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

[Ludovic Courtès]

Hi Chris,

A friendly ping.  :-)

Ludo’.

Ludovic Courtès <ludo@gnu.org> skribis:

> 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’.