Bun.serve: abort a request whose connection closed while its handler was being dispatched - #40034
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…was still being dispatched A RequestContext subscribes to its connection's close (uWS onAborted) only when it outlives the dispatch: in to_async(), after the handler returned and its microtask checkpoint ran. Both of those can run the event loop (a synchronous wait on a promise, such as bun:test's .resolves/.rejects), and a nested run can dispatch the connection's close. uWS then destroys the response's callbacks, so the later subscription is a no-op: request.signal never fires, the context stays pending, and a Promise<Response> that settles later renders into the socket uSockets has freed (heap-use-after-free). set_abort_handler() now delivers the close itself when the socket is already closed, and on_response() checks for it once the checkpoint is over, before anything is rendered, by taking the request async right there. AnyResponse gains is_closed() for this; it is always false for HTTP/3, whose stream is freed by its close callback. This is what made "a request's abort listener closing an open ServerWebSocket" in websocket-server.test.ts hang under CI load: the test's continuation runs in the handler's checkpoint and waits on the aborted fetch with expect().rejects, and under load the RST is dispatched inside that wait.
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Included review availability: Your plan provides up to 5 included reviews per hour; 0 remain after this review. WalkthroughThe change adds response closed-state checks, strengthens abort-handler setup, and centralizes cleanup when connections close during request or error-handler dispatch. Regression tests cover bodyless GET requests and partially received POST bodies. ChangesDispatch-time connection closure handling
Suggested reviewers: 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
Comment |
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Updated 10:58 PM PT - Aug 21st, 2026
❌ @robobun, your commit b51e759 has 1 failures in
🧪 To try this PR locally: bunx bun-pr 40034That installs a local version of the PR into your bun-40034 --bun |
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Status: fix and tests are pushed (b51e759). Waiting for a maintainer. How it was reproduced:
CI on b51e759: every GitHub check passes and the two tests pass on every Buildkite lane. The only red test is |
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Inline comments:
In `@src/runtime/server/RequestContext.rs`:
- Around line 662-665: Move the response-state check in the request handling
flow to immediately after set_abort_handler(), before calling
resp.on_writable(), using self.resp.get().is_none() to detect the inline abort
cleanup. Preserve the existing on_abort/stream cleanup behavior and avoid
invoking on_writable when the response has been detached.
In `@test/js/bun/http/serve-pending-promise-abort-leak.test.ts`:
- Line 742: Remove the explicit 30_000 timeout argument from the test invocation
ending at the visible timeout call, leaving the test’s existing callback and
closing syntax intact so it relies on Bun’s global timeout.
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📒 Files selected for processing (5)
src/runtime/server/RequestContext.rssrc/uws_sys/Response.rssrc/uws_sys/h3.rstest/js/bun/http/serve-close-during-dispatch-late-resolve-fixture.tstest/js/bun/http/serve-pending-promise-abort-leak.test.ts
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Additional findings (outside current diff — PR may have been updated during review):
-
🟡
src/runtime/server/RequestContext.rs:668-673— The PR description says "the do_render_stream call of set_abort_handler() cannot see a closed socket any more, since on_response() checks first and no JS runs in between", but theerror()path still can:run_error_handler_with_status_code_dont_check_respondedcalls the user'serror()callback (which can perform the same synchronous event-loop wait this PR targets) and the guard afterwards checksis_aborted_or_ended()but notresp.is_closed(), so a stream-body Response fromerror()reachesdo_render_streamwith a closed socket. This is pre-existing UB that the PR strictly improves (on_abort now fires anddiscard_stream_after_abortrecovers), but per the same-class rule anis_closed()branch here (and inprocess_on_error_promise's Fulfilled arm) routing throughon_connection_closed_during_dispatchwould make the newly-documentedset_abort_handlercontract actually hold at its remaining caller.Extended reasoning...
What the gap is
The PR adds an
is_closed()check inon_response()(RequestContext.rs:2693) so that a connection closed during the handler or its microtask checkpoint is delivered as an abort before anything renders. The PR description justifies leavingdo_render_stream's call ofset_abort_handler()(line 2080) uncovered with: "The do_render_stream call of set_abort_handler() cannot see a closed socket any more, since on_response() checks first and no JS runs in between." That claim does not hold on theerror()path, and the new doc comment onset_abort_handlereven nameserror()as a place where JS can run the event loop (line 656).Concrete step-by-step trace
on_responseline 2693:resp.is_closed()is false — socket is open — so the new guard is passed.- Line 2711 (
response_value.to_error()) or line 2719 (reject_unsendable_response) or the sync-rejected-promise arm athandle_rejectcallsrun_error_handler(err). run_error_handler_with_status_code_dont_check_respondedline 3631 callson_error.call(...)— this runs the user'serror()callback. If that callback performs a synchronous wait on a promise (expect(p).resolves— the exact primitive this PR is fixing for the fetch handler), the event loop ticks and dispatches the socket's close. No abort handler is subscribed, so uWS'sHttpContext::onClosedestructs theHttpResponseDataext block without reachingon_abort;flags.abortedstays false andthis.respstaysSome.- Line 3641:
is_aborted_or_ended() || did_upgrade_web_socket()→ both false. Nois_closed()check here. error()returned aResponsewith aReadableStreambody → line 3669protect_for_body_and_render→render→do_render→do_render_with_body→do_render_stream.do_render_streamline 2044:is_aborted_or_ended()→ still false. Line 2050 captures the closedresp.- Line 2073:
render_metadata()writes status/headers viauws_res_write_status/uws_res_write_header(libuwsockets.cpp:1218, 1263), which do not checkus_socket_is_closed, so they write into the destructedHttpResponseDataext. Pre-existing UB — unchanged by this PR. - Line 2080:
set_abort_handler()— with this PR's change,is_closed()is true, soon_abortfires synchronously right here: it enters the sink branch (sink was set at 2066), aborts the sink, detachesthis.resp, setsflags.aborted, firesrequest.signallisteners.deinitis deferred viadefer_deinit_until_callback_completes(set at context creation for the sync dispatch, andhandle_rejectsets its own guard), sothisremains valid. - Line 2081:
resp.on_writable(...)on the stale localresp.uws_res_on_writable(libuwsockets.cpp:1502) also does not checkus_socket_is_closed, so it stores a callback into the destructed ext. Pre-existing UB. - Line 2088:
assign_to_streamon a sink thaton_abortjust aborted; runs the stream'spull(). - Line 2102:
this.resp.get().is_none()→ true (step 8 detached it) →discard_stream_after_abortcleans up.
process_on_error_promise'sFulfilledarm (~line 3703–3726) has the identical gap: it goes straight toprotect_for_body_and_renderaftererror()ran, with nois_closed()check.Why existing code doesn't prevent it
is_aborted_or_ended()(line 2411–2417) checksresp.is_none(),flags.aborted(),server.is_none(),server.terminated(). None of these are set by an unsubscribed close — that is exactly the bug class this PR is fixing, anderror()is a sibling site of the fetch handler that the newon_responseguard sits before, not after.Impact
The trigger is extremely narrow: an
error()handler must (a) perform a synchronous event-loop wait, (b) have the client hang up during that wait, and (c) return aResponsewith aReadableStreambody. Sync event-loop waits (expect().resolves) are a bun:test primitive, not a production pattern. And the pre-PR behavior on this exact path was already writing to the destructed uWS ext at step 7 onward — the PR does not introduce a new crash class; it actually improves this path, sinceon_abortnow fires and step 11 recovers instead of the request staying parked with a danglingresp.The remaining concern is that
do_render_streamviolatesset_abort_handler's newly-documented contract ("callers must not use it afterwards") at lines 2081+, and the PR description's stated invariant is inaccurate. Per REVIEW.md "Fix the whole class in the same PR", this is a same-class sibling of the fixed pattern that was left uncovered.Suggested fix
At line 3641 (and equivalently in
process_on_error_promise'sFulfilledarm), add the same check the PR added toon_response:if self.is_aborted_or_ended() || self.did_upgrade_web_socket() { return; } if self.resp.get().is_some_and(|r| r.is_closed()) { self.on_connection_closed_during_dispatch(server, result); return; }
That delivers the abort before
render_metadata/on_writabletouch the destructed ext, and makes theset_abort_handlercontract actually hold at line 2080.
Jarred-Sumner
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Jarred-Sumner
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can you come up with a test that fails on main without using expect().resolves?
…t the request for it set_abort_handler() now runs before the stream body is attached to the response, so do_render_stream never registers on_writable or pulls on a closed socket, and the error() path checks for the close like on_response() does. A close delivered by set_abort_handler() snapshots url and headers the way to_async() does, since the uWS request is only valid during the dispatch. Tests: a Bun.build() whose plugin setup() is async waits synchronously, so a handler that bundles on request hits the same window without bun:test; the stream bodies of the handler and of error() pulling while the connection closes; todo on Windows, where the libuv backend frees the socket inside the nested run and uWS's own dispatch crashes on it regardless of this fix.
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Yes. A plain script, no bun:test, that segfaults on main ( import { connect } from "node:net";
const server = Bun.serve({
port: 0,
async fetch(req) {
req.signal.addEventListener("abort", () => console.log("aborted"));
client.end();
const result = await Bun.build({
entrypoints: ["./entry.ts"],
plugins: [{ name: "slow-setup", setup: () => clientClosed }],
});
return new Response(await result.outputs[0].text());
},
});
const client = connect(server.port, "127.0.0.1", () => client.write("GET / HTTP/1.1\r\nHost: x\r\n\r\n"));
const clientClosed = new Promise<void>(resolve => client.once("close", () => resolve()));
await clientClosed;
await Bun.sleep(200);
console.log({ pendingRequests: server.pendingRequests });Same push also moves |
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@robobun can you come up with a test that fails without requiring a re-entrant event loop? |
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The Windows gating concern from the earlier inline comment is addressed in ba0ff76 — all new close-during-dispatch tests are now under describe.todoIf(isWindows) with a comment referencing #40021. No further issues found this pass. Given this touches RequestContext teardown/lifetime (the set_abort_handler() contract now includes "may free self"), relies on a uSockets deferred-free invariant that is explicitly platform-dependent, and reorders set_abort_handler() relative to the abort/end guards in do_render_stream, a human look is still warranted.
Checked this pass:
do_render_stream: the movedset_abort_handler()now precedesis_aborted_or_ended(); when it fireson_abortsynchronously, the subsequentis_aborted_or_ended()guard returns before anyrespuse, so no post-free access on that path.on_connection_closed_during_dispatchin bothon_responseand the error-handler tail: reached only afteris_aborted_or_ended()/did_upgrade_web_socket()return false, soserver.stop(true)-inside-handler still takes the earlier return as the PR notes.h3::Response::is_closed()returningfalsekeeps HTTP/3 on the pre-existing path; no behaviour change there.
Extended reasoning...
Overview
The PR fixes a heap-use-after-free / hang in Bun.serve: when a connection closes while the request handler (or its microtask checkpoint) is still on the stack — reachable via any synchronous-wait-on-promise (expect().resolves, Bun.build with an async plugin setup) — the request never subscribed to the close, so request.signal never fires, pendingRequests stays pinned, and a late Promise<Response> resolve renders into a freed socket. The fix adds AnyResponse::is_closed() (backed by us_socket_t::is_closed for TCP/TLS, hard-coded false for H3), checks it in on_response() and in the error-handler result path, and makes set_abort_handler() deliver on_abort itself when the socket is already closed. do_render_stream moves its set_abort_handler() call ahead of the abort/end guard so a close observed there is met before the stream is attached. ~40 lines of native change across RequestContext.rs / uws_sys/{Response,h3}.rs, plus ~260 lines of new tests and a subprocess fixture.
Prior review
My earlier inline comment (on 5ad830b) reported the new tests segfaulting on both Windows CI targets and asked for todoIf(isWindows) gating referencing #40021/#40023. The current head ba0ff76 wraps every new test in describe.todoIf(isWindows)(...) with a comment naming #40021, and imports isWindows from harness. That concern is resolved. The comment-cop and coderabbit nits (long doc comments, per-test timeout) were also addressed in 5ad830b/ba0ff76.
Security risks
None new. This is server-side connection-lifecycle bookkeeping; no auth, crypto, or untrusted-input parsing is touched. The change closes a UAF rather than introducing one, on POSIX; on Windows the exposure is pre-existing and unchanged by this PR (the tests are gated, and #40021/#40023 own the backend fix).
Level of scrutiny
High. RequestContext teardown is exactly the "most-blocked category" in REVIEW.md: reference-count balance across abort/end/error paths, a helper whose contract now includes "may free self", a reordering in do_render_stream, and correctness resting on a uSockets invariant ("a closed socket is freed only when the outermost tick ends") that the PR itself documents as false on one platform. The reasoning in the PR description is thorough and the test coverage is broad (both close windows × handler-return shapes, in-flight body, stream pull() from both fetch and error(), error() return shapes, late-resolve subprocess under ASAN, plus a non-bun:test sync-wait via Bun.build), but the invariant chain is subtle enough that a maintainer should confirm it — particularly that every remaining caller of set_abort_handler() tolerates synchronous on_abort (which can drop the last ref via defer_deinit_until_callback_completes), and that the to_async_without_abort_handler snapshot inside set_abort_handler() is safe when reached from do_render_stream (where req should already be None, so that branch is a no-op — worth a maintainer's eye).
Other factors
CI build #103189 for ba0ff76 is in flight; the previous build's Windows failures are now gated. The bug-hunting system found no issues on this revision. This is not a mechanical or config-only change, so I'm deferring rather than approving.
…dler stop(true) closes the connection synchronously, with no nested event loop. A request with its body in flight then went async on the closed socket and was parked forever: no abort, a body read that never settled, pendingRequests stuck at 1, and a stop() promise that never resolved. One without a body rendered a 204 into the closed socket. should_render_missing() now treats a closed socket like a detached one, so both reach set_abort_handler(), which delivers the abort.
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Yes, with On main, for a request whose body is still arriving ( Pushed as 9b42f9b: Why nothing else does it: uWS dispatches the request from inside the socket's own readable callback, and only the event loop dispatches that socket's FIN or RST. So without |
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okay well delte your test that requires a re-entrant event loop |
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Done in af82b7d: the tests that wait on a nested event loop are gone, along with their fixture. What remains are the two |
… error before the handler runs
2 participants
Problem
request.signalstays silent, a pending body read never settles,pendingRequestsstays at 1. If itsPromise<Response>settles later, rendering hits a freed socket:heap-use-after-free ... in us_socket_group(socket.c:77), a segfault on a release build.RequestContextsubscribes to the close (uWSonAborted) only into_async()(RequestContext.rs:2323), at the end of the dispatch. A close dispatched before that finds no subscriber, and subscribing to a closed socket is a no-op (libuwsockets.cpp:1535).server.stop(true)inside the handler closes the socket right there: aPOSTwhose body is still arriving is then parked forever and thestop()promise never resolves. Anything that runs the event loop from inside the handler does the same, which is the CI hang of "a request's abort listener closing an open ServerWebSocket" (websocket-server.test.ts): itsexpect().rejectswaits inside the handler's microtask checkpoint.Fix
set_abort_handler()runson_abortitself when the socket is already closed, after theurland headers snapshotto_async()would have made.should_render_missing()treats a closed socket like a detached one, so a request without a body reaches it too instead of rendering a 204 into the dead socket.on_response()and theerror()path check for the close once their callback has returned, before anything is rendered, anddo_render_streamsubscribes before it attaches the stream body. These cover the nested event loop case; they have no tests here (see notes).on_abortfinds the state the uWS callback would have found. Teardown inside the frame uses the existingdefer_deinit_until_callback_completespath.test/js/bun/http/serve-pending-promise-abort-leak.test.ts, "server.stop(true) inside the handler of a GET / a POST with its body in flight aborts it". Both fail on main on Linux and Windows. The flaky test: 34 of 40 loaded runs hang on a release build without this fix, 0 of 40 with it.Background
on_abortis the only teardown for a connection that dies before the response starts. It firesrequest.signal, detachesresp, and releases the refs.on_response(),error(), andto_async().AnyResponse::is_closed()is new. It is always false for HTTP/3, whereus_quic_on_closefrees the stream at once, so that path is unchanged.Notes
Tests for the nested event loop case were removed at review: they closed the connection from inside the handler and waited for it synchronously, with
expect().resolvesor withBun.build()and a plugin whosesetup()returns a promise, and they could not run on Windows, where the libuv backend frees the closed socket inside the nested run and uWS's own dispatch segfaults on it (with or without this fix, see #40021). Theon_response(),error()anddo_render_streamchecks are kept for that case. A plain script that segfaults on main and prints "aborted" with the fix:Nested event loop runs reachable from user code, checked with a timer that fires during the call:
Bun.build()with a plugin whosesetup()returns a promise (JSBundler.rs,wait_for_promise),require()of a file that uses an async macro andBun.Transpiler.transformSync()with one (Macro.rs), and bun:test'sexpect().resolves/.rejects.Bun.spawnSyncruns an isolated loop and does not count.ASAN stack on main for the late resolve:
handle_resolve->render->do_render_blob->uws_res_cork->AsyncSocket::isCorked->us_socket_group, freed byus_internal_free_closed_socketsinus_internal_loop_post.server.stop(true)path on main:on_response()returns early on the server's terminated flag.should_render_missing()then rendered a 204 into the closed socket for a request without a body, and sent one with a body in flight downto_async(), which parked it forever (itsreq.text()never settled and thestop()promise never resolved). Now both go throughset_abort_handler()and abort, like an in-flight async request does whenstop(true)closes its socket.CI: the flaky annotation lists the websocket test in 139 of the last 400 builds (8 of 8 on main) in about 18 hours, on every platform. A copy of the test with a watchdog showed the fetch rejected and then nothing: the process held three socket fds (listener, both ends of the WebSocket), uSockets had closed the
/abort-meserver socket, but the abort listener never ran. Why it is load dependent:wait_for_promiserunstick()first and polls I/O only if the promise is still pending. Unloaded, the HTTP thread's failure task has usually arrived before the wait starts, so the nested run never polls I/O and the outer loop dispatches the RST afterto_async(). Under load the nested run polls. The debug build is slow enough on the JS thread that it rarely reproduces either way (0 of 12 unfixed), which is why the A/B uses release builds of this tree with and withoutsrc/.Cost: one flag load per request in
on_response(), one more pererror()call and pershould_render_missing().Suites run on the debug build:
serve.test.ts,bun-server.test.ts,serve-http3.test.ts,bun-serve-routes,bun-serve-file,bun-serve-propagate-errors,serve-direct-readable-stream,serve-async-stream-client-abort,serve-error-handler-stream,serve-stream-body-error,serve-response-stream-sink-leak,serve-stream-reject-flush-leak,serve-body-leak,serve-response-gc-backpressure-abort,websocket-server.test.ts,bake/deinitialization.test.ts,bake/dev/request-cookies.test.ts,bun-serve-html.test.ts. Failures identical on main in this container:serve.test.tsrequestIP v6, root range port, #6583, /bun:info;bun-server.test.tssource map stream, rejected promise logging, abruptly close upload (alllocalhost/IPv6); thewebsocket-server.test.tssend() benchmark timeout (#39370), which starves its siblings on the debug build.[review] gate passed · iteration 1 · 4 files touched
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passes on PR (with fix)
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