Report native stack overflows on every thread

[robobun]

Needs oven-sh/WebKit#741: the WebKit pin is its preview build.

Problem

• A native stack overflow kills bun with exit 139 and an empty stderr, on any thread. On 1.4.0 a 300-deep directory moved into a recursive fs.watch does it.
• JSC installs its SIGSEGV/SIGBUS handler without SA_ONSTACK. Only the main thread had a sigaltstack.

Fix

• [WTF] The signal handler runs on the alternate signal stack, and a thread that runs it there can be suspended WebKit#741 installs JSC's handler with SA_ONSTACK, except in ASAN builds. Every thread that runs configure_thread gets a 512 KiB alternate stack, and the compile cache thread runs it now.
• The report says Stack overflow for a data fault inside the frame being entered, and names the thread.
• Four threads (Bundler, fs.watch, FileWatcher, CFThreadLoop) get DEFAULT_THREAD_STACK_SIZE, not 2 MiB.
• Verified: test/cli/run/run-crash-handler.test.ts, and the fs.watch, vm, Worker, cpu-prof and compile cache suites.

Background

• After a stack overflow the kernel runs only an SA_ONSTACK handler, on an alternate stack.
• With the flag alone, bun --cpu-prof hung at a WebAssembly trap: a thread suspension inside JSC's handler retried with no limit. [WTF] The signal handler runs on the alternate signal stack, and a thread that runs it there can be suspended WebKit#741 fixes that.
• Considered a bun hook that puts the flag back after the first VM. It misses bun build --bytecode and hangs the same way.
• The recursions have their PRs: fs.watch: read each directory and close it before descending #44249 (walk_subtree of fs.watch) and bundler: keep deep CSS, export star and composes graphs from overflowing the linker's stack #38966 (the linker walks).

Downsides

• Each thread makes 6 more syscalls in its life and reserves 516 KiB of address space.
• A fault on an inaccessible page inside the current frame reports as Stack overflow, with no fault address.
• Still silent: an overflow on a WTF thread, and each overflow in an ASAN build.

Notes

Related open PRs. #34772 (guard-page faults reported as a stack overflow) is superseded by this change. #34775 (per-thread sigaltstack) is closed. #38966 rewrites the linker walks (CSS @import order, export *, composes, chunk graph) with explicit stacks. An earlier revision of this PR carried a StackCheck fallback for two of those walks. It was dropped in favor of #38966. #44249 rewrites walk_subtree of fs.watch: it reads a directory to its end and closes it before it visits a subdirectory, so the walk does not recurse and holds one descriptor at any depth. Earlier revisions of this PR had an explicit stack for that walk, which kept one descriptor for each level, and two tests. They were dropped in favor of #44249.

Reproductions on 1.4.0, each exit 139 with 0 bytes on stderr:

import fs from "node:fs";
fs.mkdirSync("src/" + "a/".repeat(300), { recursive: true }); fs.mkdirSync("watched");
fs.watch("watched", { recursive: true }, () => {});
setTimeout(() => fs.renameSync("src", "watched/moved"), 200);
setTimeout(() => { console.log("survived"); process.exit(0); }, 2000);

// 8000-file CSS @import chain or 6000-file export * chain, then
await Bun.build({ entrypoints: ["c/m0.css"], outdir: "out" });

With this change a native overflow prints panic(main thread): Stack overflow with the banner and exits by SIGSEGV. The same recursion on a worker named deep prints panic(deep): Stack overflow. The two Bun.build() chains link on the 4 MiB thread. A chain that needs more than 4 MiB still overflows, now with a report, until #38966. The fs.watch case, on a debug build without ASAN: a tree of 300 or 450 levels that moves in gives all its events on the 4 MiB fs.watch thread (301 and 451). A tree of 600 levels still overflows it, now with panic(fs.watch): Stack overflow, until #44249. An ASAN build stays silent, as on main.

Classification. The handler reads pc, fp and sp of the faulting frame from the ucontext. A fault is a stack overflow when all three hold:

• it is a data access (fault address != pc),
• the address is at most 4 KiB below sp (a push, a call, the x86-64 red zone) or above sp,
• the address is below the frame pointer, and at most 256 KiB above sp.

The first revision used only the distance from sp, as ASAN's IsStackOverflow does. Two probes showed what that mislabels when the stack is shallow, and both are tests now:

fault	distance from sp only	this PR	main
call through a pointer into the stack (top of [stack] - 4 KiB)	Stack overflow	Segmentation fault at address 0x7FFD...	same as this PR
read of the first byte past the top of [stack]	Stack overflow	Segmentation fault at address 0x7FFE...	same as this PR

Real overflows keep the label. Checked with C recursions loaded through bun:ffi (clang -O0 with frame pointers, -O2 -fomit-frame-pointer, -O2 -fstack-clash-protection; frames of 1 KiB, 16 KiB, 64 KiB and 200 KiB; first write at the low end and at the high end of the frame): 12 of 12 report Stack overflow. The reason encodes as 7 in the trace string, which bun.report already decodes (the Windows EXCEPTION_STACK_OVERFLOW reason).

What remains: a data access on an inaccessible page between sp - 4 KiB and the frame pointer gets the label. That is the guard page of the thread's own stack while the thread runs in its last frame, or a page inside the current frame that something made inaccessible. The report of an overflow does not print the fault address. Code without frame pointers can keep a value in the frame pointer register that is below the fault address. A real overflow there reports as Segmentation fault at address.

Measurements. Debug builds without ASAN of main (36cd151) and of this PR.

Calls counted with an LD_PRELOAD shim around sigaltstack, sigaction, mmap, mprotect and munmap, for a script that starts N Workers one after the other and terminates each:

	sigaltstack	mmap 516 KiB	mprotect guard	munmap 516 KiB	sigaction on crash signals
main, 0 / 4 / 8 Workers	3 / 3 / 3	0	0	0	11
this PR, 0 Workers	3	0	0	0	11
this PR, 4 Workers	15	4	4	4	11
this PR, 8 Workers	27	8	8	8	11

That is 6 calls for each thread (query, set and disable of the alternate stack, map, guard, unmap). The alternate stack is address space only: no page of it is written until a signal is delivered on it.

Thread census for std::thread::Builder spawns without .stack_size: Bundler (BundleThread.rs), fs.watch inotify and kqueue readers (path_watcher.rs), FileWatcher (Watcher.rs), CFThreadLoop (fs_events.rs). Left alone: the WatchReloadGrace, create, publish, open and Windows ClosePseudoConsole helper threads, which run a few calls and exit. A bun_threading::spawn_named helper that sets the stack size and calls configure_named_thread would stop the next copy of this. clippy.toml already names it.

Tests. The main-thread overflow test sets ulimit -s in the child: the main thread's stack is unlimited on some CI machines, and then the recursion ends in the OOM killer, not on a guard page. The two tests on the classification run on Linux without ASAN (the addresses come from /proc/self/maps). This PR adds no test to fs.watch.test.ts: #44249 has the tests for deep trees.

Suites run on debug builds pinned to the preview build autobuild-preview-pr-741-242085de. On the head of this PR, without ASAN: run-crash-handler.test.ts (36 pass), fs.watch.test.ts (45 pass), test/js/node/worker_threads/worker_threads.test.ts (142 pass), test/cli/run/cpu-prof.test.ts (12 pass), test/js/web/workers/worker.test.ts (40 to 42 pass). The two tests of worker.test.ts that fail give a child process 1 second. Under the load of my machine the child took 0.7 to 3.7 s, on the build before the last change too (8 interleaved runs of each). With ASAN: run-crash-handler.test.ts (30 pass, 15 skip), fs.watch.test.ts (45 pass), the 14 test-compile-cache-*.js tests. On the revision before the last two commits, with ASAN: test/js/node/vm/vm.test.ts (307 pass), worker.test.ts (42 pass), worker_threads.test.ts (142 pass), cpu-prof.test.ts (12 pass). vm.test.ts without ASAN: 305 to 307 pass. The 1 to 3 tests that fail are bounds on memory, and they fail in the same way when an LD_PRELOAD shim removes SA_ONSTACK from JSC's handler (3 runs of each). The loops under bun --cpu-prof --cpu-prof-interval=100 complete: 20,000 WebAssembly traps (5 of 5 runs) and 20 node:vm timeouts (3 of 3 runs). For the revision before the WebKit pin: test/internal/source-lints/ (196 pass), cargo clippy on the five touched crates, bun run rust:check-all for aarch64-apple-darwin, x86_64-unknown-freebsd and x86_64-pc-windows-msvc. The stack overflow tests fail on a debug build of main, and the two classification tests fail on the first rule for the classification.

Release of the stack on macOS. The libc of Darwin returns ENOMEM for sigaltstack() with a size below MINSIGSTKSZ, also when the call disables the stack (compat-43/sigaltstk.c in Apple's Libc; the Rust standard library has a workaround for it). The thread-local destructor of an earlier revision passed size 0, ignored the result and unmapped the stack. On macOS an exiting thread then kept a registered stack with no memory behind it. Now the disable call passes the size of the stack, and a stack that the call did not disable stays mapped. I did not run this on macOS. On Linux, with an LD_PRELOAD shim that applies the rule of Darwin to sigaltstack, 50 Workers: before, 50 of 50 disable calls rejected and 50 stacks unmapped while registered. After, 0 and 0. Without the shim the calls of a thread are the same as before (8 Workers: 27 sigaltstack, 8 mmap, 8 mprotect, 8 munmap).

SA_ONSTACK comes from WebKit. Earlier revisions of this PR put the flag back from bun, in Zig__GlobalObject__create, after the first VM. bun build --bytecode creates its first VM in vmForBytecodeCache, so that process kept JSC's handler without the flag. oven-sh/WebKit#741 sets the flag where JSC installs the handler, as V8 does at its sigaction call (v8/v8@600c599bb0fb). The hook CrashHandler__keepSignalHandlersOnAltStack is gone. The flags that JSC passes to sigaction for SIGSEGV and SIGBUS, logged with an LD_PRELOAD shim for bun -e 1 and for bun build --bytecode --target=bun x.js: 0x4 on main, 0x8000004 with this PR. #44070 was the stacked follow-up for this. Its commits are in this PR now, and it is closed.

Thread suspension. On Linux, WTF suspends a thread with a signal. The handler of that signal backs off when it runs on an alternate stack, and Thread::suspend() retries with no limit. With SA_ONSTACK, JSC's fault handler runs on the alternate stack, and it waits there for locks that SamplingProfiler::takeSample() holds while it suspends the thread. With the flag alone (the first commit of oven-sh/WebKit#741, and the hook of the earlier revisions), bun --cpu-prof --cpu-prof-interval=100 on a loop of out-of-bounds WebAssembly loads hangs at the first traps. The JS thread is in jscSignalHandler, then WasmFaultSignalHandler.cpp:100, then WTF::Lock::lockSlow. The profiler thread is in SamplingProfiler::takeSample, then Thread::suspend, then Thread::yield. oven-sh/WebKit#741 fixes it: JSC's handler publishes the registers that it interrupted, and the suspension uses them when their stack pointer is in the stack of the thread. The test while the sampling profiler suspends the thread times out on the build with the flag alone (5 of 5 runs) and passes with the fix. bun 1.4.3, where the handler runs on the thread's own stack, passes it too.

WebAssembly traps. A shim wraps the handler and asks sigaltstack if it runs on the alternate stack. 1,000 out-of-bounds loads on the main thread and 1,000 in a Worker: each throws WebAssembly.RuntimeError. On main 0 of 2,000 handlers ran on the alternate stack, with this PR 2,000 of 2,000. CPU for each trap, debug builds, 7 interleaved runs of 20,000 traps: 208.5 µs (186.2 to 221.4), main 204.6 µs (183.5 to 213.5).

The two tests with a preloaded library. No input overflows the native stack in these two places. A library loaded with LD_PRELOAD recurses until the stack ends. For bun build --bytecode it does so in exit() and quick_exit(), after the bytecode is on disk. For the compile cache it does so in pthread_getattr_np on the thread named BunCompileCache. JSC calls that function on every thread that runs it, to learn the stack bounds. They need Linux and a C compiler.

build	after bun build --bytecode	on the compile cache thread
1.4.3 canary, release (367d939)	exit 139, empty stderr	exit 139, empty stderr
this PR, debug	panic(main thread): Stack overflow	panic(BunCompileCache): Stack overflow

ASAN builds. ASAN gives every thread an alternate stack of its own, about 58 KB with no guard page. The handler of a VM trap needs more in an ASAN debug build, and it overflowed that stack: 5 of 84 runs of five node:vm timeouts died with SIGSEGV (measured for oven-sh/WebKit#742, not by me). So JSC's handler keeps SA_SIGINFO alone in an ASAN build: flags 0x4, and 0 of 1,000 WebAssembly traps run their handler on the alternate stack. An ASAN build does not report a native stack overflow, as on main, and the stack overflow tests do not run there.

The compile cache thread (src/jsc/NodeCompileCache.rs) runs JSC and did not call configure_thread(). #40173 and #40174 remove that thread. The test on the compile cache thread names it, so it goes with the thread.

The proof of the tests needs the old WebKit build. The change of behaviour of the last three commits comes from the pinned WebKit build, which is outside src/. With the new pin, their tests pass with and without the src/ changes.

JSC's own threads (heap helpers, JIT worklist) are created by WTF and get no alternate stack.

+1 host function: stackOverflow in bun:internal-for-testing.

---

no test proof · iteration 4 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/cli/run/run-crash-handler.test.ts

[robobun]

^{Updated 2:57 PM PT - Sep 29th, 2026}

❌ @robobun, your commit 2f67eda has 1 failures in Build #121677 (All Failures):

• test/js/bun/s3/s3.test.ts - code 1 on 🐧 13 x64-asan
• test/js/bun/s3/s3.test.ts - code 1 on 🐧 25.04 x64
• test/js/bun/s3/s3.test.ts - code 1 on 🐧 13 x64
• test/js/bun/s3/s3.test.ts - code 1 on 🐧 3.23 x64

---

🧪   To try this PR locally:

bunx bun-pr 40169

That installs a local version of the PR into your bun-40169 executable, so you can run:

bun-40169 --bun

[robobun]

Status: ready for review. It cannot merge before oven-sh/WebKit#741: the WebKit pin is the preview build of that PR, and I swap it to the merged SHA when that PR merges.

Reproduced on 1.4.0 and on a debug build of main (Linux x64), each exit 139 with an empty stderr: a 300-deep tree renamed into a recursive fs.watch, Bun.build() on an 8000-file CSS @import chain and on a 6000-file export * chain, and a native recursion on the main thread and in a Worker. The same for a native stack overflow at the exit of bun build --bytecode and on the compile cache thread, made by a library loaded with LD_PRELOAD.

With this branch: panic(main thread): Stack overflow with the crash banner and exit by SIGSEGV, panic(deep): Stack overflow for the same recursion in a Worker named deep, and the two chains link on the 4 MiB Bundler thread. The 300-deep tree gives its 301 events on the 4 MiB fs.watch thread. A 600-deep tree still overflows that thread, now with panic(fs.watch): Stack overflow. #44249 removes that recursion, and this PR no longer changes walk_subtree.

An ASAN build stays silent on a native stack overflow, as on main: JSC's handler needs more than the alternate stack that ASAN gives a thread, so it keeps SA_SIGINFO alone there.

The report says Stack overflow only for a data fault inside the frame being entered. A call through a pointer into the stack and a read past the top of the stack keep Segmentation fault at address 0x..., as on main.

The revisions before 28b0511 must not merge, for two reasons.

• Before 976c0ed they put SA_ONSTACK back on JSC's handler from bun. With that flag bun --cpu-prof hangs at a WebAssembly trap: JSC's handler waits on the alternate stack for a lock of the sampling profiler, and the profiler retries the suspension of that thread with no limit. [WTF] The signal handler runs on the alternate signal stack, and a thread that runs it there can be suspended WebKit#741 sets the flag in WebKit and lets that suspension complete.
• Before 28b0511 an exiting thread on macOS kept a registered alternate stack with no memory behind it: Darwin's libc rejects a disable call with size 0, and the stack was unmapped anyway. Not run on macOS. Checked on Linux with a shim that applies Darwin's rule.

#44070 was the stacked follow-up. Its commits are in this PR now.

Tests: test/cli/run/run-crash-handler.test.ts. The stack overflow tests fail on a debug build of main. They do not run in an ASAN build.

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Walkthrough

The changes add Unix alternate signal-stack setup and POSIX stack-overflow reporting. They also replace recursive filesystem watcher traversal with iterative traversal and set the default thread-pool stack size for several threads.

Changes

Unix stack safety

Layer / File(s)	Summary
Alternate signal-stack installation src/bun_core/lib.rs, src/bun_core/signal_stack.rs, src/bun_core/output.rs	The Unix signal_stack module allocates and registers a guarded alternate stack, retains it for the thread, and cleans it up at thread exit. Both source configuration paths install the stack.
Native stack-overflow crash reporting src/crash_handler/lib.rs, src/bun_core/Global.rs, src/jsc/bindings/ZigGlobalObject.cpp, src/runtime/api/crash_handler_jsc.rs, test/cli/run/run-crash-handler.test.ts	POSIX crash handling captures stack registers and classifies nearby SIGSEGV and SIGBUS faults as stack overflows. Crash output includes the current thread name when available. The JavaScript crash-handler API and tests cover stack overflow and selected fault addresses.
Deep directory watcher traversal src/runtime/node/path_watcher.rs, test/js/node/watch/fs.watch.test.ts	walk_subtree traverses directories iteratively with heap-backed frames. Linux and FreeBSD path-watcher threads use the default thread-pool stack size. Tests cover recursive watching of deep trees.
Thread stack-size configuration src/bundler/BundleThread.rs, src/runtime/node/fs_events.rs, src/watcher/Watcher.rs	Bundler, FSEvents, and file-watcher threads use DEFAULT_THREAD_STACK_SIZE.

Suggested reviewers: jarred-sumner

Priority: ⬆️ High

Merge Risk: 🔵 Low · up to 35559

A stack overflow in BunCompileCache may terminate without the expected crash report, and the deep-watcher test may fail on a slow run. These are bounded risks; the overall merge risk is low.

🚥 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 and concisely summarizes the main change: reporting native stack overflows on every thread.
Description check	✅ Passed	The description clearly explains the problem, fix, tradeoffs, background, and verification results. It does not use the exact template headings, but it provides the required information in equivalent …

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• 🪄 Fix CodeRabbit comments on this PR

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In @src/bun_core/Global.rs:
- Around line 633-635: Update set_thread_name to store each thread’s name in
fixed-size thread-local storage, and have current_thread_name read that cache
instead of calling pthread_getname_np. Preserve the existing thread-name
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📒 Files selected for processing (12)

• src/bun_core/Global.rs
• src/bun_core/lib.rs
• src/bun_core/output.rs
• src/bundler/BundleThread.rs
• src/crash_handler/lib.rs
• src/jsc/bindings/ZigGlobalObject.cpp
• src/runtime/api/crash_handler_jsc.rs
• src/runtime/node/fs_events.rs
• src/runtime/node/path_watcher.rs
• src/watcher/Watcher.rs
• test/cli/run/run-crash-handler.test.ts
• test/js/node/watch/fs.watch.test.ts

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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.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

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• Unresolved: 2 minor or pre-existing.

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🟡 Minor · Install the alternate stack in the BunCompileCache thread. · output.rs:346-393

src/bun_core/output.rs:346-393
🩺 Stability & Availability | 🟡 Minor | ⚡ Quick win

Install the alternate stack in the BunCompileCache thread.

The BunCompileCache worker uses std::thread::Builder::spawn directly and calls __bun_jsc_generate_cached_bytecode, which initializes and uses a JSC VM. No shared bootstrap calls configure_thread, and JSC initialization does not install the alternate stack.

A stack-exhaustion SIGSEGV on this worker can therefore reach the SA_ONSTACK crash handler without an alternate stack and fail before the crash reporter runs. Add Output::Source::configure_thread() at the start of this thread entrypoint.

Suggested fix

                 .stack_size(16 * 1024 * 1024)
                 .spawn(move || {
+                    crate::Output::Source::configure_thread();
                     for job in rx {

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In @src/bun_core/output.rs around lines 346 - 393, Initialize the
BunCompileCache worker with Output::Source::configure_thread() at the start of
its thread entrypoint, before processing jobs or calling
__bun_jsc_generate_cached_bytecode, so the worker installs the alternate stack
and configures JSC thread state.

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Outside diff comments:
In @src/bun_core/output.rs:
- Around line 346-393: Initialize the BunCompileCache worker with
Output::Source::configure_thread() at the start of its thread entrypoint, before
processing jobs or calling __bun_jsc_generate_cached_bytecode, so the worker
installs the alternate stack and configures JSC thread state.

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

On the out-of-diff note about the BunCompileCache thread (src/jsc/NodeCompileCache.rs:907): valid. That worker runs JSC and never calls configure_thread(), so it has no alternate signal stack.

The follow-up #44070 adds the call (725599f) and a test. A native stack overflow on that thread now prints panic(BunCompileCache): Stack overflow. Before, the process exited 139 with an empty stderr.

[claude]

🔴 Anyone building bun from main after this merges gets a build that 404s on the WebKit download once oven-sh/WebKit#741 merges or closes, which the base pin never does. WEBKIT_VERSION at scripts/build/deps/webkit.ts:6 is set to the preview tag "autobuild-preview-pr-741-5586daae"; scripts/build/download.ts:319-337 documents that GitHub deletes autobuild-preview-pr-* releases when the WebKit PR merges, so every build fails at once. Fix: before merging, land oven-sh/WebKit#741 and pin WEBKIT_VERSION to the resulting 40-hex main sha (the form the base branch and .claude/commands/upgrade-webkit.md use), so the SA_ONSTACK behaviour this PR depends on ships from a permanent release.

Why this was flagged

The pin at scripts/build/deps/webkit.ts:6 is "autobuild-preview-pr-741-5586daae", a preview release of an unmerged WebKit PR (commit bb1fd02 moved the SA_ONSTACK re-add out of ZigGlobalObject.cpp into that WebKit PR). prebuiltUrl at scripts/build/deps/webkit.ts:72-74 turns it into https://github.com/oven-sh/WebKit/releases/download/autobuild-preview-pr-741-5586daae/... for every prebuilt build. The repo's own error path at scripts/build/download.ts:319-337 states that preview releases only exist while the WebKit PR is open and are deleted on merge or close, then tells the user to edit this line. On the base branch the pin is the 40-hex sha f20ce7744553c910bcf16a33faf976af208de091, a permanent release. After merge, the moment WebKit#741 merges, bun run build and CI on main fail with "WebKit preview release is gone" until someone re-pins; the source lint at test/internal/source-lints/webkit-prebuilt-url.test.ts:127 accepts any autobuild-* tag so it does not catch this.

Verification: normal — triggers as soon as oven-sh/WebKit#741 merges, closes, or is re-pushed (the tag embeds the head short sha 5586daae), after which every prebuilt-WebKit build of bun from this branch fails at download time. Mechanism verified in the checkout. Commit bb1fd02 ("JSC installs its signal handler with SA_ONSTACK: pin that WebKit build, delete the hook") changes… | normal — triggers as soon…

[robobun]

Correct, and this PR must not merge with this pin. The first line of the PR body says so. The preview build is here on purpose: it lets CI run this change while oven-sh/WebKit#741 is open. When that PR merges, I swap the pin to the merged 40-hex SHA and check that the prebuilt archives exist for each platform. I leave this thread open until then.

[claude]

Code review found no new issues

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

Still open from earlier reviews (1):

• Unresolved: 1 blocking on lines changed since (possibly already fixed).

If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.

[robobun]

A note from the work on oven-sh/WebKit#742, which gives an alternate signal stack to the threads that WTF creates. Its first revision had the same release code as Drop for Mapping in src/bun_core/signal_stack.rs here, and a review found a defect in it on macOS.

• Drop calls sigaltstack with SS_DISABLE and ss_size 0. The libc of Darwin returns ENOMEM for a size below MINSIGSTKSZ, also when the call disables the stack. The Rust standard library has a workaround for this in library/std/src/sys/pal/unix/stack_overflow.rs ("Workaround for bug in macOS implementation of sigaltstack").
• Drop ignores the result and calls munmap. So on macOS the stack stays registered and its memory is gone. A signal for an SA_ONSTACK handler on that thread, before the thread ends, has no stack to run on.

What oven-sh/WebKit#742 does now: it sets ss_size to the size of the stack in the disable call, and it does not unmap a stack that the call did not disable.

I did not run this on macOS. On Linux, with an LD_PRELOAD shim that applies the rule of Darwin to sigaltstack, the release with size 0 had 1001 of 1001 disable calls rejected and 1001 stacks unmapped while registered. With the size set, both numbers are 0.

[robobun]

#44249 changes the same function as this PR. Its walk_subtree reads a directory to its end and closes it before it visits a subdirectory, so the walk holds one descriptor at any depth and does not recurse. That covers the partial watch that the notes of this PR describe ("A tree deeper than the limit on open files gets a partial watch and no error").

If #44249 merges first, this PR can drop three things:

• the walk_subtree hunk in src/runtime/node/path_watcher.rs,
• the two fs.watch tests and removeChain in test/js/node/watch/fs.watch.test.ts (fs.watch: read each directory and close it before descending #44249 has a 600-level test with a 4 MiB stack and a 300-level test for a tree that moves in),
• the sentence "walk_subtree is iterative" in the body.

The thread stack sizes, the alternate signal stacks and the crash handler are independent of #44249.

[claude]

Code review found no new issues

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

Still open from earlier reviews (1):

• Unresolved: 1 blocking on lines changed since (possibly already fixed).

If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.

[robobun]

One more finding from oven-sh/WebKit#742 that applies here: with the alternate stacks of this PR, a node:vm timeout in a Worker does not finish when the SIGPWR handler has SA_ONSTACK. The runtime of Go adds that flag when a process loads a Go shared library. On Bun 1.4.3 the same case finishes in a Worker and does not finish on the main thread.

The measurements and the fix (one commit for Thread::signalHandlerSuspendResume(), de6728644a) are in oven-sh/WebKit#741 (comment). The WebKit build that this PR pins needs that commit.

A test here can keep the pin honest. The preloaded library of the overflow tests can have a sigaction hook that adds SA_ONSTACK when the signal is SIGPWR. With it, this script must finish:

const w = new Worker(URL.createObjectURL(new Blob([`
  const vm = require("node:vm");
  let timeouts = 0;
  for (let i = 0; i < 5; i++) {
    try { vm.runInNewContext("for (;;) {}", {}, { timeout: 20 }); } catch { timeouts++; }
  }
  postMessage(timeouts);
`], { type: "application/javascript" })));
console.log(await new Promise(r => (w.onmessage = e => r(e.data))));
process.exit(0);

With the preview build of oven-sh/WebKit#741 (242085de) it does not finish (3 of 3 runs, killed at the limit of 40 to 45 s). With a build that has de6728644a it prints 5 (3 of 3 runs).