libmunge: Fix connect failure retry for full socket queue

libmunge retries transient errors when connecting to munged.
This should handle errors arising from the listening socket's queue
being full.  However, PR #139 uncovered a bug that did not handle
EAGAIN which is returned on Linux when a nonblocking UNIX domain
socket connection cannot be completed immediately.

This commit fixes the while-loop in _m_msg_client_connect() that
retries connect() so both EAGAIN (Linux) and ECONNREFUSED (BSD)
are handled as transient errors that should be retried.

This was tested by setting "--listen-backlog=1", running munged
with the default 2 work threads, and running remunge with 64
threads.  First, the while-loop was altered so connect() errors
would not be retried.  remunge could reproduce EAGAIN on Linux,
and this behavior was dramatically more reproducible if vcpu > 1.
remunge could reproduce ECONNREFUSED on NetBSD 9.3 with vcpu=1, and
on FreeBSD 14.0 with vcpu=2.  Adding back the retry logic for EAGAIN
and ECONNREFUSED made this connect() failure difficult to reproduce
even with "--listen-backlog=1".

Tested:
- AlmaLinux 9.3, 8.9
- Arch Linux
- CentOS Linux Stream 9, Stream 8, 7.9.2009, 6.10
- Debian sid, 12.5, 11.9, 10.13, 9.13, 8.11, 7.11, 6.0.10, 5.0.10, 4.0
- Fedora 39, 38, 37
- FreeBSD 14.0, 13.3, 13.2
- NetBSD 9.3
- OpenBSD 7.4, 7.3
- openSUSE 15.5, 15.4
- Ubuntu 23.10, 22.04.4, 20.04.6, 18.04.6, 16.04.7, 14.04.6

src/libmunge/m_msg_client.c

@@ -214,10 +214,13 @@ _m_msg_client_connect (m_msg_t m, char *path)
     i = 1;
     while (1) {
         /*
-         * If a call to connect() for a Unix domain stream socket finds that
-         *   the listening socket's queue is full, ECONNREFUSED is returned
-         *   immediately.  (cf, Stevens UNPv1, s14.4, p378)
-         * If ECONNREFUSED, try again up to MUNGE_SOCKET_CONNECT_ATTEMPTS.
+         * Retry transient errors up to MUNGE_SOCKET_CONNECT_ATTEMPTS after a
+         *   linearly increasing backoff.
+         * Linux: connect() returns EAGAIN for nonblocking UNIX domain sockets
+         *   if the connection cannot be completed immediately.
+         *   [Linux man-pages 6.05.01]
+         * BSD: connect() returns ECONNREFUSED for UNIX domain stream sockets
+         *   when the listening socket's queue is full. [Stevens UNPv1]
          */
         n = connect (sd, (struct sockaddr *) &addr, sizeof (addr));
 
@@ -227,7 +230,7 @@ _m_msg_client_connect (m_msg_t m, char *path)
         if (errno == EINTR) {
             continue;
         }
-        if (errno != ECONNREFUSED) {
+        if ((errno != EAGAIN) && (errno != ECONNREFUSED)) {
             break;
         }
         if (i >= MUNGE_SOCKET_CONNECT_ATTEMPTS) {