node:stream: stop 'data' and 'end' after destroy() on a native-backed Readable by robobun · Pull Request #43739 · oven-sh/bun

robobun · 2026-09-22T02:20:45Z

Related to #36169

Problem

child.stdout.destroy() inside a 'data' listener is followed by one more 'data' event (destroyed === true), and by 'end' if EOF was due. Node emits only 'close'. Same for child.stderr and Readable.fromWeb() over a native stream.
exec() and execFile() stop a stream with destroy() at maxBuffer, as Node does. The extra chunk gives the callback up to one chunk too many.
Cause: _read() in src/js/internal/streams/native-readable.ts pushes synchronously, so a flowing Readable holds the next chunk while a 'data' listener runs. flow() emits it after destroy(). EOF follows a tick later.

Fix

destroy() drops what a flowing stream has buffered, and EOF is not pushed to a destroyed stream. The events now match Node: 'data', then 'close'.
A paused or ended stream keeps its buffer, as Node's does. A destroy from inside _read() is the source failing, and the bytes read before the error still reach 'data'.
Verified: test/js/node/stream/node-stream.test.js (six cases fail on main), test/js/node/child_process/child_process.test.ts (two fail on main). Node v26.3.0 gives every expected sequence.

Background

NativeReadable is the Readable over a native ReadableStream source. child_process uses it for stdout and stderr, Readable.fromWeb() for a native web stream.
In flowing mode, Readable.read() calls _read() before it removes the chunk it is about to emit. A synchronous push() lands behind that chunk, so the source stays one chunk ahead.
Node's child stdio (net.Socket) and fromWeb() push asynchronously, so their buffer is empty while a 'data' listener runs.

Notes

Repro from the report. Node v26.3.0 and Bun 1.3.14 print 0. Bun 1.4.0 to main print 1.

const { spawn } = require("child_process");
const c = spawn("python3", ["-c", "import os\nb=b'x'*65536\nfor i in range(48): os.write(1, b)"]);
let at = -1, n = 0, after = 0;
c.stdout.on("data", d => {
  n += d.length;
  if (at >= 0) return void after++;
  if (n > 1024 * 1024) { at = n; c.stdout.destroy(); }
});
c.on("close", () => console.log("'data' events after destroy():", after));

The extra chunk does not come from the pending pull() promise: push() already returns false once the stream is destroyed. An instrumented run shows the chunk in _readableState.buffer when destroy() runs, and flow() emits it from emitReadable_.
Deterministic form: Readable.fromWeb(new Blob([Buffer.alloc(1 << 20)]).stream()) with destroy() in the first 'data' listener. main: data, data (destroyed=true), end, close. Node and this branch: data, close. A 100 byte body on main: data, end, close.
execFile with maxBuffer 1 MiB against a 3 MiB writer, debug build: 11 of 32 callbacks over the cap on main (1,077,568 to 1,114,112 bytes), 0 of 32 on this branch. Release main with the python writer above: 7 of 16. Node: 0 of 16. child_process: latch exec/execFile maxBuffer overflow so truncated output never exceeds the cap #36169 latches the overflow in the exec handler. With this change the handler needs no latch, as in Node.
Read errors: pull() throws a stored read error from inside _read(), and the stream is destroyed with it while a chunk is still buffered. subprocess: surface stdio read and write errors instead of dropping them #41456 delivers the bytes and then the error. destroy() keeps the buffer in that case (state.sync). The new case in test/js/bun/spawn/spawn-stdio-syscall-error.test.ts counts the bytes the parent received before an injected EIO and expects all of them in 'data'. It passes on main, and it fails (about 180 KiB lost) when the state.sync check is removed.
The read path does not change. To push each pull result on a later tick also fixes this, but it costs a tick for each chunk on the hot path, and the EOF check is still needed.
Readable itself is unchanged. A pure JS Readable that pushes synchronously emits one more 'data' after destroy() in Node too, and Node keeps buffered data across destroy().
One deliberate difference from Node: chunks buffered while paused (source not ended), then resume(), then destroy() in the first 'data' listener. Node and main emit the rest of the buffered chunks after destroy(). This branch drops them. To keep them needs a record of which buffered chunks came from a pause, on the hot path.
Not changed here: Duplex.from(nativeWebStream) emits one 'data' after destroy() on main and on this branch (its own _read loop finds several chunks ready). When the source ended with two or more chunks buffered, Bun emits 'end' before 'close' after destroy() and Node does not.
No event after destroy() on main for: http client response, http server request, fs.createReadStream, zlib gunzip, net.Socket, child.stdio[3].
On Windows the child stdio cases pass with and without the fix (reads there complete asynchronously). The fromWeb cases fail without it.
Suites run on the debug build: test/js/node/child_process/, test/js/node/stream/, readline.node.test.ts, node-fetch.test.js, undici.test.ts, test/js/web/streams/streams.test.js, fetch-backpressure.test.ts, body.test.ts, process-stdin.test.ts, spawn.test.ts (stdio subset), spawn-stdio-syscall-error.test.ts, spawn-streaming-stdout.test.ts, test/regression/issue/1632.test.ts, 09555.test.ts, and about 55 test-child-process-*, test-stdio-*, test-stream-* and fromWeb files from test/js/node/test/parallel/.

… Readable child.stdout, child.stderr and a Readable.fromWeb() over a native stream push each pull result synchronously from _read(). While the stream flows, Readable therefore holds the next chunk in its buffer when a 'data' listener runs, and EOF is pushed one tick after the last chunk. destroy() stopped neither: flow() emitted the buffered chunk with destroyed === true, and 'end' followed. exec() and execFile() rely on destroy() to stop 'data' at maxBuffer, so their callback could receive up to one chunk more than maxBuffer. destroy() now drops the chunks a flowing stream has buffered, and EOF is not pushed to a destroyed stream. A paused or ended stream keeps its buffer, as Node's does. A destroy from inside _read() is the source failing, and the bytes read before the error are still delivered.

robobun · 2026-09-22T02:20:57Z

^{Updated 8:11 PM PT - Sep 21st, 2026}

✅ @robobun, your commit 6f3ee4275fd2946588e9c867290c3309044c96d5 passed in Build #119506! 🎉

🧪 To try this PR locally:

bunx bun-pr 43739

That installs a local version of the PR into your bun-43739 executable, so you can run:

bun-43739 --bun

robobun · 2026-09-22T02:21:20Z

Status: ready for review.

Reproduced on main (a2b69f7, release) with the script from the report: 4 of 4 runs print 'data' events after destroy(): 1, Node v26.3.0 prints 0. The deterministic form is Readable.fromWeb(new Blob([Buffer.alloc(1 << 20)]).stream()) with destroy() in the first 'data' listener: main gives data, data (destroyed=true), end, close, Node and this branch give data, close.

Self-reviewed: 3 concerns raised, 1 addressed in code, 2 documented in the Notes.

Addressed: a read error destroys the stream from inside _read() while a chunk is still buffered. A first draft dropped that chunk. destroy() now keeps the buffer in that case, and spawn-stdio-syscall-error.test.ts has a case that counts the bytes.
Documented, not changed: chunks buffered while paused are dropped if the stream is resumed and then destroyed in the first 'data' listener (Node emits them). Duplex.from(nativeWebStream) still emits one 'data' after destroy(), as on main.

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Walkthrough

Changes

Readable stream lifecycle

Layer / File(s)	Summary
Native readable lifecycle `src/js/internal/streams/native-readable.ts`	`NativeReadable` declares read-ahead state, centralizes deferred EOF handling, and removes eligible read-ahead data during destruction.
Spawn stream error delivery `test/js/bun/spawn/spawn-stdio-syscall-error.test.ts`	Fault-injection tests track bytes received before `EIO` and verify byte delivery and event ordering.
Stream destruction regression coverage `test/js/node/child_process/child_process.test.ts`, `test/js/node/stream/node-stream.test.js`	Tests cover destruction during data delivery, exact `execFile()` `maxBuffer` truncation, and preservation of paused `Readable.fromWeb` data.

Suggested reviewers: dylan-conway

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Merge Risk: ⚪ Minimal · up to 6f3ee

The stream lifecycle update targets Node-compatible destruction and maxBuffer behavior; no actionable merge-blocking risk is identified, so it is ready to merge with normal checks.

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✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: preventing post-destroy() 'data' and 'end' events on native-backed Readable streams.
Description check	✅ Passed	The description explains the problem, fix, affected APIs, known limitations, and verification results. It does not use the exact template headings, but it provides the required information in equivale…
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Inline comments:
In `@test/js/node/child_process/child_process.test.ts`:
- Line 1472: Update the child-process maxBuffer test around the resolve callback
to use multibyte UTF-8 output instead of ASCII, and assert the measured output
length with Buffer.byteLength(stdout, "utf8") equals maxBuffer. Preserve the
existing code and prefix assertions so the test verifies byte-based truncation
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robobun · 2026-09-24T15:57:35Z

The case this PR added, "node:child_process: stdout delivers every byte read before the error", is red on the alpine lanes (build 120303). It exposes an older bug in the pipe reader. #43900 fixes it.

### Problem - `test/js/bun/spawn/spawn-stdio-syscall-error.test.ts` is red on alpine: `"lost": -82000`, not `0` (build 120303). #43739 added the case, not the bug. - `read_loop` (`src/io/PipeReader.rs:766`) delivers the bytes read before a failed read, then the error. The consumer asks for more inside that delivery, and the reader reads the fd past the error. The error comes late, or never. ### Fix - `read_once` sets `PosixFlags::READ_FAILED` on a fatal error, and `begin_read` does not read while it is set. The request parks, then `on_reader_error` rejects it. Nothing clears the flag. - Correct: libuv `uv__read` clears `UV_HANDLE_READABLE` before it reports a read error. - Verified: `test/js/bun/spawn/spawn-stdio-syscall-error.test.ts`, 17 pass. The four new cases fail without the fix. Suites: Notes. - Self-reviewed: 15 concerns raised, 14 addressed. Not taken: a flag reset in `start()`, which nothing on main needs. ### Background - `PosixBufferedReader` reads the fd behind subprocess stdio, file streams and the shell. Its parent gets `on_read_chunk`, then `on_reader_done` or `on_reader_error`. - `FileReader` is the parent behind a `ReadableStream`. `on_read_chunk` resolves the parked `pull()`, and the reaction runs before it returns. - Considered: a repeating shim failure hides the lost error. An error stored in `FileReader` first needs a callback in every parent. - #43920 is a newer PR with the same change. Its test trigger is here. ### Downsides - After a read error, a stream that ended with `'end'` now ends with `'error'`, as in Node. With no listener the process stops. - After a read error, `bun run --filter` no longer drains that pipe at exit. - Reads that do not fail pay nothing: `begin_read` tests one more bit. <details><summary>Notes</summary> **Trace without the fix** (bun 1.4.3-canary.1+367d939d9, shim logs each `recv()`, writer `dd bs=1025`). The debug build of main at 8d36bff does the same: 3 of 8 runs, two with no `'error'` and `lost` -7714150: ``` recv #5 len=262144 -> 95325 recv #6 len=166819 -> EIO (injected) same fill_scratch call as #5 JS data 95325 recv #7 len=65536 -> 65536 pull from inside the delivery, read_into recv #8 .. #116 to EOF {"received":452025,"got":8000125,"lost":-7548100,"events":["stdout.close","close"]} ``` No `'error'` event: the reads reached EOF before `on_reader_error` ran, and a stored error is only returned by a later pull. When the reads park first, `on_reader_error` rejects that pull and `'error'` comes late. That is the CI signature: the events match and `lost` is a negative multiple of 1025. **Why alpine.** In CI the failure is injected: the shim fails only the Nth `recv()`, so a later `recv()` succeeds and the extra bytes show. The failing `recv()` must follow bytes in the same wakeup. BusyBox `head` writes 1025 bytes at a time, so a wakeup often holds a short `recv()` and then the failing one. coreutils `head` fills the buffer in one `recv()`. The same run fails on debian when the writer is slow. With a writer that copies BusyBox `head` (stdio, 1024-byte buffer), the `RECV_AT=6` case fails 7 of 40 runs on bun 1.4.3-canary (release), and 36 of 40 when the writer also spins between chunks. This branch (debug): 0 of 120, and 0 of 60 with `dd bs=1025`. **With no shim.** A child with one AF_UNIX socket as fd 0 and fd 1 writes a line to stdout and reads stdin. The peer leaves that line unread, sends 8192 bytes and closes. The kernel gives the child 8192 bytes, then `ECONNRESET`, then EOF. | Runtime | stdin events | |---|---| | Node v26.3.0 | `'error'` `ECONNRESET` after 8192 bytes | | bun 1.4.3-canary.1+367d939d9 | `'end'` after 8192 bytes, no error | | this branch | `'error'` `ECONNRESET` after 8192 bytes | **The new cases.** Each one reaches the reader from inside the delivery in a different way. Whole-file runs of the describe block: | Case | Entry | Release, no fix | Debug, no fix | Debug, guard in `read_into` only | Debug, this branch | |---|---|---|---|---|---| | `child_process`, `'data'` listener | pull, `read_into` | fails 6 of 6 | fails 5 of 5 | passes | passes | | `child_process`, `'readable'` and `read()` | `set_flowing(true)`, `read` | fails 6 of 6 | fails 5 of 5 | fails 3 of 3 | passes | | `Bun.spawn`, `lazy` | pull, `read_into` | fails 6 of 6 | fails 5 of 5 | passes | passes | | `Bun.spawn`, reader started at spawn | pull, `read_into` | fails 6 of 6 | fails 5 of 5 | passes | passes | - The first three use counts: `SPAWN_FAULT_RECV_CAP=4096` makes every `recv()` short, so `fill_scratch` calls `recv()` again in the same wakeup. `SPAWN_FAULT_RECV_EAGAIN_AT=2` ends the first read loop, so the consumer's read parks and the next read is poll-driven. `SPAWN_FAULT_RECV_AT` then fails after bytes in that wakeup. For the `'readable'` case it is 19: #3 to #18 return 64 KiB, the highWaterMark, so the reader is stopped and `read()` starts it again. - The fourth uses state, and comes from #43920: the writer waits for a line on stdin, so the first read is parked when the bytes arrive. `SPAWN_FAULT_RECV_MID_FILL=1` fails the `recv()` that follows one that returned bytes, and `SPAWN_FAULT_READS_AFTER` counts the `recv()` calls after it. Without the fix it is 1. - With a count-based trigger and the reader started at spawn, the case passed without the fix on a debug build: the buffered reader that runs before JS reads `.stdout` took the bytes and the error. That is why this case uses state. - With the BusyBox-like writer at four speeds, the whole file passes 20 of 20 on this branch. **With the fix**, `CAP=4096 EAGAIN_AT=2 RECV_AT=5`: ``` recv #3 -> 4096, recv #4 -> 4096, recv #5 -> EIO JS data 8192 close(fd) JS error EIO ``` **Node.** libuv `uv__read` (`src/unix/stream.c`): on a read error other than `EAGAIN` it clears `UV_HANDLE_READABLE | UV_HANDLE_WRITABLE`, calls `read_cb` with the error, then stops the watcher. It calls `read_cb` once for each `read()`, so it never holds bytes and an error from one batch. **Placement.** EOF and the `maxBuffer` stop have the same guard at this site: `close_if_final` closes the reader before the final chunk is delivered. An error cannot use it, because a closed reader with no stored error reads as a clean end. For the same reason `READ_FAILED` is not part of `is_done()`. **Parents** (14 `BufferedReaderParent` implementations, what each does in `on_reader_error`): - 3 release the fd: `FileReader`, `SubprocessPipeReader`, `Terminal`. - 2 drop the reader: `FileResponseStream`, shell `subproc.rs`. - 8 only do accounting: `filter_run.rs`, `multi_run.rs`, `lifecycle_script_runner.rs`, `security_scanner.rs`, `git_runner.rs`, both cron jobs, test `Worker.rs`. `lifecycle_script_runner.rs` and `cron.rs` build a new reader with `init()` for each spawn. - 1 is shared and can restart: the shell `IOReader`. Only two read the same reader after an error. `filter_run.rs` `drain_and_close_pipes` reads once more at exit. That read is now a no-op, and `deinit()` follows. Before, it could reach a second terminal callback and decrement `remaining_fds` twice. The shell `IOReader::start()` does not restart a reader after a failed read on main, because the fired one-shot poll still counts as registered. The Windows reader gets one libuv callback for each read, so it has no bytes-then-error batch. **Left open.** - The flag is permanent. #39638 and #37901 change `IOReader::start()` to restart the shell's stdin reader. After this PR they must clear `READ_FAILED` there, or a `cat` that follows a stdin read error gets no data, no EOF and no error. - Not in this PR: a batch that stops because the buffer is full is labelled `ReadState::Eof` when the poll event carries the hangup (`read_state`, `None if received_hup`). `Bun.write(file, proc.stdout)` then writes 262144 of 400000 pending bytes and resolves. It is on main and in 1.4.3-canary, and this PR does not change it. It needs its own change. - Not in this PR: release the fd on a read error once, in `PosixBufferedReader::on_error`. #41456 names it as a follow-up. #41420, #41456 and #42150 did it for one parent each. **Suites run on the debug build:** `test/js/web/streams/streams.test.js` (624 pass), `test/js/node/stream/node-stream.test.js` (112 pass), `test/js/bun/spawn/spawn-streaming-stdout.test.ts` (pass), `test/js/node/child_process/child_process.test.ts` (81 pass, 2 fail in my container for reasons outside this change: "spawn in the default shell" reads an empty `$SHELL`, and "extra stdio pipes are not double-closed on GC" needs 5.0 s in a debug build against the 5 s timeout, its script prints `OK`). With this branch's build, the test file of #43920 passes 17 of 17 in 3 runs. #43790 is open and edits the comment above the failing case. It changes the event order in `native-readable.ts`, not the reader. </details>  --- **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/bun/spawn/spawn-stdio-syscall-error.test.ts

robobun assigned Jarred-Sumner Sep 22, 2026

github-actions Bot added the claude label Sep 22, 2026

robobun mentioned this pull request Sep 22, 2026

child_process: latch exec/execFile maxBuffer overflow so truncated output never exceeds the cap #36169

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Comment thread test/js/node/child_process/child_process.test.ts

test: say why the execFile maxBuffer writer is ASCII

6f3ee42

Jarred-Sumner merged commit 80825a7 into main Sep 22, 2026
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Jarred-Sumner deleted the robobun/23bb6507/native-readable-no-data-after-destroy branch September 22, 2026 03:23

This was referenced Sep 22, 2026

child_process: deliver each stdio event after the nextTicks and promise jobs of the one before #43790

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PipeReader: do not read an fd again after a read on it fails #43900

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This was referenced Sep 24, 2026

PipeReader: do not read again after a failed read #43920

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node:stream: lock the web stream in Readable.fromWeb at once and keep the lock #43713

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node:stream: close or error the web stream when a native Readable.fromWeb ends #42584

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node:stream: stop 'data' and 'end' after destroy() on a native-backed Readable - #43739

node:stream: stop 'data' and 'end' after destroy() on a native-backed Readable#43739
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