resolved: don't flush answer RRs on CNAME redirect too early

When doing a CNAME/DNAME redirect let's first check if the answer we
already have fully answers the redirected question already. If so, let's
use that. If not, let's properly restart things.

This simply removes one call to dns_answer_reset() that was placed too
early: instead of resetting when we detect a CNAME/DNAME redirect, do so
only after checking if the answer we already have doesn't match the
reply, and then decide to *actually* follow it. Or in other words: rely
on the dns_answer_reset() call in dns_query_go() which we'll call to
actually begin with the redirected question.

This fixes an optimization path which was broken back in 7820b320eaa608748f66f8105621640cf80e483a.

(This doesn't really matter as much as one might think, since our cache
stepped in anyway and answered the questions before going back to the
network. However, this adds noise if RRs with very short TTLs are cached
– which some CDNs do – and is of course relavant when people turn off
the local cache.)

1 files changed, 4 insertions, 3 deletions

diff --git a/src/resolve/resolved-dns-query.c b/src/resolve/resolved-dns-query.c
index aa9d65d4a8..e4386c402a 100644
--- a/src/resolve/resolved-dns-query.c
+++ b/src/resolve/resolved-dns-query.c
@@ -1019,7 +1019,9 @@ static int dns_query_cname_redirect(DnsQuery *q, const DnsResourceRecord *cname)
         q->question_utf8 = TAKE_PTR(nq_utf8);
 
         dns_query_unref_candidates(q);
-        dns_query_reset_answer(q);
+
+        /* Note that we do *not* reset the answer here, because the answer we previously got might already
+         * include everything we need, let's check that first */
 
         q->state = DNS_TRANSACTION_NULL;
 
@@ -1069,8 +1071,7 @@ int dns_query_process_cname(DnsQuery *q) {
         if (r < 0)
                 return r;
 
-        /* Let's see if the answer can already answer the new
-         * redirected question */
+        /* Let's see if the answer can already answer the new redirected question */
         r = dns_query_process_cname(q);
         if (r != DNS_QUERY_NOMATCH)
                 return r;