Handling idempotend commands

[lemmi]

I have a couple of questions on handling idempotent commands, that is commands that when run on the host already are idempotent (at least when they succeed). Specifically the case of package repository updates.
For example xbps-install -S (docs, code) will, when executed, make the local copy of the repodata updated. It only downloads the new repodata if the version on the mirror is newer (it uses etags and modifications dates). xbps-install -S is "idempotent" in itself. But there is no sane way to do the same checks via some custom fact gathering and xbps does not have a working "dry-run" for this case.
The only reasonable thing to do is to execute the command and see what happens.
But here are my questions:

• Is there a way to communicate the fact that an operation did not change anything?
  • It looks to me that the fact that an operation changes anything is derived from whether it yields some commands.
  • In case of inherently idempotent commands, this makes it impossible to communicate the fact something changed or not, which in turn means that other operations can not be made conditional on that operation. They will always count as changed.
• It seems that idempotent operations are called multiple times. Which is kind of weird. What's the difference to operations that set idempotent = False other than the notice in the docs?

I haven't tried it yet, but maybe one solution is to only execute the command when host.in_op is True and to execute the command via host.run_shell_command, capturing the output. Depending on the output, either signal a noop or yield an an empty shell command like : (unless there is a direct way to set did_change etc.)