unixTerminal: input is silently queued forever when libuv threadpool completions stop arriving (CustomWriteStream has no watchdog)

[moseoh]

Environment

• node-pty 1.1.0 (CustomWriteStream write path from Investigate replacing the setTimeouts in the backpressure handling #833)
• Electron app (pty host runs in a utility process), macOS 15 / darwin arm64

Issue

CustomWriteStream.write() queues data and relies on the async fs.write completion callback to advance the queue:

• write() only starts processing when the queue transitions 0 → 1
• _processWriteQueue() issues one fs.write and continues from its callback

fs.write completes on the libuv threadpool. If the pool stops delivering completions — which we observed live in production: all 4 workers idle in pthread_cond_wait, submitted work never picked up, event loop perfectly healthy — the callback never fires, the queue head never advances, and every subsequent write() appends to the queue forever. There is no error, no timeout, and no log: from the embedder's perspective the terminal simply stops accepting input while output (event-loop reads) keeps flowing. Terminal.write() returns void, so the embedder cannot observe or mitigate this from outside.

We confirmed this mechanism while investigating Orca's long-running "terminal intermittently stops accepting input" issue (stablyai/orca#8104). In the live wedged process, _writeQueue held every lost keystroke with the first task still at offset: 0, and hot-patching write to fs.writeSync restored input instantly — the fd was fine, only the threadpool path was dead.

We're investigating the Electron/libuv threadpool wedge separately. This issue is about how node-pty behaves when it happens: input stays queued indefinitely with no error and no recovery path.

Deterministic repro

The wedge can be simulated exactly by pinning every threadpool worker with a FIFO reader-open (blocks in open() until a writer appears):

const fs = require('fs');
const pty = require('node-pty');
const { execFileSync } = require('child_process');

execFileSync('mkfifo', ['/tmp/stall.fifo']);
const p = pty.spawn('/bin/sh', ['-c', 'read line; echo "GOT:$line"'], {});
p.onData((d) => process.stdout.write(d));

setTimeout(() => {
  for (let i = 0; i < 6; i++) fs.open('/tmp/stall.fifo', 'r', () => {});
  setTimeout(() => {
    p.write('hello\n');           // never reaches the shell
    fs.stat('/tmp', () => console.log('pool alive')); // never prints
  }, 300);
}, 300);