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The fork child ran the open action as open() + dup2() + close(). open() returns the lowest free fd. When the target slot is closed, that fd is the target itself. dup2(fd, fd) does nothing, and the close() then closed the file that the action had just opened: the /dev/null of "ignore", or the file of Bun.file(path). Skip the dup2() and the close() when open() returns the target, like glibc and musl do for posix_spawn_file_actions_addopen.
StatusHead: 242c6d6. The diff is ready for a maintainer. The review threads are answered and resolved. CI (build 118937, finished: 178 jobs passed, 3 failed)
How it was reproduced (Linux x64) // closed0.cjs, run as: bun closed0.cjs < /dev/null
const fs = require("node:fs");
const { spawn } = require("node:child_process");
fs.closeSync(0);
spawn("ls", ["-l", "/proc/self/fd"], { stdio: ["ignore", "inherit", "inherit"] });
Local checks
Not checked
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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@test/js/bun/spawn/spawn.test.ts`:
- Around line 1524-1525: Replace the nested loops over file descriptors and
modes with nested describe.each() parameterization, while preserving the nine
concurrent test cases and ensuring each descriptor operation remains inside its
child fixture lifecycle.
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src/jsc/bindings/bun-spawn.cpptest/js/bun/spawn/spawn.test.ts
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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 findings, I also checked the current_max_fd move in the Open arm: on the base build it already ran on this path (after dup2 succeeded, before close), so hoisting it out of the new opened != target guard does not change which fds survive the closeRangeOrLoop before execve. The Dup2 and Close arms have no analogous open-then-close sequence, so the guard does not need mirroring there.
Extended reasoning...
The guard itself matches what glibc's spawn_do_open and musl's FDOP_OPEN do, and the error paths (open failure, dup2 failure, close failure) are unchanged. The current_max_fd hoist is behavior-preserving because the pre-fix code reached that update on every success path of this arm. The inline findings are pre-existing sibling cases of the same fd-number-collision class (socketpair or errpipe landing on a closed stdio slot) that this PR leaves in place, which is why a maintainer should decide whether they belong in this PR.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
-
🟣
src/jsc/bindings/bun-spawn.cpp— On macOS PTY/uid/gid spawns and FreeBSD, an exec failure (bad command, ENOENT) is reported as a successful spawn with a pid whose child already exited 127. The self-pipe at src/jsc/bindings/bun-spawn.cpp:242 takes the two lowest free fds after the actions were built, so errpipe[1] can be a number a later Open/Dup2 action targets. The child's dup2 at bun-spawn.cpp:396 (or :377) replaces the write end, childFailed at :271 writes into the redirect, and the parent reads EOF at :539 and returns 0. Fix: create errpipe before building actions or move it above every action target (dup to a high fd with F_DUPFD_CLOEXEC after fork) so no file action can land on errpipe[1].Extended reasoning...
The dismissing finder called this pre-existing and out of scope, but it is the same bug class the PR fixes: an fd number handed out by the kernel colliding with a stdio slot the actions target. The PR title claims closed-slot spawns now work; on macOS PTY they can still silently mis-report exec failure. Trigger with closed slots: parent has fd 0 and fd 1 closed (fs.closeSync twice, or two ended autoClose streams). Both stdio slots become Open(/dev/null) actions at spawn_process.rs:760. pipe(errpipe) at bun-spawn.cpp:242 returns [0,1]; fcntl sets CLOEXEC only on errpipe[1]=1. fork at :517, child closes errpipe[0]=0 at :520. Open(0): open returns 0, new guard at :395 skips, fine. Open(1): open returns 3 or higher, dup2(opened,1) at :396 overwrites errpipe[1] in the child, close(opened). execve fails at :456 for a missing binary. childFailed :268 writes errno to fd 1, which is now /dev/null, and exits 127. Parent: close(errpipe[1]) at :528, read(errpipe[0]) at :539 returns 0 (no writer left), res=0, *pid=child at :554. Caller gets a Subprocess for a dead child with no error. Trigger without closed…
Verification: pre-existing (the base branch fails by the same route: the dup2 in the Open arm overwrites the self-pipe write end whether
open()returned fd 0 as before or a fresh fd N as now; the PR'sopened != action.fds[0]guard does not change that). Triggering condition: a spawn that goes throughposix_spawn_bun's fork()+self-pipe path (macOS withpty_slave_fd >= 0oruid/gidset —…
With fd 1 closed in the parent and stdio ["pipe", "ignore", "inherit"], the child's end of the stdin socketpair takes fd 1. The child closes it after the dup2 onto fd 0, so the open action for "ignore" lands on fd 1 as well. The report line now has a marker, so a sanitizer warning on stderr does not fail the fd 1 cases.
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On the review note about the self-pipe ( |
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LGTM — a small, self-contained guard in the vfork child's Open file action that matches how glibc and musl handle posix_spawn_file_actions_addopen landing on its target.
What was reviewed:
- Confirmed the
current_max_fdmove is behavior-preserving: the oldif (opened != -1)was always true after the failure return, so the update was already effectively unconditional. - Checked every
Openaction producer (spawn_process.rs/dev/null, Bun.file and extra-fd paths;bun_core/util.rssync-inherit) — none passesO_CLOEXEC, so the same-fd branch does not need theFD_CLOEXECstrip the siblingDup2arm does for inherited fds. - Traced the new
["pipe", "ignore", "inherit"]case: the stdin socketpair's child end takes fd 1, the child runsdup2(1, 0); close(1), thenopen("/dev/null")returns 1 and is kept. - macOS normal spawns use libc
posix_spawn_file_actions_addopen, which opens at the target directly, so the Linux-only/proc-based test matches the only affected path (posix_spawn_bun).
Extended reasoning...
Overview
The PR changes the FileActionType::Open arm of posix_spawn_bun in /home/claude/bun/src/jsc/bindings/bun-spawn.cpp (the vfork child on Linux/FreeBSD, and on macOS for PTY/uid/gid spawns) so that when open() returns the target stdio slot itself — which happens whenever that slot is closed in the parent — the dup2(opened, target); close(opened) sequence is skipped instead of closing the fd it just opened. It also collapses a redundant int opened = -1; opened = ... and a dead if (opened != -1) check. The test in /home/claude/bun/test/js/bun/spawn/spawn.test.ts adds a Linux-only describe covering fds 0-2 with "ignore", "inherit" (of a closed slot) and Bun.file(path), plus one pipe-adjacent case, reading the child's fd table via /proc/<pid>/fd/<n> after Bun.spawn returns (vfork returns after execve, so the table is final).
Security risks
None introduced. The change only affects which fd number the child ends up with for a /dev/null or user-supplied path; it does not change permissions, credentials, or the close-range floor (current_max_fd still covers the target). I verified that no producer of Open actions passes O_CLOEXEC (src/spawn_sys/spawn_process.rs:760,766,769,913 and src/bun_core/util.rs:4621), so the kept fd is inheritable across execve as intended, and no unintended fd leaks into the child beyond what the action requested.
Level of scrutiny
Moderate: this is child-side code between vfork and execve, which warrants care, but the diff is a three-line guard with a well-known upstream precedent (glibc spawn_do_open and musl FDOP_OPEN both guard on fd != target). I confirmed the refactor is behavior-preserving — the previous if (opened != -1) was unreachable-false after the return childFailed() above it, so current_max_fd was already updated on every success path. The sibling Dup2 same-fd arm strips FD_CLOEXEC because inherited fds can carry it; a freshly open()ed fd without O_CLOEXEC cannot, so the asymmetry is correct rather than a missed sibling. The macOS normal path goes through libc posix_spawn_file_actions_addopen (src/spawn_sys/posix_spawn.rs:590), which opens directly at the target, so the Linux-only test gating is appropriate for the affected implementation.
Other factors
The test fails for the right reason on the unfixed build (readlink on the closed slot raises ENOENT, which the fixture reports as the target string, so toEqual fails). It drains stdout/stderr/exited concurrently, uses await using, and follows the file's existing describe.if(isLinux) / module-level tmp conventions. I traced the new ["pipe", "ignore", "inherit"] case through spawn_process.rs: with fd 1 closed, the stdin socketpair yields [1, N], the child end is fd 1, the child runs dup2(1, 0); close(1), and the subsequent /dev/null open returns 1 and is retained by the new guard. Neither file is covered by CODEOWNERS. The commits since my earlier comment are test restructuring and a comment trim; the pre-existing pipe-plus-inherit interaction I noted previously is unchanged and was already flagged as non-blocking, so it does not affect this decision.
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Updated 6:30 AM PT - Sep 20th, 2026
❌ @robobun, your commit 242c6d6 has 1 failures in
🧪 To try this PR locally: bunx bun-pr 43626That installs a local version of the PR into your bun-43626 --bun |
…tdio slots (#43814) ### Problem - The IPC socketpair was `SOCK_NONBLOCK` on both ends. A non-JS child's plain `write(2)` on `NODE_CHANNEL_FD` came up short: python wrote 219264 of 4000012 bytes, and the parent got 0 messages. - File actions run in slot order, so a `dup2` source whose fd number is also a lower slot was closed or replaced first. `[..., 12 x 'ignore', 'pipe']` failed with `EBADF`. `["ignore", 2, 1]` did not swap. ### Fix In `spawn_process_posix` (`src/spawn_sys/spawn_process.rs`): - Create the IPC pair blocking, with `O_NONBLOCK` on the parent's end only, like libuv. Bun and node children set it themselves. - `source_above_slots`: first duplicate such a source above the highest slot (`F_DUPFD_CLOEXEC`), as libuv does. The parent closes the copy, and exec closes it in the child. - Probe inherit slots before the first fd is created, so a pipe end on a closed slot number cannot pass for the inherited fd. Closed fds 0-2 get `/dev/null`. A closed extra slot fails with `EBADF`. - Verified: new tests in `spawn.test.ts` and `child-process-stdio.test.js` fail on 1.4.3 and pass here. Notes list the other suites. ### Background - `ipc`, `fork()` and `stdio: [..., 'ipc']` give the child a socketpair end at an extra slot, named by `NODE_CHANNEL_FD`. - A file action is a step the child runs between fork and exec: `dup2`, `close` or `open`. - Split out of #41751, whose inherited-stdio change waits for a decision. ### Downsides - A child that relies on a nonblocking IPC fd without setting the flag now blocks. node hands over a blocking fd too. - `spawnSync` never reads an `"ipc"` slot: a child that writes 1 MB to it now blocks until the `timeout` (1.4.3: `EAGAIN`). `"pipe"` there already blocks on 1.4.3. - A relocation holds one more fd during the spawn, so at the fd limit it fails with `EMFILE`. <details><summary>Notes</summary> Suites run with the debug build, all passing: `spawn.test.ts`, `spawn.ipc.test.ts`, `spawn.ipc.bun-node.test.ts`, `spawn.ipc.node-bun.test.ts`, `bun-ipc-inherit.test.ts`, `spawnSync.test.ts`, `spawn-pidfd-emfile.test.ts`, `child-process-stdio.test.js`, `child_process_ipc.test.js`, `child_process_send_cb.test.js`, `child_process_ipc_large_disconnect.test.js`. Repros (bun 1.4.3, Linux): ```js // IPC: child fd 3 was O_NONBLOCK, python wrote 219264 of 4000012 bytes, the parent received 0 messages. import { spawn } from "node:child_process"; const py = `import os,sys,fcntl,json nb = bool(fcntl.fcntl(3, fcntl.F_GETFL) & os.O_NONBLOCK) msg = json.dumps({"big": "x"*4000000}).encode() + b"\\n" n = os.write(3, msg) sys.stderr.write("child: fd3 O_NONBLOCK=%s wrote %d of %d\\n" % (nb, n, len(msg)))`; const p = spawn("python3", ["-c", py], { stdio: ["inherit", "inherit", "inherit", "ipc"] }); let msgs = 0; p.on("message", () => msgs++); const t0 = Date.now(); while (Date.now() - t0 < 1500) {} p.on("close", () => console.log("parent received messages:", msgs)); ``` ```js // EBADF: close(3)..close(14) ran before dup2(11, 15). spawn() threw it synchronously. const { spawnSync } = require("child_process"); spawnSync("true", [], { stdio: ["inherit", "inherit", "inherit", ...Array(12).fill("ignore"), "pipe"] }); ``` With this branch: `fd3 O_NONBLOCK=False wrote 4000012 of 4000012`, 1 message. `spawnSync: ok`, `exit 0`. node 26.3 gives the same. The relocated copies do not reach the child. Measured with `ls -l /proc/self/fd` as the child: for `["ignore", "pipe", "inherit", ...57 x "ignore", "pipe"]` the child holds 0, 1, 2, 60 and the directory handle of `ls`. For `["ignore", "pipe", 1, 2]` it holds 0, 1, 2, 3 and the directory handle. node 26.3 gives the same two tables. When a socketpair child end is relocated, no explicit `close` action is added for the original. It is `SOCK_CLOEXEC` on Linux and `FD_CLOEXEC` plus `POSIX_SPAWN_CLOEXEC_DEFAULT` on macOS, and an explicit close could land on a slot number that a later action targets. The extra-slot arm also pushes both socketpair ends to the cleanup guard before `set_nonblocking`, so a failure there no longer leaks them. Closed inherit slots. The first commit here had a regression that review found: with `fs.closeSync(1)`, `stdin: "pipe"` and `stdout: "inherit"`, the stdin pair's child end lands on fd 1, the relocation drops the explicit `close(1)` action, and the inherit of slot 1 then handed the child that socket as stdout. bun 1.4.3 fails that spawn with `EBADF`. Now the slots are probed before any fd is created, and the child gets `/dev/null` there. The same probe fixes a case that 1.4.3 also gets wrong: fd 2 closed, `stdout: "pipe"`, `stderr: "inherit"` wired the child's stderr to the parent's end of the stdout pair. An oversized stdio array (`2 + len` does not fit in an fd number) fails the spawn with `EMFILE` instead of a panic. Not in this PR: a `stdio: 'inherit'` child still gets an `O_NONBLOCK` stdout after the parent used `process.stdout` (`seq: write error: Resource temporarily unavailable`). That is the part that stays in #41751. The probe fixture `test/js/bun/spawn/fixtures/fd-nonblock-probe.js` reads `/proc/self/fdinfo` when it exists and otherwise calls `fcntl(F_GETFL)` through `bun:ffi`. It does not touch `process.send`, which would adopt the fd and set the flag. Related: #36653 has a save-aside for caller-supplied fds only. It does not cover the socketpair ends that bun creates, which is the `EBADF` case here. #43626 fixes the `open` file action in `bun-spawn.cpp` and does not overlap. </details> <!-- robobun:evidence:begin --> --- **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/spawn/spawn.test.ts <!-- robobun:evidence:end -->
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#43845 carries these three commits unchanged. It keeps a closed fd 0, 1 or 2 free across spawns, so without this fix the |
…ed fd 0, 1 or 2 (#43845) Part of #43844 ### Problem - After user code closes fd 0, 1 or 2 (`fs.closeSync(1)`, #33561), the next descriptor bun creates takes that number: a `"pipe"` socketpair end, a Blob's memfd, the pidfd, the waiter thread's eventfd. - `Fd::close` skips fd 0-2 (`src/sys/fd.rs:80`), so it stays open. With fds 0 and 1 closed, a stdout pipe takes both and `proc.stdout.text()` never resolves. - A child that inherits the slot gets it: `child fd 1 -> /memfd:spawn_stdio_stdin (deleted)`. With a Blob of 8 MiB or more, the child's output overwrites the parent's Blob. ### Fix - `socketpair`, `memfd_create`, `pidfd_open` and `eventfd` in `bun_sys` return `move_above_stdio(fd)`: a descriptor numbered 0-2 moves to 3 or higher with the same `FD_CLOEXEC` state, or stays when no higher number is free. Other descriptors cost one integer compare. The waiter thread now calls `bun_sys::eventfd`. - Two spawn bugs would fire on every spawn once the closed number stays free, so they are fixed here. The first three commits are #43626 unchanged (the `Open` file action). The `fork()` path (macOS with a pty, uid or gid, and FreeBSD) keeps the write end of its error pipe above the file action targets. - Verified: 15 of the 17 new tests in `test/js/bun/spawn/spawn.test.ts` fail on a Linux debug build of main, and all pass here. The other two pin the fd limit case and the error pipe. Also ran the spawn, `child_process`, shell pipeline and `blob.test.ts` suites. - Self-reviewed: 15 concerns raised, 15 addressed (see Notes). ### Background - A new descriptor gets the lowest free number. - `Fd::close` never closes fd 0-2, to protect the real stdio. So bun must not own a descriptor there. - On Linux a Blob stdin travels as a memfd and a large Blob lives in one. A pidfd, or a waiter thread with an eventfd, reports the child's exit. - Considered guards at the six spawn call sites: they miss the Blob memfd (`LinuxMemFdAllocator`) and the shell and webview socketpairs. ### Downsides - At the fd limit, with no free number at 3 or above, the descriptor stays at the closed stdio number as on `main`. A failed move would kill a child that already started (pidfd) or panic (waiter thread). The error pipe stays in the same way, so a failed exec can still look like exit 127 there. - Not covered (#43844): `open`, usockets event loops (first `Bun.spawnSync`), PTYs, the `cgroup` option. <details><summary>Notes</summary> No user reported this. The review of #43814 found it (threads on `spawn_process.rs:751` and `:921`). Self-review: the first version of this branch guarded six call sites in the spawn path. The review showed that a large Blob's memfd comes from `LinuxMemFdAllocator` and still took the closed number, so the guard moved into the creators. It also asked for #43626 first, for tests of the hang and of the Blob, and for #43844. What sits at the closed fd on a debug build of main (dc55830, has #43814), each spawn run as the first thing after `fs.closeSync(N)`: | case | main | this branch | | --- | --- | --- | | `stdio: ["pipe", "pipe", "pipe"]`, fd 0, 1 or 2 closed | `socket:[...]` during and after | closed | | `stdio: [fd, "pipe", "pipe"]`, fd 2 closed (parent's end) | `socket:[...]` | closed | | `"pipe"` at slot 3 | `proc.stdio[3] === 1` | `proc.stdio[3] > 2` | | `ipc` | the IPC socket at fd 1, so the parent's `console.log` goes into the channel | closed | | no slot creates a descriptor | `anon_inode:[pidfd]`, or `anon_inode:[eventfd]` with the waiter thread | closed | | fds 0 and 1 closed, `stdout: "pipe"` | `proc.stdout.text()` hangs (test times out) | `"out\n"` | | `new Blob([9 MiB])`, then a child with `stdout: "inherit"` | `/memfd:memfd-num-0 (deleted)`, `blob.slice(0, 9)` is `OVERWRITE` | closed, `aaaaaaaaa` | | four children with `stdout` ignore, ignore, inherit, inherit | child fd 1: `ENOENT`, `/dev/null`, `anon_inode:[pidfd]`, `anon_inode:[pidfd]` | `/dev/null` four times | | Blob stdin, `stdout: "inherit"`, bun child | child's stdout is the stdin memfd | `/dev/null` | Why the descriptor moves and `Fd::close` stays as it is: other owners hold `Fd::stdout()` and call `close()` on it. If `close()` closed fd 0-2 after the user freed the number, a stale owner could close an unrelated descriptor that reused it. Why #43626 is in this branch: on main the first spawn after the close leaves a descriptor at the closed number, and that descriptor hides the `Open` bug from every later spawn. This change keeps the number free, so a non-bun child with `stdout: "ignore"` would get a closed fd 1 at every spawn (`echo: I/O error`). If #43626 merges first, its commits drop out of this branch on the next rebase. Callers of the four creators, all checked: `Stdio::use_memfd`, `spawn_process_posix` (stdio, extra slots, IPC, stdout/stderr memfd, pidfd), `LinuxMemFdAllocator`, the `memfd_create` test binding, shell `Pipeline`, webview `ChromeProcess` and `HostProcess`, `ParentDeathWatchdog`, the io waker, the waiter thread. None needs a number below 3. A moved descriptor keeps its `FD_CLOEXEC` state, so the `CrossProcess` memfd and `eventfd(0, 0)` stay as their callers made them. The error pipe: on macOS with a pty, uid or gid, and on FreeBSD, `posix_spawn_bun` forks and the child reports a failed exec through a pipe. With two of fd 0, 1 and 2 closed, `pipe()` gave the write end the number of a stdio slot, and the child's file action for that slot replaced it. The parent read EOF and returned a pid, and the child exited 127. On main that needs a spawn that creates no stdio descriptor. With the socketpairs moved, `"pipe"` stdio reached it too. The write end now moves above the highest file action target. The test `a failed exec is still reported when fds 0 and 1 are closed` uses `uid: process.getuid()` to take that path on macOS. No macOS machine was available for a local run, so CI is the check for it. At the fd limit, measured with `ulimit -n 64` and every descriptor held. With no free descriptor, the first `Bun.spawn` throws `EMFILE` from `pidfd_open`, or ends in `panic: Failed to start WaiterThread` under `BUN_FEATURE_FLAG_FORCE_WAITER_THREAD=1`, on 1.4.3, on main and here (#39864 turns that panic into an error). With fd 1 closed as the only free number, the spawn works on 1.4.3, on main and here: the pidfd or the eventfd stays at fd 1, because `F_DUPFD` finds no number at 3 or above. An earlier commit of this branch failed the move with `EMFILE` there, which killed the child or hit that panic. The test `at the descriptor limit a spawn still works when only the closed fd 1 is free` pins this in both modes. The spawnSync tests run one `Bun.spawnSync` before they close the fd. The first call creates the isolated event loop in usockets (C), and its epoll fd takes the lowest free number. That reproduces on 1.4.3 with `stdio: ["ignore", "ignore", "ignore"]`, and it is in #43844. `cargo check` of `bun_sys`, `bun_spawn_sys` and `bun_spawn` passes for `aarch64-apple-darwin`, `x86_64-unknown-freebsd`, `x86_64-pc-windows-msvc` and `aarch64-linux-android`. `cargo clippy` on those crates is clean. </details> <!-- robobun:evidence:begin --> --- **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.test.ts <!-- robobun:evidence:end -->
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Problem
"ignore"onstdin,stdoutorstderrgives the child a closed fd when that fd number is closed in the parent."inherit"of a closed slot andBun.file(path)do the same. The first file that the child opens becomes that fd. Node gives the child/dev/null.fs.close()of fd 0 to 2 close the fd. No user reported it.Openfile action insrc/jsc/bindings/bun-spawn.cpp:385:open(),dup2(opened, target),close(opened).open()returns the lowest free fd. With the target closed, that fd is the target.dup2(fd, fd)does nothing, andclose()closes the target.Fix
dup2()andclose()whenopen()returns the target fd. glibc and musl do the same forposix_spawn_file_actions_addopen.O_CLOEXEC, so the fd survivesexecve.test/js/bun/spawn/spawn.test.ts, fds 0 to 2 with"ignore","inherit"andBun.file(path), and"ignore"next to a pipe. All 10 cases fail on bun 1.4.3 and pass with the fix. Also ranspawnSync.test.ts,child_process.test.tsandchild-process-stdio.test.js.sleep 1000behind (see Notes).Background
Bun.spawndoes not use the libcposix_spawn.posix_spawn_buncallsvfork()and runs a list of file actions in the child beforeexecve."ignore"becomes anOpenaction for/dev/null(src/spawn_sys/spawn_process.rs:766)."inherit"of a closed slot uses the same action (:760).posix_spawnfor normal spawns, which opens at the target directly.Notes
Repro (Linux):
/dev/null/dev/null(fs.closeSync(0)did not close the fd before #33561)/proc/<pid>/fd(the directory handle oflslanded on the closed fd 0)/dev/nullA path with no explicit
fs.close():fs.createWriteStream(null, { fd: 2 })closes fd 2 when the stream ends (autoClose). A laterchild_process.spawn(cmd, args, { stdio: "inherit" })then starts the child with fd 2 closed on bun 1.4.3, and with/dev/nullon node and on this branch.With
stdout: Bun.file(path)and fd 1 closed in the parent, bun 1.4.3 starts the child with fd 1 closed, so the redirect is lost. On this branch fd 1 is the file.When the bug occurs:
Openaction runs. Usually that means the slot is closed in the parent and the spawn creates no pipe. A socketpair would take the free fd number in the parent beforevfork(), andopen()in the child would return a different number.stdio: ["pipe", "ignore", "inherit"]puts the child's end of the stdin socketpair on fd 1. The child closes it after thedup2onto fd 0, andopen()then returns 1. bun 1.4.3 leaves fd 1 closed, this branch gives/dev/null.Fd::close()skips fds 0 to 2, so the slot stays taken.bun x.js 0<&-) is not affected. Bun reopens/dev/nullon closed stdio at startup. For the same reason the child in the test issleepand not bun.The test reads
/proc/<pid>/fd/<n>of the child from the parent.vfork()returns afterexecve, so the fd table of the child is final at that point. Looped the 10 cases 10 times on the debug build with the fix: 10 of 10 pass. An earlier version of the test (6 cases) failed 6 of 6 on a debug build without the fix.The rejected self-review concern: the crash has to happen inside the fixture between
Bun.spawnandchild.kill(), where the only call is areadlinkSyncin atry.spawn.test.tsalready leaves asleep 999999behind on each run (does-not-hang.js).glibc (
sysdeps/unix/sysv/linux/spawni.c,spawn_do_open) and musl (src/process/posix_spawn.c,FDOP_OPEN) both guard thedup2+closewithfd != target. libuv keeps the fd too (close_fd >= stdio_countinuv__process_child_init).spawn_sync_inheritinsrc/bun_core/util.rsuses the same action forStdinBehavior::Ignore, so it gets the fix too. FreeBSD, and PTY or uid/gid spawns on macOS, also runposix_spawn_bun.Related open PRs, different concerns: #41707 drops
O_CREATfrom the same/dev/nullcall sites inspawn_process.rs. #41751 editsspawn_process.rsnearby. Neither touchesbun-spawn.cpp.Outside this change, not fixed here. All of these start with a stdio slot that is closed in the parent:
stdio: ["pipe", "inherit", "inherit"]throwsEBADF(bun 1.4.3 and this branch). The probe for a closed slot inspawn_process.rs:759runs after the stdin socketpair took fd 1, so the slot looks open. The action list then closes fd 1 before it inherits it. spawn: hand children blocking stdio and IPC fds, keep dup2 sources clear of stdio slots #41751 edits the same lines.Fd::close()(src/sys/fd.rs:80) skips fds 0 to 2. The next fd that bun allocates for itself lands on the free slot, and bun never closes it. Example: the pidfd of the next child stays open at fd 0 after the child exits.Bun.spawnSync, the epoll fd of its event loop takes the free slot before the spawn.stdin: "inherit"then passes that epoll fd to the child as fd 0.pipe(errpipe)inbun-spawn.cpp:242can return a stdio fd number. A laterOpenorDup2action in the child then replaces the write end, and a failedexecveis reported as a successful spawn.Local failures that do not come from this change:
child_process.test.ts"should allow us to spawn in the default shell" (the container does not exportSHELL), "extra stdio pipes are not double-closed on GC" (5 s timeout on the debug build, passes in 6 s with a longer timeout), andspawn_waiter_thread.test.ts(CPU time threshold on the debug build).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.test.ts