'exec -a' and $0 substitution

[Sergei Trofimovich]

Hello bash maintainers! nixpkgs package collection likes to wrap binaies
and shell scripts to pass extra environment variables via wrappers.

One example is dejagnu's runtest:

    $ cat `which runtest` | unnix
    #! /<<NIX>>/bash-5.2-p15/bin/bash -e
    PATH=...
    export PATH
    exec -a "$0" "/<<NIX>>/dejagnu-1.6.3/bin/.runtest-wrapped"  "$@"

Script's idea is to call .runtest-wrapped as if it had a 'runtest' name.
It's especially important for `runtest` because it internally checks for
own $0:

    mypath=${0-.}
    ...
    if [ "$target" != runtest ] ; then
        target="--target ${target}"
    else
        target=""
    fi

To the bug: looks like 'exec -a' does not work for bash scripts, but
does work for other executables. An example:

  Wrap C executable:

    $ cat b.bash
    #!/usr/bin/env bash

    exec -a "$0" $PWD/a "$@"

    $ cat a.c
    #include <stdio.h>

    int main(int argc, char * argv[])
    {
        for (int i = 0; i < argc; ++i) {
            printf ("A[%i]='%s'\n", i, argv[i]);
        }
    }
    $ gcc a.c -o a

  Result:

    $ bash -x ./b.bash 1 2 3
    + exec -a ./b.bash /home/slyfox/bugs/a 1 2 3
    A[0]='./b.bash'
    A[1]='1'
    A[2]='2'
    A[3]='3'

  Works as expected: './a' sees './b.bash' as an argv[0].

  Wrap 'bash' script:

    $ cat a.bash
    #!/usr/bin/env bash

    exec -a "$0" $PWD/foo "$@"

    $ cat foo
    #!/usr/bin/env bash

    echo "$0" "$@"

  Result:

    $ bash -x ./a.bash 1 2 3
    + exec -a ./a.bash /home/slyfox/bugs/foo 1 2 3
    /home/slyfox/bugs/foo 1 2 3

  For some reason '-a ./a.bash' did not take an effect.

Such a discrepancy makes these wrappers to break occasionally like in
https://sourceware.org/PR30052.

I fear it's a side-effect of the way 'bash' gets executed via shebang by
kernel. But maybe not? Somehow direct script execution still manages to
preserve script's name. Is it an intended behaviour that could not be
easily changed? Or it's a bug?

Thank you!

-- 

  Sergei