bug#48129: 28.0.50; Per file-handle (stdin, stdout and stderr) process-connection-type variable

[Thomas Hisch <thomas.hisch@ims.co.at>]

I would like that the configuration options of the three file
handles (stdin, stdout and stderr), when a new process is created
in emacs, are improved.  What I'm specifically asking for is to
add support for individually specifying for each file-handle
whether it should be connected to a pty or not.  Currently the
value of the process-connection-type variable applies to all
three file handles at once, which leads to a problem with the
following use-case:

Usually a program outputs ANSI escape sequences when its stdout
file-handle is connected to a pty. It is currently not possible to
communicate with a subprocess that is created by `start-file-process' by
writing to a pty (via stdin) and avoiding that this subprocess writes
ANSI escape sequences to stdout. This use-case is needed by magit, which
currently doesn't have support for displaying ANSI escape sequences but
it needs to communicate with it's started subprocesses via a PTY
(https://github.com/magit/magit/issues/3549).

The reason why I need this feature is that ANSI escape sequences may be
output by git-hooks (scripts stored in .git/hooks) if git is run from a
terminal/pty. Magit uses (setq process-connection-type t) when creating
subprocesses s.t. it can commuicate via the stdin file-handle that is
connected to a pty with the subprocess.

Here is an small example that shows that all file handles can
either be connected to a pty (process-connection-type t) or none
of them (process-connection-type nil).

(let ((process-connection-type nil)
      (process-buf (generate-new-buffer "*temp*")))
    (apply #'start-file-process
           "isatty"
           process-buf
           "/tmp/output-isatty.py" nil))

The output-isatty.pt looks like

#!/usr/bin/env python3
import sys
print(f'in: {sys.stdin.isatty()}')
print(f'out: {sys.stdout.isatty()}')
print(f'err: {sys.stderr.isatty()}')


If the above elisp code is run

in: False
out: False
err: False

is written to the *temp* buffer if process-connection-type is nil.
And

in: True
out: True
err: True

is written to the *temp* buffer if process-connection-type is t.

In the 'subprocess' and the 'pty' modules in the python stdlib the above
use-case is already supported (see the possible values for the
stdin,stdout and stderr params). Here is a small demo that attaches a
pty to stdin and two normal pipes to stdout and stderr:

import os
import pty
import subprocess

master, slave = pty.openpty()

# see https://docs.python.org/3.5/library/subprocess.html#popen-constructor
proc = subprocess.Popen(
    ('./t.py',),
    stdin=slave,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
)

os.close(slave)
with os.fdopen(master, 'wb', 0) as stdin:
    stdin.write(b'hello\n')
    out, err = proc.communicate()

print('RETC:')
print(proc.returncode)
print('OUT:')
print(out.decode(), end='')
print('ERR:')
print(err.decode(), end='')



In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.28, cairo version 1.16.0)
 of 2021-04-29 built on NBPF1PQX4B
Repository revision: 0e8c8628854d786e2f99dd2ac35ace88a5ee7924
Repository branch: master
Windowing system distributor 'Fedora Project', version 11.0.12011000
System Description: Fedora 33 (Workstation Edition)

Configured using:
 'configure 'CFLAGS=-march=native -O3' --prefix=/opt/emacs --with-pgtk
 --with-xpm --with-jpeg --with-gif --with-tiff --with-modules
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Important settings:
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Major mode: ELisp/d

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Load-path shadows:
/home/thomas.hisch/.emacs.d/user-lisp/custom hides /opt/emacs/share/emacs/28.0.50/lisp/custom

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