Add recommended combinations of Elixir and Erlang/OTP versions to CI

[adrianomitre]

This pull request makes sure all recommended Elixir and Erlang/OTP combinations are run on CI.

Quoting José Valim:

I would still advise folks to go with:

• For each Elixir version, add a run with earliest supported Erlang/OTP
• For the last Elixir version, also test the latest supported Erlang/OTP

Please refer to the original discussion on dashbitco/broadway#188 for the full rationale.

Additional functionality: make sure there are no compilation warnings, both on application and on tests, and checks formatting.

Finally, solved some minor unused variable warnings, courtesy of improved checks of the Elixir 1.11.0 compiler, and credo strict minor stylistic issues.

In case one wonders why not use otp: N.x for OTP as well as for Elixir, please refer to actions/setup-elixir#45.

[adrianomitre]

@keathley @garthk @Qqwy @wojtekmach I am looking forward to hearing your thoughts on this pull request.

[wojtekmach]

As I believe I’ve already stated elsewhere, Im not a fan of dense CI build matrices by default, I’d only reserve them for projects that would really benefit from them (eg elixirls) and for all the others, I’d test against just oldest and just newest supported version. This way we minimize resource usage that was given to us free of charge.

Regarding warnings as errors in tests, the best solution I found is the following:

# test/test_helper.exs
if System.get_env(“CI”) do
  Code.compiler_options(warnings_as_errors: true)
end

This isn’t obvious and I should have an article about it next week or so :)

[garthk]

Oh, handy.

I've been struggling with the full CI matrix on some other projects. I wish I'd thought of running just the oldest Elixir on the oldest OTP and the youngest Elixir on the youngest OTP. It'll deliver most of the benefit in a fraction of the resources. Thanks!

[Qqwy]

As I believe I’ve already stated elsewhere, Im not a fan of dense CI build matrices by default, I’d only reserve them for projects that would really benefit from them (eg elixirls) and for all the others, I’d test against just oldest and just newest supported version. This way we minimize resource usage that was given to us free of charge.

Agreed, with one little caveat (this has bitten me in the past):

Sometimes it might be worthwhile to also test newest Elixir with one-but-newest OTP and one-but-newest Elixir with the newest OTP. Sometimes this interplay will result in (compilation) errors you might otherwise miss.