bug#70164: home-bash-service default PS1 overwrites .bashrc PS1 in login shells

[Richard Sent <richard@freakingpenguin.com>]

Hi Guix!

A common convention for where to set PS1 is in .bashrc, not
.bash_profile [1-3]. Unfortunately home-bash-service doesn't support this
convention for login shells.

home-bash-service generates a default .bash_profile that follows these
steps:

1. Source .profile
2. Source .bashrc
3. Set PS1 if guix-defaults? is truthy

This means that any PS1 configuration in .bashrc is overwritten by
.bash_profile for login shells specifically.

This is visible in a TTY, but also in WSL, which defaults to opening a
login shell. PS1 will be Guix's predefined value instead of the value
set in .bashrc.

Setting guix-defaults? to #f has many side effects, so I don't feel that
is a valid solution.

A comment in home/serivces/shells.scm suggests setting PS1 via
environment-variables since that is appended to the end of
.bash_profile. This is fine for simple prompts, but complicated prompts
are often split apart into separate bash functions and variables. Either
an implicit dependency between .bashrc and .bash_profile is created, or
.bash_profile balloons into a mega-file while the conventional wisdom is
to keep it as simple as possible.

environment-variables also exports PS1, causing it to become an
environment variable, not a shell variable. This might cause some odd
behavior when subprocesses inherit it. [4]

Some possible solutions:

1. Move default PS1 to bashrc, right after serializing %default-bashrc
2. Keep default PS1 in .bash_profile, but before loading .bashrc
3. Add a set-prompt? field to home-bash-configuration

Of the 3, I think 1 is the best and plan to submit a patch for it soon.
I'm opening the bug in case anyone thinks I missed something.

[1] https://unix.stackexchange.com/a/549075
[2] https://superuser.com/a/789465
[3] https://superuser.com/a/789454
[4] https://unix.stackexchange.com/a/44000

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.