Add tests for oven-sh/WebKit#684: a typed array vector counts as allocated once, and dead ArrayBuffers do not force full collections - #42979
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Navigate logical layers of code changes, visualize relationships, and explore their blast radius. No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info⚙️ Run configurationConfiguration used: Repository: oven-sh/bun/.coderabbit.yaml Review profile: ASSERTIVE Plan: Essentials Run ID: 📒 Files selected for processing (1)
Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 6 remain after this review. WalkthroughThe GC cadence tests add controlled child-process allocation workloads. They check collection counts for arrays, buffers, and typed-array views, including short-lived allocations and views that share a backing vector. ChangesGC cadence tests
Suggested reviewers: Priority: ⬇️ Low Merge Risk: ⚪ Minimal · up to The new GC tests introduce no identified merge-blocking issue. The selected WebKit build includes the merged fix, so the earlier preview-build concern does not block merging. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Actionable comments posted: 2
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Inline comments:
In `@scripts/build/deps/webkit.ts`:
- Line 6: Update the WEBKIT_VERSION constant from the temporary
autobuild-preview-pr-684-527d50c3 pin to the merged autobuild-<sha> release
identifier, without adding a process.versions assertion.
In `@test/js/bun/gc/gc-controller-cadence.test.ts`:
- Line 186: Replace both parameterized test.concurrent.each calls with
describe.each suites, and add a nested test.concurrent for each matrix case that
must run concurrently. Preserve the existing test cases, parameters, and
assertions while applying this structure consistently to both matrices.
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scripts/build/deps/webkit.tstest/js/bun/gc/gc-controller-cadence.test.ts
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Beyond the inline findings, I also grepped the repo for the previous WebKit sha c28156899e5f... — webkit.ts:6 is its only occurrence, so no other pin needs updating when the version moves to the merged sha. The new collectionsIn helper drains stdout, stderr and exit concurrently, so the child cannot deadlock on a full pipe.
Extended reasoning...
The two posted findings (the ephemeral preview-tag pin, which the author's own description says must move to the merged sha before landing, and the ASAN/debug lane load from nine concurrent 320 MB children) cover what blocks this PR. Separately I checked that the old WebKit sha appears nowhere else in the repository, so the eventual swap is a single-line change, and that the test helper follows the harness convention of draining pipes via Promise.all. Nothing further to add; the inline comments already signal that a human must look before merge.
Findings marked 🟡 are optional suggestions and need no follow-up push.
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Updated 1:58 AM PT - Sep 27th, 2026
✅ @robobun, your commit a2352079af91839cee4f15715cf1ca41e927a742 passed in 🧪 To try this PR locally: bunx bun-pr 42979That installs a local version of the PR into your bun-42979 --bun |
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I re-reviewed the latest push and found no bugs; the two points from my earlier review are addressed in the code (the pin is now a 40-hex sha at scripts/build/deps/webkit.ts:6, and the test block runs six children of 10000 iterations instead of nine of 20000). Because the actual behavioral change is inside the JSC heap (not visible in this diff) and I could not reach GitHub from this environment, a human look would still be worthwhile to confirm the sha is the merge commit of oven-sh/WebKit#684 and that autobuild-136fb71d... tarballs exist for every os/arch/musl/debug/lto/asan lane.
What was reviewed:
- The pin: no
autobuild-preview-pr-*tag remains; the old hashc28156899eappears nowhere else in the repo. - The new test block: pipes are drained concurrently, per-loop runs are memoized so each of the six loops spawns once, exit code is asserted with the log attached, and the thresholds (0.75x-1.25x of the array-alone loop, full <= 2, total > 10) match the fixed/unfixed numbers the description reports (39 vs 19 collections, 9 vs 0 full).
- The block is not skipped on debug, unlike the sibling eden-cadence block; the description states the counts are identical on debug+ASAN because the concurrent collector is off, which is plausible but is something the CI debug/ASAN lanes should confirm.
Extended reasoning...
Overview
The diff is two files: scripts/build/deps/webkit.ts bumps WEBKIT_VERSION from c28156899e5f... to 136fb71d6262..., and test/js/bun/gc/gc-controller-cadence.test.ts gains a describe("short-lived typed arrays and ArrayBuffers") block with six test.concurrent.each cases. The three commits since the base show the progression: the original pin to a preview tag, a test rewrite halving the workload and memoizing runs, and a final commit swapping the pin to a sha described as the merge of oven-sh/WebKit#684. Both threads I opened on the first push are addressed by those later commits, so this run's message reports the new state rather than repeating them.
Security risks
None specific to this diff. The Bun-side change is a dependency pin plus a test; no user input handling, auth, crypto, or network code changes here. The engine change (JSC heap accounting of typed-array vectors and freed ArrayBuffers) affects GC pacing, not memory safety of user-reachable paths, per the description — but that claim can only be checked against the WebKit PR itself, which is outside this repo.
Level of scrutiny
Medium-high. A WebKit bump is mechanically small in Bun but changes the JS engine for every program; the real code under review lives in oven-sh/WebKit#684, which is not part of this diff. Per the repository's dependency guidance, the pin must be a merged upstream sha and prebuilt artifacts must exist for every platform x flavor. I could not verify either from this environment (network calls were not permitted), so I cannot claim high confidence that no human needs to look. The repo does have a download-time guard for preview tags (scripts/build/download.ts:316-333), and the new value is a plain 40-hex sha, so that specific failure mode is gone.
Other factors
The test follows harness conventions (bunExe, spread bunEnv, await using proc, Promise.all over stdout/stderr/exited, exit code asserted with the log as the message). The memoization map is shared across concurrent cases, which is safe because Map.set runs synchronously before the first await. The thresholds are tied to numbers the description reports for fixed vs unfixed builds (19 vs 39 total, 0 vs 9-13 full), with headroom for the one full collection that BUN_DESTRUCT_VM_ON_EXIT lanes add. The block is not skipIf(isDebug) unlike its sibling; the description argues the counts do not depend on timing since the concurrent collector is disabled and the collection runs synchronously at the allocation that passes the budget, which is consistent with how collectIfNecessaryOrDefer behaves with useConcurrentGC=0. CI on the debug and ASAN lanes is the actual check for that, and the robobun evidence block notes the test was deferred to CI.
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The two things the last review could not check from its environment:
The debug and ASAN lanes of the previous CI run (build #116735, same test block) ran the new tests green, which confirms that the counts do not depend on the build type. |
robobun
changed the title
…ped arrays and ArrayBuffers A typed array of more than 1000 elements owns a malloc'ed vector. Reading .buffer, subarray() and Buffer's slice() wrap that vector in an ArrayBuffer. Before oven-sh/WebKit#684 the heap counted the vector as allocated a second time there, and it kept counting ArrayBuffers that an eden collection had freed, so every second collection of such a loop was a full one. main has the fix since #43022. These tests count the collections of six loops with BUN_JSC_logGC=1 and the concurrent collector off. They fail on Bun 1.4.2 and pass on main.
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This PR now adds tests only. What changed and why:
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I reviewed the latest push and found no bugs; the earlier points are addressed (the diff no longer touches scripts/build/deps/webkit.ts, and each loop now runs once per file with 5000 x 32 KB). A human look is still worthwhile because the six new tests run ungated on every lane, so the CI matrix result on Windows, macOS, debug and ASAN is what settles whether the 0.75x..1.25x and full <= 2 thresholds hold there.
What was reviewed:
- The four env knobs the children set: BUN_GC_TIMER_DISABLE (GarbageCollectionController.rs:94), BUN_GARBAGE_COLLECTOR_LEVEL (VirtualMachine.rs:4284), and BUN_JSC_* options (ZigGlobalObject.cpp:293) are all read by src/.
- Each assertion can fail against the unfixed accounting (39 vs 19 collections exceeds 1.25x; 13 full collections exceeds 2), and the memoized runs reject on a nonzero exit before any count is used.
- Pipes are drained with Promise.all and the child is released via
await usingbefore assertions.
Extended reasoning...
The change appends a 65-line describe block of six test.concurrent.each cases to test/js/bun/gc/gc-controller-cadence.test.ts and modifies no production code; the earlier WebKit preview-tag pin is gone because the checkout's WEBKIT_VERSION is already the plain sha f20ce7744553. It touches no security-sensitive surface. The env knobs the tests rely on are confirmed to be read by the runtime, and the thresholds leave a wide margin from the pre-fix counts. What keeps this from an outright approve is that, unlike the neighbouring tests in the same file, the block has no isDebug/isASAN/platform gating, no local build was available here to run it, and the PR's own evidence note says the tests were not proven on the author's machine, so CI across platforms is the real check.
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On the point that the tests were not run before CI: they were, on linux x64.
The thresholds do not depend on the lane. With the concurrent collector off, the number of collections follows from the bytes that the heap counted and from the 8 MB budget, which Bun sets on every platform. The first version of this block (10000 allocations of 16 KB, the same bytes) ran on every lane of builds #116735 and #116957 and passed there, Windows, macOS and ASAN included. |
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2 participants
Behaviour change: none
Problem
Buffer.allocUnsafe(32768).slice(0, 10)ran twice the collections of the allocation alone, and every third one was a full collection.Fix
test/js/bun/gc/gc-controller-cadence.test.ts. It changes no source file.mainsince Bump WebKit to 000c48997255 #43022. That PR movedWEBKIT_VERSIONto a commit that contains136fb71d62, the merge of [JSC] Count an adopted typed array vector once, and stop counting the ArrayBuffers an eden collection freed WebKit#684. The first version of this PR carried the same pin and is obsolete.Expected: <= 23.75, Received: 39andExpected: <= 2, Received: 13). They pass on a debug build ofmainand on canary1.4.3-canary.1+367d939d9.Background
ArrayBufferthat adopts it only when JS asks (.buffer,subarray(),slice()).Notes
What the tests measure. Each case runs a loop of 5000 allocations of 32 KB in a child with
BUN_JSC_logGC=1and counts the=> EdenCollectionand=> FullCollectionlines. The child runs withBUN_JSC_useConcurrentGC=0,BUN_GC_TIMER_DISABLE=1andBUN_GARBAGE_COLLECTOR_LEVEL=0, so nothing but JSC's allocation budget asks for a collection, and the collection runs at the allocation that passes the budget. The counts are the same in a release build and in a debug build with ASAN.mainnew Uint8Array(32768)new Uint8Array(32768).buffernew Uint8Array(32768).subarray(0, 10)Buffer.allocUnsafe(32768).slice(0, 10)new ArrayBuffer(32768)The first three tests compare a loop with its allocation-alone loop and accept 75% to 125% of its collections. Under 75% would mean that the vector is not counted at all any more. The other three accept at most 2 full collections: a build that tears the VM down at exit (
BUN_DESTRUCT_VM_ON_EXIT, the ASAN lanes) runs one. Each loop runs once for all the tests that look at it, so the block starts six children. A child frees 160 MB, under ASAN's quarantine limit.Where the block sits. It is at the end of the file, after the two serial
Bun.gc(true)tests. Consecutive concurrent tests of a file run as one group, also acrossdescribeblocks, and a serial test ends the group. The file has tests that wait on timers inside the default 5 s (idle release). At the end of the file the six children of this block do not start at the same moment as those tests.What changed in this PR. The first version pinned the build of oven-sh/WebKit#684 in
scripts/build/deps/webkit.tsand added these tests. #43022 movedmainto a WebKit that contains that merge one hour after the last push here, andmainis now atf20ce77445, which is 15 commits ahead of the merge and 0 behind. The branch is nowmainplus the tests. The loops changed from 10000 allocations of 16 KB to 5000 of 32 KB: the same bytes and the same counts, with half the iterations for the debug lanes.Local run. Debug build of
main(a4f1429) with ASAN, the six tests alone: 6 pass in 3 of 3 runs. The whole file on that build: the 12 tests that run pass, 10 skip, and 2 time out (idle release lets FTL code age out). Those 2 time out the same way onmain's copy of the file without this block. The machine had a load average over 400 on 16 cores, and with--timeout 60000the whole file passes.What the fix changed, in Bun. Measured for the first version of this PR: two release builds from the same machine, one with
WEBKIT_VERSIONatc28156899e(base, before the fix) and one with the build of oven-sh/WebKit#684 (pin). linux x64, medians of 7 interleaved runs (5 for the second and third table) on a loaded machine,BUN_GC_TIMER_DISABLE=1.chunkisBuffer.allocUnsafe(16384),sliceadds.slice(0, 10),ArrayBufferisnew ArrayBuffer(16384), all 200k times.streamis a 64 KBBuffer.allocUnsafewith twosubarray()views, 50k times.readFileSyncreads a 64 KB file 50k times (its Buffer has noArrayBuffer, so it is a second control).With 2 million small objects alive, where a full collection has something to mark:
Buffers that die old: a ring of 512 buffers of 64 KB that the loop overwrites round-robin, 50k times. Only a full collection frees an old buffer.
Buffer.allocUnsafe(65536)Buffer.allocUnsafe(65536)Buffer.allocUnsafe(65536).subarray(0, 32768)Buffer.allocUnsafe(65536).subarray(0, 32768)new ArrayBuffer(65536)new ArrayBuffer(65536)The loops that die young keep their peak RSS. The ring of
ArrayBuffers goes from 175 to 210 MB: it now runs the collections that the ring of plain Buffers runs (33 eden + 17 full), wherebaseran a full collection every second time. oven-sh/WebKit#684 has the same loops in thejscshell, where the allocator has no purge thread and the peak of the loops that die young rises too.Where this came from. A performance note said that one-shot
node:zlibsync calls got 1.5 to 3.4 times slower at 1.4.0.processChunkSyncsliced its 16 KBBuffer.allocUnsafechunk on every call: 120kzlib.gunzipSync()calls of 1 KB ran 453 eden + 151 full collections, and 319 + 0 when the bytes were copied out. #35356 loweredlargeHeapSizeto 8 MB, so collections on a small heap are four times as frequent as in 1.3, and the two accounting errors cost that much more.no test proof · iteration 2 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/gc/gc-controller-cadence.test.ts