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

[Ludovic Courtès]

Hi Chris,

Did you have a chance to look into it?

  https://issues.guix.gnu.org/53355

TIA.  :-)

Ludo’.

Ludovic Courtès <ludo@gnu.org> skribis:

> Hi Chris,
>
> Thanks for debugging this!
>
> Chris Marusich <cmmarusich@gmail.com> skribis:
>
>> From c3eea81846ae71a246e6b592be74062f4bf26474 Mon Sep 17 00:00:00 2001
>> From: Chris Marusich <cmmarusich@gmail.com>
>> Date: Sun, 13 Feb 2022 14:15:14 -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): In the script
>> executed the child shell, set PS1 to an empty value and then echo three
>> sentinel lines to try to "flush" the original PS1 value before printing the
>> environment variables.  In the parent process, read and discard all lines up
>> to and including the last sentinel line.  After that, read the remaining 
>> lines
>> as usual.
>
> [...]
>
>> +    ;; Why print "GUIX_FLUSH" a few times?  We are trying to "flush" the
>> +    ;; original PS1 value to the port so we can read it (and discard it)
>> +    ;; before we start reading the environment variables from the port.  If 
>> we
>> +    ;; don't do this, the original PS1 value can sometimes get interleaved
>> +    ;; into the output, which interferes with our parsing logic.  It's a 
>> hack,
>> +    ;; but in practice it seems to do the job.  If you know of a more 
>> graceful
>> +    ;; solution, please implement it!  See: 
>> https://issues.guix.gnu.org/51466
>> +    "PS1=; for i in 1 2 3; do echo GUIX_FLUSH_$i; done; \
>> +env || /usr/bin/env || set; echo GUIX-CHECK-DONE; read x; exit\n")
>
> So you confirm that a single “echo” is not enough, right?
>
> Perhaps we should unroll the ‘for’ loop for portability, to be on the
> safe side.  Initially I tested with Bash, Zsh, and Fish:
>
>   https://issues.guix.gnu.org/51285#0-lineno49
>
> I think Fish has a very non-POSIX syntax, hence the suggestion to avoid
> ‘for’.
>
> I realized that setting PS1 could interfere with the logic below that
> checks for PS1.  And since it doesn’t seem to help, perhaps we can
> remove “PS1=;”?
>
> Thoughts?
>
> Sorry to answer with yet more questions!
>
> Thanks,
> Ludo’.