fix(unix): pace the EAGAIN write retry so a stalled reader can't saturate the thread

[nireak (nireak)]

The problem

CustomWriteStream._processWriteQueue() retries an EAGAIN write with setImmediate, which re-attempts within microseconds:

https://github.com/microsoft/node-pty/blob/main/src/unixTerminal.ts#L362

When a pty's reader stops draining — a raw-mode child that has stopped reading — that branch never clears, so the retry becomes an unbounded busy-loop that saturates the thread it runs on.

For an embedder that multiplexes many PTYs onto one process this is not a local problem: every terminal in the process freezes — no output, no input — while the host machine looks perfectly healthy. That is how we found it. A terminal daemon pinned a core at 100% twice in one hour with ~143,000 EAGAIN/s in the syscall trace, and every session on it locked up for ~17 minutes.

Why this doesn't undo #833

#833 removed setTimeouts from the backpressure path for throughput, and I did not want to quietly put one back. So I measured whether the EAGAIN branch is on that path at all.

Writing to a pty whose child actively drains, on macOS arm64:

total written	baseline	with this change	EAGAINs seen
4 MB	240 ms	240 ms	0
32 MB	2235 ms	2450 ms	0
64 MB	4740 ms	4927 ms	0

EAGAIN never fired once while a reader was draining, at any volume I tried. A merely-slow reader applies backpressure through the normal completion path; the kernel only returns EAGAIN once the buffer is full and stays full, which requires the reader to have stopped. Throughput differences above are run-to-run noise (the baseline itself varies by more than that between runs).

So this paces a branch that only executes when throughput is already zero because nobody is reading. The fast path — write completes, queue advances from the completion callback — is untouched.

Caveat stated plainly: those measurements are macOS arm64 only. I have not measured Linux, where buffer sizing and EAGAIN behaviour could differ.

The change

-  private _writeImmediate: NodeJS.Immediate | undefined;
+  private _writeRetry: NodeJS.Timeout | undefined;

   if ('code' in err && err.code === 'EAGAIN') {
-    this._writeImmediate = setImmediate(() => this._processWriteQueue());
+    this._writeRetry = setTimeout(() => this._processWriteQueue(), EAGAIN_RETRY_DELAY_MS);
   }

Renamed the field since it no longer holds an Immediate. dispose() correspondingly uses clearTimeout.

Second, related fix: dispose() clearing the handle is not sufficient on its own. An fs.write already in flight when dispose() runs still invokes its completion callback afterwards, and on EAGAIN that callback schedules a fresh retry — writing to an fd that is now closed. This is pre-existing (setImmediate does it too), but a delayed retry widens the window, so it belongs in the same change. Disposal is now tracked explicitly and checked in _processWriteQueue() and at the top of the completion callback.

EBADF is the benign outcome of that race. The one that concerns me is descriptor recycling — in a process that opens and closes PTYs continuously, a recycled fd would accept the write silently.

Waiting for actual writability (kqueue/epoll on the master fd) remains the fully correct fix, and is what #833 gestures at with "this likely needs a C++ solution like poll". This is the small version that stops the pathology in the meantime.

Tests

Two tests added to src/unixTerminal.test.ts, both verified to fail without the fix:

test	without fix	with fix
paces retries under sustained EAGAIN	3,244 attempts / 100 ms	~20
no write issued after dispose	write escapes → fails	passes

They stub fs.write to return EAGAIN deterministically rather than trying to starve a real pty, so they are fast and not timing-flaky.

End-to-end against a real stalled pty (128 MB queued, 6 baseline / 5 patched trials): avg 49,160 – 120,608 EAGAIN/s → 175 – 178, peak CPU 85 – 101% → 3 – 18%.

npm run lint is clean.

⚠️ On the full suite: npm test on this machine (macOS 26.3, node 24, arm64) is already red before my change — 17–18 passing, 8–9 failing, and flaky run to run. The failures are all in signals in parent and child and spawn (process.kill receiving a NaN pid, and the fd-leak tests). Across four runs with my change the failing set is identical and my two new tests never appear in it; the passing count rises by exactly 2. I have not tried to fix those pre-existing failures here.

Related

• unixTerminal: input is silently queued forever when libuv threadpool completions stop arriving (CustomWriteStream has no watchdog) #939 — same component, different failure mode: the threadpool stops delivering completions, so the queue never advances at all. Not addressed here.
• Investigate replacing the setTimeouts in the backpressure handling #833 — the issue this write path came from.
• Daemon busy-waits on EAGAIN writing to /dev/ptmx — saturates main thread, freezes every terminal on the runtime stablyai/orca#11178 — the production incident, with syscall capture and a standalone reproducer.

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