perf: bound docscheck's code-span scan - #21
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Closes #18. A backtick run searches forward for the run that closes it, so a paragraph of runs that never close makes each one rescan everything after it, and the work grows with the cube of the run count. Measured on this branch before the bound: 4.4 s at 4.5 MB, 35 s at 18 MB. One crafted paragraph would spend a CI job's whole budget and surface as a timeout rather than a named failure. The search now gives up after 8 KiB, leaving the run as literal text — which is what an unmatched run already becomes, so no behaviour a real document can express changes. Reaching the ceiling needs a single span longer than 8 KiB; the widest in this repository is under a hundred bytes, and a repository test now fails if one ever approaches it, while the margin is still wide. The timing test took two attempts, and the first was worth recording. At 3000 runs the unbounded scan takes 4.4 s, so a five-second ceiling did not separate the two regimes: with the bound removed the test still passed. At 6000 runs the same input costs 61 ms bounded and 39 s unbounded, so the ceiling now sits eighty times above the passing time and seven times below the failing one. A wall-clock assertion is a poor shape for a test and is used here only because the defect was a growth rate; the size was measured rather than guessed. Closes #15. A small set of filesystem-error branches could not be reached with fstest.MapFS — a directory that fails mid-walk, a root that cannot be listed. A failingFS wrapper reaches them. These are worth testing rather than assuming: had CheckAll treated a collection failure as "no documents found", the entire gate would have passed vacuously on an unreadable tree. Both mutations confirm the tests catch that. Package coverage 94.0% to 96.9%.
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Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review. 📝 WalkthroughWalkthroughDocscheck limits each inline Markdown code-span scan to 8 KiB. Candidate delimiter runs are bounded to prevent repeated rescanning. Tests cover performance, delimiter matching, scan boundaries, multiline spans, and repository code-span sizes. ChangesDocscheck hardening
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: ⚪ Minimal · up to This PR bounds pathological code-span scanning while preserving the documented behavior for unmatched runs. No actionable merge-blocking risk remains after normal checks and review. Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches📝 Generate docstrings
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Inline comments:
In `@internal/docscheck/docscheck.go`:
- Line 837: Bound the candidate delimiter scan in the surrounding search logic
before calling leadingRun, limiting it to the bytes needed to distinguish an
exact opening-length run from a non-match and never exceeding the remaining
budget. Preserve unmatched behavior when the limit reaches that budget, and add
a regression case covering an oversized non-matching candidate run.
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PR #20 was opened for #15 five minutes before this branch's pull request and implements the same thing — an fs.FS that fails on ReadDir, exercising the walk and collection error paths. Two sessions picked up the issue in parallel. Theirs was first and is scoped to #15 alone, where this branch only carried those tests because #18 touches the same package. Removing them here resolves the conflict in docscheck_test.go and leaves this branch as what it should have been: the scan bound and nothing else. The #18 work is untouched. Package coverage returns to 95.5%, with #20 carrying the rest.
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This was referenced Aug 31, 2026
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August 31, 2026 03:08
maxCodeSpanScanBytes was spent per byte stepped over, but the candidate run a scan landed on was measured whole before that accounting ran. One oversized run therefore cost its full length no matter how little budget remained, and because runs of strictly increasing length never match each other, every run in a prefix that fits inside the budget reaches that run and repeats the read. Measure a candidate only far enough to tell an exact match from a longer run, and never past the remaining budget. A run that reaches the limit is a non-match, which is what the budget running out already produces. An input of 125 increasing runs before a 64 MiB run cost 4.5 s before and 79 ms after.
Peer review found the bounded measurement wrong in two opposite directions, both introduced by fe1e9df. A measurement that stops at the limit stops inside the run, and the search resumed at that offset, reading the remainder as a run of its own. A run of five backticks therefore closed a two-backtick opening on its last two, pulling the prose between them into a code span and out of the link and heading checks. Step over the remainder instead, under the same budget. Clamping the limit to the remaining budget also left a run of exactly opening backticks indistinguishable from a longer one, and calling that unmatched turned a closing run landing on the last of the budget into literal text. Read one past the opening length regardless: that costs the opening run's own length once per call, against a budget the next iteration is about to find spent. Both were verified against the unbounded algorithm this replaced, exhaustively over every backtick arrangement up to fourteen bytes and every two-line pair up to seven, plus a sweep of the budget boundary: zero disagreements, where the previous commit disagreed on 7432 of the two-line cases. The oversized-run bound still holds at 79 ms.
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Inline comments:
In `@internal/docscheck/docscheck.go`:
- Line 851: Update the code-span parsing logic around leadingRunAtMost so
opening delimiter measurement is capped at maxCodeSpanScanBytes + 1; reject and
return false when the run exceeds maxCodeSpanScanBytes. Add a regression case
covering two equal oversized delimiter runs, ensuring they remain literal text
rather than forming a valid span.
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The ceiling governed how far a run searched, but not how long a run could be, and the two delimiters of a span are runs. Two runs of 8 KiB with a byte between them therefore closed a span after inspecting 16 KiB, which is the span the ceiling is meant to have given up on. The cost stays linear either way, so this is the documented bound drifting from the code rather than the timeout returning, but the bound is worth keeping honest: the measurement below reads one past the opening length, so an unbounded opening run also set an unbounded floor under every candidate it compared against. Measure the opening run under the same ceiling and leave one past it as literal text. One call now reads at most twice the ceiling -- an opening run at it, then a search running the same distance -- and the const comment says so rather than presenting the ceiling as exact. Also adds the multi-line cases peer review asked for. codeSpanEnd exists for spans that close on a later line, and nothing committed covered that path; the run lengths have to match across a line break exactly as they do within one, and a blank line still ends the paragraph first.
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Inline comments:
In `@internal/docscheck/docscheck.go`:
- Around line 87-92: Enforce the maxCodeSpanScanBytes forward-search ceiling by
charging the full closing-run measurement against the remaining budget before
accepting a code span in the relevant docs-check scanning logic. Add a
regression case in internal/docscheck/docscheck_test.go:1020-1024 where the
closing run has less than its full length remaining in budget, and require
literal-text behavior; update internal/docscheck/docscheck.go:87-92 for the root
fix.
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The previous commit claimed a call reads at most twice maxCodeSpanScanBytes. Instrumenting the byte counter puts the worst case at three times it: an opening run at the ceiling, a search running the full ceiling between the delimiters, and the closing run measured at the end of it. The read that overruns is the closing measurement, and it stays uncharged deliberately. Charging it would make a closing run indistinguishable from a longer one whenever the budget no longer covered its length, which is the false negative the bound already had to give up once. So the number in the comment moves rather than the code, and a test pins the worst-case shape so the figure stays reachable rather than hypothetical.
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Docstrings generation was requested by @scottescue. * #21 (comment) The following files were modified: * `internal/docscheck/docscheck.go`
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Closes #18
The problem
A backtick run searches forward for the run that closes it. A paragraph of runs that never close makes each one rescan everything after it, so the work grows with the cube of the run count. Measured on this branch before the bound:
One crafted paragraph would spend a CI job's whole budget and surface as a timeout rather than a named failure.
The fix
The search gives up after 8 KiB, leaving the run as literal text — exactly what an unmatched run already becomes, so no behaviour a real document can express changes. Reaching the ceiling needs a single code span longer than 8 KiB; the widest in this repository is under a hundred bytes, and
TestRepositoryCodeSpansStayInsideScanBoundnow fails if one ever approaches it, while the margin is still wide.The timing test took two attempts
Worth recording, because the first version was exactly the failure mode this kind of test invites. At 3000 runs the unbounded scan takes 4.4 s, so a five-second ceiling did not separate the two regimes — with the bound removed, the test still passed. I only caught it by mutating my own code after writing the test.
At 6000 runs the same input costs 61 ms bounded and 39 s unbounded, so the ceiling sits eighty times above the passing time and seven times below the failing one. The size was measured in both regimes rather than guessed. A wall-clock assertion is a poor shape for a test generally; it's used here only because the defect was a growth rate rather than a wrong answer, and the deterministic half — a span past the bound is left unclosed — is pinned separately.
Note on scope
This branch briefly also carried #15's filesystem-error tests, because both issues touch
internal/docscheck. PR #20 was opened for #15 five minutes earlier by another session working the same issue, so those tests have been removed here — theirs is first and is properly scoped. This PR is now the scan bound alone.Verification
gofmt,go vet,go test -race -count=1 ./...cleanTestCodeSpanScanIsBoundedfail at 39 sSummary by CodeRabbit
Bug Fixes
Tests