fixes #269 nngcat unreliable with ZeroTier transport

This does a few things.  First it closes a case where a dropped
message could prevent subsequent connection attempts from getting through.

Second, it changes the rate at which we retry, and the timeout, to be
a lot more aggressive when attempting to establish a connection.  We
retry every 500 ms, for up to 2 minutes, before giving up.  This gives
a lot more resilience in the face of message loss that is typical of
ZeroTier in some environments when first establishing communication.

Third, makes the values for the connection attempts *tunable*, so
that applications can adjust for different deployment scenarios.

Fourth, it includes the ability to get the UDP socket name.  This was
needed during some debug, and may be useful for a real UDP transport
later, so we're keeping it.

Finally, we added documentation for the above items.

1 files changed, 15 insertions, 0 deletions

diff --git a/src/platform/posix/posix_udp.c b/src/platform/posix/posix_udp.c
index 61546667..8a402acc 100644
--- a/src/platform/posix/posix_udp.c
+++ b/src/platform/posix/posix_udp.c
@@ -306,4 +306,19 @@ nni_plat_udp_send(nni_plat_udp *udp, nni_aio *aio)
 	nni_mtx_unlock(&udp->udp_mtx);
 }
 
+int
+nni_plat_udp_sockname(nni_plat_udp *udp, nni_sockaddr *sa)
+{
+	struct sockaddr_storage ss;
+	socklen_t               sz;
+	int                     rv;
+
+	sz = sizeof(ss);
+	if ((rv = getsockname(udp->udp_fd, (struct sockaddr *) &ss, &sz)) <
+	    0) {
+		return (nni_plat_errno(errno));
+	}
+	return (nni_posix_sockaddr2nn(sa, &ss));
+}
+
 #endif // NNG_PLATFORM_POSIX