MINOR: server: add server_get_next_id() to find next free server ID

This was previously achieved via the generic get_next_id() but we'll soon
get rid of generic ID trees so let's have a dedicated server_get_next_id().
As a bonus it reduces the exposure of the tree's root outside of the functions.

include/haproxy/server.h

@@ -66,6 +66,7 @@ struct server *server_find_by_addr(struct proxy *px, const char *addr);
 struct server *server_find(struct proxy *bk, const char *name);
 struct server *server_find_unique(struct proxy *bk, const char *name, uint32_t rid);
 struct server *server_find_best_match(struct proxy *bk, char *name, int id, int *diff);
+uint server_get_next_id(const struct proxy *px, uint from);
 void apply_server_state(void);
 void srv_compute_all_admin_states(struct proxy *px);
 int srv_set_addr_via_libc(struct server *srv, int *err_code);

src/cfgparse.c

@@ -3726,7 +3726,7 @@ out_uri_auth_compat:
 				/* server ID not set, use automatic numbering with first
 				 * spare entry starting with next_svid.
 				 */
-				next_id = get_next_id(&curproxy->conf.used_server_id, next_id);
+				next_id = server_get_next_id(curproxy, next_id);
 				newsrv->conf.id.key = newsrv->puid = next_id;
 				eb32_insert(&curproxy->conf.used_server_id, &newsrv->conf.id);
 			}

src/server.c

@@ -866,6 +866,23 @@ static const char *srv_find_best_kw(const char *word)
 	return best_ptr;
 }
 
+/* This function returns the first unused server ID greater than or equal to
+ * <from> in the proxy <px>. Zero is returned if no spare one is found (should
+ * never happen).
+ */
+uint server_get_next_id(const struct proxy *px, uint from)
+{
+	const struct eb32_node *used;
+
+	do {
+		used = eb32_lookup_ge((struct eb_root *)&px->conf.used_server_id, from);
+		if (!used || used->key > from)
+			return from; /* available */
+		from++;
+	} while (from);
+	return from;
+}
+
 /* Parse the "backup" server keyword */
 static int srv_parse_backup(char **args, int *cur_arg,
                             struct proxy *curproxy, struct server *newsrv, char **err)
@@ -6228,7 +6245,7 @@ static int cli_parse_add_server(char **args, char *payload, struct appctx *appct
 
 	/* generate the server id if not manually specified */
 	if (!srv->puid) {
-		next_id = get_next_id(&be->conf.used_server_id, 1);
+		next_id = server_get_next_id(be, 1);
 		if (!next_id) {
 			ha_alert("Cannot attach server : no id left in proxy\n");
 			goto out;