Subject: [PATCH] tests: wait for the UDP server socket before connecting

A fixed four-second sleep does not guarantee that gnutls-serv has bound
its UDP socket on a busy builder. Poll the local IPv4 UDP endpoint using
the existing ss/netstat discovery, with the same retry budget as the TCP
helper. Fail early if the server exits, and retain the original handshake
checks in serv-udp.sh.

Use flags common to ss and BSD/Linux netstat. Match the local endpoint
and complete port number, excluding TCP, IPv6-only and peer endpoints.

--- a/tests/scripts/common.sh
+++ b/tests/scripts/common.sh
@@ -185,10 +185,55 @@
 	fi
 }
 
+check_if_udp_port_bound() {
+	local PORT=$1
+	have_port_finder
+	# Use only -an, which is shared by ss and BSD/Linux netstat. UDP has
+	# no LISTEN state. Match the local IPv4 endpoint, not a peer port or
+	# a longer port number. serv-udp.sh connects to 127.0.0.1;
+	# listen_socket() in serv.c binds IPv4 separately and requests
+	# IPV6_V6ONLY=1 for its IPv6 socket.
+	$PFCMD -an | awk -v port="$PORT" '
+		$1 == "udp" || $1 == "udp4" {
+			# ss includes a state column; netstat does not.
+			address = ($2 == "UNCONN" || $2 == "ESTAB") ? $5 : $4
+			if (address ~ ("^[0-9.]+[.:]" port "$") ||
+			    address == "*." port)
+				found = 1
+		}
+		END { exit !found }
+	'
+}
+
 wait_udp_server() {
 	local PID=$1
+	local ret
 	trap "test -n \"${PID}\" && kill ${PID};exit 1" 1 15 2
-	sleep 4
+	local i=0
+	# Use the same retry budget as wait_for_port(), but also stop if the
+	# server exits before binding its socket.
+	while test $i -lt 90; do
+		if ! kill -0 "$PID" 2>/dev/null; then
+			fail "" "UDP server $PID exited before binding port $PORT"
+		fi
+		# Contain have_port_finder's exit so a skip also stops the server.
+		if (check_if_udp_port_bound "$PORT"); then
+			return 0
+		else
+			ret=$?
+			if test "$ret" = 77; then
+				kill "$PID" 2>/dev/null || :
+				wait "$PID" 2>/dev/null || :
+				exit 77
+			fi
+		fi
+		i=$((i + 1))
+		if test $i -lt 90; then
+			echo "try $i: waiting for UDP port $PORT"
+			sleep 2
+		fi
+	done
+	fail "$PID" "UDP server $PORT did not come up"
 }
 
 create_testdir() {