gh-124405: Fix NameError in openpty

[sobolevn]

@ambv how can I test this change? This looks very platform specific, but it seems that you have a reproducer :)

• Issue: NameError in openpty after #118826 #124405

[hauntsaninja]

Thanks, this looks like the correct inlining of slave_open from #118826. Adding more test coverage sounds good though!

[sobolevn]

Adding more test coverage sounds good though!

Thanks for the review! I need help adding tests, since I cannot reproduce this on my machine :(

[hauntsaninja]

I'm going to sleep now, will look tomorrow. Based on typeshed I_PUSH should exist on Linux?

[hauntsaninja]

"highly problematic" indeed ;-) https://github.com/python/typeshed/pull/6807/files#diff-f28c4d3b27ed19db8e4154e55b1ad119f6655aad863bd3e92d9aee2d481589d7R72

[vstinner]

I tried to test this change by removing os.openpty():

import os
import pty
del os.openpty
master_fd, slave_fd = pty.openpty()
os.close(master_fd)
os.close(slave_fd)

But it fails on Linux with:

Traceback (most recent call last):
  File "/home/vstinner/python/main/x.py", line 4, in <module>
    master_fd, slave_fd = pty.openpty()
                          ~~~~~~~~~~~^^
  File "/home/vstinner/python/main/Lib/pty.py", line 34, in openpty
    master_fd, slave_name = _open_terminal()
                            ~~~~~~~~~~~~~~^^
  File "/home/vstinner/python/main/Lib/pty.py", line 58, in _open_terminal
    raise OSError('out of pty devices')
OSError: out of pty devices

On my Fedora 41, I have no /dev/pty* device, only /dev/tty* devices (97 files) that I cannot open (PermissionError) because my user is not in tty or dialout groups.

It seems like os.openpty() is implemented by communicating with /dev/ptmx device using ioctl(). strace:

openat(AT_FDCWD, "/dev/ptmx", O_RDWR)   = 3
ioctl(3, TIOCGPTN, [3])                 = 0
ioctl(3, TIOCSPTLCK, [0])               = 0
ioctl(3, TIOCGPTPEER, 0x102)            = 4
ioctl(3, FIOCLEX)                       = 0
ioctl(4, FIOCLEX)                       = 0

[vstinner]

On my Fedora 41, I have no /dev/pty* device, only /dev/tty* devices (97 files) that I cannot open (PermissionError) because my user is not in tty or dialout groups.

Same on FreeBSD. On FreeBSD, os.openpty() is implemented with (syscalls recorded by truss):

posix_openpt(O_RDWR|O_NOCTTY)                    = 3 (0x3) 
ioctl(3,TIOCPTMASTER,0x0)                        = 0 (0x0)
ioctl(3,TIOCPTMASTER,0x0)                        = 0 (0x0)
ioctl(3,TIOCPTMASTER,0x0)                        = 0 (0x0)
ioctl(3,FIODGNAME,0x820f8bb90)                   = 0 (0x0)
openat(AT_FDCWD,"/dev/pts/1",O_RDWR,00)          = 4 (0x4)
ioctl(3,FIOCLEX,0x0)                             = 0 (0x0)
ioctl(4,FIOCLEX,0x0)                             = 0 (0x0)

[vstinner]

pty._open_terminal() code is quite old, it was added in 1994 when the pty module was created by commit 23cb2a8. The code almost didn't change since 1994: it still tries to open a /dev/ptyXY device and then use /dev/ttyXY device.

I guess that os.openpty() is now commonly available, and nobody uses the _open_terminal() fallback code path anymore, which looks broken on most platforms.

We should maybe do something with this code. Deprecate it. Remove it. I don't know :-)

[vstinner]

LGTM

[sobolevn]

@vstinner thanks a lot for your help! 👍