1355597 - Passing an empty array to MozPromise::All doesn't behave nicely

Status: RESOLVED FIXED

(Core :: XPCOM, enhancement)

Description

[Nika Layzell [:nika] (ni? for response)]

The resulting promise will never resolve if MosPromise::All is passed an empty nsTArray, and in fact seems to assert (confusingly) here: http://searchfox.org/mozilla-central/rev/624d25b2980cf5f83452b040e6d664460f9b7ec5/xpcom/threads/MozPromise.h#384 when run locally. 

It should probably take one of the following behaviors:

a) Resolve with an empty nsTArray as its value upon creation.

b) MOZ_ASSERT(mResolveValues.Length() > 0) in the constructor.

Another option would be to reject, however there was no error, so that seems like a surprising behavior. I was expecting the behavior of this method to follow JS's behavior, which is a). If we decide that is undesirable I think the MOZ_ASSERT is the next best option.

I will attach a patch which implements JS's behavior, as I think this is the best option.

Comment 1

[Nika Layzell [:nika] (ni? for response)]

Attachment: Support passing empty arrays to MozPromise::All

MozReview-Commit-ID: Knhm0N9huuo

Comment 2

[Gerald Squelart (he/him) (not at Mozilla since 2022-09-15)]

Bobby is always busy these. If you don't hear from him, I would recommend :jwwang, who now has the most current experience with hacking&using MozPromise.

I'd personally prefer option 'a' (resolve with an empty array) : It's consistent with Javascript's Promise.all(), and seems logical to me.

Comment 3

[JW Wang [:jwwang] [:jw_wang]]

Comment on attachment 8857167 [details] [diff] [review]
Support passing empty arrays to MozPromise::All

Review of attachment 8857167 [details] [diff] [review]:
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LGTM.

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/89305c2348d8
Support passing empty arrays to MozPromise::All, r=jwwang

Comment 5

[Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)]

https://hg.mozilla.org/mozilla-central/rev/89305c2348d8