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…SC did not send no longer crashes the process On Linux, JSC suspends and resumes threads with SIGPWR. The signal handler assumed every delivery came from Thread::suspend() or Thread::resume() and dereferenced a null target thread when anyone else sent the signal. oven-sh/WebKit#693 makes the handler act only on a request that suspend() or resume() published. Adds tests that send SIGPWR from the process itself, from the parent, and in a flood while the GC runs.
WalkthroughThe change updates the WebKit dependency, adds a Linux-only thread suspension and signal testing binding, and introduces SIGPWR regression tests covering process-directed and thread-directed delivery. ChangesLinux SIGPWR handling
Suggested reviewers: Priority: ⬆️ High Severity of issue fixed: Medium Merge Risk: 🔵 Low · up to The current SIGPWR tests follow the required lifetime protocol, but future use of this test hook could release the shared state early and crash. The localized ownership fix is recommended. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Updated 8:30 PM PT - Sep 17th, 2026
✅ @robobun, your commit 9fa50a3d0cd99d895dd607937ab21bb5f1debc7d passed in 🧪 To try this PR locally: bunx bun-pr 43213That installs a local version of the PR into your bun-43213 --bun |
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I opened oven-sh/WebKit#693 and had started on the same bump, so I stop here and leave the Bun side to this PR. Two things that may help:
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Adds suspendThreadAndSignalForTesting to bun:internal-for-testing. It suspends the calling thread the way the GC does, sends a signal during the suspension (kill(2) or pthread_kill()), and reports whether the thread ran before the resume. The tests need the WTF change in oven-sh/WebKit#693. With the current WEBKIT_VERSION 7 of the 8 fail: 5 segfault in WTF::Thread::signalHandlerSuspendResume and 2 hang in sigsuspend().
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Inline comments:
In `@src/jsc/bindings/InternalForTesting.cpp`:
- Around line 271-273: Update the code around state->typedVector() to retain the
backing storage before launching the detached thread: obtain a thread-safe
SharedArrayBufferContents via ArrayBuffer::shareWith(ArrayBufferContents&), move
the resulting ArrayBufferContents into the WTF::Thread::create lambda, and
access the slots through that captured storage. Do not capture a Ref or RefPtr
to JSC::ArrayBuffer.
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scripts/build/deps/webkit.tssrc/js/internal-for-testing.tssrc/jsc/bindings/InternalForTesting.cppsrc/jsc/bindings/InternalForTesting.htest/js/node/process/process-sigpwr.test.ts
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…rage from the helper thread
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I worked on the same crash from a separate report and did not see this PR until now. I will not open a second PR. My branch is Summary of that branch: it wraps WTF's handler in That rule is not safe, so this PR is right to read no A The branch still has parts for the follow-up that the Background here names (a callback out of the WTF handler for the deliveries that JSC did not send):
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A note on the follow-up that the Background names ("The default action or a JS listener needs a callback out of the WTF handler"). I looked into that follow-up and I do not open a PR for it. The findings are below for whoever takes it up later. A callback at the early return of
Numbers:
So the listener and the default action are not a small hook on top of #693. They need that change to the wait loop in WebKit first. Nobody has asked for a What remains is the name. With the name in the tables, Update, 2026-09-26. oven-sh/WebKit#693 merged, and main pins a WebKit that contains it since #43616. I checked canary 1.4.3-canary.1+37da174d5: a #43500 now adds the The listener and the default action stay as described: not planned, and not safe as a callback while the wait loop checks before it parks. The WebKit that main pins has that loop. The modelBuild with #define _GNU_SOURCE
#include <pthread.h>
#include <semaphore.h>
#include <signal.h>
#include <stdatomic.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
enum { None = 0, Suspend = 1, Resume = 2, Mask = 3 };
static _Atomic uintptr_t pending;
static _Atomic pthread_t requested;
static sem_t sem;
static _Atomic long strayNone;
static _Atomic long resumeSeen;
static int doWhile;
static _Atomic int stop;
static void publish(pthread_t h, int req)
{
atomic_store(&requested, h);
uintptr_t gen = (atomic_load(&pending) >> 2) + 1;
atomic_store(&pending, gen << 2 | (uintptr_t)req);
}
static void clearReq(void) { atomic_store(&pending, atomic_load(&pending) & ~(uintptr_t)Mask); }
static int pendingForMe(void)
{
for (;;) {
uintptr_t snap = atomic_load(&pending);
int req = (int)(snap & Mask);
if (req == None)
return None;
pthread_t h = atomic_load(&requested);
if (atomic_load(&pending) != snap)
continue;
return pthread_equal(pthread_self(), h) ? req : None;
}
}
static void handler(int sig, siginfo_t* info, void* uc)
{
(void)info; (void)uc;
int req = pendingForMe();
if (req != Suspend) {
if (req == None)
atomic_fetch_add(&strayNone, 1); // where a callback for foreign signals would run
else
atomic_fetch_add(&resumeSeen, 1);
return;
}
clearReq();
sem_post(&sem);
sigset_t set;
sigfillset(&set);
sigdelset(&set, sig);
if (doWhile) {
do {
sigsuspend(&set);
} while (pendingForMe() != Resume);
} else {
while (pendingForMe() != Resume)
sigsuspend(&set);
}
clearReq();
sem_post(&sem);
}
static void* target(void* arg)
{
(void)arg;
volatile unsigned long x = 0;
while (!atomic_load(&stop))
x++;
return NULL;
}
int main(int argc, char** argv)
{
long n = argc > 1 ? atol(argv[1]) : 100000;
doWhile = argc > 2 && !strcmp(argv[2], "dowhile");
sem_init(&sem, 0, 0);
struct sigaction sa;
memset(&sa, 0, sizeof sa);
sigemptyset(&sa.sa_mask);
sigaddset(&sa.sa_mask, SIGPWR);
sa.sa_sigaction = handler;
sa.sa_flags = SA_RESTART | SA_SIGINFO;
sigaction(SIGPWR, &sa, NULL);
pthread_t t;
pthread_create(&t, NULL, target, NULL);
for (long i = 0; i < n; i++) {
publish(t, Suspend);
pthread_kill(t, SIGPWR);
while (sem_wait(&sem)) { }
publish(t, Resume);
pthread_kill(t, SIGPWR);
while (sem_wait(&sem)) { }
}
atomic_store(&stop, 1);
pthread_join(t, NULL);
printf("%s rounds=%ld foreign_senders=0 deliveries_with_no_request=%ld nested_resume_deliveries=%ld\n",
doWhile ? "do-while" : "check-first (#693)", n, atomic_load(&strayNone), atomic_load(&resumeSeen));
return 0;
} |
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State as of main 8cc0397, for whoever picks this up:
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Problem
panic(main thread): Segmentation fault at address 0x60inWTF::Thread::signalHandlerSuspendResume(Source/WTF/wtf/posix/ThreadingPOSIX.cpp:140).process.kill(pid, 30),subprocess.kill("SIGPWR"),raise(SIGPWR)andkill -PWR <pid>from a shell all reach it. Container managers send SIGPWR to PID 1 on stop (LXC's default halt signal). Fixes Segfault when a Bun process receives SIGPWR on Linux (process.kill(pid, 30), kill -PWR <pid>) #43186.Thread::suspend()orThread::resume(). It dereferencestargetThread, which only those two set. A delivery that lands while a thread is suspended also endssigsuspend()early.resume()then returns before its own signal arrives, and the thread stays insigsuspend()forever.Fix
suspend()andresume()publish the request kind and the targetpthread_tbeforepthread_kill. The handler acts only on a request for its own thread. Every other delivery returns.WEBKIT_VERSIONto the preview build of that PR (autobuild-preview-pr-693-8fb74355) so CI can run. When [WTF] The thread suspend signal handler acts only on requests from suspend() and resume() WebKit#693 merges, the pin moves to the merged sha before this PR merges.suspendThreadAndSignalForTestingtobun:internal-for-testing(Linux only). A helper thread suspends the JS thread withWTF::Thread::suspend, sends SIGPWR during the suspension, and reports whether the JS thread ran before the resume. That makes the hang deterministic.test/js/node/process/process-sigpwr.test.ts(8 tests). On the current WebKit pin 5 segfault and 2 hang. With the preview all 8 pass. Alsocpu-prof.test.ts,open-in-editor-gc.test.ts,process-on.test.ts,process-signal-listener-count.test.ts,worker.test.ts.Background
Thread::suspend()sends a signal withpthread_kill, the handler waits insigsuspend(), andThread::resume()sends the signal again.sigsuspend()returns as soon as any handler returns.src/jsc/bindings/c-bindings.cppexcludes SIGPWR from the signalsprocess.oncan install.pthread_killfrom other senders bysiginfo, and the kernel delivers both asSI_USERfrom pid 0 onceRLIMIT_SIGPENDINGis exhausted. process: guard JSC's GC suspend signal against unsolicited SIGPWR on Linux #33777 tried that and hangs in the first GC underulimit -i 0. One test here setsRLIMIT_SIGPENDINGto 0 and runs the GC.process.on("SIGPWR")stays inert, as it is today. So an operator who stops Bun with SIGPWR (LXC's default halt signal) now waits for the kill timeout, where before the process died by SIGSEGV. The default action or a JS listener needs a callback out of the WTF handler, which is a separate change.Notes
Repro.
Canary 1.4.3-canary.1+b52d51348: exit 139, 3 of 3. With this build:
survived, exit 0.The tests. Four crash cases (
process.kill, a parent'sprocess.kill,subprocess.kill("SIGPWR"),raise()throughbun:ffi). Two GC cases underBUN_JSC_collectContinuously=1: the main thread sends SIGPWR to the process while the collector suspends it, and the GC withRLIMIT_SIGPENDINGat 0. Two suspension cases through the hook:kill(2)to the process andpthread_kill()to the suspended thread, two rounds each, and the JS thread must not run during the 30 ms hold. The hook and the tests come from branchrobobun/387ebfac/sigpwr-wtf.The preview build. The first preview of oven-sh/WebKit#693 was based on WebKit
c2815689and did not haveJSC::Exception::asyncContext(oven-sh/WebKit#689), sosrc/jsc/bindings/ModuleGraph.cppdid not compile against it. I updated the PR branch with WebKit main. Later previews add anerrnosave in the handler and astatic_assertthat the request atomics are lock-free. Neither changes what the tests see.Slow suites.
error-gc-test.test.jsand twoheap-snapshot.test.tscases timed out at 5 s on my machine (load average 90 on 16 cores). Their time goes toBun.inspectandString.prototype.repeatin the debug ASAN build, not to the GC: 50 full collections take 0.6 s on the same build.