BUG/MAJOR: server: do not delete srv referenced by session

A server can only be deleted if there is no elements which reference it.
This is taken care via srv_check_for_deletion(), most notably for active
and idle connections.

A special case occurs for connections directly managed by a session.
This is for so-called private connections, when using http-reuse never
or H2 + http-reuse safe for example. In this case. server does not
account these connections into its idle lists. This caused a bug as the
server is deleted despite the session still being able to access it.

To properly fix this, add a new referencing element into the server for
these session connections. A mt_list has been chosen for this. On
default http-reuse, private connections are typically not used so it
won't make any difference. If using H2 servers, or more generally when
dealing with private connections, insert/delete should typically occur
only once per session lifetime so impact on performance should be
minimal.

This should be backported up to 2.4. Note that srv_check_for_deletion()
was introduced in 3.0 dev tree. On backport, the extra condition in it
should be placed in cli_parse_delete_server() instead.

include/haproxy/server-t.h

@@ -338,6 +338,7 @@ struct server {
 	unsigned int est_need_conns;            /* Estimate on the number of needed connections (max of curr and previous max_used) */
 
 	struct queue queue;			/* pending connections */
+	struct mt_list sess_conns;		/* list of private conns managed by a session on this server */
 
 	/* Element below are usd by LB algorithms and must be doable in
 	 * parallel to other threads reusing connections above.

include/haproxy/session-t.h

@@ -62,11 +62,16 @@ struct session {
 	struct sockaddr_storage *dst; /* destination address (pool), when known, otherwise NULL */
 };
 
-/* List of private conns managed by a session, indexed by server */
+/*
+ * List of private conns managed by a session, indexed by server
+ * Stored both into the session and server instances
+ */
 struct sess_priv_conns {
 	void *target;                   /* Server or dispatch used for indexing */
 	struct list conn_list;          /* Head of the connections list */
+
 	struct list sess_el;            /* Element of session.priv_conns */
+	struct mt_list srv_el;          /* Element of server.sess_conns */
 };
 
 #endif /* _HAPROXY_SESSION_T_H */

include/haproxy/session.h

@@ -161,6 +161,7 @@ static inline void session_unown_conn(struct session *sess, struct connection *c
 		if (pconns->target == conn->target) {
 			if (LIST_ISEMPTY(&pconns->conn_list)) {
 				LIST_DELETE(&pconns->sess_el);
+				MT_LIST_DELETE(&pconns->srv_el);
 				pool_free(pool_head_sess_priv_conns, pconns);
 			}
 			break;
@@ -176,6 +177,7 @@ static inline void session_unown_conn(struct session *sess, struct connection *c
 static inline int session_add_conn(struct session *sess, struct connection *conn, void *target)
 {
 	struct sess_priv_conns *pconns = NULL;
+	struct server *srv = objt_server(conn->target);
 	int found = 0;
 
 	BUG_ON(objt_listener(conn->target));
@@ -198,6 +200,10 @@ static inline int session_add_conn(struct session *sess, struct connection *conn
 		pconns->target = target;
 		LIST_INIT(&pconns->conn_list);
 		LIST_APPEND(&sess->priv_conns, &pconns->sess_el);
+
+		MT_LIST_INIT(&pconns->srv_el);
+		if (srv)
+			MT_LIST_APPEND(&srv->sess_conns, &pconns->srv_el);
 	}
 	LIST_APPEND(&pconns->conn_list, &conn->sess_el);
 	return 1;

src/server.c

@@ -2819,6 +2819,8 @@ struct server *new_server(struct proxy *proxy)
 	srv->agent.proxy = proxy;
 	srv->xprt  = srv->check.xprt = srv->agent.xprt = xprt_get(XPRT_RAW);
 
+	MT_LIST_INIT(&srv->sess_conns);
+
 	srv->extra_counters = NULL;
 #ifdef USE_OPENSSL
 	HA_RWLOCK_INIT(&srv->ssl_ctx.lock);
@@ -5832,6 +5834,7 @@ int srv_check_for_deletion(const char *bename, const char *svname, struct proxy
 	 * cleanup function should be implemented to be used here.
 	 */
 	if (srv->curr_used_conns || srv->curr_idle_conns ||
+	    !MT_LIST_ISEMPTY(&srv->sess_conns) ||
 	    !eb_is_empty(&srv->queue.head) || srv_has_streams(srv)) {
 		msg = "Server still has connections attached to it, cannot remove it.";
 		goto leave;

src/session.c

@@ -102,6 +102,7 @@ void session_free(struct session *sess)
 				conn_free(conn);
 			}
 		}
+		MT_LIST_DELETE(&pconns->srv_el);
 		pool_free(pool_head_sess_priv_conns, pconns);
 	}
 	sockaddr_free(&sess->src);