Do not run process_handler when exit_codes=[].

[greschd]

Currently, a process_handler that gets passed an empty exit_codes list will always run. I think it would be more consistent to never run these, and always run only when exit_codes=None.

That's certainly debatable, but since the change is simple enough I decided to make a PR to decide this over instead of making an issue first.

One could also consider emitting a warning when an empty list is passed (since the handler is then essentially dead code) - but if the list of exit codes is "dynamically determined" it could lead to false warnings.

Context: I tried removing ERROR_COMPUTING_CHOLESKY from the following, and was surprised that it then always ran:

    @process_handler(priority=590, exit_codes=[
        PwCalculation.exit_codes.ERROR_COMPUTING_CHOLESKY,
    ])
    def handle_known_unrecoverable_failure(self, calculation):

Update: Just noticed that the docstring for the exit_codes parameter supports changing the behavior (or otherwise, the docstring would need changing):

    :param exit_codes: single or list of `ExitCode` instances. If defined, the handler will return `None` if the exit
        code set on the `node` does not appear in the `exit_codes`. This is useful to have a handler called only when
        the process failed with a specific exit code.

[codecov]

Codecov Report

Merging #4380 into develop will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #4380      +/-   ##
===========================================
- Coverage    79.34%   79.33%   -0.00%     
===========================================
  Files          470      470              
  Lines        34638    34638              
===========================================
- Hits         27480    27478       -2     
- Misses        7158     7160       +2     

Flag	Coverage Δ
#django	73.19% <100.00%> (ø)
#sqlalchemy	72.40% <100.00%> (-<0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiida/engine/processes/workchains/utils.py	84.79% <100.00%> (ø)
aiida/engine/daemon/client.py	72.42% <0.00%> (-1.14%)	⬇️

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[sphuber]

Certainly is debatable, but don't feel like it now. You have made the case sufficiently ;) Thanks @greschd