resolved: don't redundantly switch DNS servers because of transaction failures

When a transaction fails and we decide to switch DNS servers, don#t do
so unconditionally. Check if the current DNS server is still the same as
when the transaction was initiated. And if not, do not do anything.

That should reduce the number of redundant DNS server switches if many
parallel transactions fail simultaneously (which is pretty likely if
DNSSEC is on).

Fixes: #17040

1 files changed, 6 insertions, 3 deletions

diff --git a/src/resolve/resolved-dns-scope.c b/src/resolve/resolved-dns-scope.c
index 245f9bfce8..c962af9aba 100644
--- a/src/resolve/resolved-dns-scope.c
+++ b/src/resolve/resolved-dns-scope.c
@@ -153,16 +153,19 @@ unsigned dns_scope_get_n_dns_servers(DnsScope *s) {
         return n;
 }
 
-void dns_scope_next_dns_server(DnsScope *s) {
+void dns_scope_next_dns_server(DnsScope *s, DnsServer *if_current) {
         assert(s);
 
         if (s->protocol != DNS_PROTOCOL_DNS)
                 return;
 
+        /* Changes to the next DNS server in the list. If 'if_current' is passed will do so only if the
+         * current DNS server still matches it. */
+
         if (s->link)
-                link_next_dns_server(s->link);
+                link_next_dns_server(s->link, if_current);
         else
-                manager_next_dns_server(s->manager);
+                manager_next_dns_server(s->manager, if_current);
 }
 
 void dns_scope_packet_received(DnsScope *s, usec_t rtt) {