[robobun]

Problem

• node-http.test.ts "a dispatch that throws while a raw write of the socket is still in its buffer (tcp) > body-to-come" flakes on darwin 26 aarch64: closed: false after 10 s.
• A read pause leaves the peer's input in the kernel. close(2) over it is a reset that drops unsent output: ECONNRESET after 2228224 of 8388608 bytes on Linux. macOS 26 can lose the reset (Notes).

Fix

• us_internal_socket_close_and_notify() drops that input before it closes the fd, at most 8 MiB. The socket must be paused or unread since its resume (resumed_unread), with its FIN out and close code 0 or 2.
• Correct: the close discards that input anyway. CloseCode promises a FIN for these codes.
• Verified: node-http.test.ts (6 tests fail on main), node-http-server-socket-end-drain.test.ts (9), serve-close-delimited-framing.test.ts (1). Also: http, net, tls suites. No macOS run.
• Self-reviewed: 27 concerns raised, 21 addressed, 6 not taken (Notes).

Background

• A read pause takes the socket out of the read poll: node:http for backed-up output or an unread body, Bun.serve at 1 MiB of unread body.
• Considered a lingering close (Bun.serve: hold a pipelined request until the response ahead of it completes instead of closing the connection #38128, Bun.serve http2: close a connection gracefully with a FIN, not with close(2) #44874). It also covers later input, but holds the fd open behind the FIN, against node:http: the server follows Node (request body, framing, response finish, lifecycle) #43557 for node:http.

Downsides

• The rule holds for every usockets owner: end() then destroy() over unread input now delivers, where Node v26.3.0 resets. This scope needs a maintainer's yes.
• One more instruction per read event. A close that drops pays 2 or more recv.
• Still a reset: input that arrives after the close, and a close with no FIN first (Notes).

Notes

Decisions for a maintainer

1. Scope. The drop is in the close path of usockets, so every owner gets it: node:http, Bun.serve, node:net, node:tls, a Bun socket after shutdown(). The nearest review statement is on usockets: drain the receive queue before closing on a peer reset #39860: "There is one correct behavior for every paused socket, so nothing per-socket needs to select it." For HTTP only, the condition takes one more bit that HttpContext sets next to hangup_closes_unsent.
2. Policy. This PR drops once and closes right behind the FIN. Bun.serve: hold a pipelined request until the response ahead of it completes instead of closing the connection #38128 (Bun.serve) and Bun.serve http2: close a connection gracefully with a FIN, not with close(2) #44874 (HTTP/2) hold the socket open behind the FIN for a bounded time, which also covers input that arrives later. If that policy wins, it replaces the call of bsd_discard_queued_input() at the same place.
3. Node parity. Node v26.3.0 gives the same result in body-to-come (all bytes, then a close) and the same JSON in the 4 cells of the new block in node-http.test.ts. It resets where the handler paused the socket itself (destroySoon(), and end() then destroy()): 65536 of 262144 bytes, then ECONNRESET, as Bun does on main. The rule cannot tell a pause of the handler from a pause of the server. The tests for those rows are in node-http-server-socket-end-drain.test.ts, not in the file that must match Node.

Why the client can miss the close

• Linux tcp_close() and XNU tcp_disconnect() (which calls tcp_drop()) answer a close over unread input with one RST and discard the send queue.
• XNU accepts an RST only when its sequence number is last_ack_sent (tcp_input.c, "bad RST"). For another number inside the window it sends a challenge ACK. That happens when the client holds data that it has not ACKed yet, so it depends on the delayed ACK.
• The kernel of the server answers the challenge ACK with a second RST, and the client accepts that one. XNU has "TCP RST duplicate suppression" since xnu-12377.41.6 (tcp_rst_rlc_compress() in tcp_subr.c, net.inet.tcp.rst_rlc_enable, default 1). It does not send a second RST for the same addresses and ports within 200 ms. The tag before it, xnu-12377.1.9, does not have it. The client then gets no reset that it accepts, and keeps the connection.
• This fits what CI shows: a retried warning on the darwin 26 aarch64 lane in 7 of 144 main builds (for example 124948 and 126172), and only in the one mode where the reset comes while the response flows.
• It was not observed on a Mac: no macOS machine was available. sysctl net.inet.tcp.rst_rlc_enable and the reset counters of netstat -s -p tcp are the check.
• Emulation on Linux: a network namespace that drops RST segments on loopback. main ends body-to-come with closed: false over tcp and tls. This change ends it with closed: true and all 8388608 bytes.

Earlier reports

• No issue reports a response that a close over paused input cut.
• test(serve): fix darwin flake in the respond-without-reading-body backpressure test #36088 found this close in a Bun.serve test on darwin and changed the test. fetch(macOS): FIN instead of RST when aborting a request mid-upload #36309 fixed a reset that macOS lost in the fetch client.
• Bun.serve does not drain-or-close a keep-alive connection when the handler responds before reading the full request body #35283 and 1.4 regression: streamed response intermittently lost with ECONNRESET when the request body finishes as the server closes the connection #39846 are about an early response on a connection that the server still reads. This PR does not change that case.

What changed since the first revision

• The first revision called a new public us_socket_discard_queued_input() from seven HTTP/1 close sites and from socket.destroySoon(). A TLS close that waited for ciphertext needed a second bit and a second drop point. end() then destroy() was not covered.
• Now usockets drops where it closes the fd. bun-uws and the node:http binding are as on main.
• The mark of a paused socket was never cleared. Now a read ends it, and us_socket_pause() is as on main.
• The 42-cell block in node-http.test.ts is now 4 cells, all with the result of Node v26.3.0. The Bun.serve fault tests are now one test with an upload that is parked unread.
• The two review findings on the first revision (a missing declaration, a destroySoon() wait with no limit) were in code that is gone. No test has a per-test timeout.

Which test fails when one clause is switched off (debug build, one clause at a time)

Clause	Tests
the drop	the 16 tests under Verified
is_paused in the condition	destroySoon() (http, https), the TLS ciphertext test, the short reads test, body-to-come (tcp, tls), socket-end (tcp, tls, unix), client-fin-only, the Bun.serve test
resumed_unread in the condition	end() then destroy() (http, https), the read-at-close test, socket-end (unix), the unread request body test
the clear of resumed_unread at a read	the read-at-close test
any pause in the condition	the read-at-close test
"our FIN is out"	"a close that does not wait for that ciphertext stays a reset"
more than one read	"the close drops the unread bytes in as many reads as it takes", socket-end (tcp, tls)
reads on after a short read	"the close drops the unread bytes in as many reads as it takes"
close codes 0 and 2 only	none

• The close code check has no test. It keeps an error close (a code above 2) from a read.
• The matrix ran before the last changes to the tests. The three "at its close" rows were one read-at-close test then, and the unread request body test was a fixture mode. The source did not change after it.
• The 42 cells of the first revision also pass with this revision (run once, not committed).

Measurements (release builds of main eb807b7 and of this change on top of it, Linux x64)

• .text 81661413 to 81661669 bytes (+256). The stripped binary is 107239136 bytes in both. No symbol added.
• Read path: one andb more in us_internal_dispatch_ready_poll. us_socket_resume has 65 instructions, before 62. us_socket_pause is unchanged.
• Syscalls of 200 requests that never pause (node:http and Bun.serve, Connection: close and keep-alive), and of 200 node:http connections that the client closes first: identical in 5 of 5 cases (close: recv 600, send 400, shutdown 400, close 403, epoll_ctl 1004).
• body-to-come: shutdown(fd, SHUT_WR), recv(fd, 16384) = 5, recv(fd, 16384) = EAGAIN, close(fd). socket-end over tcp: three reads (16384, 16384, 7232), then EAGAIN.
• TcpExt TCPAbortOnClose in a private network namespace: +1 on main and +0 with this change in each of 5 runs (socket-end and body-to-come over tcp and tls, client-fin-only). Over a unix socket main ends socket-end with ECONNRESET.
• Client in its own process (Node.js), 5 runs for each size of late input. Whole response and a clean end: 10 of 10 at 5 and at 60,000 bytes (main 3 of 10), 15 of 20 at 100,000 to 1,000,000 bytes (main 9 of 20), 0 of 5 at 4,000,000 bytes on both. A part of a larger input can still be on its way when the server closes.
• Bun.serve in one process, 4 MiB response to an upload that is parked at 1 MiB with 8 KiB, 1 MB, 3 MB or 6 MB more behind it: main cuts the response in 4 of 4, this change delivers it in 4 of 4.
• Other owners: a node:net socket with 100,000 unread bytes, destroySoon() and end() then destroy(): delivered (main and Node v26.3.0 reset). A Bun.listen socket, shutdown() then end(): delivered (main resets).

Still a reset

• Input that is not in the server's kernel at the close: it arrives later, or the client's process still holds it. Node v26.3.0 also resets for a late body of 100 KB.
• More than 8 MiB queued.
• A close with no FIN first: socket.destroy(), an idle timeout, closeIfIdle() behind server.stop(), end() of a Bun.listen socket. Such a close does not say that the output is complete. shutdown(fd, SHUT_RD) in place of the reads does not help: Linux still resets.
• An early Connection: close response on a connection that never paused, while the request body still arrives.
• The HTTP/2 connection error close (Bun.serve http2: close a connection gracefully with a FIN, not with close(2) #44874).

Review points that were not taken (second review, 27 points)

1. Carve the Bun.serve sites out. No Bun.serve code is left to carve out: the rule is in usockets. The scope is decision 1.
2. A debug counter at the fd close that the tests read. The rule now runs at that place, so the class has no second site to miss.
3. Also drop at a close with no FIN first. That close can come from an abort with output that was thrown away, and the reset is then the right answer.
4. Split a dedupe of the six shutdown and close pairs into its own PR. This revision does not touch those pairs.
5. Fix only the test. The test would pass, but the response stays cut on Linux and the client stays open on macOS.
6. Observe the cause on a Mac first. No Mac was available. The XNU source above is the evidence.

Tests

• node-http.test.ts: body-to-come (tcp, tls) now expects every byte and a close. The new block has 4 cells that fail on main.
• node-http-server-socket-end-drain.test.ts: 6 tests without fault injection and 6 with it. The fault tests run on builds with socket fault injection (ASAN builds) and not on Windows. One of them passes on main: it guards the reset of a close that sent no FIN.
• serve-close-delimited-framing.test.ts: one Bun.serve test. It sends exactly the 1 MiB at which Bun.serve stops the reads, waits 32 turns of the event loop, then sends 8 KiB more. If the server has not stopped the reads by then, the test passes without a drop. It cannot fail for that reason. serve.test.ts and bun-server.test.ts have tests that fail or time out on a local debug build, so the test is not there.
• A test that closes right after the late bytes first opens a second connection. The loopback delivers in order, so the late bytes are in the kernel of the server by then.
• On a local debug or ASAN build the fixture tests of two blocks in node-http.test.ts run one at a time: 11 to 20 debug fixture processes at once need more than the 5 s default. CI runs them at once, as before.
• Windows: the unix cell is skipped (a socket path is a named pipe there), and the tcp cell of socket-end too (plain TCP takes the whole write at once there, so nothing backs up).
• Suites on the release builds. No test fails only with this change: test/js/node/http (50 files, 0 failures), test/js/bun/http (74 files, 2 failures on both: one needs a user that is not root, one the internet), test/js/node/net, test/js/node/tls, test/js/bun/net, test/js/web/websocket and fetch.tls.test.ts (the same failures on both), test/js/first_party/ws (0 failures), and the upstream test-http*, test-https*, test-net* and test-tls* files (504 and 327 parallel, 29 sequential). 3 upstream files failed in one loaded run and pass alone on both builds.

---

no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/http/node-http.test.ts, test/js/node/http/node-http-server-socket-end-drain.test.ts

[robobun]

^{Updated 11:06 AM PT - Oct 21st, 2026}

⏳ @robobun, your commit a218627 is still building in Build #130115, but has 2 failures so far (All Failures):

• test/js/first_party/ws/ws-proxy.test.ts - code 1 on 🪟 2019 x64
• step failed outside runner - exit 1 on 🍎 27 aarch64 - test-bun

[robobun]

Status

• Reproduction on Linux: bun test/js/node/http/node-http-throw-behind-raw-write-fixture.js tcp body-to-come. With main, the server closes over 5 unread bytes. The client gets 2228224 of 8388608 bytes, then ECONNRESET, and TcpExt TCPAbortOnClose goes up by 1.
• The failure of the darwin aarch64 lanes (closed: false) shows on Linux when a private network namespace drops RST segments on loopback. With this change the same run ends with closed: true and all bytes.
• PR: usockets: drop the input a read pause left unread when a socket closes behind its FIN #45907

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• packages/bun-usockets/src/bsd.c
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Walkthrough

Socket close handling now tracks resumed unread input and discards queued data under specified conditions. Bun and Node HTTP regression tests cover response delivery and connection outcomes when request input remains unread.

Changes

Socket Close with Unread Input

Layer / File(s)	Summary
Track and discard queued socket input packages/bun-usockets/src/internal/internal.h, packages/bun-usockets/src/internal/networking/bsd.h, packages/bun-usockets/src/bsd.c, packages/bun-usockets/src/context.c, packages/bun-usockets/src/loop.c, packages/bun-usockets/src/socket.c	Socket state tracks resumed unread input. The BSD networking API discards queued data up to 8 MiB. Eligible socket closes call the discard function.
Build paced Node HTTP close fixtures test/js/node/http/node-http-throw-behind-raw-write-fixture.js, test/js/node/http/node-http-close-over-unread-input-fixture.js	Fixtures exercise socket closure with unread input and record response bytes and connection outcomes. The raw-write fixture can pace client reads.
Test Node HTTP close outcomes test/js/node/http/node-http-server-socket-end-drain.test.ts, test/js/node/http/node-http.test.ts	Tests cover response draining, pending TLS ciphertext, short reads, receive errors, and close outcomes across TCP, TLS, and Unix sockets.
Test Bun HTTP close with unread request input test/js/bun/http/serve-close-delimited-framing.test.ts	An integration test checks that a close-delimited response remains complete when request input remains unread.

Possibly related issues

• Issue 43381: The change tests Node HTTP teardown with unread input and queued output, which overlaps part of the issue’s broader teardown behavior across transports.

Suggested reviewers: jarred-sumner, cirospaciari

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to a2186

The HTTP close fix looks sound from the supplied evidence. One test still sets an explicit per-test timeout, which conflicts with the repository test guideline. Removing it is a small follow-up that does not block the merge.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and specifically summarizes the primary change: discarding unread input left by a paused socket before closing after its FIN.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
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Inline comments:
Review comments at @test/js/node/http/node-http.test.ts:
- Around line 3326-3332: Remove the per-test timeout from the fixture tests:
delete the `fixtureTimeout` constant, update `itFixture` to call `it.concurrent`
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• packages/bun-usockets/src/bsd.c
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• src/jsc/bindings/node/JSNodeHTTPServerSocket.cpp
• src/jsc/bindings/node/JSNodeHTTPServerSocketPrototype.cpp
• test/js/bun/http/serve-close-over-unread-input-fixture.js
• test/js/bun/http/serve-syscall-fault.test.ts
• test/js/node/http/node-http-close-over-unread-input-fixture.js
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[claude]

Findings marked 🟡 are optional suggestions and need no follow-up push.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline findings, I checked the rewritten usockets-only design: every socket constructor (context.c listen/connect, loop.c accept, socket.c from_fd) zeroes resumed_unread; a readable dispatch cannot clear it on a paused socket because epoll_kqueue.c masks events by us_poll_events, so a stale-false bit is not reachable; the SSL deferred close (ssl_close_after_spill) now reaches the same us_internal_socket_close_and_notify with the socket already in POLL_TYPE_SOCKET_SHUT_DOWN, so it gets the discard too; and on Windows MSG_DONTWAIT is 0 but sockets are non-blocking, so the loop ends on WSAEWOULDBLOCK. One thing to update: the PR description still describes the earlier design (HttpResponse::shutdownAndClose, us_socket_discard_queued_input, a 512 KiB cap, destroySoon() waiting for TLS ciphertext), none of which is in the current diff.

Extended reasoning...

The push since the last review moved the whole fix into the usockets close path (packages/bun-usockets/src/socket.c:323-329, bsd.c, one new bit in internal.h) and removed every change under src/jsc/bindings and packages/bun-uws, which makes the earlier inline findings on those files moot. The remaining surface is the socket close path shared by every TCP, TLS and unix connection, with no auth or parsing exposure; the per-close cost is one bit test on unpaused sockets.

[claude]

🟡 nit (optional): Maintainers get a Bun.serve test that waits a fixed count of event-loop turns for the server to pause, so under load it can pass without exercising the fix. test/js/bun/http/serve-close-delimited-framing.test.ts:121-123 loops 32 setImmediate turns and then writes late; nothing checks that the server had stopped reading, and if it had not, late is read normally and the response arrives whole on main too. Fix: wait on an observable condition or a bounded poll instead of a turn count, and assert the precondition held (that late was still unread at the close) so the test fails on the base branch for the stated reason; CLAUDE.md:107 asks for the condition, not time passing.

Why this was flagged

The test at test/js/bun/http/serve-close-delimited-framing.test.ts:86 drives an in-process Bun.serve with a 1 MiB parked upload, then at lines 121-123 awaits 32 setImmediate turns on the assumption that the server has read 1 MiB and paused in fewer turns. That assumption is not asserted. On a busy or slow runner (debug, ASAN, parallel test lanes) the server may still be reading when late is written at line 125, so the 8 KiB lands in the server's body buffer rather than the kernel, the close at line 130 has nothing to discard, and the assertion at lines 134-137 passes on the base branch as well. The node:http sibling in test/js/node/http/node-http-server-socket-end-drain.test.ts:218-220 awaits the socket's 'pause' event for the same precondition; this test has no equivalent signal and no precondition assertion, which CLAUDE.md:107 ("poll with a deadline or await the event itself") and the REVIEW.md rule to assert that setup created the precondition both call for.

Verification: The server has not yet buffered the 1 MiB and paused the socket when the 32 setImmediate turns elapse, so late is read instead of staying in the kernel. test/js/bun/http/serve-close-delimited-framing.test.ts:120-122 wait a fixed turn count and line 124 writes late with nothing asserting that the server has stopped reading; the kernel sends no RST and the response arrives whole on main.

[claude]

🟣 pre-existing, not blocking: nit: On hosts whose socket receive buffers are tuned above 8 MiB, a close behind a FIN with more than that unread still resets and loses the unsent response, now after up to 512 extra recv calls. bsd_discard_queued_input stops at the fixed left = 8 * 1024 * 1024 cap at packages/bun-usockets/src/bsd.c:939 while input is still queued, so the close(2) at socket.c:331 sends the RST this change exists to avoid. Fix: bound the loop by what this socket can actually hold (bsd_socket_buffer_size(fd, 1, 0, &size) plus one buffer of slack, or read until recv reports nothing) instead of a constant, so a tuned net.ipv4.tcp_rmem or SO_RCVBUF does not defeat the discard.
A small fix can ride a push you are already making; otherwise a short reply is enough.

Why this was flagged

Trigger: an operator raises net.ipv4.tcp_rmem's max (or a socket's SO_RCVBUF) above 8 MiB, and a client uploads more than 8 MiB on a connection the server paused (Bun.serve pauses at REQUEST_BODY_HIGH_WATER_MARK via src/runtime/server/RequestContext.rs:4331). The handler answers with Connection: close, which runs shutdown then close(0) and reaches the new branch at packages/bun-usockets/src/socket.c:323-328. bsd_discard_queued_input at packages/bun-usockets/src/bsd.c:939 returns once left reaches 0 with bytes still queued; us_internal_poll_close_fd at socket.c:331 then closes over unread input and the kernel sends an RST that discards the unsent response, which is what the base branch does too, only now after up to 512 recv calls of 16 KiB. The comment at bsd.c:937 sizes the cap from the default 6 MiB Linux and 4 MiB macOS buffers, and the PR calls the cost bounded; that holds only at default sysctls, since the kernel queues up to the tuned rcvbuf.

Verification: A paused connection whose kernel receive queue holds more than 8 MiB when the handler answers with Connection: close reaches packages/bun-usockets/src/bsd.c:939-946, which exits with input still queued; close(2) at socket.c:331 over unread input sends an RST that discards the unsent response. Compared to the base branch nothing gets worse; the delta is up to 512 extra recv calls before the same reset.