MINOR: resolvers: add unique numeric id to nameservers

When we want to avoid keeping pointers on a nameserver struct, it's not
always convenient to refer as a nameserver using it's text-based unique
identifier since it's not limited in length thus it cannot be serialized
and deserialized safely.

To address this limitation, we add a new ->puid member in dns_nameserver
struct which is a parent-unique numeric value that can be used to refer
to the dns nameserver within its parent resolver context.

To achieve this, we reused the resolver->nb_nameserver member that wasn't
used. Each time we add a new nameserver to a resolver: we set ns->puid to
the current number of nameservers within the resolver and we increment
this number right away.

Public helper function find_nameserver_by_resolvers_and_id() was added to
help retrieve nameserver pointer from (resolver X nameserver puid)
combination.

include/haproxy/dns-t.h

@@ -136,6 +136,7 @@ struct dns_session {
 struct dns_nameserver {
 	char *id;                       /* nameserver unique identifier */
 	void *parent;
+	unsigned int puid;              /* parent-unique numeric id */
 	struct {
 		const char *file;       /* file where the section appears */
 		int         line;       /* line where the section appears */

include/haproxy/resolvers-t.h

@@ -134,7 +134,7 @@ struct resolv_response {
 struct resolvers {
 	__decl_thread(HA_SPINLOCK_T lock);
 	unsigned int accepted_payload_size; /* maximum payload size we accept for responses */
-	int          nb_nameservers;        /* total number of active nameservers in a resolvers section */
+	int          nb_nameservers;        /* total number of nameservers in a resolvers section */
 	int          resolve_retries;       /* number of retries before giving up */
 	struct {                            /* time to: */
 		int resolve;                /*     wait between 2 queries for the same resolution */

include/haproxy/resolvers.h

@@ -34,6 +34,7 @@ extern struct list sec_resolvers;
 extern unsigned int resolv_failed_resolutions;
 
 struct resolvers *find_resolvers_by_id(const char *id);
+struct dns_nameserver *find_nameserver_by_resolvers_and_id(struct resolvers *parent, unsigned int id);
 struct resolv_srvrq *find_srvrq_by_name(const char *name, struct proxy *px);
 struct resolv_srvrq *new_resolv_srvrq(struct server *srv, char *fqdn);
 struct resolv_answer_item *find_srvrq_answer_record(const struct resolv_requester *requester);

src/resolvers.c

@@ -170,6 +170,20 @@ struct resolvers *find_resolvers_by_id(const char *id)
 	return NULL;
 }
 
+/* Returns a pointer to the nameserver matching numerical <id> within <parent>
+ * resolver section. NULL is returned if no match is found.
+ */
+struct dns_nameserver *find_nameserver_by_resolvers_and_id(struct resolvers *parent, unsigned int id)
+{
+	struct dns_nameserver *ns;
+
+	list_for_each_entry(ns, &parent->nameservers, list) {
+		if (ns->puid == id)
+			return ns;
+	}
+	return NULL;
+}
+
 /* Returns a pointer on the SRV request matching the name <name> for the proxy
  * <px>. NULL is returned if no match is found.
  */
@@ -3371,7 +3385,9 @@ static int parse_resolve_conf(char **errmsg, char **warnmsg)
 		newnameserver->parent = curr_resolvers;
 		newnameserver->process_responses = resolv_process_responses;
 		newnameserver->conf.line = resolv_linenum;
+		newnameserver->puid = curr_resolvers->nb_nameservers;
 		LIST_APPEND(&curr_resolvers->nameservers, &newnameserver->list);
+		curr_resolvers->nb_nameservers++;
 	}
 
 resolv_out:
@@ -3428,6 +3444,7 @@ static int resolvers_new(struct resolvers **resolvers, const char *id, const cha
 	r->timeout.resolve = 1000;
 	r->timeout.retry   = 1000;
 	r->resolve_retries = 3;
+	r->nb_nameservers = 0;
 	LIST_INIT(&r->nameservers);
 	LIST_INIT(&r->resolutions.curr);
 	LIST_INIT(&r->resolutions.wait);
@@ -3572,8 +3589,10 @@ int cfg_parse_resolvers(const char *file, int linenum, char **args, int kwm)
 		newnameserver->parent = curr_resolvers;
 		newnameserver->process_responses = resolv_process_responses;
 		newnameserver->conf.line = linenum;
+		newnameserver->puid = curr_resolvers->nb_nameservers;
 		/* the nameservers are linked backward first */
 		LIST_APPEND(&curr_resolvers->nameservers, &newnameserver->list);
+		curr_resolvers->nb_nameservers++;
 	}
 	else if (strcmp(args[0], "parse-resolv-conf") == 0) {
 		err_code |= parse_resolve_conf(&errmsg, &warnmsg);