resolved: implement query coalescing

Implement dns_transaction_make_packet_mdns(), a special version of
dns_transaction_make_packet() for mDNS which differs in many ways:

a) We coalesce queries of currently active transaction on the scope.
   This is possible because mDNS actually allows many questions in a
   to be sent in a single packet and it takes some burden from the
   network.

b) Both A and AAAA query keys are broadcast on both IPv4 and IPv6
   scopes, because other hosts might only respond on one of their
   addresses but resolve both types.

c) We discard previously sent packages (t->sent) so we can start over
   and coalesce pending transactions again.

src/resolve/resolved-dns-transaction.c

@@ -624,38 +624,6 @@ static int on_transaction_timeout(sd_event_source *s, usec_t usec, void *userdat
         return 0;
 }
 
-static int dns_transaction_make_packet(DnsTransaction *t) {
-        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
-        int r;
-
-        assert(t);
-
-        if (t->sent)
-                return 0;
-
-        r = dns_packet_new_query(&p, t->scope->protocol, 0, t->scope->dnssec_mode == DNSSEC_YES);
-        if (r < 0)
-                return r;
-
-        r = dns_scope_good_key(t->scope, t->key);
-        if (r < 0)
-                return r;
-        if (r == 0)
-                return -EDOM;
-
-        r = dns_packet_append_key(p, t->key, NULL);
-        if (r < 0)
-                return r;
-
-        DNS_PACKET_HEADER(p)->qdcount = htobe16(1);
-        DNS_PACKET_HEADER(p)->id = t->id;
-
-        t->sent = p;
-        p = NULL;
-
-        return 0;
-}
-
 static usec_t transaction_get_resend_timeout(DnsTransaction *t) {
         assert(t);
         assert(t->scope);
@@ -762,6 +730,130 @@ static int dns_transaction_prepare_next_attempt(DnsTransaction *t, usec_t ts) {
         return 1;
 }
 
+static int dns_transaction_make_packet_mdns(DnsTransaction *t) {
+
+        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
+        DnsTransaction *other;
+        unsigned qdcount;
+        usec_t ts;
+        int r;
+
+        assert(t);
+        assert(t->scope->protocol == DNS_PROTOCOL_MDNS);
+
+        /* Discard any previously prepared packet, so we can start over and coaleasce again */
+        t->sent = dns_packet_unref(t->sent);
+
+        r = dns_packet_new_query(&p, t->scope->protocol, 0, false);
+        if (r < 0)
+                return r;
+
+        r = dns_packet_append_key(p, t->key, NULL);
+        if (r < 0)
+                return r;
+
+        qdcount = 1;
+
+        /*
+         * For mDNS, we want to coalesce as many open queries in pending transactions into one single
+         * query packet on the wire as possible. To achieve that, we iterate through all pending transactions
+         * in our current scope, and see whether their timing contraints allow them to be sent.
+         */
+
+        assert_se(sd_event_now(t->scope->manager->event, clock_boottime_or_monotonic(), &ts) >= 0);
+
+        LIST_FOREACH(transactions_by_scope, other, t->scope->transactions) {
+
+                /* Skip ourselves */
+                if (other == t)
+                        continue;
+
+                if (other->state != DNS_TRANSACTION_PENDING)
+                        continue;
+
+                if (other->next_attempt_after > ts)
+                        continue;
+
+                if (qdcount >= UINT16_MAX)
+                        break;
+
+                r = dns_packet_append_key(p, other->key, NULL);
+
+                /*
+                 * If we can't stuff more questions into the packet, just give up.
+                 * One of the 'other' transactions will fire later and take care of the rest.
+                 */
+                if (r == -EMSGSIZE)
+                        break;
+
+                if (r < 0)
+                        return r;
+
+                r = dns_transaction_prepare_next_attempt(other, ts);
+                if (r <= 0)
+                        continue;
+
+                ts += transaction_get_resend_timeout(other);
+
+                r = sd_event_add_time(
+                                other->scope->manager->event,
+                                &other->timeout_event_source,
+                                clock_boottime_or_monotonic(),
+                                ts, 0,
+                                on_transaction_timeout, other);
+                if (r < 0)
+                        return r;
+
+                other->state = DNS_TRANSACTION_PENDING;
+                other->next_attempt_after = ts;
+
+                qdcount ++;
+        }
+
+        DNS_PACKET_HEADER(p)->qdcount = htobe16(qdcount);
+        DNS_PACKET_HEADER(p)->id = t->id;
+
+        t->sent = p;
+        p = NULL;
+
+        return 0;
+}
+
+static int dns_transaction_make_packet(DnsTransaction *t) {
+        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
+        int r;
+
+        assert(t);
+
+        if (t->scope->protocol == DNS_PROTOCOL_MDNS)
+                return dns_transaction_make_packet_mdns(t);
+
+        if (t->sent)
+                return 0;
+
+        r = dns_packet_new_query(&p, t->scope->protocol, 0, t->scope->dnssec_mode == DNSSEC_YES);
+        if (r < 0)
+                return r;
+
+        r = dns_scope_good_key(t->scope, t->key);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return -EDOM;
+
+        r = dns_packet_append_key(p, t->key, NULL);
+        if (r < 0)
+                return r;
+
+        DNS_PACKET_HEADER(p)->qdcount = htobe16(1);
+        DNS_PACKET_HEADER(p)->id = t->id;
+
+        t->sent = p;
+        p = NULL;
+
+        return 0;
+}
+
 int dns_transaction_go(DnsTransaction *t) {
         usec_t ts;
         int r;