Ensure WorkChain does not exit unless stepper returns non-zero value

[sphuber]

Fixes #1944

The engine exposes the possibility to the user of aborting a running
workchain from within its outline step functions, by simply returning
a non-zero integer or ExitCode instance with a non-zero status.
Any other return value should be ignored in terms of aborting the
running of the WorkChain. Due to a logical flaw in the _do_step
implementation of the WorkChain this heuristic was not being
respected and returning an ExitCode instance with zero status, would
cause the WorkChain to exit the control flow.

[codecov-io]

Codecov Report

Merging #1945 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1945   +/-   ##
========================================
  Coverage    67.55%   67.55%           
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  Files          321      321           
  Lines        33275    33275           
========================================
  Hits         22479    22479           
  Misses       10796    10796

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[giovannipizzi]

Apart for my comment, it's ok

[giovannipizzi]

Is this the right way to check also in py2/py3? (e.g. long in py2, ...)

[sphuber]

No clue to be honest. Surely @dev-zero would be able to tell us? :)

[dev-zero]

if you suspect that stepper_result could be a long in Python 2, the future-proof ways to check are:

import six

if isinstance(stepper_result, six.integer_types) and stepper_result != 0:

or

import numbers  # Python built-in since 2.6

if isinstance(stepper_result, numbers.Integral) and stepper_result != 0:

but for stepper_result to be a long in Python 2, you either have to set it explicitly using long(123), the ...L literal suffix or that it grows bigger than sys.maxint (which is in Python 2: 9223372036854775807) by itself.

[sphuber]

Since it can be a value that is returned by a user (return value from a WorkChain step function) and the spec is that it should allow positive integers greater than zero (which therefore technically should include longs), I will go with the six variant. Thanks @dev-zero