bug#65325: "make check" hangs on Cygwin

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 18 Aug 2023 18:02:46 -0400
> From: Ken Brown <kbrown@cornell.edu>
> Cc: 65325@debbugs.gnu.org
> 
> > I can reproduce the hang.  But I don't get the hang in the default build 
> > on my system (no configure options).  I'll have to play with the 
> > configure options and try to figure out what's going on.
> 
> OK, the hang in server-tests.el [and a similar hang in 
> keyboard-tests.el] occurs when getchar is called at keyboard.c:3978. 
> This call only occurs if dbus, file notification, and threads are all 
> disabled, i.e., if the configure options without-dbus, 
> without-file-notification, and without-threads are all specified or 
> implied.  I don't understand the keyboard code well enough to know (a) 
> why we reach that call to getchar in the context of the hanging tests 
> and (b) why getchar is only called in the absence of dbus, file 
> notification, and threads.

Because we have no other way to read input in such a build.  The code
we have there obviously cannot handle anything but character input
from stdin, so it is a small wonder that it hangs in an ERT test.

I think this is a red herring: we should skip these tests if none of
those 3 features is available.  (We could also rewrite the tests to
support such a build, but I have very little sympathy for people who
build Emacs --without-all and still want to run all the test suite
successfully, and thus I don't think this would be a worthwhile
investment of our energy and resources.)

> I can work around the problem by always avoiding that getchar call on 
> Cygwin (i.e., by adding "&& !defined CYGWIN" at line 3972).  With that 
> change, all tests pass on Cygwin with Bruno's configure options.  But I 
> don't want to make that change without understanding what's going on. 
> Can anyone help?

I don't think it's a Cygwin-only failure.  But if it is, it is also OK
to skip these tests only on Cygwin, in addition to testing the absence
of those 3 features.

Thanks.