MEDIUM: session: close new idle conns if server in maintenance

Previous patch ensures that a backend connection going into idle state
is rejected and freed if its target server is in maintenance.

This patch introduces a similar change for connections attached in the
session. session_check_idle_conn() now returns an errorl if connection
target server is in maintenance, similarly to session max idle conns
limit reached. This is sufficient to instruct muxes to delete the
connection immediately.

Author: a-denoyelle

src/session.c

@@ -705,6 +705,8 @@ int session_add_conn(struct session *sess, struct connection *conn)
  */
 int session_check_idle_conn(struct session *sess, struct connection *conn)
 {
+	struct server *srv = objt_server(conn->target);
+
 	/* Connection must be attached to session prior to this function call. */
 	BUG_ON(!conn->owner || conn->owner != sess);
 
@@ -715,7 +717,8 @@ int session_check_idle_conn(struct session *sess, struct connection *conn)
 	/* Ensure conn is not already accounted as idle to prevent sess idle count excess increment. */
 	BUG_ON(conn->flags & CO_FL_SESS_IDLE);
 
-	if (sess->idle_conns >= sess->fe->max_out_conns) {
+	if (sess->idle_conns >= sess->fe->max_out_conns ||
+	    (srv && (srv->cur_admin & SRV_ADMF_MAINT))) {
 		session_unown_conn(sess, conn);
 		conn->owner = NULL;
 		return -1;