1833402 - Returning promise from C++ automatically blocks microtasks on Gecko but not on Blink?

Status: RESOLVED FIXED

(Core :: DOM: Core & HTML, defect)

Description

[Kagami Rosylight [:saschanaz] (they/them)]

Attachment: repro.html

During investigation in bug 1819086 I found that somehow returning Promises from C++ can automatically trigger nsAutoMicroTask via PromiseBinding (then again via CallbackObject::CallSetup). Is this a Gecko quirk or is it in any spec?

Comment 1

[Kagami Rosylight [:saschanaz] (they/them)]

Both Blink and WebKit does run microtask in the above code (logs "shouldn't have run microtasks yet" ... because the code is copied from bug 1612308), while Gecko logs "Did not run microtask".

Hi Peter, is this something you can answer? Thanks!

Comment 2

[Kagami Rosylight [:saschanaz] (they/them)]

(For now I marked it as defect as it's a web-visible difference, but not sure it really is a defect)

Comment 3

[Olli Pettay [:smaug][[e-mail]]]

What does "blocks" mean in this context?

Comment 4

[Kagami Rosylight [:saschanaz] (they/them)]

"Prevents microtask runs (but let it run later, of course)" it means.

Comment 5

[Olli Pettay [:smaug][[e-mail]]]

The testcase talks about fetch., but I don't see any fetch usage.
Anyhow, JS shouldn't be able to trigger microtask checkpoint in middle of JS execution.
Microtasks run at the end of the outermost JS execution or at the end of the innermost task (if there is no JS running)

Comment 6

[Kagami Rosylight [:saschanaz] (they/them)]

That's my fault and those are just copied from the testcase in bug 1612308. 😬

Anyhow, JS shouldn't be able to trigger microtask checkpoint in middle of JS execution.

Practically yes but I don't think that's what others do here, what spec actually forces it?

Comment 7

[Olli Pettay [:smaug][[e-mail]]]

https://html.spec.whatwg.org/#clean-up-after-running-script

Comment 8

[Kagami Rosylight [:saschanaz] (they/them)]

That's supposed to be called when running script, but in the repro code permissions.query() does not call any script, right?

Comment 9

[Olli Pettay [:smaug][[e-mail]]]

It does call a script, the then callback, no? So there is script on the stack and microtask checkpoint shouldn't happen.

Comment 10

[Kagami Rosylight [:saschanaz] (they/them)]

(In reply to Olli Pettay [:smaug][[e-mail]] from comment #9)

It does call a script, the then callback, no?

Not sure I understand, yes it does call a script but that's because of .then quirk of promise resolution, isn't it? Is there anything left for the IDL to do at that point?

Comment 11

[Kagami Rosylight [:saschanaz] (they/them)]

The behavior changed here to match Chrome on 2023-07-20 but not sure what fixed it. The range: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=898b3ffbdca34afc5fd0304f07f769e3d243265e&tochange=0bc898fe076442e9a552e9918f1b50ece1c87859