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

[Ken Brown]

The hang can actually be triggered by evaluating the following, extracted from 
the test in the subject:

(setq socket-name "/tmp/socktest/socket")
(delete-file socket-name)
(make-network-process :name "server"
                      :server 10
                      :buffer nil
                      :service socket-name
                      :family 'local
                      :coding 'no-conversion
                      :noquery t)
(make-network-process :name "client"
                      :service socket-name
                      :family 'local
                      :coding 'no-conversion
                      :noquery t)

The reason for the hang is that Cygwin does a blocking handshake in 
accept/connect to exchange credentials.  This can hang in certain situations in 
which connect is called before a listening socket is ready to accept connections.

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?

In GNU Emacs 28.0.50 (build 9, x86_64-pc-cygwin, GTK+ Version 3.22.28, cairo 
version 1.17.4)
 of 2021-07-09 built on moufang2
Repository revision: 68276f6d30bbdc09cc26fb49d7f0c3aa4bce35f2
Repository branch: master
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