Bun.file().stream(): poll a dup of a stdio pipe, socket or terminal by robobun · Pull Request #46807 · oven-sh/bun

robobun · 2026-10-24T20:43:51Z

Decision needed

Should a second stream over a stdio pipe share the bytes (this PR) or fail with a named error, as in Node?
Sharing lets the later reader win: it silently took all 35 bytes from a listening process.stdin in 5 of 5 runs.

Problem

On Linux since 1.4.3, a second Bun.file(0).stream() reader over a stdin pipe rejects with EEXIST: file already exists, epoll_ctl. So does the second bun test --isolate file that reads process.stdin.
Lazy::open_file_blob (src/runtime/webcore/FileReader.rs:145) hands the reader the stdio fd itself. epoll refuses a second registration of one fd number. Release the poll and surface the error when a Bun.file() stream read fails #41420 made that a stream error.

Fix

A reader that polls a stdio fd takes a dup, like every other fd.
Off Linux only a socket gets one. With no descriptor left, the reader keeps the stdio fd.
Verified: test/js/web/streams/streams.test.js (17 new, 15 fail on main), test/js/node/process/process-stdin.test.ts (1 new).
Self-reviewed: 35 concerns raised, 32 addressed. Left out: ending a finished --isolate file's reader at the swap.

Background

FileReader, the native source of Bun.file().stream() and process.stdin, polls pipes, sockets and terminals.
Considered a per-VM claim on fd 0: 66 lines that save one fcntl and one close.

Downsides

Per stream over a stdio pipe or socket: +1 fcntl, +1 close, +1 descriptor (9 to 11 syscalls, gdb catch syscall). .text: +256 bytes.
A reader that outlives its owner keeps its descriptor: one per --isolate file or --hot reload that reads stdin.
Not fixed: pipe readers on kqueue, Windows (Windows: refuse a second reading pipe over a stdio fd instead of blocking in uv_pipe_open #40295), two Bun.stdin.text(), two Bun.serve responses over one fd.

Notes

There is no user report for this. It was found during other stdin work.

Measured on Linux x64. "main" is a debug build of 6292c65, "PR" a debug build of this branch. Stdin is the socketpair that Bun.spawn gives a child, unless a row says otherwise.

Case	1.4.2	1.4.3 and main	PR
Two `Bun.file(0).stream()` readers, input arrives while both wait	the second never settles	the second rejects `EEXIST epoll_ctl`	both end, each byte goes to one
`process.stdin.on("data")`, then a stream, 35 bytes	`process.stdin` 35, stream never settles	`process.stdin` 35, stream rejects	stream 35, `process.stdin` 0, no error (5 of 5 runs, release builds)
`process.stdin` reads, `pause()`, a stream reads, `resume()`	`process.stdin` emits `EEXIST epoll_ctl`	the same	both read, both end
`bun test --isolate`, 6 files that listen on `process.stdin`	6 pass	1 pass, 5 fail (unhandled `EEXIST`)	6 pass, and 1 to 6 descriptors held
`Bun.connect({ fd: 0 })`, then a stream	the stream never settles	stream rejects `EEXIST epoll_ctl`	both read
A stream, then `Bun.connect({ fd: 0 })`	connect rejects `ECONNREFUSED`	the same	both read
Two `Bun.stdin.text()` calls	second rejects `EEXIST` in 2 of 3 runs	3 of 3 (1.4.3)	2 of 3 (not changed)
Two `Bun.serve` responses `new Response(Bun.file(0))` at once	both connections are reset	one or both are reset	one 200, one reset, as on main (not changed)
A stream, then one `Bun.serve` response over fd 0	the response is reset	the response is reset, the stream gets the bytes	the response gets the bytes, the stream none

Node v26.3.0, the same socketpair: process.stdin.on("data") then new net.Socket({ fd: 0 }) throws EEXIST open, and process.stdin gets all 35 bytes. With fs.createReadStream(null, { fd: 0 }) as the second reader (stdin from child_process.spawn), that stream fails with EAGAIN read, and process.stdin again gets all 35.

Cost, main vs PR (gdb catch syscall on the debug builds, markers around one stream from getReader() to EOF):

Stdin is a pipe: 2 fstat, 5 ioctl, EPOLL_CTL_ADD, EPOLL_CTL_DEL on both. The PR adds fcntl(0, F_DUPFD_CLOEXEC, 3) on the JS thread and close of the dup on a pool thread. The pool task exists on main too: there it ends in close(0) SKIPPED.
Stdin is a regular file: the same 7 syscalls on both, no dup.
Stdin is a terminal that reopens: not measured. The reader already gets its own fd there (open_as_nonblocking_tty), and the new lines only act on fd 0, 1 or 2.
One descriptor per live stream over a stdio pipe or socket. 6 readers that were started and released without cancel() hold 6 descriptors until stdin ends, also after Bun.gc(true). Two Bun.file(3).stream() readers do the same on main.
Release builds of main and of the PR (size -A): .text is 82253657 and 82253913 bytes, +256. FileReader::on_start for a sole reader of a piped stdin runs 783 and 833 user instructions (gdb stepi over its 6th call, 2 runs each).
No new field, no new type, 0 lines in src/io.

What stays as it is

kqueue (macOS, FreeBSD), pipe or terminal on stdin: no dup, so a second EV_ADD still replaces the first and the first reader stops waking. A dup alone is not enough there: one write wakes both registrations, and off Linux the reader that lost the bytes calls read(2) with no second check (PosixBufferedReader::sys_read, src/io/PipeReader.rs:702), so it would block the JS thread until the next write. This is from the code. I have no macOS machine. A socket on stdin does get a dup there. On the darwin aarch64 lane the 13 new tests that run off Linux pass, and so does process-stdin.test.ts (23 pass).
Windows: not touched. Windows: refuse a second reading pipe over a stdio fd instead of blocking in uv_pipe_open #40295 proposes EBUSY for the second reader there.
Two Bun.stdin.text() calls register with the io thread's epoll and still collide.
Bun.serve responses that return new Response(Bun.file(fd)): do_sendfile (src/runtime/server/RequestContext.rs:1854) gives the Blob's fd to FileResponseStream with no dup, for fd 0 and for fd 3 alike. Two such responses at once still collide on main and with this PR: one gets the bytes and the other connection is reset (ECONNRESET, 3 of 3 runs each, release builds). 1.4.2 fails too, so this is not part of the 1.4.3 regression. A dup there adds an fcntl and a close to each fd-backed response over a pipe or socket.
Under bun test --isolate the reader of a finished file stays alive, so each file that listens on stdin adds a descriptor, and that reader still takes input that arrives before the next file starts its own. Ending it at the global swap is the stdin sibling of test --isolate: end the outgoing file's process stdio sinks at the global swap #38008.
bun --hot is the same: a reload that starts a stdin stream keeps the reader of the replaced generation. Open descriptors over 4 generations: 8, 8, 8, 8 on main with EEXIST on every reload, and 9, 10, 11, 12 with this PR.

Designs weighed

A per-VM claim: the first reader keeps fd 0 and only a later one takes a dup. I built it first (66 source lines, a new type in rare_data.rs). A reader with stdin to itself then pays nothing. It was dropped because a stream on fd 0 still collides with Bun.connect({ fd: 0 }), and because the saving is one fcntl and one close per stream.
A refusal with a named error on every platform: the second reader still fails, and process.stdin after pause() plus a later stream is such a pair.
A dup only after EEXIST: kqueue reports no collision, and the reader would change its fd at a re-arm.
One shared registration per fd for every reader: it adds work at each re-arm of every pipe reader.

Other open PRs near this code: #44499 and #41504 edit open_file_blob at other lines. #40295 is the Windows side. #38008 ends the stdio sinks at the --isolate swap.

Tests. In streams.test.js, block "Bun.file().stream() readers that share a stdio fd":

6 pairs of readers (Bun.file(0).stream(), Bun.stdin.stream(), process.stdin, Response.body, Bun.readableStreamToText), a fetch() upload body, and Bun.connect({ fd: 0 }) in both orders.
Linux only: a shell pipe, a named FIFO whose writer stays open (with and without RWF_NOWAIT), and a pty master.
Descriptor counts for a socket and for a regular file, the descriptor limit, and two --isolate files.
15 of the 17 fail on main. The regular-file row and the descriptor-limit test pass on both: they pin that a file is not dup'd and that a failed dup is not an error.

In process-stdin.test.ts: the pause() and resume() case is new. The test for a reader cancelled at the highwater backstop now waits for the reader's descriptor to close, so that the next reader gets the same fd number. Without that wait it could no longer see a registration that was left behind.

Also run on the PR debug build, all pass: the whole of streams.test.js (744 pass), spawn-stdin-readable-stream, console-iterator, bun-stdin-slice, bun-file-fd-read, filesink, process-stdio, process-stdin-stale-hup, spawn-socketpair-shutdown, body-clone, bun-serve-file. bun run rust:check-all passes for the 12 targets.

Failing on a debug build with and without this change: stdin-fixtures.test.ts (its runner kills the child after 1 s, and a debug child needs about 1.1 s) and one test in streams-leak.test.ts (5 s timeout). With the dup turned off in the same binary through an LD_PRELOAD shim, stdin-fixtures fails at the same rate.

Self-review. Left out: (1) ending the stdin reader of a finished --isolate file at the global swap, which is a change to the test runner next to #38008, (2) an assertion in the --isolate test that the next file gets every byte, which needs (1), (3) tracking issues for the collisions that stay, which this section lists.

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/web/streams/streams.test.js, test/js/node/process/process-stdin.test.ts

Lazy::open_file_blob hands a stdio fd that did not reopen as a terminal to the reader as it is. Every other branch gives the reader an fd of its own. Two readers over stdio then registered one fd number with the event loop: epoll refused the second registration with EEXIST, and kqueue replaced the first. A reader that polls such an fd now takes a dup, which it owns and closes. Off Linux only a socket gets one, because the reader of a pipe there reads again with no check after a wake-up. With no descriptor left, the reader polls the stdio fd itself.

robobun · 2026-10-24T20:43:57Z

Status: waiting for a maintainer to answer the question at the top of the description (share or refuse).

Reproduction, with stdin a pipe (printf 'hello world\n' | bun two-readers.mjs):

const a = Bun.file(0).stream().getReader();
const b = Bun.file(0).stream().getReader();
const show = p => p.then(r => (r.done ? "done" : "read " + r.value.length), e => "REJECTS " + e.code + " " + e.syscall);
console.log("first :", await show(a.read()));
console.log("second:", await show(b.read()));
process.exit(0);

1.4.2: first : done, second: read 12. With no input ready at the start, the second reader never settles.
1.4.3 and main: first : read 12, second: REJECTS EEXIST epoll_ctl.
This branch: first : read 12, second: done.

PR: #46807

robobun · 2026-10-24T20:44:05Z

^{Updated 4:33 PM PT - Oct 24th, 2026}

✅ @robobun, your commit cb6d7755d972c48b3134a45874a508064646db75 passed in Build #132233! 🎉

🧪 To try this PR locally:

bunx bun-pr 46807

That installs a local version of the PR into your bun-46807 executable, so you can run:

bun-46807 --bun

coderabbitai · 2026-10-24T20:51:31Z

No actionable comments were generated in the recent review.

Recent review info

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Repository: oven-sh/bun
Branch: robobun/4319c8ba/stdio-stream-second-reader
File: .coderabbit.yaml
Commit: cb6d775
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Reviewing files that changed from the base of the PR and between 7703857 and cb6d775.

Files selected for processing: 1

src/runtime/webcore/FileReader.rs

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 1 remain after this review.

Walkthrough

On Unix, Lazy::open_file_blob duplicates selected pollable stdio descriptors. Tests cover concurrent readers, descriptor cleanup, and stdin backed by pipes, FIFOs, terminals, sockets, or regular files.

Changes

Stdio file streams

Layer / File(s)	Summary
Descriptor selection and cleanup `src/runtime/webcore/FileReader.rs`, `test/js/web/streams/streams.test.js`, `test/js/node/process/process-stdin.test.ts`	`Lazy::open_file_blob` duplicates selected pollable stdio descriptors with a minimum descriptor number of 3 and uses the original descriptor if duplication fails. Tests check descriptor counts, descriptor limits, and duplicate cleanup after cancellation.
Concurrent stdio readers `test/js/web/streams/streams.test.js`, `test/js/node/process/process-stdin.test.ts`	Tests cover concurrent `Bun.file(0)` readers, stdin paired with fetch uploads or sockets, paused `process.stdin`, and separate test files that both listen on stdin.
Pipe, FIFO, and terminal coverage `test/js/web/streams/streams.test.js`	Linux-only tests cover shell pipes, FIFO event-loop behavior, and terminal detection, line-ending translation, and read errors after the terminal slave closes.

Possibly related PRs

oven-sh/bun#43747: Both changes address reader handoff on process.stdin; this PR duplicates descriptors for Bun.file(0).stream(), while the related PR changes native stdin flowing and reader-lock handling.
oven-sh/bun#46800: Both changes concern duplicate descriptors for pollable stdio sources and their cleanup after cancellation.

Suggested reviewers: jarred-sumner, dylan-conway, alii

Priority: Normal

Merge Risk: Low · up to cb6d7

A benign build diagnostic can make the descriptor-limit test fail and block validation on debug or ASAN builds.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: polling duplicated stdio descriptors for Bun.file().stream().
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description follows the repository template. It has both required sections: "What does this PR do?" (as "Decision needed", "Problem", "Fix", "Background", "Downsides") and "How did you verify your…

Full details: Description check

Explanation

The description follows the repository template. It has both required sections: "What does this PR do?" (as "Decision needed", "Problem", "Fix", "Background", "Downsides") and "How did you verify your code works?" (as "Tests" and the verification notes, including the platform and build details). It is detailed and specific, with measured syscall counts, a behavior table across versions, named test files and counts, and an explicit list of what is not fixed. The verification section does admit that platform-specific tests (streams.test.js and process-stdin.test.ts non-Linux cases) were not run on the author's machine and are deferred to CI. It also reports a darwin aarch64 run, so this is a partial gap, not a missing section. The template headings are reworded rather than kept verbatim, but the required information is present. The description is mostly complete.

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claude

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

claude · 2026-10-24T20:57:27Z

+        let fd = if two_can_poll && this.pollable && fd.stdio_tag().is_some() {
+            sys::dup_at_least(fd, 3).unwrap_or(fd)


🔴 Programs with a live process.stdin listener now silently stop receiving input once any other stream over fd 0 starts, where the base raised EEXIST. src/runtime/webcore/FileReader.rs:249 gives each pollable stdio reader its own dup, so two readers hold two epoll registrations on one pipe and whichever wakes first drains it (src/io/PipeReader.rs:690); the other gets EAGAIN and re-arms. On Linux the newer registration wakes first, so the earlier consumer gets nothing and no error. Fix: a second live reader over the same stdio file must either fail with a named error or get a copy of the bytes through one shared registration, not race for them. The description lists this as a decision still open; this is the cost of the sharing option. [also at: src/runtime/webcore/FileReader.rs:249 - After merging, a program listening on process.stdin can silently receive nothing once any other code opens a stream over fd 0; today that second stream fails loudly with EEXIST.]

Why this was flagged

A program registers process.stdin.on("data", ...) (reader A), then later something starts Bun.file(0).stream(), Bun.stdin.stream(), a Response(Bun.file(0)).body or a fetch upload body (reader B) while A is still listening. On the base branch B's register_with_fd (src/io/PipeReader.rs:547) fails with EEXIST from epoll_ctl and B's stream errors, so A keeps every byte and the program sees a loud error. After this change src/runtime/webcore/FileReader.rs:249-250 hands B dup_at_least(fd, 3), so A and B each hold an EPOLLONESHOT registration on the same open file description. One write readies both; the reader Bun services first calls read_nowait (src/io/PipeReader.rs:690) and takes the whole pipe, the other gets EAGAIN, maps it to Stop::WouldBlock (src/io/PipeReader.rs:728) and re-arms. Linux wakes the most recently added epoll entry first, so B (added later) is serviced first on every write and A's listener receives nothing, with no error or warning on either side.

Verification: The PR description's "Decision needed" section and its table row record exactly this outcome, and the new test at test/js/web/streams/streams.test.js:3010-3023 pins only that the concatenated text of both readers equals the input ("each byte goes to one"), not that the earlier listener keeps receiving.

This is correct, and it is the question at the top of the description. I measured the same result. On a release build of this branch the later stream takes all 35 bytes from a listening process.stdin in 5 of 5 runs, with no error. On main process.stdin keeps all 35 and the stream rejects with EEXIST: file already exists, epoll_ctl.

The costs of each answer, as far as I measured or can read from the code:

Share (this PR). The later reader usually wins, with no error. Two Bun.file(fd > 2).stream() readers behave this way on main.

Refuse with a named error. The earlier reader keeps every byte. A stream that starts after process.stdin.pause() or rl.close() is refused too, because that reader is paused and not closed. The second bun test --isolate file that reads process.stdin keeps failing, with a clearer error. kqueue reports no collision, so a refusal needs a per-VM record of which reader polls fd 0, 1 or 2. The first shape of this change had that record. I can put it back with a refusal in place of the dup.

A copy for each reader through one registration. A reader that is left without cancel() then holds every later byte in memory, or it stops the other readers.

I leave this thread open for a maintainer to choose.

…imit exits The leak check of an ASAN build opens /proc/self/task when the process exits. The child of the descriptor-limit test still held every descriptor then, so the check failed and the child aborted. The child now closes them after the stream has opened, and prints how many descriptors stdin has, which shows that the stream kept fd 0. The two waits for a reader to close its descriptor now stop after 2 s and throw with what they saw.

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Actionable comments posted: 1

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Inline comments:
Review comments at @test/js/web/streams/streams.test.js:
- Line 3115: Remove the empty-stderr expectation from the test near the `run()`
call while keeping `run()` draining child stderr; continue asserting the
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test/js/node/process/process-stdin.test.ts
test/js/web/streams/streams.test.js

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claude

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

Still open from earlier reviews (1):

🔴 src/runtime/webcore/FileReader.rs:250 — Programs with a live process.stdin listener now silently stop receiving input once any other stream over fd 0 starts, w…

If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.

claude

Code review found no new issues

No new issues were found in this update; 1 finding from earlier reviews is still open above.

Still open from earlier reviews (1):

🔴 src/runtime/webcore/FileReader.rs:250 — Programs with a live process.stdin listener now silently stop receiving input once any other stream over fd 0 starts, w…

If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.

github-actions Bot added the claude label Oct 24, 2026

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Comment thread test/js/web/streams/streams.test.js Resolved

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Comment thread src/runtime/webcore/FileReader.rs Resolved

Comment thread src/runtime/webcore/FileReader.rs Resolved

FileReader: cut the comments at the stdio dup to one line each

cb6d775

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This was referenced Oct 26, 2026

streams: stop a stdio source when its reader is released #46954

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ReadFile: keep each read inside the blob slice and do not wait on an empty slice #47375

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Bun.file().stream(): poll a dup of a stdio pipe, socket or terminal - #46807

Bun.file().stream(): poll a dup of a stdio pipe, socket or terminal#46807
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		let fd = if two_can_poll && this.pollable && fd.stdio_tag().is_some() {
		sys::dup_at_least(fd, 3).unwrap_or(fd)

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