bug#53355: guix shell --check: confusing error message

[Ludovic Courtès]

Hi Chris,

Chris Marusich <cmmarusich@gmail.com> skribis:

> One alternative method I tried successfully in a variety of shells was
> to use shell redirection (see attached).  I like this approach.
> However, this will only work in shells that support redirection.  I
> recall testing with bash, ash (busybox's shell), dash, zsh, fish, ksh,
> and csh.  I recall that only csh failed, since it doesn't support
> redirection.

That’s a good success list in my view; not being a POSIX shell, (t)csh
was already excluded from the list in the original submission I think:
<https://issues.guix.gnu.org/51285>.

> I personally like the attached patch better than what I proposed
> earlier.  The earlier patch just echoes a few times.  Presumably, it
> only works because the PS1 prompt is likely (but not guaranteed) to be
> emitted before the last of the echo commands finishes printing.  I'd
> rather just control the desired output and ignore PS1 entirely, and that
> is what the attached patch accomplishes using FDs.  However, if support
> for shells without redirection is a requirement, then maybe the original
> hack (echo a few times) is OK, or perhaps we need something else.
>
> How would you like to proceed?  Is it OK to rely on shell redirection?

Yeah, I think so.  This new approach looks more robust.

> From 9a1cef589abf01b61e22656f44c76441f4c50ffd Mon Sep 17 00:00:00 2001
> From: Chris Marusich <cmmarusich@gmail.com>
> Date: Fri, 11 Mar 2022 00:20:12 -0800
> Subject: [PATCH] environment: Prevent PS1 from clobbering output in 'check'.
>
> Fixes: <https://issues.guix.gnu.org/51466>.
>
> * guix/scripts/environment.scm (child-shell-environment) [shell-pipe]
> [shell-pipe-in, shell-pipe-out]: New local variables.
> [script]: Redirect the stdout of each command to the file descriptor of the
> shell-pipe-out port.
> [lines]: In the child, close shell-pipe-in before starting the shell.  In the
> parent, close shell-pipe-out before sending the script to the shell.  Read
> lines from shell-pipe-in, not port, so that the shell's PS1 prompt cannot
> clobber the lines.  Close shell-pipe-in just before waiting on the child.

LGTM, please push!

Ludo’.