as of now, the module(main.js).[[CycleRoot]] is still null as InnerModuleEvaluation(main.js) exits abnormally.

Then

1. AsyncModuleExecutionFulfilled(tla.js)
2. GatherAvailableAncestors(tla.js)
3. for each m in module.[[AsyncParentModules]] // main.js
4. access m.[[CycleRoot]] --> still null

GatherAvailableAncestors

Step 1.a. If execList does not contain m and m.[[CycleRoot]].[[EvaluationError]] is empty

Both Spidermonkey and V8 have fixed this in their implementations
SM fix

V8 fix

CCing @nicolo-ribaudo

[nicolo-ribaudo]

This sentence in the description of [[CycleRoot]] is wrong:

Once Evaluate has completed, a module's [[DFSAncestorIndex]] is the depth-first traversal index of its [[CycleRoot]].

Because if Evalute() completes with an error, [[CycleRoot]] is empty. Node that if a module's [[Status]] is ~evaluated~ and it has an error it could still have [[CycleRoot]] set, if the module rejected asynchronously.

I'm going through all the reads of [[CycleRoot]] to see if we are missing anything else.

---

1. Evaluate() step 3.a is already guarded for EMPTY
2. InnerModuleEvaluation() step 11.iv.1 is good. requiredModule has already gone through InnerModuleEvaluation and we know that it's not EVALUATING (thanks to 11.iii), which means it's either EVALUATED or EVALUATED-ASYNC. It's not in an error state, otherwise step 11.b would have returned early, so its [[CycleRoot]] is not empty.
   • this benefits from an assertion
3. InnerModuleEvaluation() step 16.b.viii (where we set [[CycleRoot]]) always sets it overriding an EMPTY and not an already set value. This is because the elements in stack are all in their EVALUATING state, so [[CycleRoot]] has not being set yet (because it happens after updating their state to EVALUATED/EVALUATING-ASYNC)
   • this also benefits from an assertion
4. GatherAvailableAncestors() step 1.a is the bug described in this issue.
5. AsyncModuleExecutionFulfilled() step 7.a is ok. If a module's status is EVALUATING-ASYNC it's because its cycle did not synchronously throw (otherwise it would be already EVALUATED), so [[CycleRoot]] has been set.
6. AsyncModuleExecutionFulfilled() step 12.c.ii.3.a is ok. If the module (or one of its deps) had errored, it would be EVALUATED and thus we would have gone in step 12.a instead of 12.c.
7. AsyncModuleExecutionRejected() step 9.a ok, same as AsyncModuleExecutionFulfilled() step 7.a.