mimalloc: a forced purge waits for the pass in progress, and what is freed during a pass is purged by the next by robobun · Pull Request #42571 · oven-sh/bun

robobun · 2026-09-13T06:38:28Z

Both fork PRs are merged: oven-sh/mimalloc#38 and oven-sh/mimalloc#39. The pin 707d90be is on bun-dev3-v2 (they were merged with merge commits, so the sha did not change). This PR now carries both bumps and both tests; it replaces #42543.

Two fixes in the fork

1. A forced purge waits for the pass in progress (oven-sh/mimalloc#38).

Bun.gc(true) ends with mi_collect(true) since Bun.gc(true) and gc() return what the collection freed to the OS before they return #42428. mimalloc dropped that purge when another thread held its purge guard, and the purge thread holds it for 36 to 47 ms while it purges what was already freed. Bun.gc(true) then returned with all of it still resident.
test/js/bun/spawn/spawn-pipe-leak.test.ts fails on main because of it: Peak RSS: warmup 46 MB -> main 312 MB (573%), 7 of 10 runs pass. With this pin: 25 of 25.
Only a forced collect waits. bun's one caller is garbage_collect_from_js. It takes 29 to 47 ms in place of 1 to 2 ms when it collides with a pass over 384 MB, and the memory is back with the OS when it returns.
Test: test/js/bun/gc/gc-controller-cadence.test.ts, "Bun.gc(true) returns what is free to the OS while the purge thread is at work". Fails 3 of 3 on main's release build (38 to 42 MB released, expected more than 300).

2. What is freed during a purge pass is purged by the next pass (oven-sh/mimalloc#39). The rest of this description.

Problem

Memory freed while mimalloc's purge thread (the scavenger) is in a pass can stay resident until the event loop goes idle or something forces a collection. With one arena (Malloc=1) a busy script keeps 110 of 384 MB: {"released":262.6,"purged":274.4}.
In the fork, _mi_arenas_try_purge (src/arena.c) keeps its old subproc->purge_expire during a pass, so a free in that time arms the arena only. The scavenger then clears the value, and no later free sets it again.
It reaches bun's default configuration. JSC's structure heap is a second arena that rarely purges, which hides the case of one busy thread. With several busy threads and no ticking event loop it shows each time: 8 workers that churn ArrayBuffers and then spin hold 578 MB RSS on main's release build and fall to 91 MB with this pin.

Fix

Bump MIMALLOC_COMMIT to 707d90be (arena: what is freed during a purge pass is purged by the next pass mimalloc#39, which contains arena: a forced purge waits for the pass in progress instead of being dropped mimalloc#38). The bump also picks up 0fecc729, a 32-bit shift fix in src/prof.c that was already on bun-dev3-v2.
In the fork a pass now resets subproc->purge_expire first and puts back what is pending at its end, so a free behind the pass schedules the next one.
Verified: the new test in test/js/bun/jsc/heapStats-mimalloc.test.ts fails 3 of 3 on main's release build, passes 20 of 20 with this pin. bun bd test skips it (ASAN).

Background

A purge returns freed mimalloc arena memory to the OS (madvise), 100 ms after the free.
The scavenger is the fork's thread that runs those purges. It sleeps until subproc->purge_expire.
The first free into an arena after a pass arms it (arena->purge_expire). Only that free looks at subproc->purge_expire.

Notes

The two commits of oven-sh/mimalloc#39.

1b894afe arena: what is freed during a purge pass is purged by the next pass (the fix)
707d90be init: release the purge guard at process exit on Windows

The free path (mi_arena_schedule_purge) is unchanged. Only the pass and the scavenger loop change.

The defect in full. A free arms its arena (arena->purge_expire) and, only if the arena was not armed yet, arms the subprocess (subproc->purge_expire, 0 -> set), which is what the scavenger sleeps on. A pass resets the arena expire when it goes into the arena, but it kept its old subprocess expire until its end. The first free into the arena behind the pass armed the arena again, found the subprocess expire still set, and left it alone. Then _mi_arenas_try_purge skipped its update (the break for max_purge_count also fires on the last arena, so a pass in which each arena purged ended with all_visited = false), and mi_scavenger_run set subproc->purge_expire to 0. The arena was armed and the subprocess was not. Each later free into that arena saw an armed arena and stopped there. The 30 s timeout of the scavenger did not help: with nothing scheduled it only waits again.

Reach in bun. JSC registers its structure heap as an exclusive mimalloc arena (mi_manage_os_memory_ex in StructureAlignedMemoryAllocator.cpp). A pass over two arenas in which only the main one purges ends with all_visited = true, reads the re-armed expire of the main arena after its walk, and installs it. So the same script that loses 128 MB with one arena gets its second pass on time with two (traced on main's release build: first pass at 107 to 123 ms, second at 207 ms). It shows when both arenas purge in the same pass, when a process has more arenas (more than 1 GiB of mimalloc memory) and each purges, and with Malloc=1. The event loop's mi_on_thread_idle_start on each epoll_wait heals it when the sweep runs to its end. Under load the owner wakes first and the sweep stops before _mi_arenas_purge_now.

The state is absorbing: once an arena is armed and the subprocess is not, nothing but a park or a forced collect ends it. So a low chance per pass is enough for a process whose threads stay busy. Measured with stock two arena bun (release, Linux x64): 8 workers allocate and free 1 to 8 MB ArrayBuffers for 3 s (transfer(0) frees at once), free everything, and spin, while the main thread sits in Atomics.wait. After a quiet 2.5 s RSS is 578 MB on main and 91 MB with this pin (arena_count 2 in both). The same happens with only the one line fix for all_visited in the fork, which is why the pass resets the subprocess expire first.

The test. A child frees 256 MB with ArrayBuffer.prototype.transfer(0) (no collection involved), spins until RSS fell by 64 MB (the purge thread is in its pass), frees 128 MB more, and spins without an await until RSS fell by 336 MB, for at most 2 s. It reports that drop and the growth of heapStats().mimalloc.purged. Both have to be at least 336 MB. The drop in RSS is taken before the last heapStats() call, because that call polls the allocator (mi_collect(false)), and a poll runs a pass that is due by itself. Malloc=1 makes JSC allocate through malloc, which is mimalloc on Linux, so there is one arena. Linux only (the wait reads RSS) and not ASAN (malloc is not mimalloc there).

build	result
release build of main (`6a92015fc`, pin `6a64e1ba`)	fails 3 of 3: `{"released":262.6,"purged":274.4}`, expected >= 336
`bun run build:release` with this pin	passes 20 of 20, about 450 ms
`bun bd test` with this pin (debug, ASAN)	new test skipped, the other 4 tests in the file pass

The change is under scripts/, so a checkout of src/ from main does not undo it. The fail-before check needs a release build of main.

About 707d90be. 1b894afe alone failed the 5 Windows jobs of the fork's CI. Its new test returns from main while the scavenger is in its last pass. On Windows mi_process_done runs in the process detach callback, after the system terminated every other thread, so the purge guard stays held, and the forced collect at exit waits for it without end (that wait is what oven-sh/mimalloc#38 adds). 707d90be releases the guard there. bun builds mimalloc with MI_NO_PROCESS_DETACH and MI_SKIP_COLLECT_ON_EXIT, so bun does not reach that code on any platform. Fork CI at 707d90be: 15 of 15 jobs pass (Windows x64 and arm64, mingw, macOS, Linux x64 and arm64 with ASAN, UBSAN, TSAN and guarded builds, FreeBSD).

In the fork. New test/test-purge-behind-pass.c: 5 of 5 runs fail before (266 of 320 MiB), 30 of 30 pass after. A stress harness with 8 to 16 busy threads: 15 of 15 rounds end stuck before, with about 530 MiB of free committed ranges unpurged. After: 750 rounds, 0 stuck. Details are in oven-sh/mimalloc#39.

Effect on purge activity. Ranges that stayed resident by accident are now purged 100 ms after the free, which is what purge_delay promises.

Suites run with a release build of this branch (Linux x64). test/js/bun/jsc/heapStats-mimalloc.test.ts, test/js/bun/gc/gc-controller-cadence.test.ts (11 pass), test/js/bun/spawn/spawn-pipe-leak.test.ts (3 pass), test/js/bun/spawn/spawn-noread-leak.test.ts, test/js/bun/jsc/bun-jsc.test.ts (38 pass). Nothing here was run on macOS or Windows.

Now pins the merged upstream sync

MIMALLOC_COMMIT is ab13501334a8da2b64ab2e5f4f552c3a39b96350: the merge of oven-sh/mimalloc#37 on bun-dev3-v2. It contains 707d90be (both fixes above), so everything in this description still holds.

What it adds beyond oven-sh/mimalloc#38 and oven-sh/mimalloc#39

microsoft/mimalloc dev3 through v3.5.2 (301 commits): always-on statistics (MI_STATS=1), sampled profiler hooks (mi_profiler_t, MI_PROFILE=1), codegen work in free/zalloc/memzero, double-free detection through the padding canary.
The fork's own heap profiler is gone (src/prof.c, mi_prof_*, MIMALLOC_PROF_*); upstream's hooks replace it. bun referenced none of it.
fork(): no arena purge pass runs across it any more. That was the macOS Debug SIGBUS of a forked child in the fork's CI, and a whole class of torn purge state in the child.
mi_heap_destroy of a heap that still has a block on a page's thread-free list no longer reports corrupted meta-data.
Two slips in upstream's free.c are fixed: mi_usable_size always took its slow path, and an overflow could be reported as a double free.
The malloc/free fast paths update upstream's packed used count with one instruction (decw / addq on memory).

One new define: MI_FREE_USE_PAGEMAP=1

Upstream's new default keeps page meta data at 256 MiB boundaries so that mi_free needs no page map. Every arena then has to start on such a boundary.
JSC gives mimalloc its structure heap as an arena (mi_manage_os_memory_ex(start + 16 KiB, ...), under a RELEASE_ASSERT), and halves that reservation when address space is short. With the new default mi_manage_os_memory_ex refuses it once the reservation is 256 MiB or less.
Measured with a release build of the new pin without the define: ulimit -v 3000000; bun -e 1 aborts on startup, and so does BUN_JSC_structureHeapSizeInKB=262144. The old pin runs both. With the default 4 GiB reservation it works but loses the first 256 MiB of structure address space.
With the define, mi_free looks the page up through the page map as it does today, and the structure heap works down to 64 MiB as before. The fork's CI runs this configuration (debug and release) on Linux x64 and arm64.
The aligned layout is worth another 1.1% of instructions on the malloc/free benchmark below. Getting it needs WebKit to hand mimalloc an aligned region (mi_arena_min_alignment()), which is a separate change.

Not changed: MI_PROFILE stays at its default of 1. Nothing in bun attaches a profiler yet, and MI_PROFILE=0 would save about 1.4% of instructions on the same benchmark. The hooks are what a Bun.pprof.heap API would build on, so they stay compiled in.

Bindings. Every mi_* function that src/mimalloc_sys/mimalloc.rs and the C++ side declare is exported with an unchanged declaration. mi_heap_area_t has the same layout. mi_option_t 0..46 did not move (47 is collect_merges_stats now, snapshot_on_exit 48); bun sets option 0 only. The heapStats().mimalloc keys the tests read (purged, page_bins, pages, committed) are emitted as before.

Allocator micro benchmark (25 M mi_malloc + 25 M mi_free of 16..1039 bytes, bun's compile flags, user-mode instructions):

	instructions
old pin (`707d90be`)	2.2463 G
this pin, `MI_FREE_USE_PAGEMAP=1` (what this PR builds)	2.2403 G (-0.3%)
this pin, upstream's aligned layout	2.2153 G (-1.4%)

bun, release build, old pin against this commit (same tree, only the pin and the define differ; 5 interleaved runs pinned to 8 cores; user-mode instructions and peak RSS, medians):

	old pin	this commit
`bun -e 1`	9.17 M, 25.9 MB	9.16 M (-0.06%), 25.7 MB
`Bun.serve` hello, per request (20000 sequential `fetch`)	51748, 48.2 MB	51787 (+0.08%), 48.5 MB
`JSON.stringify` + `JSON.parse` of a 200-user object, 3000 times	5102.3 M, 42.6 MB	5102.9 M (+0.01%), 42.6 MB
string concat + `Buffer.from` + `split`, 300 x 5000	1939.8 M, 58.9 MB	1904.3 M (-1.8%, the spread between runs is 2%), 57.8 MB
idle after start (`Bun.sleep`), RSS from `smaps_rollup`	32.4 MB (anon 3.85)	32.5 MB (anon 3.96)

Nothing is slower by more than 0.1%.

Tests with a release build of this commit (Linux x64)

test/js/bun/spawn/spawn-pipe-leak.test.ts: 25 of 25 runs pass. test/js/bun/jsc/heapStats-mimalloc.test.ts and test/js/bun/gc/gc-controller-cadence.test.ts: 10 of 10 each.
137 test files, 4280 tests pass: the two files of this PR, spawn-noread-leak, require-cache, heap-snapshot, bun-jsc, socket-retention, serve.test.ts, all of test/js/node/child_process, node-net, bunshell, fetch.test.ts, bun-install.test.ts, bundler_edgecase, all of test/js/web/workers and test/js/node/worker_threads, test/js/bun/resolve, test/js/bun/jsc, the --compile and bundler files, test/bake/dev.
8 tests fail, and the same 8 fail with a release build of the old pin: they need a non-root user (a port below 1024 that must be refused, EPERM after dropping privileges, unreadable files and directories).
Small structure heaps: BUN_JSC_structureHeapSizeInKB = 262144, 131072, 65536 and ulimit -v = 3000000, 2000000, 1500000 all start.
Debug (ASAN) build: heapStats-mimalloc, gc-controller-cadence, test/js/web/workers/worker.test.ts pass; child_process.test.ts has the one root-only failure.
src/static.c compiles with the new define and bun's flags for macOS arm64/x64 and Windows x64/arm64 (release and debug). Nothing here was run on macOS or Windows.

no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/jsc/heapStats-mimalloc.test.ts, test/js/bun/gc/gc-controller-cadence.test.ts

robobun · 2026-09-13T06:38:42Z

^{Updated 7:53 AM PT - Sep 13th, 2026}

✅ @robobun, your commit 7e83bb8a4b37d4fb12faecaeb40f3cf5d93ee36c passed in Build #115162! 🎉

🧪 To try this PR locally:

bunx bun-pr 42571

That installs a local version of the PR into your bun-42571 executable, so you can run:

bun-42571 --bun

robobun · 2026-09-13T06:39:09Z

Status (2026-09-13 16:40 UTC)

Head is 7e83bb8. CI is green on it (build 115162). The one red job of the build before (test/cli/run/require-cache.test.ts on x64-asan, a 60 s timeout) was not from this PR: that file takes 52 to 60 s on that lane in unrelated builds too. I reported it for triage.
The pin is ab135013 (the merge of Sync upstream dev3 (2026-09-12, 636510a3, v3.5.2) mimalloc#37 on bun-dev3-v2), with arena: a forced purge waits for the pass in progress instead of being dropped mimalloc#38 and arena: what is freed during a purge pass is purged by the next pass mimalloc#39 as ancestors. The purge code of Allow build to output code that runs on Node.js #39 is unchanged in it.
How I reproduced it: memory freed while the purge thread is at work ... in test/js/bun/jsc/heapStats-mimalloc.test.ts fails 3 of 3 on a release build of main ({"released":262.6,"purged":274.4}, expected >= 336) and passes on a release build of this branch, together with the Bun.gc(true) test in gc-controller-cadence.test.ts (6 of 6 runs). bun bd test skips both, because the debug build is ASAN and they need mimalloc as malloc.
Reach, corrected in the description: stock two arena bun shows it with several busy threads and no ticking event loop. 8 workers that churn ArrayBuffers and then spin hold 578 MB RSS on main's release build and fall to 91 MB with this pin.
Self-reviewed after the merge of arena: what is freed during a purge pass is purged by the next pass mimalloc#39: 4 points raised. 3 are addressed: the idle pull-forward in _mi_arenas_purge_now now uses the CAS paths, the fork test has a two arena part (both in arena: the idle pull-forward uses the CAS paths, and a two arena test for the purge schedule mimalloc#42, no pin bump needed for it), and the reach wording here is corrected. 1 is rejected: a debug check that an armed arena implies an armed subprocess can fail without a defect, because a free arms the arena first and the subprocess second.
All review threads are resolved.
Nothing here was run on macOS or Windows locally. The purge tests are Linux only.

Next: a maintainer merges. oven-sh/mimalloc#42 can ride with the next pin bump.

coderabbitai · 2026-09-13T06:42:14Z

Walkthrough

The mimalloc pin and build settings changed. Tests cover JSC structure-heap reservations and memory release during active purge and forced garbage collection on Linux without ASAN.

Changes

mimalloc purge behavior

Layer / File(s)	Summary
Update mimalloc pin and build settings `scripts/build/deps/mimalloc.ts`	The pinned mimalloc commit changed to `ab13501334a8da2b64ab2e5f4f552c3a39b96350`. The build now defines `MI_FREE_USE_PAGEMAP`.
Add purge regression coverage `test/js/bun/jsc/heapStats-mimalloc.test.ts`, `test/js/bun/gc/gc-controller-cadence.test.ts`	The tests verify JSC structure-heap reservations and memory release during active purge and forced garbage collection. Linux-only tests skip ASAN runs and assert successful subprocess completion.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to 6d690

The structure-heap regression can pass under ASAN without testing the allocator configuration it is intended to protect. Skip it under ASAN before relying on this coverage.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the two primary mimalloc purge fixes. It is specific and related to the main changes, although it does not mention the pin update or pagemap define.
Description check	✅ Passed	The description provides detailed coverage of the PR purpose, implementation, test changes, verification results, platform limits, and known test failures. It does not use the template headings, but i…

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Inline comments:
In `@test/js/bun/jsc/heapStats-mimalloc.test.ts`:
- Line 156: Update the heapStats test to assert the RSS release value before
asserting purged, using the released field from the parsed final result; keep
the existing purged threshold assertion afterward so the test verifies release
before the final collection.

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robobun · 2026-09-13T06:54:53Z

Review round on 67f2910, handled in d57cf2c:

Test assertion order (heapStats-mimalloc.test.ts): taken. The wait loop keeps the last held - rss() it read, so the drop in RSS is known before the final heapStats() call polls the allocator. The test asserts that drop first and the purged counter after it. Both thresholds are 336 MB (the fix ends at about 369 MB released of 384 MB freed, main's release build at 262.6). 20 of 20 runs pass with the new pin, 3 of 3 fail on main's release build. Thread resolved.
Pin on an unmerged PR head (scripts/build/deps/mimalloc.ts): correct, and it is the reason for the first line of the description. The pin moves to a commit on bun-dev3-v2 after arena: a forced purge waits for the pass in progress instead of being dropped mimalloc#38 and arena: what is freed during a purge pass is purged by the next pass mimalloc#39 merge. That thread stays open until then.

I also rewrote the description into the usual Problem / Fix / Background shape. The earlier text is in its Notes block.

claude

Code review found no new issues

No new issues were found in this update; 1 finding from earlier reviews is still open above.

Bumps mimalloc to 707d90be (oven-sh/mimalloc#39, which is stacked on oven-sh/mimalloc#38). The allocator's purge thread (the scavenger) hands freed arena memory back to the OS 100 ms after the free. What a thread freed while the scavenger was in the middle of a pass could be left out for good: the arena was marked as scheduled, the scavenger was not, and no later free into that arena scheduled it again. The memory stayed resident until the event loop went idle or something forced a collection. It took a pass in which each arena had something to purge. JSC's structure heap is a mimalloc arena of its own that rarely has, which hides most of it in bun. With one arena (Malloc=1) each such free was lost. The pass now resets the schedule before it looks at the arenas, so a free behind it schedules the next pass, and puts back what is still pending at its end. The second commit of oven-sh/mimalloc#39 (707d90be) releases the purge guard in mi_process_done on Windows, where the process detach callback runs after the system terminated the purge thread and the forced collect at exit would wait for that guard without end. bun builds mimalloc with MI_NO_PROCESS_DETACH and never runs mi_process_done, so that part is for the fork's own tests.

heapStats() calls mi_collect(false), and that poll runs a purge pass that is due by itself. The drop in RSS is now taken in the wait loop, before the last heapStats() call, and asserted first. The counter is asserted after it.

…hread is in a pass The mimalloc bump in this branch makes a forced collect wait for a purge pass in progress instead of being dropped. This is the test from #42543, which bumped to the first of the two mimalloc commits only. No-Verification-Needed: test-only commit (test/js/bun/gc/gc-controller-cadence.test.ts)

claude

Code review found no issues

No high-confidence issues detected in this change.

Pins ab13501334a8, the merge of oven-sh/mimalloc#37 on bun-dev3-v2. On top of the two purge fixes this branch already carried (#38, #39) it brings: - microsoft/mimalloc dev3 through v3.5.2 (301 commits): always-on statistics (MI_STATS=1), sampled profiler hooks (mi_profiler_t, MI_PROFILE=1; the fork's own prof.c and mi_prof_* are gone, bun used none of it), codegen work in free/zalloc/memzero, double-free detection through the padding canary - fork(): no arena purge pass runs across it any more (the macOS Debug SIGBUS of a forked child, and a whole class of torn purge state in the child) - mi_heap_destroy of a heap with a block still on a page's thread-free list no longer reports corrupted meta-data; two slips in upstream's free.c fixed (mi_usable_size always took its slow path; an overflow could be reported as a double free) - malloc/free fast paths update the packed used count with one instruction: 25 M malloc + 25 M free with bun's flags execute 2.2403 G instructions in this configuration, 2.2463 G with the old pin MI_FREE_USE_PAGEMAP=1 keeps the page-map lookup in mi_free. Upstream's new default puts page meta data at 256 MiB boundaries and needs every arena to start on one; mi_manage_os_memory_ex then cannot take JSC's structure heap when its reservation is 256 MiB or less, and bun aborted on startup under `ulimit -v 3000000` or BUN_JSC_structureHeapSizeInKB<=262144. With the define that works down to 64 MiB as before. The aligned layout is worth another 1.1% of instructions on that benchmark (2.2153 G) once WebKit hands mimalloc an aligned region. Every mi_* function and mi_option number bun declares is unchanged (options 0..46; 47 is collect_merges_stats now, snapshot_on_exit 48). Release build, old pin against this one, 5 interleaved runs, user-mode instructions / peak RSS: bun -e 1 9.17 M / 25.9 MB 9.16 M / 25.7 MB Bun.serve hello 51748 / request 51787 / request (+0.08%), 48.2 MB / 48.5 MB JSON round trips 5102.3 M / 42.6 MB 5102.9 M / 42.6 MB string concat 1939.8 M / 58.9 MB 1904.3 M / 57.8 MB (run to run spread 2%)

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Inline comments:
In `@test/js/bun/gc/gc-controller-cadence.test.ts`:
- Line 315: Update the loop around Bun.gc(true) to record whether RSS crosses
the 32 MiB threshold before the deadline, and only invoke Bun.gc(true) when that
crossing was observed. If the loop times out without observing it, fail or retry
the round before the forced GC call; do not infer success from held - rss()
after the loop.

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claude

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

… at work The wait for the purge thread ends when RSS fell by 32 MB or after one second. A round that ran into the deadline passed as well, although Bun.gc(true) then purged all of it by itself and the round said nothing about the two at once. Each round now records whether the drop was seen before Bun.gc(true) ran, and the test asserts it.

JSC hands its structure heap to mimalloc as an arena of its own and halves the reservation when address space is short. Without MI_FREE_USE_PAGEMAP the allocator refuses a reservation of 256 MiB or less and bun aborts on startup. BUN_JSC_structureHeapSizeInKB=131072 takes that path: a release build without the define exits with 134 (abort() called), with it the script runs.

robobun · 2026-09-13T13:58:16Z

Review round on 308125b, two test commits on top of it (no change to the pin or the build):

4c5fafd, gc-controller-cadence.test.ts: each round records whether RSS fell by 32 MB before Bun.gc(true) ran, and the test asserts it. A round that ran into the one second deadline used to pass although it said nothing about the purge thread and Bun.gc(true) at once.
6d690f9, heapStats-mimalloc.test.ts: bun starts with BUN_JSC_structureHeapSizeInKB=131072. A release build of 308125b without MI_FREE_USE_PAGEMAP aborts there (exit 134, also for 262144 and under ulimit -v 3000000). With the define it runs down to 65536.

I also checked the new pin. _mi_arenas_try_purge and the scavenger loop at ab135013 are the same as in oven-sh/mimalloc#39, the fork's purge tests pass there, and both purge tests of this PR pass 6 of 6 runs on a release build of 308125b.

This push restarts CI. Sorry for that if a merge was close.

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In `@test/js/bun/jsc/heapStats-mimalloc.test.ts`:
- Line 117: Update the test named “starts with a small structure heap
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…imalloc#41) On Linux mimalloc counts an arena slice as committed the first time it hands it out. A purge returns the slice to the OS with MADV_DONTNEED, which keeps it committed, and then forgets that it was handed out so that the next mi_zalloc can skip its memset. The next allocation counted the slice again, and nothing counted it down. `committed` grew by the size of every free-and-reuse cycle, forever: 16 x 1 MB freed and taken again 8 times read 18 MB, 36, 53, 141 MB against 30 MB resident. That is the number the crash report prints as `Commit` (28 GB after 8.8 days in #42652) and `heapStats().mimalloc.committed`. Bump to oven-sh/mimalloc ac33e6a7 (oven-sh/mimalloc#41, two commits on top of the ab135013 that #42571 pinned): the purge debits the slices it forgets, under the condition the allocation uses to credit them, and an arena that is not committed up front is counted on its commit bits alone. Test in test/js/bun/jsc/heapStats-mimalloc.test.ts: 8 rounds of 16 x 1 MB, freed and purged by the purge thread. `committed` has to hold one round's worth at the top of each round and come back down after the purge. Fails on 1.4.2 (35 MB after round 1, 141 MB after round 7), passes with the bump (20 MB held, 3 MB after).

…ix two CI test deadlocks (#42665) ### What does this PR do? Three of the tests that have been turning builds red over the last few days, by root cause. (The fourth and largest, `spawn-pipe-leak.test.ts`, was #42428 and is already fixed on main by #42571.) **1. `Bun.build` left all but one bundler `Worker` alive after the build** (`bun-build-api.test.ts` "bytecode: repeated builds don't retain the generated code", flaky in 129 of 198 builds on 09-13) - At the end of a build each pool thread gets an idle task that tears down its `Worker` (its AST heap, its transpiler), and `wake_for_idle_events()` wakes the pool. It woke every parked thread but posted one notification. One thread consumed it, left `Event::wait` and drained its idle queue. The rest saw `WAITING` again and went back into the futex without looking at their queues. - So after a `Bun.build` every worker but one stayed allocated until its thread next ran a task. With `bpftrace` on main's release build: 13 to 19 `get_worker_slow`, one `Worker::deinit`. RSS after bundling one 2.5 MB file and `Bun.gc(true)`: **~160 MB in 8 of 10 runs, ~60 MB in the 2 where the lucky thread was the one that parsed the file**. With this PR: 57 to 63 MB in 10 of 10. - The IO pool that reads files on macOS and Windows was never woken, so its threads kept theirs too. - The test compares RSS after three bytecode builds against a baseline taken right after one plain build. Since #42428 `Bun.gc(true)` returns freed memory at once, so the baseline became bimodal (60 or 160) and the comparison failed whenever it came out low. Same fixture, 24 runs, 12 at a time: main fails 2 (deltas 96, 101), this branch 0 (max 31). Fix, in `src/threading/ThreadPool.rs`: the upper bits of the `Event` word count `wake_all()` calls, and a waiter drains its idle queue after publishing `WAITING` and before each sleep. A `wake_all()` that lands between the drain and the sleep changes the word `Futex::wait` expects, so nothing is lost; no notification is posted, so every waiter goes back to waiting. `shutdown()` is a `fetch_or` (SHUTDOWN is both state bits) and `notify()` no longer overwrites SHUTDOWN. The bundler's pool wakes its IO pool as well. `bundlerWorkerLiveCount()` is added to `bun:internal-for-testing` (same shape as `sslCtxLiveCount`). New test: build, then the count must reach 0; run with and without `BUN_FEATURE_FLAG_FORCE_IO_POOL`. Without the `Event` change it prints `[15,16]`; without the IO pool wake the forced-IO-pool row stays at 1. **2. `fetch-backpressure.test.ts` "stalled no consumer drains the full body"** (90 s timeout, windows 11 aarch64 only, ~65% of builds flaky, ~5% red; since #40965) Not a fetch bug: the test's handshake can deadlock. `settled()` waits for the server to have written more than 8 chunks, the client waits for `settled()` before it reads, and a client nothing reads pauses its socket after 256 KiB (`BODY_HIGH_WATER_MARK`, 4 chunks). Where loopback buffers take fewer than 5 more chunks, the server is waiting for `'drain'` at 8 chunks or fewer and nobody moves. Reproduced on Linux with main's CI release build in a netns with `tcp_rmem`/`tcp_wmem` capped at 64 KiB: the three h1 "no consumer" cases time out. The threshold is now what a client guarantees to take (the mark). Whole file, patched: 70/70 at 16 KiB, 64 KiB, 96 KiB and default buffers. **3. `bun-patch.test.ts` "packages whose label is longer than 1024 bytes"** (Windows, ~60% of builds flaky, ~4% red) Nothing to do with the label length. `scripts/runner.node.mjs` exports `BUN_INSTALL_CACHE_DIR`, which wins over the `cache` the harness writes to each test's bunfig, and two of the three `test.concurrent` cases install the same tarball spec, so they extract into the same `@T@<hash>` folder. The test now pins the cache per project, as `bun-dedupe`, `bun-install-patch` and `bun-install-offline` already do. That leaves the product side, which this PR does not change: on Windows the loser of that race renames the winner's folder away and `delete_tree`s it (`extract_tarball.rs`, the retry branch), where POSIX does an atomic `renameat2(EXCHANGE)`. The winner then sees `failed to resolve cache dir: EBADF`, `"package.json" failed to open: ENOENT`, or a failed hardlink. A local tarball is always re-extracted and keyed only by its spec string, so "reuse what is there" is not available for it. ### How did you verify your code works? - New test fails with the `Event` change gutted (`[15,16]`), and its IO pool row fails with the IO pool wake removed; both pass with the fix on debug+ASAN and release. - `bytecode: repeated builds don't retain the generated code`: 10/10 on this branch's release build, 7/10 on main's. - 40 rounds each (default and forced IO pool) of 4 concurrent `Bun.build`s + 16 `readFile`s + 4 `Bun.password.hash` on the same pool, release and debug: no worker left, no hang. `bun build` CLI (owned pool, shutdown + join) exits. - `fs.promises.stat` in a loop, which goes through `Event::notify` on every call, interleaved against main's release build on 8 cores: 13.9/13.9/14.2 vs 13.3/15.0/14.0 µs/op. - `test/js/node/fs/fs.test.ts`, `test/bundler/bundler_splitting.test.ts`, `test/bake/dev/bundle.test.ts`, `test/bake/deinitialization.test.ts`, `test/cli/install/bun-patch.test.ts`, `test/js/web/fetch/fetch-backpressure.test.ts` on the debug build. - The Windows halves of 2 and 3 are from CI logs and code; this PR's Windows lanes are the check.

Both tests are new in oven-sh#42571 and gate on !isLinux || isASAN. On OHOS the forced release works (Bun.gc(true) frees ~384 MB) and the purge counter reaches ~387 MB, but madvise'd pages are not deducted from RSS, so the RSS-delta waits never observe the background purge pass. Failures are identical with the pre-merge pin, so this is platform accounting, not a regression.

…freed during a pass is purged by the next (oven-sh#42571) Both fork PRs are merged: oven-sh/mimalloc#38 and oven-sh/mimalloc#39. The pin `707d90be` is on `bun-dev3-v2` (they were merged with merge commits, so the sha did not change). This PR now carries both bumps and both tests; it replaces oven-sh#42543. ### Two fixes in the fork **1. A forced purge waits for the pass in progress (oven-sh/mimalloc#38).** - `Bun.gc(true)` ends with `mi_collect(true)` since oven-sh#42428. mimalloc dropped that purge when another thread held its purge guard, and the purge thread holds it for 36 to 47 ms while it purges what was already freed. `Bun.gc(true)` then returned with all of it still resident. - `test/js/bun/spawn/spawn-pipe-leak.test.ts` fails on main because of it: `Peak RSS: warmup 46 MB -> main 312 MB (573%)`, 7 of 10 runs pass. With this pin: 25 of 25. - Only a forced collect waits. bun's one caller is `garbage_collect_from_js`. It takes 29 to 47 ms in place of 1 to 2 ms when it collides with a pass over 384 MB, and the memory is back with the OS when it returns. - Test: `test/js/bun/gc/gc-controller-cadence.test.ts`, "Bun.gc(true) returns what is free to the OS while the purge thread is at work". Fails 3 of 3 on main's release build (38 to 42 MB released, expected more than 300). **2. What is freed during a purge pass is purged by the next pass (oven-sh/mimalloc#39).** The rest of this description. ### Problem - Memory freed while mimalloc's purge thread (the scavenger) is in a pass can stay resident until the event loop goes idle or something forces a collection. With one arena (`Malloc=1`) a busy script keeps 110 of 384 MB: `{"released":262.6,"purged":274.4}`. - In the fork, `_mi_arenas_try_purge` (`src/arena.c`) keeps its old `subproc->purge_expire` during a pass, so a free in that time arms the arena only. The scavenger then clears the value, and no later free sets it again. - It reaches bun's default configuration. JSC's structure heap is a second arena that rarely purges, which hides the case of one busy thread. With several busy threads and no ticking event loop it shows each time: 8 workers that churn `ArrayBuffer`s and then spin hold 578 MB RSS on main's release build and fall to 91 MB with this pin. ### Fix - Bump `MIMALLOC_COMMIT` to `707d90be` (oven-sh/mimalloc#39, which contains oven-sh/mimalloc#38). The bump also picks up `0fecc729`, a 32-bit shift fix in `src/prof.c` that was already on `bun-dev3-v2`. - In the fork a pass now resets `subproc->purge_expire` first and puts back what is pending at its end, so a free behind the pass schedules the next one. - Verified: the new test in `test/js/bun/jsc/heapStats-mimalloc.test.ts` fails 3 of 3 on main's release build, passes 20 of 20 with this pin. `bun bd test` skips it (ASAN). ### Background - A purge returns freed mimalloc arena memory to the OS (`madvise`), 100 ms after the free. - The scavenger is the fork's thread that runs those purges. It sleeps until `subproc->purge_expire`. - The first free into an arena after a pass arms it (`arena->purge_expire`). Only that free looks at `subproc->purge_expire`. <details><summary>Notes</summary> **The two commits of oven-sh/mimalloc#39.** - `1b894afe` arena: what is freed during a purge pass is purged by the next pass (the fix) - `707d90be` init: release the purge guard at process exit on Windows **The free path (`mi_arena_schedule_purge`) is unchanged.** Only the pass and the scavenger loop change. **The defect in full.** A free arms its arena (`arena->purge_expire`) and, only if the arena was not armed yet, arms the subprocess (`subproc->purge_expire`, 0 -> set), which is what the scavenger sleeps on. A pass resets the arena expire when it goes into the arena, but it kept its old subprocess expire until its end. The first free into the arena behind the pass armed the arena again, found the subprocess expire still set, and left it alone. Then `_mi_arenas_try_purge` skipped its update (the break for `max_purge_count` also fires on the last arena, so a pass in which each arena purged ended with `all_visited = false`), and `mi_scavenger_run` set `subproc->purge_expire` to 0. The arena was armed and the subprocess was not. Each later free into that arena saw an armed arena and stopped there. The 30 s timeout of the scavenger did not help: with nothing scheduled it only waits again. **Reach in bun.** JSC registers its structure heap as an exclusive mimalloc arena (`mi_manage_os_memory_ex` in `StructureAlignedMemoryAllocator.cpp`). A pass over two arenas in which only the main one purges ends with `all_visited = true`, reads the re-armed expire of the main arena after its walk, and installs it. So the same script that loses 128 MB with one arena gets its second pass on time with two (traced on main's release build: first pass at 107 to 123 ms, second at 207 ms). It shows when both arenas purge in the same pass, when a process has more arenas (more than 1 GiB of mimalloc memory) and each purges, and with `Malloc=1`. The event loop's `mi_on_thread_idle_start` on each `epoll_wait` heals it when the sweep runs to its end. Under load the owner wakes first and the sweep stops before `_mi_arenas_purge_now`. The state is absorbing: once an arena is armed and the subprocess is not, nothing but a park or a forced collect ends it. So a low chance per pass is enough for a process whose threads stay busy. Measured with stock two arena bun (release, Linux x64): 8 workers allocate and free 1 to 8 MB `ArrayBuffer`s for 3 s (`transfer(0)` frees at once), free everything, and spin, while the main thread sits in `Atomics.wait`. After a quiet 2.5 s RSS is 578 MB on main and 91 MB with this pin (`arena_count` 2 in both). The same happens with only the one line fix for `all_visited` in the fork, which is why the pass resets the subprocess expire first. **The test.** A child frees 256 MB with `ArrayBuffer.prototype.transfer(0)` (no collection involved), spins until RSS fell by 64 MB (the purge thread is in its pass), frees 128 MB more, and spins without an `await` until RSS fell by 336 MB, for at most 2 s. It reports that drop and the growth of `heapStats().mimalloc.purged`. Both have to be at least 336 MB. The drop in RSS is taken before the last `heapStats()` call, because that call polls the allocator (`mi_collect(false)`), and a poll runs a pass that is due by itself. `Malloc=1` makes JSC allocate through `malloc`, which is mimalloc on Linux, so there is one arena. Linux only (the wait reads RSS) and not ASAN (malloc is not mimalloc there). | build | result | | --- | --- | | release build of main (`6a92015fc`, pin `6a64e1ba`) | fails 3 of 3: `{"released":262.6,"purged":274.4}`, expected >= 336 | | `bun run build:release` with this pin | passes 20 of 20, about 450 ms | | `bun bd test` with this pin (debug, ASAN) | new test skipped, the other 4 tests in the file pass | The change is under `scripts/`, so a checkout of `src/` from main does not undo it. The fail-before check needs a release build of main. **About `707d90be`.** `1b894afe` alone failed the 5 Windows jobs of the fork's CI. Its new test returns from `main` while the scavenger is in its last pass. On Windows `mi_process_done` runs in the process detach callback, after the system terminated every other thread, so the purge guard stays held, and the forced collect at exit waits for it without end (that wait is what oven-sh/mimalloc#38 adds). `707d90be` releases the guard there. bun builds mimalloc with `MI_NO_PROCESS_DETACH` and `MI_SKIP_COLLECT_ON_EXIT`, so bun does not reach that code on any platform. Fork CI at `707d90be`: 15 of 15 jobs pass (Windows x64 and arm64, mingw, macOS, Linux x64 and arm64 with ASAN, UBSAN, TSAN and guarded builds, FreeBSD). **In the fork.** New `test/test-purge-behind-pass.c`: 5 of 5 runs fail before (`266 of 320 MiB`), 30 of 30 pass after. A stress harness with 8 to 16 busy threads: 15 of 15 rounds end stuck before, with about 530 MiB of free committed ranges unpurged. After: 750 rounds, 0 stuck. Details are in oven-sh/mimalloc#39. **Effect on purge activity.** Ranges that stayed resident by accident are now purged 100 ms after the free, which is what `purge_delay` promises. **Suites run with a release build of this branch (Linux x64).** `test/js/bun/jsc/heapStats-mimalloc.test.ts`, `test/js/bun/gc/gc-controller-cadence.test.ts` (11 pass), `test/js/bun/spawn/spawn-pipe-leak.test.ts` (3 pass), `test/js/bun/spawn/spawn-noread-leak.test.ts`, `test/js/bun/jsc/bun-jsc.test.ts` (38 pass). Nothing here was run on macOS or Windows. </details> ### Now pins the merged upstream sync `MIMALLOC_COMMIT` is `ab13501334a8da2b64ab2e5f4f552c3a39b96350`: the merge of oven-sh/mimalloc#37 on `bun-dev3-v2`. It contains `707d90be` (both fixes above), so everything in this description still holds. **What it adds beyond oven-sh/mimalloc#38 and oven-sh/mimalloc#39** - microsoft/mimalloc `dev3` through v3.5.2 (301 commits): always-on statistics (`MI_STATS=1`), sampled profiler hooks (`mi_profiler_t`, `MI_PROFILE=1`), codegen work in free/zalloc/memzero, double-free detection through the padding canary. - The fork's own heap profiler is gone (`src/prof.c`, `mi_prof_*`, `MIMALLOC_PROF_*`); upstream's hooks replace it. bun referenced none of it. - `fork()`: no arena purge pass runs across it any more. That was the macOS Debug SIGBUS of a forked child in the fork's CI, and a whole class of torn purge state in the child. - `mi_heap_destroy` of a heap that still has a block on a page's thread-free list no longer reports corrupted meta-data. - Two slips in upstream's `free.c` are fixed: `mi_usable_size` always took its slow path, and an overflow could be reported as a double free. - The malloc/free fast paths update upstream's packed used count with one instruction (`decw` / `addq` on memory). **One new define: `MI_FREE_USE_PAGEMAP=1`** - Upstream's new default keeps page meta data at 256 MiB boundaries so that `mi_free` needs no page map. Every arena then has to start on such a boundary. - JSC gives mimalloc its structure heap as an arena (`mi_manage_os_memory_ex(start + 16 KiB, ...)`, under a `RELEASE_ASSERT`), and halves that reservation when address space is short. With the new default `mi_manage_os_memory_ex` refuses it once the reservation is 256 MiB or less. - Measured with a release build of the new pin without the define: `ulimit -v 3000000; bun -e 1` aborts on startup, and so does `BUN_JSC_structureHeapSizeInKB=262144`. The old pin runs both. With the default 4 GiB reservation it works but loses the first 256 MiB of structure address space. - With the define, `mi_free` looks the page up through the page map as it does today, and the structure heap works down to 64 MiB as before. The fork's CI runs this configuration (debug and release) on Linux x64 and arm64. - The aligned layout is worth another 1.1% of instructions on the malloc/free benchmark below. Getting it needs WebKit to hand mimalloc an aligned region (`mi_arena_min_alignment()`), which is a separate change. **Not changed: `MI_PROFILE` stays at its default of 1.** Nothing in bun attaches a profiler yet, and `MI_PROFILE=0` would save about 1.4% of instructions on the same benchmark. The hooks are what a `Bun.pprof.heap` API would build on, so they stay compiled in. **Bindings.** Every `mi_*` function that `src/mimalloc_sys/mimalloc.rs` and the C++ side declare is exported with an unchanged declaration. `mi_heap_area_t` has the same layout. `mi_option_t` 0..46 did not move (47 is `collect_merges_stats` now, `snapshot_on_exit` 48); bun sets option 0 only. The `heapStats().mimalloc` keys the tests read (`purged`, `page_bins`, `pages`, `committed`) are emitted as before. **Allocator micro benchmark** (25 M `mi_malloc` + 25 M `mi_free` of 16..1039 bytes, bun's compile flags, user-mode instructions): | | instructions | | --- | --- | | old pin (`707d90be`) | 2.2463 G | | this pin, `MI_FREE_USE_PAGEMAP=1` (what this PR builds) | **2.2403 G** (-0.3%) | | this pin, upstream's aligned layout | 2.2153 G (-1.4%) | **bun, release build, old pin against this commit** (same tree, only the pin and the define differ; 5 interleaved runs pinned to 8 cores; user-mode instructions and peak RSS, medians): | | old pin | this commit | | --- | --- | --- | | `bun -e 1` | 9.17 M, 25.9 MB | 9.16 M (-0.06%), 25.7 MB | | `Bun.serve` hello, per request (20000 sequential `fetch`) | 51748, 48.2 MB | 51787 (+0.08%), 48.5 MB | | `JSON.stringify` + `JSON.parse` of a 200-user object, 3000 times | 5102.3 M, 42.6 MB | 5102.9 M (+0.01%), 42.6 MB | | string concat + `Buffer.from` + `split`, 300 x 5000 | 1939.8 M, 58.9 MB | 1904.3 M (-1.8%, the spread between runs is 2%), 57.8 MB | | idle after start (`Bun.sleep`), RSS from `smaps_rollup` | 32.4 MB (anon 3.85) | 32.5 MB (anon 3.96) | Nothing is slower by more than 0.1%. **Tests with a release build of this commit (Linux x64)** - `test/js/bun/spawn/spawn-pipe-leak.test.ts`: 25 of 25 runs pass. `test/js/bun/jsc/heapStats-mimalloc.test.ts` and `test/js/bun/gc/gc-controller-cadence.test.ts`: 10 of 10 each. - 137 test files, 4280 tests pass: the two files of this PR, `spawn-noread-leak`, `require-cache`, `heap-snapshot`, `bun-jsc`, `socket-retention`, `serve.test.ts`, all of `test/js/node/child_process`, `node-net`, `bunshell`, `fetch.test.ts`, `bun-install.test.ts`, `bundler_edgecase`, all of `test/js/web/workers` and `test/js/node/worker_threads`, `test/js/bun/resolve`, `test/js/bun/jsc`, the `--compile` and bundler files, `test/bake/dev`. - 8 tests fail, and the same 8 fail with a release build of the old pin: they need a non-root user (a port below 1024 that must be refused, `EPERM` after dropping privileges, unreadable files and directories). - Small structure heaps: `BUN_JSC_structureHeapSizeInKB` = 262144, 131072, 65536 and `ulimit -v` = 3000000, 2000000, 1500000 all start. - Debug (ASAN) build: `heapStats-mimalloc`, `gc-controller-cadence`, `test/js/web/workers/worker.test.ts` pass; `child_process.test.ts` has the one root-only failure. - `src/static.c` compiles with the new define and bun's flags for macOS arm64/x64 and Windows x64/arm64 (release and debug). Nothing here was run on macOS or Windows.  --- **no test proof** · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/jsc/heapStats-mimalloc.test.ts, test/js/bun/gc/gc-controller-cadence.test.ts  --------- Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>

…ix two CI test deadlocks (oven-sh#42665) ### What does this PR do? Three of the tests that have been turning builds red over the last few days, by root cause. (The fourth and largest, `spawn-pipe-leak.test.ts`, was oven-sh#42428 and is already fixed on main by oven-sh#42571.) **1. `Bun.build` left all but one bundler `Worker` alive after the build** (`bun-build-api.test.ts` "bytecode: repeated builds don't retain the generated code", flaky in 129 of 198 builds on 09-13) - At the end of a build each pool thread gets an idle task that tears down its `Worker` (its AST heap, its transpiler), and `wake_for_idle_events()` wakes the pool. It woke every parked thread but posted one notification. One thread consumed it, left `Event::wait` and drained its idle queue. The rest saw `WAITING` again and went back into the futex without looking at their queues. - So after a `Bun.build` every worker but one stayed allocated until its thread next ran a task. With `bpftrace` on main's release build: 13 to 19 `get_worker_slow`, one `Worker::deinit`. RSS after bundling one 2.5 MB file and `Bun.gc(true)`: **~160 MB in 8 of 10 runs, ~60 MB in the 2 where the lucky thread was the one that parsed the file**. With this PR: 57 to 63 MB in 10 of 10. - The IO pool that reads files on macOS and Windows was never woken, so its threads kept theirs too. - The test compares RSS after three bytecode builds against a baseline taken right after one plain build. Since oven-sh#42428 `Bun.gc(true)` returns freed memory at once, so the baseline became bimodal (60 or 160) and the comparison failed whenever it came out low. Same fixture, 24 runs, 12 at a time: main fails 2 (deltas 96, 101), this branch 0 (max 31). Fix, in `src/threading/ThreadPool.rs`: the upper bits of the `Event` word count `wake_all()` calls, and a waiter drains its idle queue after publishing `WAITING` and before each sleep. A `wake_all()` that lands between the drain and the sleep changes the word `Futex::wait` expects, so nothing is lost; no notification is posted, so every waiter goes back to waiting. `shutdown()` is a `fetch_or` (SHUTDOWN is both state bits) and `notify()` no longer overwrites SHUTDOWN. The bundler's pool wakes its IO pool as well. `bundlerWorkerLiveCount()` is added to `bun:internal-for-testing` (same shape as `sslCtxLiveCount`). New test: build, then the count must reach 0; run with and without `BUN_FEATURE_FLAG_FORCE_IO_POOL`. Without the `Event` change it prints `[15,16]`; without the IO pool wake the forced-IO-pool row stays at 1. **2. `fetch-backpressure.test.ts` "stalled no consumer drains the full body"** (90 s timeout, windows 11 aarch64 only, ~65% of builds flaky, ~5% red; since oven-sh#40965) Not a fetch bug: the test's handshake can deadlock. `settled()` waits for the server to have written more than 8 chunks, the client waits for `settled()` before it reads, and a client nothing reads pauses its socket after 256 KiB (`BODY_HIGH_WATER_MARK`, 4 chunks). Where loopback buffers take fewer than 5 more chunks, the server is waiting for `'drain'` at 8 chunks or fewer and nobody moves. Reproduced on Linux with main's CI release build in a netns with `tcp_rmem`/`tcp_wmem` capped at 64 KiB: the three h1 "no consumer" cases time out. The threshold is now what a client guarantees to take (the mark). Whole file, patched: 70/70 at 16 KiB, 64 KiB, 96 KiB and default buffers. **3. `bun-patch.test.ts` "packages whose label is longer than 1024 bytes"** (Windows, ~60% of builds flaky, ~4% red) Nothing to do with the label length. `scripts/runner.node.mjs` exports `BUN_INSTALL_CACHE_DIR`, which wins over the `cache` the harness writes to each test's bunfig, and two of the three `test.concurrent` cases install the same tarball spec, so they extract into the same `@T@<hash>` folder. The test now pins the cache per project, as `bun-dedupe`, `bun-install-patch` and `bun-install-offline` already do. That leaves the product side, which this PR does not change: on Windows the loser of that race renames the winner's folder away and `delete_tree`s it (`extract_tarball.rs`, the retry branch), where POSIX does an atomic `renameat2(EXCHANGE)`. The winner then sees `failed to resolve cache dir: EBADF`, `"package.json" failed to open: ENOENT`, or a failed hardlink. A local tarball is always re-extracted and keyed only by its spec string, so "reuse what is there" is not available for it. ### How did you verify your code works? - New test fails with the `Event` change gutted (`[15,16]`), and its IO pool row fails with the IO pool wake removed; both pass with the fix on debug+ASAN and release. - `bytecode: repeated builds don't retain the generated code`: 10/10 on this branch's release build, 7/10 on main's. - 40 rounds each (default and forced IO pool) of 4 concurrent `Bun.build`s + 16 `readFile`s + 4 `Bun.password.hash` on the same pool, release and debug: no worker left, no hang. `bun build` CLI (owned pool, shutdown + join) exits. - `fs.promises.stat` in a loop, which goes through `Event::notify` on every call, interleaved against main's release build on 8 cores: 13.9/13.9/14.2 vs 13.3/15.0/14.0 µs/op. - `test/js/node/fs/fs.test.ts`, `test/bundler/bundler_splitting.test.ts`, `test/bake/dev/bundle.test.ts`, `test/bake/deinitialization.test.ts`, `test/cli/install/bun-patch.test.ts`, `test/js/web/fetch/fetch-backpressure.test.ts` on the debug build. - The Windows halves of 2 and 3 are from CI logs and code; this PR's Windows lanes are the check.

github-actions Bot added the claude label Sep 13, 2026

coderabbitai Bot reviewed Sep 13, 2026

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Comment thread test/js/bun/jsc/heapStats-mimalloc.test.ts Outdated