0008: Pipe I/O is non-blocking, deadlined and waited with poll

Since 3.1.0 (deadlines), 5.0.0 (poll). Code: read_with_timeout and write_all_with_deadline in c_src/py_nif.h, the pipe setup in c_src/py_thread_worker.c and c_src/py_callback.c.

Situation

Callback responses are written by Erlang processes on dirty I/O schedulers into pipes read by Python threads. A blocking write to a stalled reader pinned a dirty scheduler for good; a short read or write left the framed protocol out of phase with no way back. Later, a CI run with more than 1024 open files showed that select() cannot watch such a descriptor at all, and the first thread worker created past that point took every later thread callback down with it.

Decision

Write ends are O_NONBLOCK; every write is write_all_with_deadline and every read read_with_timeout, both waiting with poll(). A partial frame is never recovered in band: the thread worker is poisoned and replaced, and the context pipe is closed only when its thread has been joined. The coordinator reports a failed ready signal instead of leaving Python to time out.

Consequences

• No scheduler thread waits on Python without a bound.
• A desynchronised pipe costs one worker, not a hang.
• select() and <sys/select.h> do not appear in the NIF; a review that sees them come back should ask why.