MEDIUM: resolvers: split resolving and dns message exchange layers.

This patch splits recv and send functions in two layers. the
lowest is responsible of DNS message transactions over
the network. Doing this we could use DNS message layer
for something else than resolving. Load balancing for instance.

This patch also re-works the way to init a nameserver and
introduce the new struct dns_dgram_server to prepare the arrival
of dns_stream_server and the support of DNS over TCP.

The way to retry a send failure of a request because of EAGAIN
was re-worked. Previously there was no control and all "pending"
queries were re-played each time it reaches a EAGAIN. This
patch introduce a ring to stack messages in case of sent
failure. This patch is emptied if poller shows that the
socket is ready again to push messages.

Author: EmericBr

include/haproxy/dns-t.h

@@ -95,6 +95,9 @@ extern struct pool_head *resolv_requester_pool;
 /* DNS header size */
 #define DNS_HEADER_SIZE  ((int)sizeof(struct dns_header))
 
+#define DNS_TCP_MSG_MAX_SIZE 65535
+#define DNS_TCP_MSG_RING_MAX_SIZE (1 + 1 + 3 + DNS_TCP_MSG_MAX_SIZE) // varint_bytes(DNS_TCP_MSG_MAX_SIZE) == 3
+
 /* DNS request or response header structure */
 struct dns_header {
 	uint16_t id;
@@ -196,6 +199,12 @@ struct resolvers {
 	} conf;                             /* config information */
 };
 
+struct dns_dgram_server {
+	struct dgram_conn conn;  /* transport layer */
+	struct ring *ring_req;
+	size_t ofs_req;           // ring buffer reader offset
+};
+
 /* Structure describing a name server used during name resolution.
  * A name server belongs to a resolvers section.
  */
@@ -207,8 +216,8 @@ struct dns_nameserver {
 		int         line;       /* line where the section appears */
 	} conf;                         /* config information */
 
-	struct dgram_conn      *dgram;  /* transport layer */
-	struct sockaddr_storage addr;   /* IP address */
+	int (*process_responses)(struct dns_nameserver *ns); /* callback used to process responses */
+	struct dns_dgram_server *dgram;  /* used for dgram dns */
 
 	EXTRA_COUNTERS(extra_counters);
 	struct dns_counters *counters;

include/haproxy/dns.h

@@ -53,5 +53,6 @@ int stats_dump_resolvers(struct stream_interface *si,
                          struct list *stat_modules);
 void resolv_stats_clear_counters(int clrall, struct list *stat_modules);
 int resolv_allocate_counters(struct list *stat_modules);
+int dns_dgram_init(struct dns_nameserver *ns, struct sockaddr_storage *sk);
 
 #endif // _HAPROXY_DNS_H