[Python-Dev] Clarifying Cygwin support in CPython

[Erik Bray]

Hi folks,

As some people here know I've been working off and on for a while to
improve CPython's support of Cygwin.  I'm motivated in part by a need
to have software working on Python 3.x on Cygwin for the foreseeable
future, preferably with minimal graft.  (As an incidental side-effect
Python's test suite--especially of system-level functionality--serves
as an interesting test suite for Cygwin itself too.)

This is partly what motivated PEP 539 [1], although that PEP had the
advantage of benefiting other POSIX-compatible platforms as well (and
in fact was fixing an aspect of CPython that made it unfriendly to
supporting other platforms).

As far as I can tell, the first commit to Python to add any kind of
support for Cygwin was made by Guido (committing a contributed patch)
back in 1999 [2].  Since then, bits and pieces have been added for
Cygwin's benefit over time, with varying degrees of impact in terms of
#ifdefs and the like (for the most part Cygwin does not require *much*
in the way of special support, but it does have some differences from
a "normal" POSIX-compliant platform, such as the possibility for
case-insensitive filesystems and executables that end in .exe).  I
don't know whether it's ever been "officially supported" but someone
with a longer memory of the project can comment on that.  I'm not sure
if it was discussed at all or not in the context of PEP 11.

I have personally put in a fair amount of effort already in either
fixing issues on Cygwin (many of these issues also impact MinGW), or
more often than not fixing issues in the CPython test suite on
Cygwin--these are mostly tests that are broken due to invalid
assumptions about the platform (for example, that there is always a
"root" user with uid=0; this is not the case on Cygwin).  In other
cases some tests need to be skipped or worked around due to
platform-specific bugs, and Cygwin is hardly the only case of this in
the test suite.

I also have an experimental AppVeyor configuration for running the
tests on Cygwin [3], as well as an experimental buildbot (not
available on the internet, but working).  These currently rely on a
custom branch that includes fixes needed for the test suite to run to
completion without crashing or hanging (e.g.
https://bugs.python.org/issue31885).  It would be nice to add this as
an official buildbot, but I'm not sure if it makes sense to do that
until it's "green", or at least not crashing.  I have several other
patches to the tests toward this goal, and am currently down to ~22
tests failing.

Before I do any more work on this, however, it would be best to once
and for all clarify the support for Cygwin in CPython, as it has never
been "officially supported" nor unsupported--this way we can avoid
having this discussion every time a patch related to Cygwin comes up.
I could provide some arguments for why I believe Cygwin should
supported, but before this gets too long I'd just like to float the
idea of having the discussion in the first place.  It's also not
exactly clear to me how to meet the standards in PEP 11 for supporting
a platform--in particular it's not clear when a buildbot is considered
"stable", or how to achieve that without getting necessary fixes
merged into the main branch in the first place.

Thanks,
Erik



[1] https://www.python.org/dev/peps/pep-0539/
[2] https://github.com/python/cpython/commit/717d1fdf2acbef5e6b47d9b4dcf48ef1829be685
[3] https://ci.appveyor.com/project/embray/cpython