1797947 - Opening DevTools triggers PerformanceObserver (with entryTypes: ["resource"]) callback

Status: VERIFIED FIXED

(DevTools :: Inspector, defect)

Description

[Benoît Zugmeyer]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:106.0) Gecko/20100101 Firefox/106.0

Steps to reproduce:

1. Navigate to https://firefox-duplicated-performancetiming-for-css.benoitzugmeyer.repl.co/ or any page with a few stylesheets included via <link> tags

2. Open the devtools Inspector panel (ex: right click > Inspect element)

3. Many PerformanceTiming entries are reported to PerformanceObserver callbacks with their .name property referencing stylesheets URLs multiple times. In the demo page above, many ENTRY lines are showing up for the two stylesheets, even if the stylesheets are downloaded only once.

I tried a few anterior Firefox versions and the issue occurs way more since Firefox 89. Before that, PerformanceTiming entries were still repeated but just a few times.

Actual results:

Many PerformanceTiming are reported for stylesheets even if they are requested only once

Expected results:

Only one PerformanceTiming is reported for each requested stylesheet.

Comment 1

[Benoît Zugmeyer]

Attachment: Screenshot 2022-10-28 at 11.56.05.png

Comment 2

[Benoît Zugmeyer]

Comment on attachment 9300736 [details]
Screenshot 2022-10-28 at 11.56.05.png

Screenshot showing the issue on https://firefox-duplicated-performancetiming-for-css.benoitzugmeyer.repl.co/ on Firefox 106. As we can see, PerformanceTiming entries are repeated many times.

Comment 3

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

The Bugbug bot thinks this bug should belong to the 'DevTools::Inspector' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Comment 4

[Nicolas Chevobbe [:nchevobbe]]

Thanks for the report Benoît.
I think we got a similar bug recently as DevTools are fetching resources from the network, but I can't find it at the moment.

Comment 5

[Julian Descottes [:jdescottes]]

This seem related to this fetch call: https://searchfox.org/mozilla-central/rev/73ff5f1ac45202c4621a1bcee4b35d20027e71ad/devtools/server/actors/utils/stylesheets-manager.js#457, which is done on behalf of the page (same window/principal)

Comment 6

[Julian Descottes [:jdescottes]]

Try to use cacheKey as in the debugger: https://searchfox.org/mozilla-central/rev/d1fe4b6dd7027a54c17ce3030948a1be354598ab/devtools/server/actors/utils/sources-manager.js#440

Comment 7

[Julian Descottes [:jdescottes]]

Using cacheKey doesn't seem to help here.

As mentioned in comment #5 we use both the window and principal options for this fetch call. The PerformanceObserver seems triggered as long as we set the window option. But using the same window as the page is still required for Bug 1405342.

So on one hand we want to request the stylesheet as if it was requested from the actual page, but we still would like to have a way to hide it from the PerformanceObserver. Markus, do you know if there is anything we could do to hide the fetch call at https://searchfox.org/mozilla-central/rev/73ff5f1ac45202c4621a1bcee4b35d20027e71ad/devtools/server/actors/utils/stylesheets-manager.js#457 from the PerformanceObserver

Comment 8

[Markus Stange [:mstange]]

I don't know the PerformanceObserver code all that well, forwarding to Sean.

Comment 9

[Sean Feng [:sefeng]]

I don't think we could do this from the javascript side, however I think we can utilize IsInDevToolsContext to filter these fetch requests out. Though I am not sure if it'll break existing PerformanceObserver usage in the devtools (if we have some), probably need to check IsInDevToolsContext + content principal I think.

I'll write a write a patch for this next week.

Comment 10

[Sean Feng [:sefeng]]

Attachment: Bug 1797947 - Make devtools generated fetch requests excluded from resource timing r=jdescottes,valentin

Devtools can generate fetch request on behalf of the content document,
reporting these requests in PerformanceObserver would cause confusion,
so we should ignore them.

Comment 11

[Sean Feng [:sefeng]]

I wrote the patch, so removing my NI.

Comment 12

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/b3760215fcc0
Make devtools generated fetch requests excluded from resource timing r=jdescottes,valentin,necko-reviewers

Comment 13

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/b3760215fcc0

Comment 14

[Ardelean Oana, Desktop QA [:oardelean]]

Reproducible on a 2022-10-28 Nightly build on macOS 12.
Verified as fixed on Firefox 109.0b6(build ID: 20221222190305) and Nightly 110.0a1(build ID: 20221226154927) on macOS 12, Windows 10, Ubuntu 22.