[robobun]

Problem

• A child that queues more than 2 GiB of process.send() data panics with int cast: TryFromIntError(PosOverflow) in SendQueue::on_write_complete (Sentry BUN-4T5Q).
• on_write_complete (src/runtime/ipc.rs:1564) cast the head item's length to i32. Messages queued behind an in-flight message coalesce into one item, so its length can pass i32::MAX.

Fix

• Compare the write count with the lengths as usize. No length is cast.
• write caps each request at MAX_WRITE_LEN (i32::MAX) on both transports. When a whole capped request is taken and bytes remain, on_write_complete starts the next request at once.
• Verified: test/js/bun/spawn/spawn.ipc.test.ts (stock bun fails it with the panic). A local build with a 64 KiB cap delivers 50 messages through 695 continuations, and 0 of 50 without the continuation.
• Self-reviewed (first shape): 2 concerns raised, 2 addressed.

Background

• SendQueue is the outgoing side of an IPC channel. A queue item owns a Vec<u8> and a cursor of bytes already written.
• The transport reports the count it took as an i32. usockets arms the writable poll only after a short write (socket.c:513), and a libuv pipe write has no writable event.
• Considered a Windows-only continuation: it leaves a POSIX stall when the kernel takes exactly i32::MAX bytes.

Downsides

• Per write request: one min. Per partial write: one comparison. No allocation, no syscall.
• The queue still grows without bound, as in Node.
• No automated test runs the continuation on Windows. It ran there by hand only for this PR's first shape.

Notes

• Repro (release, Linux): a forked child does process.send("x".repeat(8 MiB)) 300 times while the parent spins for 20 s. The child prints the panic and exits with SIGABRT. All sends happen in one synchronous tick, so the parent does not need to block for the bug to trigger.
• How the item forms on current main: Release the consumed prefix of StreamBuffer so S3 uploads do not keep the whole payload in memory #42731 added last.data.cursor == 0 to the append rule in start_message (ipc.rs:1297). The in-flight first message stays its own item, and the messages queued behind it coalesce into the next item. Before Release the consumed prefix of StreamBuffer so S3 uploads do not keep the whole payload in memory #42731 every message coalesced into the head item.
• The test queues 22 x 100 MiB in one tick. The first message is in flight and the other 21 (2100 MiB) coalesce. The parent waits for the second message, which is the first one inside the coalesced item, then stops the child. On stock bun the child aborts in the callback that finishes message 1, and the test fails with child exited (null, SIGABRT) before the second message.
• The test has a 60 s ceiling. In advanced mode, which the test uses, one message cannot pass i32::MAX bytes in a robust way: the structured clone buffer is capped at 2^31 - 1 bytes (SerializedScriptValue.cpp:643, a 2^31 - 1 byte Buffer fails with DataCloneError). So at least two messages must coalesce, and the child copies each message three times to queue it (serialize, frame, append). JSON mode was not measured. Measured under the debug ASAN build: 6.5 to 7 s on an idle machine, 8 to 11 s on a loaded one. The default is 5 s. The sibling 2 GiB tests (shelloutput, spawnSync, blob-oom, streams-string-limit) carry 60 to 120 s for the same reason.
• The test is gated on 16 GiB of memory (the smaller of os.totalmem() and process.constrainedMemory(), the same shape as blob-oom.test.ts). Peak child RSS measured at the first shape: 3.95 GiB under the debug ASAN build, 5.59 GiB under the release build.
• Capped write experiment, local build only, MAX_WRITE_LEN = 64 * 1024: a child sends 50 messages of 1 MiB. Advanced mode: 695 capped continuations, 398 writable events, all 50 arrive in order. JSON mode: 678 continuations, 432 writable events, all 50 arrive. With the Done::Capped arm changed to not continue: 2 capped writes, then no writable event, 0 of 50 arrive after 20 s.
• Windows: bun run rust:check-all x86_64-pc-windows-msvc passes. The run by hand was at commit c5f6aae, where the continuation was a #[cfg(windows)] arm: the coalesced item was 2306867508 bytes, the first request completed with 2147483647 bytes, the queue then issued the 159383861-byte remainder, and the parent received all 22 messages (49 s, child peak RSS 10.45 GiB because write copies each request). It was not run again for the current shape. On Windows both shapes make the same call: a completed capped request reports i32::MAX.
• The test skips Windows because the continuation there runs only after the parent drains a whole 2 GiB request, and the child's 10.45 GiB peak is too close to the 16 GiB of the Windows test machines.
• Not changed here: us_socket_t::write (src/uws_sys/us_socket_t.rs:342) clamps to i32::MAX for every caller. A caller other than IPC that passes a buffer over 2 GiB and waits for a writable event after the short count has the same exposure when the kernel takes the whole clamped chunk. No repro exists for it.
• Other casts in this file: ipc.rs:414 (u32::try_from(serialized.data().len())) is the 4 GiB wire-format limit of one message. ipc.rs:1803 (i32::try_from(write_len), Windows) is bounded by the cap.
• Other suites run with the debug build: spawn.ipc, spawn.ipc.bun-node, spawn.ipc.node-bun, spawn-ipc-gc, bun-ipc-inherit, child_process_ipc, child_process_ipc_handle, child_process_ipc_large_disconnect, child_process_send_cb (42 tests), and the node tests test-child-process-ipc, test-child-process-send-cb, test-child-process-send-utf8, test-child-process-fork, test-child-process-fork-net-socket, test-child-process-fork-net-server, test-child-process-send-keep-open, test-child-process-fork-closed-channel-segfault, test-cluster-basic, test-cluster-advanced-serialization.
• First shape of this PR (commits c5f6aae, b231b14): a match on usize::try_from(n) and a Windows-only continuation. Review pointed out the POSIX stall and the re-indented comments. The current shape keeps the original if chain.

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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.ipc.test.ts

[robobun]

^{Updated 3:55 PM PT - Oct 1st, 2026}

✅ @robobun, your commit 7b3d97707a7827d4e51392bdb3503e8483c8afce passed in Build #122561! 🎉

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bunx bun-pr 41155

That installs a local version of the PR into your bun-41155 executable, so you can run:

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[robobun]

Status

• Reproduced on a release build of main: a child calls process.send() with a 100 MiB string 22 times in one tick. It prints panic: int cast: TryFromIntError(PosOverflow) and exits with SIGABRT.
• The test in test/js/bun/spawn/spawn.ipc.test.ts fails the same way on that build and passes with this branch.
• The capped-write continuation ran on Linux in a local build that lowers the cap to 64 KiB. The Windows path ran by hand only for the first shape of this PR (c5f6aae).

[coderabbitai]

Navigate logical layers of code changes, visualize relationships, and explore their blast radius.

Walkthrough

The IPC send queue now validates signed write counts against remaining bytes. It handles full, partial, zero-progress, and invalid writes. Windows resumes partial writes immediately. A gated regression test covers advanced IPC queues exceeding 2 GiB.

Changes

IPC write queue handling

Layer / File(s)	Summary
Write completion handling src/runtime/ipc.rs	SendQueue::on_write_complete validates signed counts against remaining bytes. It handles invalid and zero-progress writes, and resumes partial writes on Windows.
Large queue regression coverage test/js/bun/spawn/spawn.ipc.test.ts	A gated test queues 22 100 MiB advanced-serialization messages, verifies delivery, stops the child, and checks successful termination.

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

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to aa3e9

The IPC fix has no established runtime blocker. Remove the regression test’s explicit timeout to comply with repository requirements; the remaining merge risk is bounded.

🚥 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 identifies the main change: handling IPC send-queue items larger than 2 GiB with capped write requests.
Description check	✅ Passed	The description explains the problem, fix, verification steps, platform behavior, test coverage, and known limitations. It does not use the exact template headings, but it provides the required inform…

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@test/js/bun/spawn/spawn.ipc.test.ts`:
- Line 308: Add a separate Windows IPC regression test near the skipped “send
queue past 2 GiB” case that forces a write larger than the i32::MAX cap and
requires a second capped write, then verify the first message is delivered and
the process exits cleanly after stop. Keep the existing non-Windows test
behavior unchanged and use the surrounding spawn/IPC test helpers and
assertions.

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[claude]

LGTM — straightforward widening of the write-count comparison from i32 to usize, removing a user-reachable .expect() panic.

What was reviewed:

• Match arm ordering in on_write_complete: full-write / zero / partial / error map to the same outcomes as before, and negative n (Err from usize::try_from) still hits Done::Error.
• The Windows Done::Partial → continue_send branch: write_in_progress is cleared before the call, continue_send(OnWritable) bypasses the cursor-!=-0 guard, and the libuv write is async so there's no recursion risk.
• Test: awaits the ipc event via withResolvers, wires onExit to reject, asserts payload before exit code, uses Buffer.alloc(...).toString(), and gates on both totalmem() and constrainedMemory().

Extended reasoning...

Overview

This PR fixes an integer-overflow panic in SendQueue::on_write_complete (src/runtime/ipc.rs). When many process.send() payloads coalesce behind a partial write into a single buffer exceeding 2 GiB, the old code's i32::try_from(first.data.list.len()).expect("int cast") panicked. The fix converts the socket's i32 write count to usize once and compares against the already-usize to_send_len, restructured as match arms. It also adds a Windows-only continue_send call on Done::Partial, since libuv completes writes in full and emits no follow-up writable event — without it, a >2 GiB item would stall after the first i32::MAX-byte chunk. A new test in the existing spawn.ipc.test.ts queues 22 × 100 MiB in a child and asserts the parent receives a full message and the child exits cleanly.

Security risks

None. This is an outgoing IPC write-completion path between a parent and its own spawned child. No untrusted input parsing, no auth, no crypto. The change removes a panic (DoS on self) and does not introduce any new trust boundary.

Level of scrutiny

Moderate. The core change is a mechanical type widening that removes a user-reachable .expect() — exactly what REVIEW.md's "User-reachable failures are recoverable errors, never panics" rule asks for. I traced each match arm against the old if/else chain: Ok(written) == to_send_len → full write (unchanged); Ok(0) → NoProgress (unchanged); Ok(written) < to_send_len → Partial with cursor advance (unchanged, and the new bound to_send_len is tighter and more correct than the old list.len()); Err(_) (negative n) or written > to_send_len (impossible in practice) → Error. The Windows #[cfg] branch is the only new behavior; I verified write() caps at i32::MAX on Windows (line 1715), that write_in_progress is cleared before continue_send runs so the debug_assert at line 1513 holds, and that the libuv write is queued async so re-entering continue_send cannot recurse unboundedly.

Other factors

The test follows harness conventions closely: added to the existing IPC test file, uses bunExe/bunEnv, await using on the subprocess, Buffer.alloc().toString() for the large payload, Promise.withResolvers awaiting the actual ipc event with onExit wired to reject, and asserts the message length before the exit code. It's memory-gated at 16 GiB using both os.totalmem() and process.constrainedMemory() (matching the pattern cited from blob-oom.test.ts) and skipped on Windows. The 60 s per-test timeout is a ceiling for a genuine outlier (multi-GB buffer under debug ASAN, measured at 6.5 s). The Windows branch has no automated coverage because the test can't run there under the memory gate, but the PR records a manual verification with concrete byte counts, and the branch is strictly an improvement over both the old panic and a stall. No CODEOWNERS cover these paths, no prior reviewer objections exist, and the bug hunt exited on dry_streak with no findings.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
Review comments at @test/js/bun/spawn/spawn.ipc.test.ts:
- Around line 399-433: Remove the per-test timeout from the test declared with
“keeps sending after more than 2 GiB is queued before the first writable event”:
delete the timeout constant and omit the third timeout argument to `it`, while
preserving the test body.

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[claude]

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

[claude]

I re-reviewed after 7b3d977 and found no bugs; the capped-write resume is now shared by both transports, so the earlier platform split is gone. Because this is the core IPC send path and the new test is heavy (child peaks at 4-6 GB RSS, ~7 s on debug ASAN, gated at 16 GiB), a human look at whether that fixture belongs in the default lane is still worthwhile.

What was reviewed:

• on_write_complete arms: written is now usize on every branch, negative n maps to Done::Error as before, and written > to_send_len still errors; Done::Capped fires only when written == MAX_WRITE_LEN with bytes remaining, and continue_send(OnWritable) passes the cursor != 0 guard.
• Checked Done::Capped against us_socket_write in packages/bun-usockets/src/socket.c: the writable poll is rearmed only when written != length, so a full i32::MAX send indeed produces no writable event and the immediate resume is needed on POSIX too; a 0-byte follow-up write rearms normally.
• Both write branches cap at MAX_WRITE_LEN, so the Windows i32::try_from(write_len) in windows_on_write_complete cannot overflow.
• Test fixture: start_message only coalesces onto a head with cursor == 0, so the first message is in flight and the next 21 form the >2 GiB item as the comment states; onExit rejects the awaited promise and the memory gate handles constrainedMemory() === 0.

Extended reasoning...

The change is confined to SendQueue::on_write_complete and SendQueue::write in src/runtime/ipc.rs (about 20 logic lines) plus one new gated test block in test/js/bun/spawn/spawn.ipc.test.ts. It touches kernel-reported write counts and length arithmetic on the process IPC channel, but no auth, crypto, or injection surface. The latest commit replaced the Windows-only #[cfg(windows)] resume with a platform-agnostic Done::Capped arm, and I traced the usockets rearm logic to confirm the resume is correct and necessary on POSIX. I deferred rather than approved because it is a core runtime native path and the new test's resource footprint (multi-GB RSS, several seconds on debug builds) is a maintainer judgment call; the per-test timeout thread from the prior run remains open.

[Jarred-Sumner]

This is Claude (an AI assistant) commenting on Jarred's behalf.

This fix went in with #46021, which is merged. Closing.