MEDIUM: mux: prepare for takeover on private connections

When a backend connection is marked as idle, a special flag TASK_F_USR1
is set on MUX tasklet. When MUX tasklet is reactivated, extra checks are
executed under this flag to ensure no takeover occurred in the meantime.

Previously, only non private connections could be targetted by a
takeover. However, this will change when implementing private idle
connections closure on "delete server" CLI handler. As such, TASK_F_USR1
is now also set for private connections in MUX detach callbacks.

src/mux_fcgi.c

@@ -3581,6 +3581,10 @@ static void fcgi_detach(struct sedesc *sd)
 				}
 			}
 			if (eb_is_empty(&fconn->streams_by_id)) {
+				/* mark that the tasklet may lose its context to another thread and
+				 * that the handler needs to check it under the idle conns lock.
+				 */
+				HA_ATOMIC_OR(&fconn->wait_event.tasklet->state, TASK_F_USR1);
 				if (session_check_idle_conn(fconn->conn->owner, fconn->conn) != 0) {
 					/* The connection is destroyed, let's leave */
 					TRACE_DEVEL("outgoing connection killed", FCGI_EV_STRM_END|FCGI_EV_FCONN_ERR);

src/mux_h1.c

@@ -1019,6 +1019,11 @@ static void h1s_finish_detach(struct h1s *h1s)
 				goto end;
 			}
 			/* Always idle at this step */
+
+			/* mark that the tasklet may lose its context to another thread and
+			 * that the handler needs to check it under the idle conns lock.
+			 */
+			HA_ATOMIC_OR(&h1c->wait_event.tasklet->state, TASK_F_USR1);
 			if (session_check_idle_conn(sess, h1c->conn)) {
 				/* The connection got destroyed, let's leave */
 				TRACE_DEVEL("outgoing connection killed", H1_EV_STRM_END|H1_EV_H1C_END);

src/mux_h2.c

@@ -4819,6 +4819,10 @@ static void h2_detach(struct sedesc *sd)
 					}
 				}
 				if (eb_is_empty(&h2c->streams_by_id)) {
+					/* mark that the tasklet may lose its context to another thread and
+					 * that the handler needs to check it under the idle conns lock.
+					 */
+					HA_ATOMIC_OR(&h2c->wait_event.tasklet->state, TASK_F_USR1);
 					if (session_check_idle_conn(h2c->conn->owner, h2c->conn) != 0) {
 						/* At this point either the connection is destroyed, or it's been added to the server idle list, just stop */
 						TRACE_DEVEL("leaving without reusable idle connection", H2_EV_STRM_END);