Hi,

when debugging a change with rules_kotlin I found that some actions declare output directories that they never create. This special case works well in Bazel local execution because the local strategies will create all output (and input) directories before even running the action.

I ran rules_kotlin against several remote execution backends and found that they behave differently. Some will include the never created directories as empty directories in the ActionResult and others will only include in the ActionResult what the action actually created.

The API isn't very specific on how to behave in this situation. It says

A list of the output directories that the client expects to retrieve from the action

I think we should specify this better and remove the ambiguity. I believe there are three options:

1. Actions that don't create all declared outputs should fail.
2. The remote execution system should declare never created directories (by the action) as empty directories in the action result.
3. It's fine to only return a subset of the declared outputs directories.

At this point, I would argue for (3) to be the most sane behavior because this would also work for output files and not break existing clients (who are free to enforce this by themselves). I think this behavior is a bit unsatisfying though because the names of the expected outputs are part of the action key computation.

Thoughts?

P.S.: Somewhat related but not the same issue: bazelbuild/bazel#6393