1706265 - Order of ports in MessageEvent.ports is random

Status: RESOLVED FIXED

(Core :: DOM: postMessage, defect, P3)

Description

[guanzo91]

Attachment: ff_bug.png

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.128 Safari/537.36

Steps to reproduce:

Reproduction links
Parent window: https://codesandbox.io/s/wonderful-dew-ktw1v?file=/index.html
Iframe window: https://codesandbox.io/s/hardcore-elbakyan-ou60y?file=/index.html

On the parent window link, click the refresh icon until you see "ERROR: ports swapped".

TL;DR

Parent window:
$iframe.contentWindow.postMessage(null, null, [port1, port2])

Iframe window:
window.addEventListener('message', async event => {
const [port1, port2] = event.ports // Sometimes it's this order
const [port2, port1] = event.ports // Sometimes it's this order
})

1. Create two MessageChannels. postMessage each channels "port2" to an iframe.
2. Setup message listeners on each channels "port1"
3. Inside the iframe, retrieve ports from event.ports, and for each port, postMessage back to parent window.

Actual results:

Messages are sometimes received from the wrong channel. This is because sometimes, the order of ports passed to postMessage is not the same order when iframe retrieves ports from event.ports.

Expected results:

The order of ports in event.ports should stay the same after being postMessaged.

Comment 1

[guanzo91]

Attachment: 2021-04-20 01-11-53.flv

Comment 2

[BugBot [:suhaib / :marco/ :calixte]]

The Bugbug bot thinks this bug should belong to the 'Core::DOM: Core & HTML' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

Comment 4

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

hmm, is this coming from the JS engine, https://searchfox.org/mozilla-central/rev/489e82dcc1e5afbe691ff3b1c982382914637e38/js/src/vm/StructuredClone.cpp#593-595

Comment 5

[Steve Fink [:sfink] [:s:]]

(In reply to Olli Pettay [:smaug] from comment #4)

hmm, is this coming from the JS engine, https://searchfox.org/mozilla-central/rev/489e82dcc1e5afbe691ff3b1c982382914637e38/js/src/vm/StructuredClone.cpp#593-595

Yes, definitely.

Comment 6

[Steve Fink [:sfink] [:s:]]

Attachment: Bug 1706265 - retain ordering of transferable objects

Comment 7

[Steve Fink [:sfink] [:s:]]

Hm, I see why I didn't have a test for this. Within the JS engine, the order of Transferables is not visible. The only Transferable type is ArrayBuffer.

MessagePorts do a weird thing during transferring where they get appended to an mTransferredPorts array and exposed to the caller.

Comment 8

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/af672c9161d7
retain ordering of transferable objects r=jonco,smaug

Comment 9

[Noemi Erli[:noemi_erli]]

https://hg.mozilla.org/mozilla-central/rev/af672c9161d7