[musl] getusershell should ignore comments and empty lines.

[Collin Funk <collin.funk1@gmail.com>]

Hello Musl maintainers,

The getusershell function behaves differently from Glibc and FreeBSD.
I believe that it should follow those implementations by ignoring
comments and empty lines.

I wrote a test for Gnulib that catches this issue. You may find it
helpful for testing [1].

On FreeBSD I have the following etc/shells:

==============================================
# List of acceptable shells for chpass(1).
# ftpd(8) will not allow users to connect who are not using
# one of these shells.



/bin/sh
/bin/csh
/bin/tcsh
/usr/local/bin/bash
/usr/local/bin/rbash
/usr/local/libexec/git-core/git-shell
==============================================

And I run the following in a Gnulib checkout:

  $ rm -rf testdir1 && ./gnulib-tool --create-testdir --dir testdir1 getusershell
  $ cd testdir1
  $ ./configure
  $ make
  $ ./gltests/test-getusershell
  /bin/sh
  /bin/csh
  /bin/tcsh
  /usr/local/bin/bash
  /usr/local/bin/rbash
  /usr/local/libexec/git-core/git-shell

GNU libc behaves the same way. I have not checked the other BSDs but I
assume they use the same code derived from 4.3BSD or 4.4BSD.

Using an Alpine Linux virtual machine with Musl Version 1.2.4_git20230717
and a few packages installed I have the default /etc/shells:

==============================================
# valid login shells
/bin/sh
/bin/ash
/bin/bash
==============================================

Using the same commands listed earlier I run:

   $ ./gltests/test-getusershell 
   test-getusershell.c:54: assertion 'ptr[0] != '#'' failed
   Aborted

And after adding an empty line before the comment:

  $ ./gltests/test-getusershell 
  test-getusershell.c:55: assertion 'ptr[0] != '\0'' failed
  Aborted

Let me know if you have any questions. The FreeBSD shells(5) man page
is pretty good and might be helpful [2]. Here is a link to their
implementation incase that helps too [3].

Collin

[1] https://git.savannah.gnu.org/cgit/gnulib.git/tree/tests/test-getusershell.c
[2] https://man.freebsd.org/cgi/man.cgi?query=shells&sektion=5&manpath=freebsd-release
[3] https://github.com/freebsd/freebsd-src/blob/main/lib/libc/gen/getusershell.c