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A handshaking TLS socket that waits in the low-priority queue has its reads off. When both directions of such a socket are down, the loop closed it on the hangup event and discarded what the peer had sent. The dispatcher now defers the hangup of a socket in the queue, like it does for a paused socket. The queue registers the socket again and the read loop reads to the end of the stream. A socket that the kernel refuses at that point closes with the error.
A socket in the queue that this dispatch did read waits for the queue too, like a paused socket.
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Status How to reproduce, on main (9f70da0) and on 1.4.2, Linux x64:
With this PR all of them pass. Not in this PR: a peer reset still closes a socket in the queue unread (22 of 32, as on main). That is a change of the error path and the next PR. |
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Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 5 remain after this review. WalkthroughThe socket loop now handles readable-poll restoration failures and defers EOF for sockets parked in the low-priority queue. New TLS tests cover burst delivery and a poll-start ChangesTLS hangup handling
Suggested reviewers: Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to The change preserves buffered TLS data for queued sockets and adds burst-delivery and poll-failure coverage. No concrete merge-blocking risk is evident in the inspected paths. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
Review comments at @packages/bun-usockets/src/loop.c:
- Around line 359-364: Update the kqueue branch of us_poll_change to return the
result of kqueue_change instead of discarding it, so registration failures reach
the low-priority socket handler.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline finding, I also checked that on Linux a parked socket the dispatcher DELs from epoll cannot be stranded: us_internal_loop_pre drains the low-priority queue before every wait (epoll_kqueue.c:453/:485), and each re-added hung-up socket fires EPOLLHUP at once, so the remaining queue keeps getting ticked five at a time. The us_poll_change failure path in the queue handler closes the socket synchronously with the errno, matching us_socket_resume, and the ENOMEM fixture exercises it.
Extended reasoning...
The change is a ~10-line edit to the EOF arm of us_internal_dispatch_ready_poll and to us_internal_handle_low_priority_sockets in packages/bun-usockets/src/loop.c, plus four new tests and a fault-injection fixture. It touches the core event-loop hangup path for TLS sockets (no auth, crypto, or injection surface). One confirmed finding on the kqueue backend is posted inline, so a human look is already warranted; the note above records what else was traced on the epoll path.
…in the queue A socket in the queue that this dispatch did read takes the eof at once, as on main. The close of a socket that the kernel refuses runs its handler after the caller picked the timeout of the tick, so the loop is woken.
The sockets stop reading until the relay delivered, so every socket is readable in one iteration on every backend. The tally now holds the closes per iteration: 5, 5, 5, 1 shows that the queue held 11 of the 16 sockets.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline finding, I also traced the !(events & LIBUS_SOCKET_READABLE) clause that 738afd1 added to reads_are_off in packages/bun-usockets/src/loop.c: when the budget is spent, the parking gate's break leaves the switch before the EOF block, so the clause only decides the budget-available path where the read loop already drained to recv()==0 — acting on that eof loses nothing. One note: the PR description's self-review still says that readable-bit clause "is gone", which no longer matches the code.
Extended reasoning...
The change touches the usockets EOF/hangup arm and the low-priority handshake queue in loop.c plus new burst tests; no security-sensitive surface. Examined the new clause added after the prior review and the close path in us_internal_handle_low_priority_sockets (socket is unlinked from the queue head before close_raw runs, so the loop's head-walk is unaffected).
The closes per iteration matched any array off Linux, so a burst that the queue did not hold passed there. The bound holds for every split that the queue can give. A unix socket on kqueue whose peer closed takes the error path, which has no queue: that case expects all 16 in one iteration.
On the Windows lanes the 16 accepted sockets read all and close in one iteration, with and without shutdown(). The clients there close 5 per iteration, as on Linux and macOS. The cause is not known, so the bound is not asserted for that case.
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Merged main into this branch (5e9e8c3) #42351 merged to main at 22:09Z. It edits the same lines of
Tests on a debug build with ASAN of the merge, Linux x64:
A socket that is paused, in the queue and hung up The tests of the two PRs do not reach this state. The burst tests of this PR resume every socket before the loop parks it, and the fixture of #42351 has no hangup. So I ran a probe for it (the script is below).
Three debug builds that differ in
With the merge, 6, 64 and 200 connections give the same result, and ASAN reports nothing. After The path with the merge:
Not run: macOS and Windows. The probeIt imports // Probe: a TLS socket that is paused AND parked in the low-priority handshake queue AND hung up.
//
// BUN_FEATURE_FLAG_INTERNAL_FOR_TESTING=1 bun bd test/js/bun/net/probe.ts <pauseAfter: 1 | 2> [connections]
//
// CONNECTIONS Bun.connect clients of a TLS 1.2 server call shutdown() when their last handshake flight left. A relay
// holds the server's last flight, its data and its FIN until the FIN of every client arrived.
// 1. The clients pause. The relay delivers to all of them. Both directions of each socket are down now.
// 2. The clients resume in one tick. The next iteration reads BUDGET sockets and parks the others.
// 3. pauseAfter = 1: an immediate of that iteration pauses the parked sockets.
// pauseAfter = 2: an immediate of the iteration after it pauses them. By then the loop saw the hangup of the
// sockets that are still in the queue.
// 4. The loop turns long enough to drain the queue several times over. No handler may run for a paused socket.
// 5. The clients resume. Every socket must get its handshake, the data and its close.
import type { Socket } from "bun";
import { getEventLoopStats } from "bun:internal-for-testing";
import { tls as certs } from "harness";
import { once } from "node:events";
import net from "node:net";
import { createServer } from "node:tls";
const PAUSE_AFTER = Number(process.argv[2] ?? 1);
const CONNECTIONS = Number(process.argv[3] ?? 16);
// MAX_LOW_PRIO_SOCKETS_PER_LOOP_ITERATION in packages/bun-usockets/src/loop.c.
const BUDGET = 5;
const STEP_DEADLINE_MS = 60_000;
const iteration = () => getEventLoopStats().iteration;
type Observed = { events: string[]; paused: boolean; whilePaused: string[]; closed: boolean };
const clients: Socket<Observed>[] = [];
let done = false;
function fail(error: unknown) {
// The teardown of the relay resets connections.
if (done) return;
console.error(error);
process.exit(1);
}
function outcomes() {
const result: Record<string, number> = {};
for (const { data } of clients) {
const outcome = data.events.join(", ") || "nothing";
result[outcome] = (result[outcome] ?? 0) + 1;
}
return result;
}
async function until(step: string, cond: () => boolean) {
const deadline = Date.now() + STEP_DEADLINE_MS;
while (!cond()) {
if (Date.now() > deadline) {
console.log(JSON.stringify({ error: `timed out waiting for ${step}`, outcomes: outcomes() }));
process.exit(1);
}
await new Promise(r => setImmediate(r));
}
}
function afterIterations(n: number) {
const target = iteration() + n;
return until(`${n} loop iterations`, () => iteration() >= target);
}
const server = createServer({ ...certs, maxVersion: "TLSv1.2" }, socket => {
socket.on("error", fail);
socket.end("last words");
});
server.on("tlsClientError", fail);
await once(server.listen(0, "127.0.0.1"), "listening");
const deliveries: (() => Promise<void>)[] = [];
const relayed: net.Socket[] = [];
const serversEnded = Promise.withResolvers<void>();
const clientsEnded = Promise.withResolvers<void>();
let serverFins = 0;
let clientFins = 0;
const relay = net.createServer({ allowHalfOpen: true }, fromClient => {
const toServer = net.connect({ port: (server.address() as net.AddressInfo).port, host: "127.0.0.1" });
relayed.push(fromClient, toServer);
// What the server sends after the client's second flight is its last flight.
let sawServerHello = false;
let holding = false;
const held: Buffer[] = [];
fromClient.on("data", data => {
holding = sawServerHello;
toServer.write(data);
});
toServer.on("data", data => {
sawServerHello = true;
if (holding) held.push(data);
else fromClient.write(data);
});
toServer.on("end", () => ++serverFins === CONNECTIONS && serversEnded.resolve());
fromClient.on("end", () => ++clientFins === CONNECTIONS && clientsEnded.resolve());
fromClient.on("error", fail);
toServer.on("error", fail);
deliveries.push(() => new Promise<void>(written => fromClient.end(Buffer.concat(held), () => written())));
});
await once(relay.listen(0, "127.0.0.1"), "listening");
// A connection that the peer answers: what the relay wrote before it is in the receive buffers by the time it resolves.
async function roundTrip() {
const echo = net.createServer(socket => socket.end("x"));
await once(echo.listen(0, "127.0.0.1"), "listening");
const socket = net.connect((echo.address() as net.AddressInfo).port, "127.0.0.1");
await once(socket, "data");
socket.destroy();
echo.close();
}
function record(socket: Socket<Observed>, event: string) {
socket.data.events.push(event);
if (socket.data.paused) socket.data.whilePaused.push(event);
}
for (let i = 0; i < CONNECTIONS; i++) {
clients.push(
await Bun.connect<Observed>({
hostname: "127.0.0.1",
port: (relay.address() as net.AddressInfo).port,
tls: { rejectUnauthorized: false },
data: { events: [], paused: false, whilePaused: [], closed: false },
socket: {
open() {},
handshake(socket, success) {
record(socket, `handshake ${success}`);
},
data(socket, data) {
record(socket, `data ${data}`);
},
close(socket, error) {
record(socket, error ? `close ${(error as NodeJS.ErrnoException).code}` : "close");
socket.data.closed = true;
},
error(_socket, error) {
fail(error);
},
},
}),
);
}
const closedCount = () => clients.filter(client => client.data.closed).length;
// Step 1.
await serversEnded.promise;
for (const client of clients) {
client.shutdown();
client.pause();
}
await clientsEnded.promise;
await Promise.all(deliveries.map(deliver => deliver()));
await roundTrip();
const closedBeforeFirstResume = closedCount();
// Step 2.
for (const client of clients) client.resume();
// Immediates run after an iteration's dispatch: the first one that sees the counter advance runs with the budget
// spent and the other sockets parked.
await afterIterations(1);
const closedAfterOneIteration = closedCount();
// Step 3.
if (PAUSE_AFTER === 2) await afterIterations(1);
const closedBeforePause = closedCount();
let pausedCount = 0;
for (const client of clients) {
if (client.data.closed) continue;
client.pause();
client.data.paused = true;
pausedCount++;
}
// Step 4.
await afterIterations(4 * Math.ceil(CONNECTIONS / BUDGET) + 8);
const whilePaused: Record<string, number> = {};
for (const { data } of clients) {
for (const event of data.whilePaused) {
const kind = event.split(" ")[0];
whilePaused[kind] = (whilePaused[kind] ?? 0) + 1;
}
}
const closedWhilePaused = closedCount() - closedBeforePause;
// Step 5.
const resumedAt = iteration();
for (const client of clients) {
if (!client.data.paused) continue;
client.data.paused = false;
client.resume();
}
await until("the close of every client", () => closedCount() === CONNECTIONS);
console.log(
JSON.stringify({
pauseAfter: PAUSE_AFTER,
connections: CONNECTIONS,
closedBeforeFirstResume,
closedAfterOneIteration,
closedBeforePause,
paused: pausedCount,
whilePaused,
closedWhilePaused,
iterationsAfterResume: iteration() - resumedAt,
outcomes: outcomes(),
}),
);
done = true;
for (const socket of relayed) socket.destroy();
relay.close();
server.close(); |
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
-
🟣
packages/bun-usockets/src/loop.c— pre-existing: a TLS socket waiting in the handshake queue still loses everything its peer sent before a reset, while the same bytes now survive a FIN or hangup. drain_for_error at packages/bun-usockets/src/loop.c:654 excludes low_prio_state == 1, so an EPOLLERR (or kqueue EV_EOF with fflags) on a parked socket skips the read loop and goes straight to the SO_ERROR close at loop.c:970-985. Fix: drain a queued socket on an error event too, the way the readable path already reads a state-1 socket when budget remains (loop.c:667), and only then close with the error; us_internal_socket_close_raw already unlinks a state-1 socket from the queue. The PR lists this as a downside (22 of 32 close unread, Node reads all 32) but does not close it.Why this was flagged
A burst of more than MAX_LOW_PRIO_SOCKETS_PER_LOOP_ITERATION (5, loop.c:335) handshaking TLS peers send their last flight plus data and then reset (resetAndDestroy(), or a Linux close() with unread bytes in their own receive buffer, which sends RST). The sockets past the budget are parked at loop.c:699 with reads off. The reset arrives as EPOLLERR|EPOLLHUP; epoll_kqueue.c:271-277 dispatches it with error=1. At loop.c:654 drain_for_error is false because s->flags.low_prio_state == 1, so the read loop at loop.c:655 does not run, the low-prio gate at loop.c:664 is skipped (error set), and the socket reaches loop.c:970-985, which closes it with SO_ERROR without a single recv(). The kernel keeps the receive queue on a reset (loop.c:645-646), so the peer's flight and data were readable. The base branch does the same; this PR's new deferral at loop.c:898-899 only covers eof without error, so a FIN/hangup on a parked socket is now read to the end while a reset still discards the data. The PR description records this gap (22 of 32 close unread, Node reads all 32) as a downside rather than…
Verification: pre-existing; acknowledged in diff: the PR description states "A peer reset still closes a socket in the queue unread: 22 of 32, as on main. Node reads all 32." and the new RST tests at test/js/bun/net/socket.test.ts:4379-4382 say "Which sockets read what their peer sent before the reset is not pinned" — the stated bound (same as main) is accurate, but it is the author flagging the sibling…
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About the reset case in the last review: it stays out of this PR, because the change needs a choice that I do not want to make inside this diff.
The tests for the follow-up are here already. The 4 reset tests expect one |
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This change also repairs a test that is red on main: The cause is the EOF block that this PR changes, on kqueue:
I checked the clause on Linux with the kqueue report emulated on epoll ( The fixture and its test are on |
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1 participant
Problem
closeonly when its peer's last flight, data and FIN arrive. In a burst of 32, 22 sockets lose 4096 bytes each. No user reported it.us_internal_dispatch_ready_poll(packages/bun-usockets/src/loop.c) closes a socket that epoll reports as hung up, with no read. In reach: a TCP socket aftershutdown(), and a unix socket whose peer closed.Fix
us_internal_handle_low_priority_socketsregisters the socket again, and the read loop reads to the end.socket.test.ts(12 new),node-tls-duplex-end-verify.test.ts(1 new, passes on Node v26.3.0),socket-syscall-fault.test.ts(1 new). Self-reviewed: 14 concerns raised, 12 addressed.Background
recv() == 0in the block. It leaves the socket open, and nothing wakes it.Downsides
epoll_ctlcalls and 1recvfrom, and closes up to 5 iterations later. A TCP burst with noshutdown()has equal counts..text: 58148853 B before and after. Two functions grow by 115 B.Notes
Where this comes from
A review of the EOF block for #44192 found it. #44192 documents when a TLS socket closes with no
handshakecall, and it changes no behaviour.Why the queue registers the socket and does not read it
A read from
us_internal_handle_low_priority_socketssaves theepoll_ctlcalls. It runs the JS callbacks of every deferred socket before the loop polls, and it is a branch for epoll only. The registration uses the dispatch that every other socket takes.Order
us_internal_handle_low_priority_sockets. The merge keeps the earlycontinueof usockets: keep a socket paused when the TLS handshake queue drains it #42351 for a paused socket, then setslow_prio_state = 2, then makes the checkedus_poll_changeof this PR. A probe for a socket that is paused, in the queue and hung up gives 16 of 16 complete sockets with no callback while paused (comment).socket.test.tsandnode-tls-duplex-end-verify.test.ts. Both sets of tests stay.Bursts, debug builds of main (9f70da0) and of this PR, Linux x64
N
Bun.connectclients of a TLS 1.2 server callshutdown()when their last flight left. A relay holds the server's last flight and 4096 bytes of data until the FIN of every client arrived. Then it delivers all of it, with a FIN, to every client in one tick.handshake, 4096 bytes,closecloseand 0 bytescloseand 0 bytescloseand 0 bytesshutdown()shutdown(), the relay closeshandshake(false),closeand 0 bytes1.4.2 gives the same counts as main. The limit of 10 is the budget of two loop iterations: the first reads 5 sockets and parks the others, the second takes 5 out of the queue. The hangup of the sockets that are still in the queue arrives in that second iteration.
The same happens to the accepted sockets of
Bun.listen, and to the sockets of atls.Serverthat callend(): 10 of 16 emitsecureConnectionon main, 16 of 16 with this PR and on Node v26.3.0.Mechanism
loop.c, thelow_prio_stategate).shutdown()of its own. epoll reports EPOLLHUP for both, with the readable bit masked out, so the read loop does not run.us_socket_raw_write). If its handshake is complete by then, the read loop runs for it, and the end of the stream that it reads is behind the data. The block acts on that, as on main. No test covers this clause: the state needs a handshake that completes in the writable half of a dispatch.Tests
socket.test.ts, "in a burst of connections", 16 sockets per test (3 iterations of 5, and 1 more):Bun.connectclients orBun.listenaccepted sockets, TCP or unix socket, with or withoutshutdown(). The sockets stop reading until the relay delivered, then all read again in one tick. Each test expects that every socket reads all, and that no iteration closes more than 5 sockets, which shows that the queue held the burst. Two cases do not have that bound, see "Other platforms". On Linux the closes must take 4 iterations: 5, 5, 5, 1. On main 6 of the 8 fail with 10 complete sockets and closes of 5 and 11. The 2 TCP tests with noshutdown()are controls on Linux.closecall. They pass on main on Linux. They have no platform gate, because a deferred hangup on kqueue and libuv was not measured.Syscalls, burst of 32, whole process (gdb
catch syscall, hits divided by 2)shutdown()shutdown()shutdown()shutdown()epoll_ctlepoll_pwait2recvfromshutdowncloseThe 3
epoll_ctlcalls per socket are the DEL when the hangup is deferred, the MOD of the queue that fails with ENOENT, and the ADD that follows it.straceandperfare not installed on the test machine.Size, release builds from the same toolchain, only
loop.cdiffers.text(llvm-size -A bun-profile)bunus_internal_dispatch_ready_pollus_internal_handle_low_priority_socketsThe new test of the flags is inside the block for an event that carries an EOF hint. An event with no hint runs the same instructions as before.
The registration that fails
tls-parked-hangup-fixture.tsarms thepoll_startfault once before the relay delivers. With this PR 15 sockets read all and 1 closes withENOMEM. Without the close inus_internal_handle_low_priority_socketsthe fixture hangs: nothing reports that socket again. On main the fixture prints 10 complete sockets and 6 withcloseonly.us_wakeup_loop. Measured with the peers in a child process, 6 runs each on debug builds: asetImmediatefrom that close handler runs after 2.6 to 3.9 ms with the wake-up, and after 172 to 588 ms without it. No test covers the wake-up: another timer of the process always ends the wait, so only the delay shows it.us_poll_changereturns 0 on kqueue and libuv.Other platforms
I ran Linux x64 (epoll) only. The macOS and Windows numbers below are from the CI lanes of this PR (x64 and aarch64 each). What main does there was not run.
kqueue_change,EV_CLEARwithNOTE_LOWAT) and reports the peer's FIN once with the readable bit masked out, also for a socket that did not shut down.EV_EOFinepoll_kqueue.c), and the error path has no queue. The test expects 16 there.Bun.connectclients: no iteration closes more than 5 sockets, and every socket reads all. So the queue holds the burst on libuv too. By reading, libuv maps the AFD disconnect to the EOF hint for a socket that shut down, and arms it again on the next poll change, which the queue makes.Bun.listen: all 16 read all and close in one iteration, with and withoutshutdown(). I do not know the cause, and I cannot run Windows. The tests do not pin a number for that case, so on Windows they do not show the queue for accepted sockets.A peer reset, not in this PR
drain_for_errorinloop.c), and the error close that follows readsSO_ERRORwith norecv().closewithECONNRESETand 0 bytes, with and withoutshutdown(), on main and with this PR. Node v26.3.0 reads 4096 bytes on all 32.shutdown(), so it is the next PR. It is a change of the error path, not of the EOF block.erroris set, so a burst of resets then runs the handshake of each queued socket in one iteration. The other defers the reset until the queue hands the socket back: it keeps the budget, and it needs a second report of the error from each backend. That choice is the reason for a PR of its own (comment).Sibling sites that this PR leaves as they are
loop.c,drain_for_error: the reset case above.epoll_kqueue.c, the write-sideEV_EOFof a unix socket on kqueue: it is treated as a hangup for a paused socket only. Not measured.socket.c,us_internal_rearm_writable: it puts the readable poll back for a socket in the queue. The gate of the readable dispatch handles that case (thelow_prio_state == 1check).Self-review
drain_for_erroron main. The PR for the reset case removes that exclusion, and it rewrites the comment then.loop.c. It needs a push of a build without the fix.us_poll_changediscards the result ofkqueue_change, for every caller. A fix changesus_socket_resumefor every kqueue user and needs a run on macOS.Related
us_socket_resumeinto the loop. The close in the queue function can use its entry point when it lands.tlsClientErrorof atls.Serverfor a handshake that the peer left.Suites, debug build with ASAN,
--timeout 60000because of the load of the test machinetest/js/node/tls/: 451 pass, 0 fail.test/js/bun/net/: 303 pass, 2 fail.should not call drain before handshakeneeds DNS forwww.example.com.active TCP socket wrapper survives GC until closedpasses when its file runs alone, and it fails in the same run on a build of main.no test proof · iteration 1 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/net/socket.test.ts, test/js/bun/net/socket-syscall-fault.test.ts