napi: implement uv_async_t, uv_queue_work and uv_default_loop on posix - #39652
napi: implement uv_async_t, uv_queue_work and uv_default_loop on posix#39652robobun wants to merge 5 commits into
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On Linux and macOS every loop-related uv_* symbol an addon can link against is a stub that aborts the process with "unsupported uv function". Addons that use libuv for cross-thread wakeups (uv_async_init on uv_default_loop() or on the loop from napi_get_uv_event_loop) or for pool work (uv_queue_work) die in process.dlopen or in a constructor. src/runtime/napi/uv_posix.rs implements the loop-backed functions on top of the VM's event loop: - A uv_loop_t* is a UvLoop, one per VM, embedded in its RuntimeState. Its first word is the addon's data slot, as in uv_loop_t. napi_get_uv_event_loop returns the env's VM's loop (it used to return a pointer to Bun's internal EventLoop struct, whose first bytes an addon writing loop->data would have overwritten); uv_default_loop() returns the main thread's from any thread. - uv_async_init, uv_async_send, uv_close, uv_ref, uv_unref, uv_has_ref, uv_is_active, uv_is_closing. The handle's public fields are where uv.h puts them; the private fields hold a KeepAlive (the ref), libuv's pending flag and busy counter. uv_async_send sets pending and posts at most one dispatch task per loop through the VM's VmHandle; the task walks the loop's live handles on the JS thread. uv_close unregisters the handle, waits out senders, and delivers close_cb from a later loop turn. - uv_queue_work and uv_cancel, as a bun_jsc::Job whose off-thread half is the request itself; a state word in the request's private area decides between after_work_cb(0) and after_work_cb(UV_ECANCELED). uv-posix-polyfills.c gains the functions that need only the headers: uv_version, uv_version_string, uv_handle_size, uv_req_size, the type names, the handle, request and loop data getters and setters, and uv_get_osfhandle / uv_open_osfhandle. process.versions.uv now comes from uv_version_string() on every platform (1.51.0, the version of the vendored headers and of the libuv linked on Windows; posix used to report 1.48.0). The 28 implemented symbols are removed from the generated stubs, the generator's list and the stub test addon. Tests: test/napi/uv.test.ts (uv-stub-stuff/uv_impl.c), each in a child process since the process exit is part of what is checked; test-resolve-async.js of Node's test_callback_scope suite now runs on posix.
WalkthroughChangesThe PR adds a VM-backed POSIX libuv compatibility layer for N-API. It adds C polyfills, updates symbol generation and retention, exposes the runtime libuv version, and adds async, work-queue, cancellation, loop, and metadata tests. POSIX libuv compatibility
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Updated 6:35 AM PT - Aug 19th, 2026
❌ @robobun, your commit c466dee has 3 failures in
🧪 To try this PR locally: bunx bun-pr 39652That installs a local version of the PR into your bun-39652 --bun |
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Status: ready for review. All review threads are resolved. Three CI runs (101244, 101261, 101268) passed the uv tests on every lane; each run was red only on tests this change does not touch (a darwin TLS timeout, GitHub API 504s in install tests, a memrmem segfault in filesystem_router.test.ts on musl aarch64, two sql tests on a lane without docker, the pre-existing bake/deinitialization.test.ts segfault on Windows), all reported to main-break triage. The retrigger is used up, so the remaining red lanes need a maintainer's judgement. Reproduction, before this change, with the released bun on Linux:
Older PRs for parts of this, superseded: #35475 (uv_async_t only) and #34156 (napi_get_uv_event_loop failing on posix). |
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…irst check libuv's uv__async_spin stores pending = 1 before it waits for the busy counter. A uv_async_send that starts after the close then returns at its first load and never touches the handle again. uv_close used the dispatch pass's take_pending, which cleared the flag, so such a send went on to increment busy, post a dispatch task and decrement busy, and the decrement could land after close_cb had freed the handle. Split the two paths: take_pending for the dispatch pass, stop_sends for uv_close. The close test now also sends after the close and expects 0 and no callback. Drop an unused field from the work test struct.
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Review sweep for the two pushes since the last status:
CI for c591639 is running. The previous run (101244) was green apart from |
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Inline comments:
In `@src/jsc/VirtualMachine.rs`:
- Around line 801-805: Update VirtualMachine::init and global_exit to publish
MAIN_THREAD_VM only after initialization succeeds, clear or otherwise invalidate
it before teardown, and add a reader lifetime protocol in get_main_thread_vm so
callers cannot observe partially initialized or destroyed state.
In `@test/napi/node-napi-tests/test/node-api/test_callback_scope/do.test.ts`:
- Around line 11-15: Update the explanatory comment above test.todoIf to
explicitly document both test.js and test-resolve-async.js as Windows
exclusions, or remove test.js from the condition if it is no longer required;
keep the test-async-hooks.js exclusion unchanged.
In `@test/napi/uv-stub-stuff/uv_impl.c`:
- Line 482: Check and handle the return value of uv_async_init in
test_async_close_with_send_pending, test_async_stress, and test_async_ref,
reusing the existing checked initialization approach from test_async. On
failure, propagate the named error and stop before calling uv_async_send,
uv_close, or uv_has_ref on the uninitialized handle.
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src/jsc/VirtualMachine.rssrc/jsc/bindings/BunProcess.cppsrc/jsc/bindings/libuv/generate_uv_posix_stubs.tssrc/jsc/bindings/libuv/generate_uv_posix_stubs_constants.tssrc/jsc/bindings/uv-posix-polyfills.csrc/jsc/bindings/uv-posix-stubs.csrc/runtime/jsc_hooks.rssrc/runtime/napi/mod.rssrc/runtime/napi/napi_body.rssrc/runtime/napi/uv_posix.rstest/napi/node-napi-tests/test/node-api/test_async_cleanup_hook/do.test.tstest/napi/node-napi-tests/test/node-api/test_callback_scope/do.test.tstest/napi/uv-stub-stuff/plugin.ctest/napi/uv-stub-stuff/uv_impl.ctest/napi/uv.test.ts
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… null states of_vm returns null before init_runtime_state has run and after destroy has cleared the field (which it does before freeing the state), so its doc says that instead of "never freed". The test addon now throws when uv_async_init fails instead of going on to use the handle, and the callback-scope todo comment names the pre-existing test.js exclusion too.
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Inline comments:
In `@test/napi/uv-stub-stuff/uv_impl.c`:
- Around line 455-456: In the async initialization failure paths, release the
reporter reference and allocation before returning: update
test/napi/uv-stub-stuff/uv_impl.c lines 455-456, 496-497, and 564-565 to destroy
test->reporter, free test, then return NULL. Apply the same cleanup in each
affected caller of init_async.
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I reviewed this PR and didn't find any bugs this pass; the earlier nit about the test.js Windows-todo comment is addressed. Because this adds ~725 lines of new unsafe Rust that must match libuv's ABI byte-for-byte and implements a cross-thread lock-free dispatch/busy-spin protocol against addon-owned memory, a human look at the concurrency and lifetime design would still be worthwhile.
What was reviewed:
uv_async_send/stop_sendsbusy-counter protocol vs. libuv'suv__async_spin, and that no&UvAsyncis formed while other threads hold the atomics.UvLoop::of_vmnull-safety at both ends ofruntime_state's lifetime and the widenedget_main_thread_vmvisibility.run_passlist-swap vs.uv_closefrom inside a callback;dispatchingdrained back on termination.uv_queue_work/uv_cancelstate word race and the compile-time offset/size asserts againstuv.h.
Extended reasoning...
Overview
This PR implements uv_async_t, uv_queue_work, uv_default_loop and ~25 header-only libuv functions on posix, replacing crash stubs. The core is a new 725-line src/runtime/napi/uv_posix.rs that defines #[repr(C)] structs matching libuv's public field offsets (asserted at compile time), stores Bun's private state in the tail room of addon-allocated uv_async_t/uv_work_t, and drives callbacks through VmHandle::post and Job::schedule. A per-VM UvLoop is embedded in RuntimeState (jsc_hooks.rs). napi_get_uv_event_loop on posix now returns that instead of Bun's own EventLoop*. VirtualMachine::get_main_thread_vm is widened from pub(crate) to pub. BunProcess.cpp now reads uv_version_string() on all platforms. 15 new tests in test/napi/uv.test.ts exercise send/coalesce/close ordering, ref/unref keepalive, a 200-round + 2000-burst stress test, Worker-local loops, and uv_cancel.
Security risks
None in the traditional sense — this is native-addon compat surface, not user-input handling. The risk class is memory safety: addon-allocated structs are read/written through raw pointers with a documented thread contract (only uv_async_send is any-thread), a spin-wait in stop_sends waits out concurrent senders before close_cb may free the handle, and UvLoop is reached cross-thread through an AtomicPtr<VirtualMachine>. All of this is carefully argued in SAFETY comments and mirrors libuv's own protocol, but it is exactly the category the repo's review rules call "the most-blocked".
Level of scrutiny
High. This is new FFI-exported unsafe Rust with ABI-layout guarantees, a lock-free three-state dispatch machine borrowed from ThreadSafeFunction, per-handle atomics that coexist with loop-thread plain-field writes, and integration into VM init/teardown. It also makes design choices a maintainer should sign off on: embedding UvLoop directly in RuntimeState (address stability), using bun_io::js_vm_ctx() for the KeepAlive in uv_ref/uv_unref (which reads the current thread's VM context — correct because these are loop-thread-only per libuv, but worth a maintainer's eye), and the Posted::Refused handling when a VM is gone.
Other factors
Test coverage is thorough and well-designed (child-process exit is part of the assertion, a stress test would hang on a lost wakeup, both uv_cancel race outcomes are accepted). All prior review threads (mine, CodeRabbit's, comment-cop's) are resolved; the author responded to each with either a fix commit or a stated reason. CI on the previous push was green apart from an unrelated darwin TLS test. Still, per the approval guidelines, a change of this size and unsafe-density in a critical compat path should have a human reviewer's approval rather than an automated one.
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Closed #35475 and #34156 in favor of this PR. The 13 symbols #35475 un-stubs are all in this PR's list, and each of its test cases has a counterpart in uv.test.ts here. #34156 returned a failure from napi_get_uv_event_loop, which this PR replaces with a real loop. For whoever lands second: the open loop-free PRs overlap this one on a few header-only functions. #34155 shares uv_version, uv_version_string, uv_handle_size, uv_req_size, uv_handle_type_name, uv_req_type_name, uv_get_osfhandle and uv_open_osfhandle, plus the process.versions.uv change and the OS(FREEBSD) fix in the stub generator. #31696 shares uv_version, uv_version_string, uv_get_osfhandle and uv_open_osfhandle. The duplicates drop out on rebase, and the stub files regenerate with |
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Problem
uv_default_loop,uv_async_init,uv_queue_workand helpers likeuv_version_stringare crash stubs (src/jsc/bindings/uv-posix-stubs.c). An addon that calls one at load time takes the process down:panic: unsupported uv function: uv_async_init(six Sentry issues, in Notes). On Windows the same addons work.napi_get_uv_event_loopon posix returns Bun's ownEventLoopstruct (src/runtime/napi/napi_body.rs). An addon that writesloop->dataoverwrites it.Fix
src/runtime/napi/uv_posix.rs. Auv_loop_t*is aUvLoop, one per VM in itsRuntimeState.napi_get_uv_event_loopreturns the env's,uv_default_loop()the main thread's.uv_async_sendsets the handle's pending flag and posts one dispatch task per loop through the VM'sVmHandle; the task runs the callbacks on the JS thread.uv_closeunregisters the handle, waits out senders and runsclose_cbon a later turn.uv_queue_workis abun_jsc::Job,uv_cancela state word in the request.uv.hoffsets and the private state fits insizeof, both asserted at compile time. The semantics are libuv's (Notes).uv-posix-polyfills.cgets the header-only functions;process.versions.uvnow usesuv_version_string()too. 28 stubs go away.test/napi/uv.test.ts, 15 new tests, all 15 fail on the stock bun. Node'stest-resolve-async.jsnow runs on posix.Background
uv_async_tanduv_work_titself and readsdata,loopandtypefrom them. The rest of each struct belongs to the library.uv_async_sendruns off the loop thread. A handle is valid untilclose_cbhas run, a request untilafter_work_cbhas run. The code depends on this.VmHandle(src/jsc/VmHandle.rs) posts a task to a VM's thread from any thread.Job(src/jsc/job.rs) runs a body on the pool and a completion on the JS thread.KeepAlive(src/io/keep_alive.rs), stored in the handle, is its ref on the loop.Notes
Sentry issues: BUN-492M (
uv_async_init), BUN-44KV (uv_version_string), BUN-4ADE (uv_get_osfhandle), BUN-49Z2 (uv_handle_size), BUN-4AJG (uv_default_loop), BUN-4ADH (uv_queue_work).Semantics kept from libuv: a handle is active and ref'd from init; sends before the loop turns coalesce into one callback; a send from inside the callback gives one more callback;
close_cbis never called synchronously;uv_ref/uv_unrefare idempotent;after_work_cbalways gets 0 orUV_ECANCELED;uv_cancelanswersUV_EBUSYonce the work started andUV_EINVALfor other request types;uv_async_initwith a null loop anduv_queue_workwithout awork_cbanswerUV_EINVAL. Closing twice is a no-op (libuv asserts).uv_has_refreports theKeepAlive, so it reads 0 afteruv_close, where libuv keeps reporting the flag.Sentry, 90 days: about 500 events name
uv_default_loop, about 190uv_async_init, thenuv_check_init,uv_timer_init,uv_version_string,uv_get_osfhandle,uv_interface_addresses,uv_cwd,uv_cond_init,uv_thread_self,uv_handle_size. The ones after the first two are not in this change; the handle functions here switch ontype, so more handle types can be added to them.Layout.
offsetof/sizeofforuv_handle_t,uv_async_t,uv_req_t,uv_work_tare identical with the vendored headers (libuv 1.51.0) and Node 26.3.0's (libuv 1.52.1): handle prefix 96 bytes (data0,loop8,type16,close_cb24,flags88),uv_async_t128 withasync_cbat 96,uv_req_t64 (typeat 8),uv_work_t128 (loop64,work_cb72,after_work_cb80). Bun's private state is aKeepAlive,busyandpendingbehindasync_cb, and oneAtomicU32where libuv keepsstruct uv__work.Dispatch.
UvLoop.dispatch_stateis the Idle/Pending/Running machine ofThreadSafeFunction::schedule_dispatch: Idle to Pending posts the task; a send during a pass makes the final Running to Idle exchange fail and the pass runs again. A pass swaps the live list intodispatchingand moves each handle back before its callback, like libuv'suv__async_io, so auv_closefrom any callback removes the handle from whichever list holds it. No function forms a&UvAsync, because other threads may be insideuv_async_sendon the handle: fields go through the raw pointer and only the atomics are borrowed.Teardown. The loop dies with the VM's
RuntimeState. The main thread's is never freed, like libuv's default loop. A Worker's addon has to close its handles before the Worker exits, as with libuv (Node aborts otherwise); a later send is refused by theVmHandle. Once the VM stops running script, async callbacks are skipped like a threadsafe function's, close callbacks still run, and a work completion that lands then is released like every otherJob, soafter_work_cbdoes not run in that one case.Exceptions. A JS exception a callback leaves pending (a throwing function called through
napi_call_function) is reported as uncaught after that callback and the pass continues. Tested.Tests.
uv_impl.creports events through a JS callback. The child's exit is part of each test: a ref'd handle or a queued request must keep it alive after the script returned, an open unref'd handle must let it exit. The stress test does 200 send/callback round trips with a thread, then 2000 sends racinguv_close, and expects exactly 201 callbacks. The Worker test checks the Worker's loop differs fromuv_default_loop(), works, and lets the Worker exit.uv_cancelright after queueing won 12 of 12 runs here; the test accepts both outcomes, and every work test checks theUV_EBUSYanswer for a finished request.process.versions.uvwas the literal "1.48.0" on posix anduv_version_string()(1.51.0) on Windows; it is 1.51.0 everywhere now, the version of the vendored headers.Found on the way and handed off separately: env teardown does not wait for async cleanup hooks (
test_async_cleanup_hooknow reaches its assertion instead of crashing inuv_async_init; its todo comment is updated), andtest-resolve-async.jsfails on Windows with the real libuv (the process exits beforeafter_work_cb), so it stays todo there.#35475implementeduv_async_talone, in C with three Rust shims, and keptEventLoop*as theuv_loop_t;#34156madenapi_get_uv_event_loopreturn a failure on posix. Neither was reviewed; this change covers both.uv_stub.test.ts: one of its 46 sampled stubs timed out once under load in this container (one crash child takes 3.9 s on the ASAN build here) and passed alone.Suites run:
test/napi/uv.test.ts(5 times),uv_stub.test.ts,test_callback_scope,test_async_cleanup_hook --todo,test/internal/source-lints,cargo clippy -p bun_runtime -p bun_jsc,bun scripts/rust-check-all.ts,cargo fmt --check,clang-format:check, prettier.no test proof · iteration 1 · Platform-specific test(s) that do not run on this machine. Deferring to CI, which covers all platforms: test/napi/node-napi-tests/test/node-api/test_callback_scope/do.test.ts test/napi/uv.test.ts