Avoid generating an unused Date header fallback by marcus-campbell · Pull Request #13299 · aio-libs/aiohttp · GitHub
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Avoid generating an unused Date header fallback - #13299

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bdraco merged 6 commits into
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marcus-campbell:perf-avoid-unused-date-fallback
Aug 24, 2026
Merged

Avoid generating an unused Date header fallback#13299
bdraco merged 6 commits into
aio-libs:masterfrom
marcus-campbell:perf-avoid-unused-date-fallback

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@marcus-campbell

@marcus-campbell marcus-campbell commented Aug 1, 2026 •
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What do these changes do?

headers.setdefault(hdrs.DATE, rfc822_formatted_time()) evaluates the formatter even when the response already has a Date header, so the clock read (and possible cache refresh) happens and the result is discarded. This switches to a presence check so the formatter only runs when its value is used, and adds a regression test plus a small _prepare_headers() benchmark with and without an explicit Date.

Locally, a paired A/B run showed ~0.45 µs (~2.4%) per response on the explicit-Date path and no measurable change on the generated-Date path.

Follow-up to #9898: that benchmark only constructed Response objects and never reached _prepare_headers(), which is why CodSpeed showed no change.

Are there changes in behavior for the user?

No. A user-supplied Date is still retained.

Is it a substantial burden for the maintainers to support this?

No.

Related issue number

N/A (see #9898 above)

Checklist

  • I think the code is well written
  • Unit tests for the changes exist
  • Documentation reflects the changes (no user-facing documentation change is
    required for this internal performance cleanup)
  • If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • Add a new news fragment into the CHANGES/ folder

AI assistance

Drafted with help from GPT-5.6 Sol (benchmarking, parts of this description); code by me. I've reviewed all of the code and remain responsible for the submission.

@pyba-chronographer pyba-chronographer Bot added the bot:chronographer:provided There is a change note present in this PR label Aug 1, 2026
@codecov

codecov Bot commented Aug 1, 2026 •
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@codspeed

codspeed Bot commented Aug 1, 2026 •
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Merging this PR will degrade performance by 1%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
❌ 1 (👁 1) regressed benchmark
✅ 90 untouched benchmarks
🆕 2 new benchmarks
⏩ 83 skipped benchmarks1

Performance Changes

Benchmark BASE HEAD Efficiency
⚡ test_list_element_pattern_redos_payload[whitespace_run_after_content] 33.9 ms 30.9 ms +9.91%
🆕 test_prepare_web_response_headers[explicit-date] N/A 7.6 ms N/A
🆕 test_prepare_web_response_headers[generated-date] N/A 8.1 ms N/A
👁 test_read_base64_part 14.3 ms 16 ms -10.83%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

Comparing marcus-campbell:perf-avoid-unused-date-fallback (0993f17) with master (394959f)

Open in CodSpeed

Footnotes

  1. 83 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

@marcus-campbell
marcus-campbell marked this pull request as ready for review August 1, 2026 04:08
@marcus-campbell

marcus-campbell commented Aug 1, 2026

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A note on the CodSpeed report above: "will not alter performance" is expected here, because the two benchmarks that exercise this change (test_prepare_web_response_headers[explicit-date] and [generated-date]) are new in this PR. There's no BASE measurement to compare against, so they show N/A rather than an improvement. The 84 untouched benchmarks don't cover _prepare_headers(), which is the gap that also affected #9898's experiment.

The evidence for the change is the local paired A/B run described in the PR body.

@bdraco bdraco added backport-3.14 Trigger automatic backporting to the 3.14 release branch by Patchback robot backport-3.15 Trigger automatic backporting to the 3.15 release branch by Patchback robot labels Aug 24, 2026
bdraco
bdraco reviewed Aug 24, 2026
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bdraco enabled auto-merge (squash) August 24, 2026 01:19
@greptile-apps

greptile-apps Bot commented Aug 24, 2026 •
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Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Reviews (3): Last reviewed commit: "Merge branch 'master' into perf-avoid-un..." | Re-trigger Greptile

Dreamsorcerer
Dreamsorcerer reviewed Aug 24, 2026
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@bdraco
bdraco merged commit 5fa8584 into aio-libs:master Aug 24, 2026
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@patchback

patchback Bot commented Aug 24, 2026 •
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Backport to 3.15: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.15/5fa85842951ad68b2cba0bbc511dad1eca90cd3a/pr-13299

Backported as #13543

🤖 @patchback
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my source is open — https://github.com/sanitizers/patchback-github-app.

@patchback patchback Bot mentioned this pull request Aug 24, 2026
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@patchback patchback Bot mentioned this pull request Aug 24, 2026
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Dreamsorcerer pushed a commit that referenced this pull request Aug 25, 2026
@patchback @marcus-campbell
[PR #13299/5fa85842 backport][3.15] Avoid generating an unused Date h…
e97ff7e 
…eader fallback (#13543)

**This is a backport of PR #13299 as merged into master
(5fa8584).**

---------

Co-authored-by: Marcus Campbell <21266489+marcus-campbell@users.noreply.github.com>
Dreamsorcerer pushed a commit that referenced this pull request Aug 25, 2026
@patchback @marcus-campbell
[PR #13299/5fa85842 backport][3.14] Avoid generating an unused Date h…
e3640af 
…eader fallback (#13544)

**This is a backport of PR #13299 as merged into master
(5fa8584).**

---------

Co-authored-by: Marcus Campbell <21266489+marcus-campbell@users.noreply.github.com>
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@bdraco bdraco bdraco approved these changes
@Dreamsorcerer Dreamsorcerer Dreamsorcerer left review comments
@webknjaz webknjaz Awaiting requested review from webknjaz webknjaz is a code owner
@asvetlov asvetlov Awaiting requested review from asvetlov asvetlov is a code owner

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backport-3.14 Trigger automatic backporting to the 3.14 release branch by Patchback robot backport-3.15 Trigger automatic backporting to the 3.15 release branch by Patchback robot bot:chronographer:provided There is a change note present in this PR

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