🏗️🚀：read a message without segmenting every line by DerekNonGeneric · Pull Request #913 · OpenINF/.github

DerekNonGeneric · 2026-09-08T23:19:33Z

validateCommitMessage cost 25–30× more than it needed to, and allocated
enough that a large message could get the process killed. The commit queue
runs it over a message assembled from whatever a pull request's commits say,
while holding a token that can merge, so that is the exposure #906 is about.

One correction to the report: measured as one call to a process, which is how
the queue does it, the reading is linear at every size, not super-linear.
The blow-up in the issue's table is heap pressure from repeated in-process
calls, and the kill at 800,000 lines is memory, not exponent. The fix is the
same either way — stop doing avoidable work per line — but it changes what a
test has to look for.

What was paying for it

Every line segmented into graphemes to measure its width. No grapheme is
fewer than one code unit, so a line of ≤72 code units is within the limit
whatever it holds. This was the bulk of it.
Seven regexes compiled per line of the last paragraph, which a message
can make the whole message. Built once now.
The message rebuilt and re-split to find the trailer block, with the
paragraphs already in hand.
paragraphsOf joined and re-split the lines twice over. It walks them.

Reading 200,000 lines now costs 2–6× looking at every line once, against
33–124× before. 800,000 lines fit in 183 MB rather than 674 MB.

Why the tests are rebuilt

The two they replace asserted a wall-clock budget at a single size. That
cannot separate linear from quadratic, and it went red twice on a loaded
runner over code that was fine.

Each new one is a ratio between two measurements taken moments apart on the
same machine, one of them work known to be linear — so a slow or busy
machine raises both and leaves the ratio alone. The floor is written out in
the test rather than borrowed from the module, since sharing linesOf would
let a change that made splitting quadratic raise the floor by as much as the
measurement and hide in it.

They were checked against the code they must reject:

implementation	fails
the reading this replaces	3 of 4, at 39–163× the floor
trailing newlines stripped by a regex	2 of 4, at 1,230–1,973×
`Assisted-by` pattern with its `^` dropped	1 of 4, at 3,063× its control

The fixed code sits at 2–6× and 1.05×. Ten runs on a machine carrying four
times as many spinning processes as cores produced no failures.

The same file is byte-identical in openinf.github.io; the matching change
goes there for #1883.

Fixes: #906

🤖 Generated with Claude Code

Summary by CodeRabbit

Performance
- Improved commit-message validation efficiency for long messages and body lines.
- Reduced unnecessary processing during trailer and line-length checks.
Tests
- Replaced fixed-duration performance limits with machine-independent ratio-based benchmarks.
- Added coverage for messages containing many lines, blank lines, trailing newlines, and long trailer values.

coderabbitai · 2026-09-08T23:19:50Z

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📝 Walkthrough

Walkthrough

validateCommitMessage now reuses parsed paragraphs and trailer blocks, avoids unnecessary grapheme segmentation, and uses ratio-based performance tests against a linear baseline.

Changes

Commit message validation

Layer / File(s)	Summary
Linear paragraph and trailer parsing `build/shared/commit-message.mts`	`paragraphsOf` scans lines directly. `trailerBlockOf` centralizes trailer extraction. `readTrailers` and `checkTrailers` reuse parsed paragraphs and precomputed trailer-token patterns.
Conditional body line segmentation `build/shared/commit-message.mts`	Body lines are segmented into graphemes only when their code-unit length exceeds `BODY_MAX`.
Ratio-based performance validation `build/shared/commit-message.test.mts`	Performance tests compare validation with a linear line-reading baseline for large bodies, blank lines, trailing newlines, and failing `Assisted-by` values.

Priority: ⬆️ High

Estimated code review effort: 3 (Moderate) | ~20 minutes

Severity of issue fixed: High

Merge Risk: 🟡 Moderate · up to 435a2

Commit-message validation now avoids repeated parsing and segmentation work, but its performance checks may still miss super-linear growth for increasingly large pull-request-controlled messages. This could allow validation latency to grow unexpectedly in the commit-verification path.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the primary optimization: reading messages without segmenting every line. It is concise and related to the changes.
Linked Issues check	✅ Passed	The implementation addresses issue `#906` by avoiding unnecessary grapheme segmentation, repeated message reconstruction, and repeated paragraph joining and splitting. The tests replace fixed wall-clock…
Out of Scope Changes check	✅ Passed	The production and test changes are directly related to improving validateCommitMessage performance and verifying linear scaling. No unrelated changes are identified.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.

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Commit unit tests in branch infra/linear-commit-message

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Actionable comments posted: 3

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Inline comments:
In `@build/shared/commit-message.test.mts`:
- Around line 440-443: Update the benchmark loop around baseline and measured
timing so each round alternates execution order, running measured work first on
alternating rounds and baseline first on the others. Keep the existing spentOn,
Math.min, and ROUNDS aggregation behavior unchanged.
- Line 389: Rename the constants RUNS, ROUNDS, LINES, and CEILING to camelCase
names that satisfy the Biome naming rule, and update every reference to each
constant throughout the affected tests.
- Around line 476-480: Update the performance test around costOfReading to
measure both small and large messages, rather than relying on a single
fixed-size input. Compare validator scaling relative to the readEveryLine
baseline across those sizes so an O(n log n) regression cannot pass solely
because it remains under the fixed 20x ceiling.

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The port of OpenINF/.github#913, where the reasoning is written out. This repository's commit queue is the one in daily use, so the exposure here is real rather than theoretical: the queue reads a message assembled from whatever a pull request's commits say, while holding a token that can merge. Reading cost 25 to 30 times what it needed to, and allocated enough that a large message could get the process killed -- 800,000 lines took 674 MB. Four costs, all of them per line: every line segmented into graphemes to measure its width, seven regexes compiled for each line of the last paragraph, the message rebuilt and re-split to find the trailers with the paragraphs already in hand, and a paragraph split that joined the lines and split the result twice more. Reading 200,000 lines now costs 2 to 6 times looking at every line once, against 33 to 124 before, and 800,000 lines fit in 183 MB. The two tests that claimed to guard this asserted a wall-clock budget at a single size, which cannot tell linear from quadratic and which went red on a loaded runner over code that was fine. Each is now a ratio between two measurements taken moments apart on the same machine, one of them work known to be linear. Both files are byte-identical to the ones in OpenINF/.github but for the package the test imports from. Signed-off-by: Derek Lewis <DerekNonGeneric@inf.is> Assisted-by: Claude-Code:claude-opus-5 Fixes: #1883 Refs: OpenINF/.github#913

The commit queue reads a message assembled from whatever a pull request's commits say, while holding a token that can merge, so how long that can be made to take is a security property. Issue #906 reported the reading as super-linear. Measured again one call to a process, which is how the queue does it, it is linear at every size -- but 25 to 30 times dearer than it needs to be, and it allocates enough to be killed for it: 800,000 lines took 674 MB, and on a smaller heap the process simply died. That is the denial of service the issue is about, reached by weight rather than by exponent. Four things were paying for it, all of them per line: - Every line was segmented into graphemes to measure its width. No grapheme is fewer than one code unit, so a line of 72 code units or fewer is within the limit whatever it holds and the segmenter need never see it. This was the bulk of the cost. - Seven regexes were compiled for each line of the last paragraph, which a message can make the whole message. Built once now. - The trailer block was found by rebuilding the message from its lines and splitting it all over again, with the paragraphs already in hand. - Splitting into paragraphs joined the lines and split the result twice more. It walks the lines it is given instead. Reading 200,000 lines now costs 2 to 6 times looking at every line once, against 33 to 124 before, and 800,000 lines fit in 183 MB. The tests are rebuilt, because the pair that claimed to guard this could not. They asserted a wall-clock budget at a single size, which cannot tell linear from quadratic, and which went red twice on a loaded runner over code that was fine. Each is now a ratio between two measurements taken moments apart on the same machine, one of them work known to be linear, so a slow or busy machine raises both and leaves the ratio where it was. The floor is written out in the test rather than borrowed from the module, since sharing it would let a change that made splitting quadratic raise the floor by as much as the measurement and go unseen. They were checked against the code they have to reject: the reading this replaces fails three of them, restoring the regex that strips the trailing newlines fails two at over a thousand times the floor, and dropping the `^` from the Assisted-by pattern fails the fourth at three thousand times its control. Ten runs on a machine carrying four times as many spinning processes as it has cores produced no failures. Signed-off-by: Derek Lewis <DerekNonGeneric@inf.is> Assisted-by: Claude-Code:claude-opus-5 Fixes: #906

The port of OpenINF/.github#913, where the reasoning is written out. This repository's commit queue is the one in daily use, so the exposure here is real rather than theoretical: the queue reads a message assembled from whatever a pull request's commits say, while holding a token that can merge. Reading cost 25 to 30 times what it needed to, and allocated enough that a large message could get the process killed -- 800,000 lines took 674 MB. Four costs, all of them per line: every line segmented into graphemes to measure its width, seven regexes compiled for each line of the last paragraph, the message rebuilt and re-split to find the trailers with the paragraphs already in hand, and a paragraph split that joined the lines and split the result twice more. Reading 200,000 lines now costs 2 to 6 times looking at every line once, against 33 to 124 before, and 800,000 lines fit in 183 MB. The two tests that claimed to guard this asserted a wall-clock budget at a single size, which cannot tell linear from quadratic and which went red on a loaded runner over code that was fine. Each is now a ratio between two measurements taken moments apart on the same machine, one of them work known to be linear. Both files are byte-identical to the ones in OpenINF/.github but for the package the test imports from. Signed-off-by: Derek Lewis <DerekNonGeneric@inf.is> Assisted-by: Claude-Code:claude-opus-5 Fixes: #1883 Refs: OpenINF/.github#913

The port of OpenINF/.github#913, where the reasoning is written out. This repository's commit queue is the one in daily use, so the exposure here is real rather than theoretical: the queue reads a message assembled from whatever a pull request's commits say, while holding a token that can merge. Reading cost 25 to 30 times what it needed to, and allocated enough that a large message could get the process killed -- 800,000 lines took 674 MB. Four costs, all of them per line: every line segmented into graphemes to measure its width, seven regexes compiled for each line of the last paragraph, the message rebuilt and re-split to find the trailers with the paragraphs already in hand, and a paragraph split that joined the lines and split the result twice more. Reading 200,000 lines now costs 2 to 6 times looking at every line once, against 33 to 124 before, and 800,000 lines fit in 183 MB. The two tests that claimed to guard this asserted a wall-clock budget at a single size, which cannot tell linear from quadratic and which went red on a loaded runner over code that was fine. Each is now a ratio between two measurements taken moments apart on the same machine, one of them work known to be linear. Both files are byte-identical to the ones in OpenINF/.github but for the package the test imports from. Signed-off-by: Derek Lewis <DerekNonGeneric@inf.is> Assisted-by: Claude-Code:claude-opus-5 PR-URL: #1896 Fixes: #1883 Refs: OpenINF/.github#913

DerekNonGeneric mentioned this pull request Sep 8, 2026

🏗️🚀：read a message without segmenting every line OpenINF/openinf.github.io#1896

Merged

coderabbitai Bot reviewed Sep 8, 2026

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Comment thread build/shared/commit-message.test.mts

DerekNonGeneric force-pushed the infra/linear-commit-message branch from 9b907ea to 80ce069 Compare September 8, 2026 23:26

DerekNonGeneric force-pushed the infra/linear-commit-message branch from 80ce069 to 435a2dd Compare September 9, 2026 00:45

DerekNonGeneric force-pushed the infra/linear-commit-message branch from 435a2dd to b4627d7 Compare September 9, 2026 01:20

DerekNonGeneric added the 🚀 Status: Commit Queue Land this pull request when its checks pass label Sep 9, 2026

openinf-commit-queue Bot merged commit 1ac4cb4 into main Sep 9, 2026
10 checks passed

openinf-commit-queue Bot removed the 🚀 Status: Commit Queue Land this pull request when its checks pass label Sep 9, 2026

openinf-commit-queue Bot deleted the infra/linear-commit-message branch September 9, 2026 02:05

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🏗️🚀：read a message without segmenting every line - #913

🏗️🚀：read a message without segmenting every line#913
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Why the tests are rebuilt

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