make the shell configurable (default to bash)

[marten-seemann]

Setting the shell is mandatory: https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#runsstepsshell

[marten-seemann]

This PR doesn't work. It looks like we can't specify the shell here using variables. It seems like only strings are allowed (although this isn't properly documented).
Unrecognized named-value: 'inputs'. Located at position 1 within expression: inputs.shell

[aschmahmann]

This seems to be a composite action restriction actions/runner#835. There's a hack/workaround in the issue we might be able to leverage (e.g. creating a file or symlink on the OS which proxies to our shell)

[marten-seemann]

That's annoying. multiple-go-modules was supposed to be a fairly independent action, I'd hate to couple it that tightly to the rest of our shell logic.

[marten-seemann]

I thought that alternatively, we could duplicate the step here and add an if: ${{ inputs.shell == 'bash' }} to if: ${{ inputs.shell == 'msys2' }} to each step, respectively, but that doesn't work either. Apparently, you can't use if in an Action.

[marten-seemann]

After thinking about this for a bit, maybe the hack that issue is not that terrible.
I modified the PR, and we're now checking not for the OS, but for ${{ inputs.shell }}. That makes this action (at least a little bit) more general purpose.

[aschmahmann]

We can make it more general purpose if we need to by making a bash script that forwards the command along. Essentially making the yaml point at custom-shell {0} and making a script called custom-shell that just takes its arguments and forwards them to whatever the shell name in inputs.shell is.

[marten-seemann]

I simplified the setup a bit by using ifs (which GitHub now finally allows).
The alternative to this would be to just duplicate the step:

    - shell: bash
      if: inputs.shell == 'bash'
      working-directory: ${{ inputs.working-directory }}
      run: |
        <code>

once for bash and once for msys2 {0} (unfortunately, GH Actions doesn't support YAML anchors, so we can't use that to avoid the duplication). Not sure which one is less ugly.

@mvdan @aschmahmann Can you take another look?

[mvdan]

Took me a minute to understand what the symlink code is doing, but I agree with the approach :)

What's with the {0} by the way? Ideally we'd strip that, especially given the weird characters and the space.

[marten-seemann]

What's with the {0} by the way? Ideally we'd strip that, especially given the weird characters and the space.

That would be nice, but I think I tried this at some point, and it didn't work. Also, it seems to be the standard way of doing this, see for example https://github.com/msys2/setup-msys2 or https://ariya.io/2020/07/on-github-actions-with-msys2.