Add selective.reset command

[joan38]

Our CI does the follow before running all tests with selective.run:

  Try(sh("git reset --hard Last-Successful-Release")) match
    case Success(_) => ev.evaluate("selective.prepare")
    case Failure(_) => os.write(workspaceRoot / "out" / "mill-selective-execution.json", "")
  sh("git reset --hard $SHA")

We can also see that the mill-selective-execution.json file is being created in the mill CI:

mill/.github/actions/pre-build-setup/action.yml

Lines 49 to 50 in fe778e0

	- run: mkdir out && touch out/mill-selective-execution.json
	shell: bash

So I propose to have an actual task for creating an empty mill-selective-execution.json so that we can run selective.run.

[lihaoyi]

could you flesh out the PR description, I have no idea what this is about

[joan38]

@lihaoyi Done.

Are the tests failing because of my change?

[lihaoyi]

@joan38 probably not, likely just flakiness

[joan38]

@lihaoyi @lefou do we have agreement on adding this feature?

[lihaoyi]

@joan38 I still don't quite understand your PR description

[joan38]

Basically this PR is adding the selective.reset command that simply creates the out/mill-selective-execution.json file.

Why the need for this command?

We cannot run selective.run without that json file. It's prompting to run selective.prepare.
But there are instances were we cannot prepare because we don't know the relative change. Therefore we would like to run everything.

The documentation suggests:

selective.run relies on an out/mill-selective-execution.json file generated by selective.prepare in order to work, and will report an error if that file is missing. You can also zero out that file to explicitly tell selective.run to run all given tasks non-selectively, which is convenient if you want to conditionally disable selective execution (e.g. perhaps you want to perform selective execution on pre-merge on pull requests but not post-merge on the main branch)

The "zero out that file" can be done with selective.reset.

If you have a better task name, I'll take it.

[lihaoyi]

Maybe something like selective.prepare --empty or --none? selective.reset doesn't really fit what you describe, since it's not resetting anything to a previous state

[joan38]

Like this?

[joan38]

@lihaoyi the empty flag does not play very well with the var arg of tasks.
Should I rollback to the initial version? Or you have a better idea?

[lihaoyi]

Maybe a different name then? selective.prepareEmpty perhaps?