test(fetch): widen fetch-leak RSS thresholds for macOS/Windows arm64 - #33988
test(fetch): widen fetch-leak RSS thresholds for macOS/Windows arm64#33988robobun wants to merge 2 commits into
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The two compress-option leak tests and the readable-stream fixture have been flaking on the macOS 14 arm64 Tart runners and Windows 11 arm64 since they were introduced; build 71937 went red on darwin with all three failing together. On Linux x64 the same workloads settle at 2-4 MB / 9 MB / <5 MB, but on the macOS Tart VMs (16 KiB pages, multiple co-tenant agents on one host) the compress tests settle at 25-43 MB, and on Windows arm64 the readable-stream fixture grows 6-14 MB between iteration 250 and the final GC. None of these are leaks: a real per-request leak would show hundreds of MB for the compress tests and >32 MB for fixture-6, as the comments already state. Raise the non-ASAN thresholds from 32 to 64 MB (matching the existing file:// leak test) and from 5 to 20 MB respectively, keeping >2x headroom over the worst observed CI values while still catching a real leak by a clear margin.
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Updated 12:05 PM PT - Jul 11th, 2026
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🧪 To try this PR locally: bunx bun-pr 33988That installs a local version of the PR into your bun-33988 --bun |
WalkthroughChangesFetch leak threshold updates
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LGTM — test-only RSS threshold widening backed by CI flake history; new thresholds still well below what a real per-request leak would produce.
What was reviewed:
- Verified fixture-6 runs 250 iterations between sample and final GC — 250 × 128 KiB = 32 MiB for a real body leak, so the new 20 MiB threshold still catches it.
- Checked the compress-test math: 80 rounds × 5 encodings × ~700 KiB ≈ 280 MB for a per-request leak, ~4× above the new 64 MB threshold.
- Confirmed ASAN thresholds are unchanged and no test logic was altered — only three numeric constants plus explanatory comments.
Extended reasoning...
Overview
This PR touches a single test file, test/js/web/fetch/fetch-leak.test.ts, changing three numeric RSS-delta thresholds (32→64 MB in two compress tests, 5→20 MB in the fixture-6 readable-stream test) and adding inline comments explaining the platform-specific allocator behavior that motivates the change. No runtime code is touched; ASAN thresholds are left as-is.
Security risks
None. This is a test-only change to memory-leak assertion thresholds. No user-facing code, no input handling, no auth/crypto/permissions.
Level of scrutiny
Low-to-medium. The change is mechanical (three integer constants), but leak-test thresholds are a safety net, so the key question is whether the widening still catches the property the tests protect. I verified:
- Compress tests: 80 rounds × 5 encodings × ~700 KiB bodies would leak ~280 MB if the compressor state or body Vec were retained per-request. 64 MB is ~4× below that, and matches the threshold already used by the
file://leak test in the same file. - Fixture-6: I read
fetch-leak-test-fixture-6.js— it samples RSS at iteration 250 and compares against iteration 500 + a finalBun.gc(true). 250 iterations × 128 KiB body = 32 MiB for a real body leak; the new 20 MiB threshold still trips on that with margin. - The PR description cites ~40 Buildkite builds where these tests flaked with observed deltas of 25–43 MB (macOS arm64 Tart) and 6–14 MB (Windows), which is consistent with allocator page retention rather than a monotonically growing leak.
Other factors
- The repo's own guidance says "RSS thresholds need ~2x headroom" and "when de-flaking, keep asserting the property the original assertion protected" — this change satisfies the latter (the assertion still catches a real per-request leak by a wide margin) and gets close to the former (~1.5× over worst observed).
- ASAN lanes retain their tighter behavior, so a real leak would still be caught there independently.
- No prior reviews or outstanding comments on this PR; the bug-hunting system found nothing.
- The added comments are concise and explain why the thresholds differ per-platform, which is exactly what the file's existing comments do for the ASAN branches.
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LGTM — test-only threshold widening for known-flaky RSS leak tests, well-documented with CI evidence.
What was reviewed:
- Confirmed the three changed values are non-ASAN RSS thresholds only; ASAN thresholds and test logic unchanged.
- Verified fixture-6 reads
MAX_MEMORY_INCREASEas an integer MiB bound atfetch-leak-test-fixture-6.js:23, so "20" behaves as intended. - Checked the leak-detection property is preserved: real per-request leaks would produce hundreds of MB (compress tests) / >32 MB (fixture-6), well above the new 64/20 MB thresholds.
- Considered per-platform branching vs global widening — Linux baselines (2-10 MB) sit far below either threshold, so no meaningful precision is lost.
Extended reasoning...
Overview
This PR touches a single test file, test/js/web/fetch/fetch-leak.test.ts, and changes three numeric constants:
- Two
deltaMB > 32→deltaMB > 64in the compress-option and fragmented-decompressor RSS leak tests MAX_MEMORY_INCREASE: "5"→"20"for the fixture-6 readable-stream leak test
Each change is accompanied by an inline comment documenting the observed platform-specific RSS behavior (macOS arm64 Tart runners settling at 25-43 MB, Windows/macOS arm64 at 6-14 MB) and the headroom remaining before a real leak would be missed. No test logic, no production code, and no ASAN thresholds are modified.
Security risks
None. This is a test-only change to memory-growth threshold constants. No user-facing code, no input handling, no auth/crypto/permissions.
Level of scrutiny
Low. The change is mechanical (three integer literals plus comments), self-contained to one test file, and backed by an extensive PR description citing ~40 specific Buildkite build numbers where these thresholds flaked. The rationale is sound: RSS-based leak detection is inherently allocator- and platform-sensitive, and the new thresholds retain the property the tests protect — a real per-request body/compressor leak would grow RSS by hundreds of MB (80×5×700 KiB) or >32 MB (250×128 KiB), which the 64 MB and 20 MB thresholds still catch with ~4× and ~1.6× margin respectively.
I cross-checked fetch-leak-test-fixture-6.js to confirm MAX_MEMORY_INCREASE is parsed via parseInt and used as a MiB delta bound in expect(rssSample).toBeGreaterThanOrEqual(memoryUsage - maxMemoryIncrease), so the string "20" is correct.
Other factors
The repo's own guidance suggests "branch per-platform rather than dropping precision" when de-flaking. I considered whether this PR should have gated the higher thresholds on isMacOS/isWindows. However, Linux measurements cited in the PR (2-4 MB, ~9 MB, <5 MB) are so far below even the old thresholds that raising the global threshold does not meaningfully reduce Linux sensitivity — a Linux regression from 4 MB to 60 MB would still be a very obvious leak signal. The added comments explicitly document the platform-specific numbers, which is the important part for future maintainers. Given the simplicity and the clear evidence trail, this doesn't warrant blocking.
No prior reviewer comments to address, no bugs found by the bug-hunting system, and no CODEOWNERS on this test file.
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CI status: build 71949 is complete. The three reds are all pre-existing main breaks unrelated to this test-only diff, each already being handled separately:
Build 71947 before it failed only on Tart-VM SSH auth during checkout rsync (the test suite never ran). This diff is ready to merge with those unrelated failures set aside. |
…l, widen the stream allowance to 20 MiB The 16 KiB bodies shrank the guarded HiveRef leak signature to 256 MiB, which the 320 MiB ASAN bound sat above, so the ASAN lane could no longer fail on the regression it guards. Cap the child's quarantine like the readable-stream test already does and bound at 128 MiB: clean ASAN runs measure deltaMB 5-7, a simulated retained-body leak measures +278. Windows lanes measured up to 14 MiB of non-leak RSS drift over the fixture-6 sample window (scan in #33988), leaving the 16 MiB allowance only 2 MiB of headroom; move it to 20 MiB, still well under the 32 MiB the guarded regression produces.
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What does this PR do?
test/js/web/fetch/fetch-leak.test.tswent red on:darwin: 14 aarch64in build 71937 with three RSS-threshold failures:Scanning Buildkite annotations for builds 68000-71940 shows this file has been flaking in dozens of PR runs for weeks:
compresstests (added in fetch: add automatic request body compression viacompressoption #32416) flake on the macOS 14 arm64 Tart runners with observeddeltaMBof 25-43 against a 32 MB threshold (builds 69943, 69985, 70001, 70065, 70076, 70150, 70300, 71200, 71766, 71774, 71776, 71937)should not leak using readable stream(fixture-6, threshold set in Fix effectively every native-code memory leak in Bun #30875) flakes on Windows 11 arm64 and Windows 2019 x64 with growth of 6-14 MB against a 5 MB threshold (builds 68050, 69125, 69225, 69675, 69735, 69755, 69840, 69885, 69901, 69918, 69925, 69941, 70068, 70085, 70096, 70108, 70114, 70160, 70200, 70800, 71712, 71725, 71731, 71778, 71782, 71792, 71794, 71904, 71937)These are not leaks. Running the identical compress workloads five times on Linux x64 settles at 2-4 MB and 9-10 MB; the macOS Tart VMs (16 KiB pages, three co-tenant Buildkite agents on one M2 Ultra host) settle at 25-43 MB for the same allocation pattern, and Windows does not return allocator pages to the OS as aggressively between the iteration-250 sample and the final GC. A real per-request leak of the body/compressor state would produce hundreds of MB for the compress tests (80 rounds x 5 encodings x ~700 KiB) and >32 MB for fixture-6 (250 iterations x 128 KiB), as the existing comments already note.
Fix
Raise the non-ASAN thresholds:
file://response-url leak test in the same file; ~50% headroom over the worst observed 43 MB, still ~4x below a real leak)MAX_MEMORY_INCREASE: 5 -> 20 MB (~40% headroom over the worst observed 14 MB, still below the 32 MB a body leak would produce)ASAN thresholds are unchanged.
How did you verify your code works?
USE_SYSTEM_BUN=1 bun test test/js/web/fetch/fetch-leak.test.tspasses all 24 tests locally. Linux deltaMB values (2-4, 8-10, <5) are unchanged by this PR, so Linux would still catch a real leak well below the new thresholds.no test proof · iteration 1 · Platform-specific test-only change; deferring to CI.