MINOR: listener: add listener_get_next_id() to find next free listener 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 listener_get_next_id().
As a bonus it reduces the exposure of the tree's root outside of the functions.

include/haproxy/listener.h

@@ -82,6 +82,12 @@ int relax_listener(struct listener *l, int lpx, int lli);
  */
 void stop_listener(struct listener *l, int lpx, int lpr, int lli);
 
+/* This function returns the first unused listener ID greater than or equal to
+ * <from> in the proxy <px>. Zero is returned if no spare one is found (should
+ * never happen).
+ */
+uint listener_get_next_id(const struct proxy *px, uint from);
+
 /* This function adds the specified listener's file descriptor to the polling
  * lists if it is in the LI_LISTEN state. The listener enters LI_READY or
  * LI_FULL state depending on its number of connections. In daemon mode, we

src/cfgparse.c

@@ -4290,7 +4290,7 @@ init_proxies_list_stage2:
 					if (prev_li->luid)
 						next_id = prev_li->luid + 1;
 				}
-				next_id = get_next_id(&curproxy->conf.used_listener_id, next_id);
+				next_id = listener_get_next_id(curproxy, next_id);
 				listener->conf.id.key = listener->luid = next_id;
 				eb32_insert(&curproxy->conf.used_listener_id, &listener->conf.id);
 			}

src/listener.c

@@ -399,6 +399,23 @@ void stop_listener(struct listener *l, int lpx, int lpr, int lli)
 		HA_RWLOCK_WRUNLOCK(PROXY_LOCK, &px->lock);
 }
 
+/* This function returns the first unused listener ID greater than or equal to
+ * <from> in the proxy <px>. Zero is returned if no spare one is found (should
+ * never happen).
+ */
+uint listener_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_listener_id, from);
+		if (!used || used->key > from)
+			return from; /* available */
+		from++;
+	} while (from);
+	return from;
+}
+
 /* This function adds the specified <listener> to the protocol <proto>. It
  * does nothing if the protocol was already added. The listener's state is
  * automatically updated from LI_INIT to LI_ASSIGNED. The number of listeners