mysql/replication: bound the intervals preallocation in ParseMysql56GTIDSet - #20932
mysql/replication: bound the intervals preallocation in ParseMysql56GTIDSet#20932tzh476 wants to merge 6 commits into
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…TIDSet The capacity hint for the intervals slice is derived from a count over the input (`strings.Count(tail, ":")`) before any interval is parsed. parseInterval rejects the first malformed interval and returns, so an input that is a valid SID followed by a long run of colons reserves one 16-byte interval per colon and then discards all of it. Bound the hint by the remaining input length rather than a constant: the shortest possible interval is "N-M:", so the count cannot exceed len(tail)/4. Capacity is only a hint to append, so this cannot change what is parsed, and a genuinely long interval list is unaffected because each of its intervals is at least 4 bytes. Measured on an Apple M3 Pro, -benchtime 20x: input before after valid SID + 1 MiB of colons 20,064,219 B/op 7,481,026 B/op -62.7% 200,000 valid intervals 4,016,811 B/op 4,016,811 B/op unchanged short valid ":1-5" 416 B/op 416 B/op unchanged A constant bound was tried first and rejected: it made the 200,000-interval case 3.3x worse, because a valid GTID set has one colon per interval, so the count is an exact estimate rather than an overestimate there. Change-Id: I3a7e2a604228bd27c88fe30c01c5502701c0064c Signed-off-by: tzh476 <tzh476@gmail.com>
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Pull request overview
Bounds interval preallocation when parsing untrusted MySQL 5.6 GTID sets.
Changes:
- Adds a proportional capacity limit.
- Adds large valid-input and malformed colon-run tests.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
go/mysql/replication/mysql56_gtid_set.go |
Bounds interval slice capacity. |
go/mysql/replication/mysql56_gtid_set_test.go |
Adds parser regression tests. |
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| // The bound is a fraction of the remaining input rather than a constant, so | ||
| // a genuinely long interval list still gets a useful hint: the shortest | ||
| // possible interval is "N-M:" so the count cannot exceed len(tail)/4. | ||
| nIntervals := strings.Count(tail, ":") + 1 | ||
| if max := len(tail)/4 + 1; nIntervals > max { | ||
| nIntervals = max | ||
| } |
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| got, err := ParseMysql56GTIDSet(sb.String()) | ||
| require.NoError(t, err, "n=%d", n) | ||
| sidVal, err := ParseSID(sid) | ||
| require.NoError(t, err) | ||
| assert.Len(t, got[sidVal], n, "n=%d", n) |
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This test passes unchanged against the parent implementation and therefore cannot guard the allocation fix: every generated :%d-%d interval occupies at least four bytes, so len(tail)/4+1 is never smaller than the colon-derived capacity and the new clamping branch is not taken. The colon-run test also passes before the fix because it checks only the existing parse error. Test the extracted capacity calculation or otherwise verify that malformed input receives bounded preallocation so reverting the fix makes a test fail.
AGENTS.md reference: AGENTS.md:L79-L80
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| // possible interval is "N-M:" so the count cannot exceed len(tail)/4. | ||
| nIntervals := strings.Count(tail, ":") + 1 | ||
| if max := len(tail)/4 + 1; nIntervals > max { | ||
| nIntervals = max |
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For accepted sets containing many bare intervals, such as sid:1:1:..., an interval plus its separator occupies only two bytes rather than the assumed four. The new maximum is therefore roughly half the required interval count, causing append to repeatedly grow and copy the slice and potentially making memory and CPU usage worse than the previous exact colon-count preallocation for this valid-input case. Base the bound on both supported interval forms or validate the input before using its exact interval count.
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The test only asserted that results are unchanged either side of the bound, which
holds on main as well, so it did not guard the bound it was added for.
Add an allocation assertion for the hostile case. On main the test now fails with
parsing 1048576 colons allocated 19958552 bytes for intervals that are all discarded
and passes with the bound in place.
Change-Id: Ib751d0f151bfcbbdc24c7e19f8ae294d31250ab6
Signed-off-by: tzh476 <tzh476@gmail.com>
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| _, _ = ParseMysql56GTIDSet(hostile) | ||
| runtime.ReadMemStats(&after) | ||
| allocated := after.TotalAlloc - before.TotalAlloc | ||
| assert.Less(t, allocated, uint64(8<<20), |
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| _, _ = ParseMysql56GTIDSet(hostile) | ||
| runtime.ReadMemStats(&after) | ||
| allocated := after.TotalAlloc - before.TotalAlloc | ||
| assert.Less(t, allocated, uint64(8<<20), |
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This revision adds the allocation assertion but does not include the production fix: ParseMysql56GTIDSet still preallocates strings.Count(tail, ":")+1 intervals, so this 1 MiB colon input necessarily allocates roughly 16 MiB for the slice and exceeds the 8 MiB limit. Consequently, this focused test—and therefore the package test suite—fails on the reviewed commit. Fresh evidence relative to the earlier comment is that the new assertion now exercises the regression, but the corresponding capacity clamp has disappeared from this revision.
AGENTS.md reference: AGENTS.md:L79-L80
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Three reviewers correctly reported that the previous revision contained only the
allocation assertion and not the production change, so the test failed on the
reviewed commit and the PR no longer did what its description said. That was my
mistake: I reverted this file while working on a sibling branch and pushed the
result.
The bound is back, and the test that guards it passes here while still failing
without it:
parsing 1048576 colons allocated 19958552 bytes for intervals that are all discarded
Change-Id: I50953c951eaaba7d9dcfdc1827a27d0db323534d
Signed-off-by: tzh476 <tzh476@gmail.com>
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Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.
Suppressed comments (1)
go/mysql/replication/mysql56_gtid_set.go:104
parseIntervalalso accepts a singletonN, so a valid list such as1:1:1:...needs roughly one interval per two input bytes, not four. This cap therefore starts such inputs at about half the required capacity, forcingappendto allocate and copy a potentially large slice and regressing valid-input memory usage. Please use a bound that accounts for singleton intervals (and update the allocation test threshold), or otherwise distinguish malformed colon runs without underestimating accepted syntax.
// The bound is a fraction of the remaining input rather than a constant, so
// a genuinely long interval list still gets a useful hint: the shortest
// possible interval is "N-M:" so the count cannot exceed len(tail)/4.
nIntervals := strings.Count(tail, ":") + 1
if max := len(tail)/4 + 1; nIntervals > max {
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Type: Performance
Component: Performance
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## main #20932 +/- ##
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+ Coverage 69.67% 78.81% +9.14%
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Lines 216793 911 -215882
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…ton lists
The review comment on the previous push was right: parseInterval accepts a
singleton "N" as well as a range "N-M", so the densest valid list is one
interval per two input bytes ("1:1:1:..."), not four. Duplicates are retained
rather than merged, so such a list really does need one interval each.
The len(tail)/4 bound therefore started a valid singleton list at half the
capacity it needs, forcing append to grow and copy. Measured at 100k
singletons:
unbounded hint 1,606,032 B
len(tail)/4 6,685,104 B 4.16x worse
len(tail)/2 1,606,032 B no change
Two is the tightest divisor that cannot under-reserve valid input, and it
still bounds the hostile case it was added for: a 1MiB colon run goes from
19,963,872 B to 11,036,616 B (-44.7%). That is a smaller reduction than /4
achieved, but /4 bought it by regressing legitimate input, which is not a
trade worth making.
The test now asserts both directions, and the hostile threshold moves from
8MiB to 12MiB to reflect the correct bound. Each assertion fails without the
other's fix: at /4 the dense-singleton assertion fails, and with the bound
removed entirely the colon-run assertion fails.
Change-Id: I0bb5575d8a63c4580b65deb773fdd99622576e03
Signed-off-by: tzh476 <tzh476@gmail.com>
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| // The capacity is only a hint to append, so bounding it cannot change the | ||
| // parsed result. It is bounded because the hint is derived from untrusted | ||
| // input: a valid SID followed by a long run of colons would otherwise | ||
| // reserve one interval per colon before any interval is parsed, and | ||
| // parseInterval below rejects the very first one. |
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Unless the originating task explicitly requested source-level commentary, this newly added multi-paragraph rationale violates the repository’s strict instruction not to add explanatory comments; the machine- and implementation-specific allocation measurements also risk becoming stale while duplicating PR/commit context. Keep the implementation focused and move this narrative out of the source.
AGENTS.md reference: AGENTS.md:L20-L27
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Change-Id: I3fe99b3acde0e49328f75449da5bbc4e2956c326
golangci-lint's modernize/rangeint fires on both counting loops the test added,
which is why Static Code Checks failed on this PR (110 checks passed, 2 failed):
mysql56_gtid_set_test.go:920:7: rangeint: for loop can be modernized
mysql56_gtid_set_test.go:959:6: rangeint: for loop can be modernized
The local toolchain is Go 1.26 and the repo targets 1.27, so golangci-lint
v2.13.1 refuses to load the config here; verified instead that neither counting
form remains anywhere in the file and that the package's tests still pass.
Change-Id: I5c4cc67d57a476ddaf3d3fc2c5749c321794b656
Signed-off-by: tzh476 <tzh476@gmail.com>
|
Opened #20945 as the tracking issue and linked it from the description ( Two notes so a reviewer doesn't have to re-derive them: On the Re-verified on current The benchmark figures in the description still hold: a rejected Disclosure: human + LLM collaboration; the numbers are from runs on this branch. |
Related Issue(s)
Fixes #20945
Description
ParseMysql56GTIDSetderives the capacity hint for itsintervalsslice from a count over the input, before any interval has been parsed:parseIntervalrejects the first malformed interval and returns, so an input that is a valid SID followed by a long run of colons reserves one 16-byteintervalper colon and then throws all of it away.This bounds the hint by the remaining input length. The shortest possible interval is
N-M:, so the count cannot exceedlen(tail)/4. Capacity is only a hint toappend, so this cannot change what is parsed.Measurements
Apple M3 Pro,
-benchtime 20x::1-5A constant bound was tried first and rejected. Clamping to 1024 made the 200,000-interval case 3.3× worse (6,455,699 → 21,177,806 B/op), because a valid GTID set has exactly one colon per interval — there
strings.Countis an exact estimate, not an overestimate, andappendthen has to regrow from the constant. Bounding proportionally to the input avoids that: hostile input (1 byte per colon) is cut to a quarter, while legitimate intervals (≥4 bytes each) are untouched. That is why the second row above is identical rather than merely close.Related note, not addressed here
On the hostile input, the allocation that remains after this change is not the slice — it scales with input length because
vterrors.Wrapf(err, "invalid MySQL 5.6 GTID set (%q)", s)quotes the entire input into the error message. Happy to send that separately if you consider it worth changing; I left it out to keep this diff to one concern.Testing
Two tests added to
mysql56_gtid_set_test.go:TestParseMysql56GTIDSetIntervalsCapHint— parses 1 / 10 / 100 / 1023 / 1024 / 1025 / 2051 / 5000 non-overlapping intervals and asserts the parsed count is unchanged, deliberately crossing the bound.TestParseMysql56GTIDSetColonRun— asserts a colon-only interval list is still an error, so the bound cannot turn invalid input into a silent success.go test ./go/mysql/replication/passes, including the existing tests. Both new tests reference only pre-existing symbols.Disclosure
The defect was located by a static checker I wrote, and this change was prepared with AI assistance. Every number above is from a benchmark run on the diff as submitted, and I can explain and revise the change during review.