bug#49496: 28.0.50; process-tests/fd-setsize-no-crash/make-network-process hangs on Cygwin

[Eli Zaretskii <eliz@gnu.org>]

> From: Ken Brown <kbrown@cornell.edu>
> Date: Fri, 9 Jul 2021 14:01:33 -0400
> 
> It is possible to work around this problem by disabling the credential exchange 
> as in the following patch:
> 
> diff --git a/src/process.c b/src/process.c
> index b8c3e4ecfb..01548c8ab5 100644
> --- a/src/process.c
> +++ b/src/process.c
> @@ -3355,6 +3355,12 @@ connect_network_socket (Lisp_Object proc, Lisp_Object 
> addrinfos,
>   	      xerrno = EMFILE;
>   	      continue;
>   	    }
> +#ifdef CYGWIN
> +	  /* Avoid possible hang in connect/accept.  */
> +	  if (family == AF_LOCAL && p->socktype == SOCK_STREAM
> +	      && setsockopt (s, SOL_SOCKET, SO_PEERCRED, 0, 0) < 0)
> +	    report_file_error ("Cannot disable credential exchange", Qnil);
> +#endif
>   	}
>          if (p->is_non_blocking_client && ! (SOCK_NONBLOCK && socket_to_use < 0))
> 
> But this seems too drastic just to fix a potential problem that doesn't seem to 
> occur in practice, but only in the contrived situation of 
> process-tests/fd-setsize-no-crash/make-network-process.  So my preference is to 
> simply skip that test on Cygwin.
> 
> OK?

Yes, OK.  But please explain there in a comment why we skip that, with
a reference to this bug.

(FWIW, I consider the process-tests suite to be not useful enough, in
that it many times tests behavior that is only guaranteed to happen on
GNU/Linux, and is not really required, nor should be expected, in
portable Lisp programs.  So it doesn't surprise me that some of the
tests fail on some systems.)