[New-bugs-announce] [issue41818] Lib/pty.py major revision

[Soumendra Ganguly]

New submission from Soumendra Ganguly <soumendra at tamu.edu>:

The current pty library has the following issues:

  1. Does not set slave termios. Documented in the source.

  2. Does not set initial slave window size. Documented in the source. Does not handle SIGWINCH. See bpo-41494, bpo-41541. This is essential in the following practical scenarios: i. creating split windows/panes while using a terminal multiplexer; ii. when resizing GUI terminal emulator window, especially relevant when using tiling window managers; iii. resizing an ansi-term window created inside a GNU Emacs frame.

  3. Does not perform signal handling. Signals must be blocked during sensitive portions of code.

  4. Hangs on FreeBSD. See bpo-26228.

  5. Includes deprecated functions pty.master_open(), pty.slave_open().

  6. In pty.fork(), the fallback code should try using TIOCSCTTY first. It is still using the old method of opening a tty to make it the controlling tty. Currently even SysV based systems provide TIOCSCTTY. See https://stackoverflow.com/questions/51593530/code-explanation-for-glibc-login-tty-function-openttyname-immediately-f

The current version of pty.spawn() uses pty.fork() internally. However, pty.fork() closes slave and only returns (pid, master_fd). To update winsize, access to slave is necessary. Further, slave termios must be properly set. The proposed modifications do this by implementing a login_tty(3) based function ( tty.login() ), and using that in pty.spawn() instead of pty.fork(). tty.login() tries TIOCSCTTY before falling back to the old SysV method because Python currently does not provide an interface to the native login_tty(3).

  7. tty.setraw() is called right after tty.tcgetattr(). This increases redundancy of code because tty.setraw() itself makes an identical tty.tcgetattr() call.

  8. Requires testing/porting to more platforms. Solaris, Illumos, macOS, Cygwin, etc. Windows ConPTY?

  9. There should be an option in pty.spawn() to turn off slave's ECHO flag. For example, when it is being used in a pipe. See https://github.com/karelzak/util-linux/commit/1eee1acb245a8b724e441778dfa9b858465bf7e5 and https://github.com/karelzak/util-linux/commit/75ccd75a2fa1194c6415c47b0024a438e26f1ad7#diff-3834a3d25eeaf20d9d0dcb05a46995f6

  10. Tests are incomplete. Tests consider OSes such as Tru64 but not {Free/Net/Open/...}BSD.

Find ongoing work here: https://github.com/8vasu/pypty2

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components: FreeBSD, Library (Lib), Tests, Windows, macOS
messages: 377195
nosy: koobs, ned.deily, paul.moore, ronaldoussoren, soumendra, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: Lib/pty.py major revision
versions: Python 3.10, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41818>
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