shell(IOReader): drain readers instead of iterating a snapshot of the list

[robobun]

What

IOReader.on_reader_done_cb and on_reader_error iterated a clone of readers while the loop body runs the Yield trampoline. The trampoline drives shell state machines that can call add_reader() on the same IOReader (the root stdin reader is shared across every command that inherits stdin). Two problems:

• readers appended mid-loop are never notified, because the snapshot was taken before they existed
• already-notified entries stay in the list, so add_reader's contains() dedup can match a freed-then-reused NodeId and silently skip registering the next reader

Both loops are replaced by drain_readers(), which pops each entry via swap_remove(0) before dispatching, so neither hazard applies and readers appended mid-drain are still picked up. A draining re-entrancy flag makes nested calls (reached from inside the trampoline) no-ops so the outermost loop owns the drain and Yield::run nesting stays bounded.

Windows

The new test exposed a separate pre-existing Windows crash: the BufferedReader nulls its source before invoking on_reader_done_cb, so when a reader's done-handler starts the next cat and calls start() from inside that callback, start_with_current_pipe() unwraps a None source and panics. echo | (cat && cat) had never been tested, so this never fired.

start() on Windows now checks reader().is_done() and drains the late-registered reader(s) instead of restarting the finished pipe. Because of the re-entrancy guard, this covers both call sites: reached from inside a drain (sync stdout write) it's a no-op and the outer loop handles the new reader; reached after the drain returned (async stdout, on_io_writer_chunk fires later) it starts a fresh drain. Without the fresh-drain path the test hung for 90s on Windows 11 aarch64 in CI.

Test

Added echo hi | (cat && cat && … ×12) to the pipeline-stack tests in bunshell.test.ts. Every cat shares the subshell's stdin IOReader; each done-handler appends the next reader and calls start() on the already-done reader. Crashes on Windows without this change; passes with it on all three Windows lanes plus debian-asan.

Rebase note

Earlier revisions of this PR changed both src/runtime/shell/IOReader.zig and src/runtime/shell/IOReader.rs. The Zig shell source has since been deleted from main (the Zig→Rust port completed), so the .zig half of the change was dropped during the rebase and the PR now touches only IOReader.rs and the test. The .rs logic is unchanged from what reviewers already looked at.

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• src/runtime/shell/IOReader.rs

---

Walkthrough

The change updates IOReader to safely drain re-entrant listeners and adds a regression test for twelve sequential cat commands sharing subshell input.

Changes

Shell reader draining

Layer / File(s)	Summary
IOReader completion and listener draining src/runtime/shell/IOReader.rs	IOReader tracks active draining, handles completed readers on Windows, and uses shared listener draining for completion and error callbacks.
Sequential pipeline regression coverage test/js/bun/shell/bunshell.test.ts	The test runs twelve sequential cat commands and verifies successful completion with exactly hi\n.

Possibly related PRs

• oven-sh/bun#35337: Changes cat file-reading behavior and related shell pipeline tests.
• oven-sh/bun#37743: Uses related shell pipeline behavior involving cat.
• oven-sh/bun#37752: Modifies IOReader.rs listener draining and adds a related sequential cat pipeline test.

Suggested reviewers: jarred-sumner, dylan-conway

Mergeability Score: ⚪ Minimal · up to d9a7b

This localized reader-draining change has no actionable merge-blocking risk in the supplied evidence and is merge-ready after normal checks and review.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main IOReader change from snapshot iteration to reader draining.
Description check	✅ Passed	The description explains the change, Windows-specific behavior, regression test, verification results, and rebase context.
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.

---

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[robobun]

^{Updated 10:05 PM PT - Aug 13th, 2026}

❌ @robobun, your commit d9a7bcc has some failures in Build #95150 (All Failures)

---

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[claude]

LGTM — straightforward defensive fix that mirrors the existing index-based iteration in onReadChunk.

Extended reasoning...

Overview

This PR changes two loops in src/shell/IOReader.zig (onReaderDone and onReaderError) from for (this.readers.slice()) |r| to index-based while (i < this.readers.len()) iteration that re-fetches each element via this.readers.get(i).* before calling .run(). It also adds one regression test to bunshell.test.ts that pushes the shared-stdin reader list past the SmolList inline→heap and heap-realloc boundaries.

Security risks

None. This is purely an internal iteration-pattern change in the shell's IO reader; no auth, crypto, parsing, or user-input handling is touched.

Level of scrutiny

Low. The new code is a near-verbatim copy of the pattern already used in onReadChunk immediately above (same file, same SmolList container). I verified that SmolList.get() returns *T (so the .* copy is correct) and that SmolList.append() can promote .inlined → .heap or realloc the heap buffer, which is exactly the hazard the captured-slice loop was vulnerable to. Behavior is unchanged for the current single-reader case and strictly safer when the list mutates mid-iteration.

Other factors

• No CODEOWNERS entry covers src/shell/.
• The PR description openly notes the bug isn't reproducible today (background & not yet supported), so this is hardening; the added test still exercises the new loop end-to-end without changing any other test.
• No outstanding human review comments; only bot/build comments on the timeline.
• Diff is ~14 lines of code plus comments and a single new test case.

[claude]

Thanks — 9aef7d3 addresses both of my earlier comments (the late-registration hang via drainReaders() in start() with the draining re-entrancy guard, and the stale test comment). I don't see further issues, but the Yield-trampoline re-entrancy and Windows-specific EOF path are subtle enough that a human pass is still worthwhile.

Extended reasoning...

Overview

This PR fixes a use-after-free hazard in IOReader.onReaderDone/onReaderError where iterating this.readers.slice() while the loop body drives the Yield trampoline could leave the slice dangling if a callback appends to the same SmolList. The fix introduces drainReaders() which pops each reader via swapRemove(0) before dispatching, plus a draining re-entrancy flag, and makes the Windows start() path drain late-registered readers when the underlying pipe is already done (instead of null-unwrapping in startWithCurrentPipe). A regression test with 12 sequential cats sharing a subshell stdin was added.

What changed since my last review

My two prior inline comments on cf14425 were both addressed in 9aef7d3:

• The 🔴 (late-registration hang when start() is reached after drainReaders() has returned, e.g. via a deferred onIOWriterChunk) is fixed by calling drainReaders() from start() when isDone() is true, guarded by a new draining: bool so nested calls from inside the trampoline are no-ops and the outermost loop owns the drain.
• The 🟡 (stale test comment referencing the abandoned index-based loop / SmolList growth) was rewritten to describe the drain pattern.

I traced the late-registration path and the re-entrancy guard bounds .run() nesting to one level; the tail-position return stdin.fd.start() in cat.next() doesn't touch this after start() returns, so destroying the cat inside drainReaders() before next() unwinds looks safe.

Security risks

None — this is internal shell interpreter IO bookkeeping with no auth, crypto, or untrusted-input parsing changes.

Level of scrutiny

Medium-high. The change is small in line count but sits at the intersection of the shell's Yield trampoline, SmolList reallocation semantics, Windows BufferedReader lifecycle, and allocator address-reuse behavior. The re-entrancy guard and the "call onReaderDone on a reader whose next() is still on the stack" pattern are correct as far as I can trace, but they're the kind of invariant a maintainer familiar with the Yield machinery should sign off on.

Other factors

The bug hunter found no issues on this revision. CI failures on the previous commit were unrelated (bun-create.test.ts, dev-and-prod.test.ts, fetch-http2-client.test.ts). The PR went through three iterations responding to review feedback, which gives me reasonable confidence in the final shape, but I'm deferring rather than approving given the subtlety of the control flow.

[Jarred-Sumner]

The test doesn't fail on main?

[robobun]

The test doesn't fail on main?

It does, on Windows only — all 3 Windows lanes (x64, x64-baseline, aarch64) crash on main with:

panic(main thread): attempt to use null value

in WindowsBufferedReader.startWithCurrentPipe() (this.source.? where source was nulled by closeImpl before onReaderDone fired). First cat finishes → trampoline starts the second cat → stdin.start() → startWithCurrentPipe() on a null source → panic. Confirmed in build #49370 where the test ran against 2714745 (which only had the iteration change, not the start() guard). echo | (cat && cat) had never been tested before so this path never fired.

On Posix the test passes on main: start() there re-registers the poll on the still-open fd rather than touching source, so each subsequent cat gets another EOF callback.

The original slice-iteration hazard that prompted the request (readers list promoting/reallocing mid-for (slice)) isn't reachable from JS today because & isn't supported yet — only one cat is ever registered at a time. That part is hardening ahead of background jobs; the Windows start()-after-EOF fix is what the test actually validates.

[claude]

Thanks — the swapRemove-before-dispatch drain, the draining re-entrancy guard, and the start()-after-EOF drain all look right now and close out everything I'd flagged. Deferring to a human for final sign-off since the safety of calling drainReaders() from inside a reader's own next() (cat2 draining itself via start()) hinges on the tail-return-no-this-deref invariant up the Yield trampoline, which is the kind of cross-file invariant worth a maintainer's eyes.

Extended reasoning...

Overview

This PR rewrites how IOReader notifies registered child readers when the underlying pipe finishes. The old for (this.readers.slice()) |r| r.onReaderDone(...).run() loop in onReaderDone/onReaderError is replaced by a shared drainReaders() helper that pops each entry via swapRemove(0) before dispatching, guarded by a draining: bool re-entrancy flag. On the Windows start() path, a new reader.isDone() check calls drainReaders() instead of restarting a pipe whose source has already been nulled. A regression test (echo hi | (cat && cat && … ×12)) is added to bunshell.test.ts.

The PR went through three rounds of my own feedback (stale-pointer dedup hazard → fixed by the swapRemove drain; start()-after-EOF returning .suspended without draining → fixed by calling drainReaders() there with the re-entrancy guard; stale test comment → updated). All three are resolved in the final diff and the bug-hunter found nothing new.

Security risks

None. This is internal shell-interpreter plumbing for fd/pipe reader lifecycle; no parsing, auth, network, or untrusted-input surface is touched.

Level of scrutiny

Moderate-to-high. The change is small (~40 LoC in one Zig file plus a test) but lives in concurrency-sensitive runtime code: the Yield trampoline can re-enter addReader()/start() on the same IOReader while a drain is in flight, and on Windows start() can now synchronously dispatch onReaderDone to the very reader whose next() is still on the call stack. The author's argument that this is safe — every frame between cat.next() and the trampoline tail-returns the Yield without touching this afterward, and the IOReader itself is async-deinit'd — checks out from what I traced through cat.zig/Builtin/Binary, but it is a cross-file invariant rather than something locally enforced, so it deserves a maintainer's confirmation rather than a bot approval.

Other factors

• All three of my earlier inline comments are marked resolved and the diff reflects each fix.
• The added test demonstrably crashes on Windows main (null-unwrap in startWithCurrentPipe) per the author's CI links, and previously hung on Windows aarch64 before the late-drain fix, so it is a real regression guard.
• A human reviewer is already engaged (the "doesn't fail on main?" exchange), so deferring rather than approving keeps the existing review loop intact.
• This does not fit the "simple/mechanical/obvious" bar for bot auto-approval; it took three iterations to converge and the correctness argument is non-local.

[claude]

All my earlier concerns (stale-pointer dedup, late-registered readers on Windows, stale test comment) have been addressed — nothing further from me, but the re-entrancy and Yield-trampoline interactions here are subtle enough that a human should sign off.

Extended reasoning...

Overview

This PR replaces the for (this.readers.slice()) iteration in IOReader.onReaderDone/onReaderError with a drainReaders() helper that pops each reader via swapRemove(0) before dispatching .run(), adds a draining re-entrancy guard, and adds a Windows-specific start() fast-path that drains late-registered readers when the underlying BufferedReader is already done. The same changes are mirrored in the Rust port (IOReader.rs). A regression test (echo hi | (cat && cat && … ×12)) is added to bunshell.test.ts.

Security risks

None. This is internal shell IO plumbing — no auth, crypto, untrusted-input parsing, or permission boundaries are touched.

Level of scrutiny

Medium-high. The change is small in line count but the correctness argument is subtle: it depends on Yield-trampoline re-entrancy, allocator address-reuse hazards in addReader's pointer-equality dedup, the Windows BufferedReader lifecycle (source nulled before onReaderDone fires), and the interaction between drainReaders() being called from inside vs. after an existing drain. The PR description and code comments document these invariants well, and the author confirmed the Windows hang was reproduced in CI (build 49421) before the fix. Still, this is exactly the kind of change where a second pair of human eyes on the re-entrancy guard and the start()-after-EOF path is warranted — a mistake here manifests as a hang or UAF, not a clean test failure.

Other factors

• I left three inline comments on earlier revisions (stale-pointer dedup collision, start() returning .suspended without draining when reached after the drain loop exits, and a stale test comment). All three were addressed in cf14425 and 9aef7d3 and are marked resolved.
• The bug-hunting system found no issues on the current revision.
• The Rust port changes faithfully mirror the Zig changes.
• The added test is a valid regression guard for the Windows crash and the addReader-during-drain path, though (as noted in the prior discussion) it doesn't exercise the deferred-IOWriter path because TestBuilder captures stdout to a buffer.

Given the subtlety of the re-entrancy semantics and that this is core shell IO on a hot path, I'm deferring rather than approving.

[claude]

All prior feedback is addressed and the Rust port mirrors the Zig drainReaders() + start()-after-EOF fix cleanly — deferring to a human for final sign-off given the re-entrancy and lifetime subtleties across two implementations.

Extended reasoning...

Overview

This PR replaces the captured-slice iteration in IOReader.onReaderDone/onReaderError with a drainReaders() helper that swapRemove(0)-pops each reader before dispatching, guarded by a draining re-entrancy flag, and adds a Windows-only start() fast path that drains late-registered readers when the underlying BufferedReader is already done. The change is applied symmetrically to src/runtime/shell/IOReader.zig (spec) and src/runtime/shell/IOReader.rs (live implementation), plus a regression test in bunshell.test.ts.

Security risks

None identified. This is internal shell-interpreter plumbing (fd reader fan-out); no user-controlled input parsing, auth, crypto, or permission surface is touched. The change actually removes a UAF-shaped hazard (iterating a SmolList slice that could be reallocated mid-loop) and a Windows null-unwrap panic.

Level of scrutiny

High. The correctness argument depends on non-local invariants: Yield-trampoline re-entrancy, addReader dedup vs. freed-then-reused pointers/NodeIds, Arc/asyncDeinit keepalive across run_yield, and divergent Posix/Windows BufferedReader teardown ordering. Earlier revisions of this PR had two real issues (stale-pointer dedup collision, and start() returning .suspended after the drain had exited), both of which the author fixed in cf14425 and 9aef7d3; the current revision additionally ports those fixes to the Rust implementation (commits 0fb5060/ea19bf1). That iteration history alone argues for a maintainer who owns the shell interpreter to give it a final look.

Other factors

• All three of my prior inline comments are resolved and the test comment was updated to match the final mechanism.
• The bug-hunting pass on the latest revision found nothing.
• The Rust drain_readers() correctly re-derives self.state() each iteration (no long-lived &mut State held across run_yield), resets draining on exit, and the two callers (on_reader_done_cb/on_reader_error) both hold a _keepalive Arc across the call so the trailing self.state().draining = false write is safe even if the last external ref drops mid-drain. The Windows start() path reads self.reader().is_done() outside any vtable callback, so the documented reader() aliasing constraint is respected.
• The added test is a real regression guard (crashes on Windows main), though per the author it does not exercise the deferred-IOWriter path that motivated the start()→drainReaders() change — that was validated by a CI hang on an intermediate commit.

Net: I believe the change is correct, but it is not simple or mechanical, and it touches a core shell IO state machine in two languages. A human reviewer should confirm the re-entrancy reasoning and the Zig↔Rust parity before merge.

[robobun]

CI status for ea19bf1 (current head):

• cargo clippy: passes (initial failure was a vendor-fetch infra error — lol_html_c_api path dep missing before any linting ran; green on re-run).
• BuildKite #57367: 284 jobs passed, 0 test failures, no error annotations, nothing shell-related. The only red lane is darwin-14-aarch64-test-bun, which Expired (agent never picked the job up) — same infra issue that hit several earlier builds on this branch and others.
• All shell lanes (debian-asan, windows x64 / x64-baseline / aarch64) pass, including the added many readers on shared stdin IOReader test, which crashes on Windows without this change.

The diff itself is green; the remaining red is agent availability, not code. Review bots have signed off and all review threads are resolved — this just needs a maintainer's final look.

[robobun]

CI status for d727df0 (current head, rebased on a988615):

The only failing test in build 61334 is test/cli/install/bunx.test.ts > "should handle package that requires node 24", on both debian-asan and windows-x64. That test runs bunx --bun @angular/cli@latest --help against the live npm registry, so it breaks whenever Angular's latest moves its node engine requirement; it does not touch the shell. No bunshell.test.ts or IOReader-related failures on any lane.

All shell lanes pass, including the added many readers on shared stdin IOReader regression test (which crashes on Windows without this change). Diff is green; the red is an external-registry test unrelated to this PR.

[robobun]

CI status for 8ceed53 (main merged in by branch update):

Build 61672 produced zero test failures — no error annotations at all. 11 build-stage jobs (build-cpp/build-rust/build-bun across darwin/linux/freebsd/windows) Expired before an agent picked them up, which cascaded into 164 downstream test jobs never running. Pure agent-availability infra; nothing executed my diff and failed.

The previous repo-wide blocker (bunx.test.ts "requires node 24") is now skipped on main (#32042), so that's cleared. The 111 jobs that did run all passed, and prior builds already validated the shell lanes (debian-asan + all three Windows variants) green with the regression test. The diff is unchanged and needs only a re-run with available agents, or a maintainer's merge.

[robobun]

Rebased onto main (fb24aac). The conflict was non-trivial: src/runtime/shell/IOReader.zig has been deleted from main (the Zig to Rust shell port completed), so the Zig half of this PR no longer applies. I dropped it and kept the IOReader.rs fix, which is the live implementation and is unchanged in substance from what was already reviewed.

The PR is now a single commit touching only src/runtime/shell/IOReader.rs and test/js/bun/shell/bunshell.test.ts. Verified after the rebase:

• cargo check -p bun_runtime clean on both the host target and x86_64-pc-windows-msvc
• bun bd test test/js/bun/shell/bunshell.test.ts -t "pipeline stack": 19/19 pass, including the new many readers on shared stdin IOReader test

PR title and description updated to match.

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[robobun]

CI status for 176da33 (current head).

Build 66819 finished 280 passed / 6 failed. None of the failures involve this PR's code (src/runtime/shell/IOReader.rs, used only by the Bun.$ interpreter's cat builtin) and bunshell.test.ts passed on every lane:

• darwin 26 aarch64 - test-bun: buildkite-agent artifact download timed out after 120s before any test ran. Same agent failed the same way on the previous build; pure infra.
• test/js/bun/util/v8-heap-snapshot.test.ts (SIGKILL, ubuntu) and test/js/node/test/parallel/test-net-connect-memleak.js (alpine): both are currently failing on several other branches' builds too (66856-66871), so they are repo-wide flakes, not from this change.
• test/js/bun/terminal/terminal.test.ts (darwin 14 x64) and test/regression/issue/20965.test.ts (darwin 14 aarch64): both 90s timeouts in unrelated subsystems (Bun.spawn({terminal}) PTY and Bun.serve file-stream abort respectively). Neither goes anywhere near the shell IOReader; 88/90 terminal tests passed on the same lane.

The diff itself is green on every shell lane, review threads are all resolved, and the PR is mergeable. This just needs a maintainer's review.

[coderabbitai]

Note

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/runtime/shell/IOReader.rs`:
- Around line 234-238: In the EOF branch of the reader polling method, ensure
the mutable State reference held in s is no longer live before calling
drain_readers(). Restructure the preceding state access or limit its scope so
drain_readers() can safely obtain its own mutable State reference, while
preserving the late-reader EOF notification and suspended yield behavior.

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Reviewing files that changed from the base of the PR and between 18391f6 and 4b3df9b.

📒 Files selected for processing (2)

• src/runtime/shell/IOReader.rs
• test/js/bun/shell/bunshell.test.ts

[claude]

I reviewed the current head (d9a7bcc) and didn't find any bugs. All earlier review threads are addressed — the drain pattern, the draining re-entrancy guard, the Windows is_done() → drain_readers() path, the test-comment sync, and the &mut State borrow-scope hygiene all look correct in the final diff. Because this is unsafe Rust with UnsafeCell interior mutability and re-entrant Arc-refcounted callbacks, a maintainer familiar with the shell's aliasing contracts should still give it a final look.

Checked this run:

• drain_readers(): the initial s borrow's last use is s.interp before the loop; every loop iteration re-derives self.state(), so no overlapping &mut State across run_yield.
• Windows start() → drain_readers() without a local _keepalive: ruled out — reached either re-entrantly (outer on_reader_done_cb/on_reader_error already holds one) or via a caller that holds the stdin Arc<IOReader> for the duration of the call.
• on_reader_error() now sets raw_err before draining and re-derives state per access, matching on_reader_done_cb; drain_readers() reads raw_err per iteration so the error propagates.

Extended reasoning...

Overview

The PR replaces the clone-and-iterate pattern in IOReader::on_reader_done_cb and on_reader_error with a shared drain_readers() helper that swap_remove(0)s each entry before dispatching, adds a draining re-entrancy flag so nested calls are no-ops, and on Windows makes start() check reader().is_done() and drain late-registered readers instead of restarting a nulled pipe. A regression test (echo hi | (cat && cat && … ×12)) is added to bunshell.test.ts; per the timeline it crashes on all three Windows lanes on main and passes with the fix.

Since my last inline comment (2026-06-29, test-comment naming), three commits landed: 176da33 (test comment re-sync to Rust names), 4b3df9b (trim four multi-line source comments to one line each per the comment-cop bot), and d9a7bcc (re-derive self.state() per access in start() and on_reader_error() instead of holding a named &mut State across drain_readers()). All are narrow cleanups; the core logic reviewed in earlier rounds is unchanged.

Security risks

None. This is internal shell-interpreter I/O plumbing with no user-controlled parsing, no auth/crypto, and no new external surface. The only "attacker" is a shell script the user is already executing.

Level of scrutiny

High. The file uses UnsafeCell with a hand-maintained aliasing contract (split state/reader cells, reader() forbidden inside vtable callbacks), Arc self-keepalive across re-entrant run_yield, and platform-gated code paths. The change itself is small (~50 lines net) and follows the file's existing patterns exactly, but the correctness argument depends on borrow-death-before-re-derive (Stacked Borrows), on which callers hold a strong ref across drain_readers(), and on the Windows BufferedReader lifecycle. That is not the kind of change I should approve without a maintainer's sign-off.

Other factors

• Every prior review thread (mine, comment-cop, CodeRabbit) is resolved with a corresponding fix commit visible in the diff.
• The bug-hunting system found nothing this run; two finder candidates about start() lacking a keepalive around drain_readers() were examined and refuted by verifiers.
• CI has been repeatedly green on all shell lanes (debian-asan + all three Windows variants) across several rebases; the remaining red in the most recent build is documented as unrelated repo-wide flakes and infra expiries.
• The author has explicitly asked for a maintainer's final look in multiple status comments; deferring aligns with that.

[robobun]

CI status for d9a7bcc (current head).

Build 95150: 177 passed, 0 test failures, no error annotations. The only red is two darwin 14 aarch64 - test-bun slots that expired before an agent picked them up (the same darwin agent-availability issue as the last several builds); nothing ran and failed. The shell lanes that did run, including the many readers on shared stdin IOReader regression test, all passed.

Review state: CodeRabbit's borrow-scope finding is fixed in d9a7bcc and acknowledged, Comment Cop reports nothing left to flag, and all review threads are resolved. The PR is mergeable and needs a maintainer's review.