spawn: enforce maxBuffer when lazy is set

[robobun]

Problem

• Bun.spawn({ lazy: true, maxBuffer }) never kills a child that writes past maxBuffer while nothing reads its pipes. The child blocks in write() on the full stdout socketpair, and proc.exited never settles.
• lazy starts the PipeReader paused (PosixFlags::IS_PAUSED, SubprocessPipeReader.rs:192; on Windows it defers uv_read_start). MaxBuf::on_read_bytes (src/io/MaxBuf.rs:147) runs only from the read path, so on_max_buffer_overflow (subprocess.rs:678) never runs. timeout has its own timer and still works.

Fix

• spawn_maybe_sync starts the stdout and stderr readers eagerly when maxBuffer is set (js_bun_spawn_bindings.rs:1699). Without maxBuffer, lazy behaves as before.
• Correct because lazy exists to stop an unread pipe from buffering without bound, and maxBuffer already bounds it: reading stops at maxBuffer plus one 64 KiB read for each pipe (spawn: stop reading once maxBuffer is exceeded #33309).
• node:child_process does not change. It passes lazy: true, but it never passes maxBuffer to the async Bun.spawn.
• Verified: test/js/bun/spawn/spawn-maxbuf.test.ts (two new tests, both time out on stock bun). Also spawn.test.ts, child_process.test.ts, and the five test-child-process-*-maxbuf.js tests.

Background

• maxBuffer is a documented kill limit: a process that outputs more bytes is killed with killSignal. A MaxBuf holds the remaining byte budget of one pipe, and each read charges it.
• lazy: true defers the pipe reads until JS first pulls from proc.stdout or proc.stderr. Until then the kernel pipe buffer blocks the child (child_process: apply kernel backpressure to stdout/stderr pipes #34971).
• A reader can count only the bytes that it reads. A byte limit and a deferred reader cannot both hold, so one option must yield.

Notes

Repro (run as bun x.mjs hang | touch | eager):

const mode = process.argv[2], t0 = Date.now();
const p = Bun.spawn({ cmd: ["sh", "-c", "head -c 300000 /dev/zero; sleep 1; exit 3"], maxBuffer: 1000, ...(mode === "eager" ? {} : { lazy: true }) });
p.exited.then(c => console.log(`[+${Date.now() - t0} ms] exited ->`, c, p.signalCode));
if (mode === "touch") setTimeout(() => p.stdout.getReader().read().then(r => console.log(`[+${Date.now() - t0} ms] read ->`, r.value.length, "B")), 1500);
setTimeout(() => { console.log("guard: exitCode", p.exitCode); p.kill(9); }, 5000).unref();

mode	1.4.3-canary.1+4ff919377	this branch (debug build)
eager	[+2 ms] exited -> 143 SIGTERM	[+24 ms] exited -> 143 SIGTERM
touch	[+1502 ms] read -> 66536 B, then [+1502 ms] exited -> 143 SIGTERM	[+25 ms] exited -> 143 SIGTERM, then [+1525 ms] read -> 16384 B
hang	guard: exitCode null at +5003 ms, child never killed	[+19 ms] exited -> 143 SIGTERM

On stock bun, strace shows that the only EPOLL_CTL_ADD after the spawn is the pidfd. The stdout socketpair fd is never registered or read until the first pull. At the first pull, recvfrom(8, ..., 66536, MSG_DONTWAIT) = 66536 is followed at once by kill(<child>, SIGTERM).

The other shape for this fix is to reject lazy: true together with maxBuffer as an argument error. I did not choose it. It breaks a caller that forwards both options, and the eager reader is already bounded by the limit. Node has the same model: maxBuffer exists only on exec and execFile, and those always read.

maxBuffer: Infinity, 0, a negative number, or NaN does not set a limit, so lazy still applies with those values.

Suites run against the debug build:

• test/js/bun/spawn/spawn-maxbuf.test.ts: 18 pass.
• test/js/bun/spawn/spawn.test.ts: 0 fail.
• test/js/node/child_process/child_process.test.ts: 65 pass, 2 fail. should allow us to spawn in the default shell fails the same way on stock bun in this container ($SHELL is empty in the child). extra stdio pipes are not double-closed on GC runs 20 debug children in sequence and exceeds the 5 s budget under ASAN. Neither test sets maxBuffer, and with no maxBuffer the new expression is equal to the old one.
• test-child-process-{exec,execfile,execfilesync,execsync,spawnsync}-maxbuf.js: all exit 0.

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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-maxbuf.test.ts

[robobun]

^{Updated 7:24 PM PT - Sep 10th, 2026}

✅ @robobun, your commit d750cf3b3aa156ef6a5e8a97eb0bd07a5043d2aa passed in Build #114075! 🎉

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bunx bun-pr 42256

That installs a local version of the PR into your bun-42256 executable, so you can run:

bun-42256 --bun

[robobun]

Status

• Reproduced on 1.4.3-canary.1+4ff919377 with the script in the Notes block of the description. The hang mode prints guard: exitCode null at +5003 ms: the child is never killed.
• Fail before: USE_SYSTEM_BUN=1 bun test test/js/bun/spawn/spawn-maxbuf.test.ts -t "lazy: true" gives 2 fail (both time out after 5000 ms).
• Pass after: bun bd test test/js/bun/spawn/spawn-maxbuf.test.ts gives 18 pass, 0 fail.
• Review threads: both are addressed and resolved (46cfe7a moves the exit status assertion last, d750cf3 cuts the code comment to one line).
• The red TypeScript types check does not come from this diff. The errors are in globals.d.ts, overrides.d.ts, and the test fixtures, and the same job fails on every branch that runs it since 2026-09-10 17:14 UTC. bun-types test: check against the newest @types/node release, not the latest dist-tag #42230 fixes it.

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• src/runtime/api/bun/js_bun_spawn_bindings.rs
• test/js/bun/spawn/spawn-maxbuf.test.ts

Walkthrough

The spawn implementation disables lazy readers when maxBuffer is configured or execution is synchronous. Documentation describes this behavior, and tests verify process termination and later access to buffered output.

Changes

Spawn reader buffering

Layer / File(s)	Summary
Normalize spawn reader laziness src/runtime/api/bun/js_bun_spawn_bindings.rs	Stdout and stderr readers now use normalized laziness. Readers start eagerly for synchronous spawns and when maxBuffer is configured.
Document and test maxBuffer behavior packages/bun-types/bun.d.ts, test/js/bun/spawn/spawn-maxbuf.test.ts	The declaration documents that lazy is ignored with maxBuffer. Tests verify process termination and later reads of buffered output.

Suggested reviewers: dylan-conway

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to 14083

The maxBuffer lazy-reader behavior and its coverage are ready to merge.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly describes the main change: enforcing maxBuffer when lazy is enabled for spawn operations.
Description check	✅ Passed	The description explains the problem, implementation, behavior, rationale, tests, and verification results. It does not use the template headings exactly, but it provides the required information and …

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[coderabbitai]

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Inline comments:
In `@test/js/bun/spawn/spawn-maxbuf.test.ts`:
- Around line 171-174: Update the test around proc[fd].bytes() so both buffered
stream assertions run immediately after await proc.exited; move the
exitCode/signalCode toEqual assertion after those stream reads, preserving the
existing expected values.

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• packages/bun-types/bun.d.ts
• src/runtime/api/bun/js_bun_spawn_bindings.rs
• test/js/bun/spawn/spawn-maxbuf.test.ts

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[claude]

I reviewed this and didn't find any bugs — the fix is small and the boolean rewrite is provably equivalent to the old expressions when max_buffer.is_none(). Deferring only because packages/bun-types/ has a CODEOWNER.

What was reviewed:

• Verified let lazy = !is_sync && lazy && max_buffer.is_none() reduces to the exact prior start(..., !is_sync && lazy) and is_sync || !lazy gates when no maxBuffer is set, so behavior without maxBuffer is unchanged.
• Confirmed max_buffer is Some only for finite positive values with at least one piped stdio (line 767-775), so lazy still applies with Infinity/0/negative/NaN as the PR states.
• Test follows the file's existing describe.each shape, uses test.concurrent, Buffer.alloc(n, fill).toString(), awaits proc.exited (no sleeps), and the child's setInterval guarantees a hang on the unfixed build.

Extended reasoning...

Overview

This PR fixes Bun.spawn({ lazy: true, maxBuffer }) never killing a child that exceeds maxBuffer while its pipes are unread. The fix is a single new let lazy = !is_sync && lazy && max_buffer.is_none(); binding in spawn_maybe_sync (src/runtime/api/bun/js_bun_spawn_bindings.rs:1702), plus mechanical simplification of the four surrounding start()/read_all() conditionals to use that binding. A 3-line JSDoc note is added to packages/bun-types/bun.d.ts, and a new describe.each block in test/js/bun/spawn/spawn-maxbuf.test.ts covers stdout and stderr.

Security risks

None. This changes only when the pipe reader starts eagerly vs. lazily for a subprocess the caller already asked to spawn. No new inputs are parsed, no auth/crypto/permissions paths are touched, and the eager reader's buffering is bounded by the existing maxBuffer machinery (MaxBuf::on_read_bytes).

Level of scrutiny

Low-to-moderate. The Rust change is five lines with a clear mechanism, and I checked the boolean algebra by hand: with max_buffer.is_none(), the new lazy equals the old !is_sync && lazy, and !lazy equals the old is_sync || !lazy, so the no-maxBuffer path is byte-for-byte equivalent. With max_buffer.is_some(), lazy becomes false and both readers start eagerly — exactly the intended fix. I also confirmed at lines 767-775 that max_buffer is only Some for finite positive values with at least one piped stdio, matching the PR's claim that Infinity/0/negative/NaN still leave lazy in effect.

Other factors

The test is well-constructed per REVIEW.md conventions: it lives in the existing spawn-maxbuf.test.ts, uses describe.each over both pipes, test.concurrent, bunExe()/bunEnv, Buffer.alloc(300000, 65).toString() (not .repeat()), and awaits proc.exited directly rather than sleeping. The child's setInterval(() => {}, 1e9) keeps it alive indefinitely, so on the unfixed build proc.exited never resolves and the test times out — it fails for the right reason. The byte-count assertion (> 1000 and <= 1000 + 64*1024) matches the "one extra 64KB read" overshoot documented in the existing tests. The comment on the new let lazy line is load-bearing (explains why the eager reader is safe: the budget bounds it). I would have approved outright, but packages/bun-types/ is CODEOWNER'd, so deferring for that sign-off on the JSDoc addition.

[claude]

Code review found no issues

No high-confidence issues detected in this change.