Support for asynchronous Garbage Collection in host-defined $262.gc()

[jcscottiii]

During the integration of Test262 into Web Platform Tests (WPT) (PR), we noted that $262.gc() is expected by Test262 tests to be synchronous. However, in many modern web browsers, garbage collection is inherently asynchronous or non-blocking for performance reasons, and WPT's TestUtils.gc() is often asynchronous.

Test262 tests that rely on $262.gc() usually assume it is synchronous and that garbage collection has occurred before the next line of code executes.

Would Test262 consider allowing $262.gc() to return a Promise or provide some other completion signal for hosts where GC is asynchronous?

Or is there a recommended pattern for handling async GC in Test262 tests?

[ptomato]

I thought there would be too many tests that are written to expect a sync $262.gc(). But I checked and to my surprise there aren't any, except in staging? I guess gc() was only used in FinalizationRegistry tests and when we removed cleanupSome it went away (#4239), and even there it was awaited.

Unless I missed some place where it is used...

I'd suggest we rewrite the staging tests to handle async GC, and amend INTERPRETING.md to be clear about what's expected. If some of the staging tests really can't be written without sync GC, then we can reevaluate what to do.

[ptomato]

@gibson042 and I discussed this today. We think that it's safe to change INTERPRETING.md to say that $262.gc() should be async. It won't break anything to change $262.gc() to await $262.gc() even if the function is sync, so consumers' test262 harnesses can adapt at their own pace.

From a quick scan, most of the uses of $262.gc() can simply be changed to await $262.gc(). The problematic one is in test/staging/sm/generators/iterator.js where we should probably either delete the GC stanza entirely, or add a comment that async GC is probably not testing anything important there; it's arguable whether sync GC was in the first place.

@jcscottiii Are you interested in writing a PR for this?