`bytes_to_read()` raises `SystemError: buffer overflow` on Python 3.14 (`FIONREAD` undersized buffer)

[RemiDesgrange]

Hi,

Same root cause as #1038 (fixed for _pty_size) but in terminals.bytes_to_read(), which was not touched by that fix.

On Python 3.14, fcntl.ioctl rejects undersized mutable buffers (cpython#144206). bytes_to_read passes a 2-byte buffer to FIONREAD, which writes a C int (4 bytes):

fionread = fcntl.ioctl(input_, termios.FIONREAD, b"  ")
return int(struct.unpack("h", fionread)[0])

which worked by accident on ≤3.13 thanks to the 1024-byte static copy buffer

Repro: any Connection.run() from a TTY-attached process on 3.14 crashes the handle_stdin thread

Do you want me to make a PR a fix with something like fionread = fcntl.ioctl(input_, termios.FIONREAD, b"\x00\x00\x00\x00") in https://github.com/pyinvoke/invoke/blob/main/invoke/terminals.py#L246-L247

Thx.

[tagilman]

Independently reproduced on Python 3.14.6 — same root cause you identified:
FIONREAD returns a C int (4 bytes) but bytes_to_read() passes a
2-byte buffer, which 3.14's stricter fcntl.ioctl now rejects with
SystemError: buffer overflow (silently truncated on ≤3.13).

Since this has been open a little while, I've put up a PR with the
int-sized-buffer fix plus a regression test that exercises the real ioctl
against a pty (it fails with SystemError on 3.14 without the fix). Credit
to you in the PR/changelog for the report and analysis — happy to hand off
or defer if you'd rather land your own. Thanks!

PR: #1077