BUG/MINOR: check: only try connection reuse for http-check rulesets

In 3.2, a new server keyword "check-reuse-pool" has been introduced. It
allows to reuse a connection for a new check, instead of always
initializing a new one. This is only performed if the check does not
rely on specific connection parameters differing from the server.

This patch further restricts reuse for checks only when an HTTP ruleset
is used at the backend level. Indeed, reusing a connection outside of
HTTP is an undefined behavior. The impact of this bug is unknown and
depends on the proxy/server configuration. In the case of an HTTP
backend with non-HTTP checks, check-reuse-pool would probably cause a
drop in reuse rate.

Along this change, implement a new diagnostic warning on servers to
report that check-reuse-pool cannot apply due to an incompatible check
type.

This must be backported up to 3.2.

doc/configuration.txt

@@ -17829,11 +17829,12 @@ check-reuse-pool
   This option permits checks to reuse idle connections if available instead of
   opening a dedicated one. The connection is reinserted in the pool on check
   completion. The main objective is to limit the number of connections opening
-  and closure on a specific server.
+  and closure on a specific server. This feature is compatible only with
+  http-check rulesets. It is silently ignored for other check types.
 
-  Note that for configuration simplicity, this option is silently ignored if
+  For configuration simplicity, this option is silently ignored if any specific
-  any specific check connect option is defined, either on the server line or
+  check connect option is defined, either on the server line or via a custom
-  via a custom tcp-check connect rule.
+  tcp-check connect rule.
 
   This option is automatically enabled for servers acting as passive reverse
   HTTP gateway, as for those servers connect is only supported through reuse.

src/cfgdiag.c

@@ -65,6 +65,19 @@ static void srv_diag_cookies(int *ret, struct server *srv, struct eb_root *cooki
 	}
 }
 
+/* Reports a diag if check-reuse-pool is active while backend check ruleset is
+ * non HTTP.
+ */
+static void srv_diag_check_reuse(int *ret, struct server *srv, struct proxy *px)
+{
+	if (srv->do_check && srv->check.reuse_pool) {
+		if ((px->tcpcheck_rules.flags & TCPCHK_RULES_PROTO_CHK) != TCPCHK_RULES_HTTP_CHK) {
+			diag_warning(ret, "parsing [%s:%d] : 'server %s': check-reuse-pool is ineffective for non http-check rulesets.\n",
+			             srv->conf.file, srv->conf.line, srv->id);
+		}
+	}
+}
+
 /* Perform a series of diagnostics on every servers from the configuration. */
 static void run_servers_diag(int *ret)
 {
@@ -74,8 +87,10 @@ static void run_servers_diag(int *ret)
 	struct server *srv;
 
 	for (px = proxies_list; px; px = px->next) {
-		for (srv = px->srv; srv; srv = srv->next)
+		for (srv = px->srv; srv; srv = srv->next) {
 			srv_diag_cookies(ret, srv, &cookies_tree);
+			srv_diag_check_reuse(ret, srv, px);
+		}
 
 		/* clear the cookies tree before passing to the next proxy */
 		while ((cookie_node = ebpt_first(&cookies_tree))) {

src/tcpcheck.c

@@ -1270,9 +1270,12 @@ enum tcpcheck_eval_ret tcpcheck_eval_connect(struct check *check, struct tcpchec
 	struct tcpcheck_rule *next;
 	struct buffer *auto_sni = NULL;
 	int status, port;
+	int check_type;
 
 	TRACE_ENTER(CHK_EV_TCPCHK_CONN, check);
 
+	check_type = check->tcpcheck_rules->flags & TCPCHK_RULES_PROTO_CHK;
+
 	next = get_next_tcpcheck_rule(check->tcpcheck_rules, rule);
 
 	/* current connection already created, check if it is established or not */
@@ -1309,8 +1312,8 @@ enum tcpcheck_eval_ret tcpcheck_eval_connect(struct check *check, struct tcpchec
 		}
 	}
 
-
+	/* For http-check rulesets connection reuse may be used (check-reuse-pool). */
-	if (!(check->state & CHK_ST_AGENT) && check->reuse_pool &&
+	if (check_type == TCPCHK_RULES_HTTP_CHK && check->reuse_pool &&
 	    !tcpcheck_use_nondefault_connect(check, connect) &&
 	    !srv_is_transparent(s)) {
 		struct ist pool_conn_name = IST_NULL;