Option to fail if mixin not found?

[emersonknapp]

I'm creating a workflow where I dynamically choose a mixin and invoke colcon, and would like to fail the build if the given mixin name isn't found for the verb.

As is, this is just a warning, and I'm not sure how I could exit execution early in this case

➜  colcon build --mixin aarch64-gcc
WARNING:colcon.colcon_mixin.mixin.mixin_argument:Mixin 'aarch64-gcc' is not available for 'build'

This starts as a question: is there a way I could currently enforce that all selected mixins exist? If not, would it be reasonable to add a feature for enforcing this? My instinct is that the default case should be an error, because the user intentionally asked for something, and by analogy --packages-up-to exits with an error if the package is not found (though conversely --packages-select does not, it just builds 0 packages and emits a warning)

[dirk-thomas]

The choice to make this a warning was made in the very first iteration of this around 2 years ago. I don't recall a specific reason why making it a warning was chosen.

I completely agree with your point that it is similar to passing a not existing package name - the user passed something explicitly which doesn't exist. So converting this case into an error which aborts the execution sounds absolutely viable to me.

Please see #26 for the proposed change.