[Python-Dev] Update on Cygwin support (was: Clarifying Cygwin support in CPython)

Erik Bray erik.m.bray at gmail.com
Wed Jul 25 12:13:24 EDT 2018

On Wed, Nov 8, 2017 at 3:39 PM Erik Bray <erik.m.bray at gmail.com> wrote:
> 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

Apologies for responding to a months old post, but rather than repeat
myself verbatim I'll just mention that all of the above is still true
and relevant, and I am still interested in getting Python somewhere
closer to "stable" on Cygwin.

Part of the problem with my previous approach is that I was trying to
fix every last test failure before asking to add Cygwin to CPython's
CI fleet.  While I believe all failures *should* be fixed (or skipped
as appropriate) this is not practical to do in a short amount of time,
and not having CI implemented for a platform means new bugs are added
faster than we can fix the existing bugs.  For example, between 3.6
and 3.7 two new bugs have caused Python to be unbuildable on Cygwin:


This is in addition to an older issue that I was hoping to have fixed
in Python 3.7, as the PR was "green" for some time well before its


However, I fear part of why it was never merged is that lack of CI for
Cygwin, which is in turn because it was impossible to get a "stable"
test pass on Cygwin in the first place--something of a catch-22.

I think a new approach that might be more practical for actually
getting this platform re-supported, is to go ahead and add a CI build,
and just skip all known failing test modules.  This is what I've done
in a new PR to add a Cygwin build on AppVeyor:


This is far from ideal of course, and should not mean the platform is
"supported".  But now I and others can go through and fix the
remaining test failures, re-enable those modules in the CI
configuration, and actually obtain some meaningful results, which will
hopefully encourage the core committers to accept fixes for the

Once a few more major fixes are accepted (there are many for which I
have fixes, but have not yet made PRs) I can also add a buildbot for
Cygwin.  I already had a working experimental buildbot last year, so
now it's just a matter of getting a few critical issues (such as the
above-mentioned build issues) fixed, plus more of the major test

Thanks for your consideration,

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