🐛 improve early request collection reliability in Firefox

[BenoitZugmeyer]

Motivation

The microfrontend e2e test microfrontend > RUM > with beforeSend > expose handling stack for fetch requests › async fails ~70% of the time on firefox-pinned (10 failed / 6 flaky / 4 passed out of 20 reproductions). The same race affects any fetch resource event in production: event.context.handlingStack (and the rest of the request-derived domain context — requestInit, response, error, …) is intermittently undefined for fetch resources on Firefox.

Root cause

When a fetch resolves, two things happen around the same time:

• the browser dispatches a PerformanceResourceTiming entry to our observer
• afterSend() resumes from await responsePromise and notifies REQUEST_COMPLETED

On Firefox the observer task can fire before the resolve microtask runs, so the request isn't in the requestRegistry yet when assembleResource() looks it up via requestRegistry.getMatchingRequest(). The lookup returns undefined and the resulting resource event is missing all request-derived domain context.

This was introduced by #4285 always collect early requests (commit 2b71062e7), which switched from request-driven (performance.getEntriesByName() at REQUEST_COMPLETED time) to entry-driven matching.

CI logs of the failing case make the ordering obvious:

[failure run]
[DEBUG resourceCollection assembleResource]
  entryStartTime:90, matchedRequest:null, registrySnapshot:[]   ← runs first, registry empty
[DEBUG requestRegistry REQUEST_COMPLETED]
  url:.../ok, startRel:89, hasHandlingStack:true, nowRel:121     ← arrives too late

Changes

• startResourceCollection: for request-type entries (fetch / xmlhttprequest initiator), defer the registry lookup by REQUEST_MATCHING_DELAY (50 ms) via setTimeout. In practice the matching REQUEST_COMPLETED arrives within ~30 ms even on slow CI runners, so 50 ms is a generous bound.
• assembleResource now takes the resolved request directly instead of pulling it from the registry — the matching policy is owned by the call site where the timing context is clear.
• requestRegistry is unchanged; getMatchingRequest is still synchronous.
• Unit tests drive the new delay through mockClock.tick(REQUEST_MATCHING_DELAY) inside runTasks.

Test instructions

Repro on the e2e suite (Firefox is the most reliable, but the race is browser-agnostic):

yarn playwright test --config test/e2e/playwright.config.ts --project firefox-pinned \
  --repeat-each=20 \
  -g 'microfrontend RUM with beforeSend expose handling stack for fetch requests async'

CI validation (e2e job scoped to the failing test, --repeat-each=20):

pipeline	chromium	firefox-pinned	webkit-pinned
before fix	20/20	4/20 (10 failed, 6 flaky)	20/20
after fix	20/20	20/20	20/20

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	169.22 KiB	169.26 KiB	+39 B	+0.02%	✅
Rum Profiler	6.10 KiB	6.10 KiB	0 B	0.00%	✅
Rum Recorder	21.23 KiB	21.23 KiB	0 B	0.00%	✅
Logs	54.56 KiB	54.56 KiB	0 B	0.00%	✅
Rum Slim	127.56 KiB	127.59 KiB	+31 B	+0.02%	✅
Worker	22.99 KiB	22.99 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base CPU Time (ms)	Local CPU Time (ms)	𝚫%
RUM - add global context	0.0039	0.002	-48.72%
RUM - add action	0.0211	0.0125	-40.76%
RUM - add error	0.02	0.0118	-41.00%
RUM - add timing	0.001	0.0005	-50.00%
RUM - start view	0.0194	0.0123	-36.60%
RUM - start/stop session replay recording	0.0014	0.001	-28.57%
Logs - log message	0.0342	0.0174	-49.12%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	43.46 KiB	38.90 KiB	-4.56 KiB
RUM - add action	67.59 KiB	63.35 KiB	-4.24 KiB
RUM - add timing	42.64 KiB	36.74 KiB	-5.90 KiB
RUM - add error	75.08 KiB	69.76 KiB	-5.32 KiB
RUM - start/stop session replay recording	49.95 KiB	42.04 KiB	-7.91 KiB
RUM - start view	497.36 KiB	488.52 KiB	-8.84 KiB
Logs - log message	59.14 KiB	54.38 KiB	-4.75 KiB

🔗 RealWorld

[datadog-official]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 83.33%
• Overall Coverage: 77.50% (+0.02%)

_{This comment will be updated automatically if new data arrives.
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