Fix Bun.spawnSync + GC unbalancing the event loop's keep-alive count - #40508
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….event_loop_handle Bun.spawnSync points vm.event_loop_handle at a private uws loop for the duration of the call. A GC that ends inside spawnSync schedules FinalizationRegistry cleanup through JSCTaskScheduler::onAddPendingWork, whose +1 keep-alive folded into vm.event_loop_handle (the private loop) while the matching -1 later folded into the main loop. Each occurrence left the main loop's num_polls/active one lower; once it hit 0 the loop stopped polling I/O. EventLoop now records the uws loop it runs on (previously only on Windows) and apply_concurrent_ref_delta/wakeup/usockets_loop/uv_loop resolve through that, so a ref taken on the regular loop always lands on the regular loop regardless of what spawnSync has installed.
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WalkthroughChangesSpawnSync event-loop isolation
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…y; drop dead vm.event_loop restore in spawnSync cleanup
…edded loops' uws loop is the thread loop
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In `@test/js/bun/spawn/spawnsync-isolated-event-loop.test.ts`:
- Around line 122-149: Move the subprocess fixture logic used by the tests “GC
finishing inside spawnSync does not move the main loop's keep-alive count” and
“spawnSync under GC pressure with a worker and a server keeps the main loop
balanced and exits” into this test file, passing each inline script via bunExe()
with the -e argument. Remove the separate fixture-file references and delete
those fixture files, while preserving the existing environment, output, and
exit-code assertions.
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📒 Files selected for processing (6)
src/event_loop/SpawnSyncEventLoop.rssrc/jsc/event_loop.rssrc/runtime/api/bun/js_bun_spawn_bindings.rstest/js/bun/spawn/spawnSync-keepalive-gc-fixture.jstest/js/bun/spawn/spawnSync-keepalive-stress-fixture.jstest/js/bun/spawn/spawnsync-isolated-event-loop.test.ts
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Updated 9:36 PM PT - Aug 25th, 2026
✅ @dylan-conway, your commit 7e2b44875c90989395db87f3e316c316b02990e5 passed in 🧪 To try this PR locally: bunx bun-pr 40508That installs a local version of the PR into your bun-40508 --bun |
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What
Bun.spawnSyncpointsvm.event_loop_handleat a private uws/libuv loop for the duration of the call (SpawnSyncEventLoop::prepare/cleanup). If a GC finishes whilespawnSyncis on the stack and aFinalizationRegistryhas dead targets,JSFinalizationRegistry::reconcileWeakReferencesAtGCEnd→DeferredWorkTimer::addPendingWork→JSCTaskScheduler::onAddPendingWorktakes a keep-alive withBun__eventLoop__refKeepAlive, which folded intovm.event_loop_handle— the private loop. The matching-1(when the cleanup task runs) is queued on the regularEventLoopand folded at its next tick onto the main loop.Net per occurrence: main loop
num_polls -= 1/active -= 1. Oncenum_pollsreads 0 while polls are still registered,us_loop_run_bun_tickreturns beforeepoll_wait/keventand the process stops observing I/O and child exits (timers keep firing;Bun.servenever accepts,await proc.exitednever resolves, …). On Windows the same sequence shows up asactive_handlesdrift and a process that never exits. Regressed by #39905 (which switched this+1from a queuedVmHandleref to an immediate fold); 1.4.0 is unaffected.Fix
EventLoopnow records the uws loop it runs on (uws_loop, previously a Windows-only field): the thread's loop for the VM's regular/macro loops (set inensure_waker), the private loop for a spawnSync loop (set at creation).apply_concurrent_ref_delta,wakeup,usockets_loopandnative_loop/uv_loopresolve through that instead of throughvm.event_loop_handle, so a keep-alive taken on the regular loop lands on the regular loop regardless of what spawnSync has installed. This covers everyBun__eventLoop__refKeepAlivecaller (deferred work,MessagePort,BroadcastChannel,ScriptExecutionContext), not just the FinalizationRegistry path, and also removes an off-thread read ofvm.event_loop_handle(VmHandle→wakeup()) that raced spawnSync's swap.Also:
EventLoop::uv_loop()is now Windows-only (every caller already was) withnative_loop()as the cross-platform accessor, matchingEventLoopHandle; and spawnSync'scleanupno longer writesvm.event_loopback to the value it just read.Why this is behaviour-preserving outside spawnSync
Every writer of
vm.event_loop_handle(ensure_waker, two sites inserver/mod.rs) stores the thread's loop (bun_io::Loop::get()), except spawnSync'sprepare/cleanup. For the regular and macroEventLoops,uws_loopisuws::Loop::get(), anduws_to_native(uws::Loop::get()) == bun_io::Loop::get()on every platform (Windows:uws_get_loop_with_native(uv::Loop::get()); nowdebug_asserted inensure_waker). So old and new resolve to the same pointer everywhere except betweenprepareandcleanup— which is exactly the broken window.This was checked mechanically: a temporary build computed both the old (
vm.event_loop_handle) and new (self.uws_loop) pointer in each touched method and aborted if they differed outside the spawnSync window. ~14k tests across 39 directories (js/bun/spawn,js/node/child_process,js/web/workers,js/bun/shell,cli/run,js/bun/http/serve,js/web/fetch,js/node/fs,js/node/http, …) on a Linux release build: 0 divergences outside the window. Inside the window the only divergent callers were, by backtrace,reconcileWeakReferencesAtGCEnd → onAddPendingWork → ref_keep_alive(the bug) and→ scheduleWorkSoon → VmHandle::post → wakeup(previously woke the private loop, now the main one), both on the JS thread.Tests
spawnSync-keepalive-gc-fixture.js: 5000 FinalizationRegistry targets,spawnSync, tick, assertnumPollsnever drops below its starting value; run underBUN_JSC_collectContinuously=1so a collection reliably ends insidespawnSync.spawnSync-keepalive-stress-fixture.js: the same under a message-flooding Worker and a listeningBun.serve; after each round the server must still answer afetch,numPollsmust not move, and the process must exit on its own at the end.11fb73032spawnsync-isolated-event-loop,spawnSync,spawn,child_process,39900, macro-test,worker,message-channel,broadcastchannel,worker_threads,serve¹
stdin/stdout … not affected by spawnSyncfails on the Windows box used here on 1.4.0/canary/this PR alike (it spawnsecho); passes in CI.