[Avocado-devel] Make some avocado.utils.process APIs compatible with macOS

[Lucas Meneghel Rodrigues]

Let's discuss this particular topic a little bit.

So I dedicated some time to make avocado self tests to run on macOS (see
https://github.com/avocado-framework/avocado/pull/1834). This would be a
first step to make macOS (new name of what was known as Mac OS X) a 2nd
tier supported platform. This would interesting from a project health
perspective since attracting macOS developers would expand user base and
mind share.

Now, some of the avocado.utils.process APIs (example, get_children_pids)
rely on executing 'ps' as a subprocess. We implemented those functions
contradicting our own rules of avoiding relying on subprocesses, but it was
the pragmatic choice at the moment.

macOS does have coreutils, but they all come from BSD, not GNU. The macOS
version of 'ps' does not support the command line arguments that make the
APIs to return the correct values. It stands to reason that, if we want to
make avocado more portable among platforms, we should look for, well, more
portable implementations.

For that particular case, the excellent 3rd party 'psutil' library gives us
all the facilities implemented in a portable way so that it would just work
under macOS (and possibly other systems). Summarizing the choices:

1) Use 'psutil' but then introduce an EPEL dependency on RHEL platforms. I
know some of the groups using avocado have been pushing for a core program
that does not rely on EPEL, so for those guys, that solution is a no-go.

2) 'borrow' the psutil implementations and put them in avocado, which is
frowned upon by Fedora and RHEL and pretty much every distro packager ever.
I don't like that idea.

3) Remove the offending APIs. That would be a valid solution, although
capping existing functionality is not something that I fancy. We are still
with a liability of having avocado libs relying on parsing external
subprocess outputs, which is bad for a number of reasons and it should be
addressed at some point.

4) Simply say the APIs are not supported on macOS. It's also valid but then
it would be interesting to start delimiting linux specific APIs to a
namespace (think avocado.utils.linux, avocado.utils.darwin,
avocado.utils.nt) to make things clear from the get go.

5) Forget about the whole 'support non-linux' idea.

Let me know your thoughts on the subject.

Cheers,

Lucas
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