resolved: rework dns server lifecycle logic

Previously, there was a chance of memory corruption, because when
switching to the next DNS server we didn't care whether they linked list
of DNS servers was still valid.

Clean up lifecycle of the dns server logic:

- When a DnsServer object is still in the linked list of DnsServers for
  a link or the manager, indicate so with a "linked" boolean field, and
  never follow the linked list if that boolean is not set.

- When picking a DnsServer to use for a link ot manager, always
  explicitly take a reference.

This also rearranges some logic, to make the tracking of dns servers by
link and globally more alike.

1 files changed, 2 insertions, 7 deletions

diff --git a/src/resolve/resolved-conf.c b/src/resolve/resolved-conf.c
index 8b49d46a59..fe88006fb7 100644
--- a/src/resolve/resolved-conf.c
+++ b/src/resolve/resolved-conf.c
@@ -29,8 +29,8 @@
 
 int manager_add_dns_server_by_string(Manager *m, DnsServerType type, const char *word) {
         union in_addr_union address;
-        DnsServer *first, *s;
         int family, r;
+        DnsServer *s;
 
         assert(m);
         assert(word);
@@ -39,13 +39,8 @@ int manager_add_dns_server_by_string(Manager *m, DnsServerType type, const char
         if (r < 0)
                 return r;
 
-        first = type == DNS_SERVER_FALLBACK ? m->fallback_dns_servers : m->dns_servers;
-
         /* Filter out duplicates */
-        LIST_FOREACH(servers, s, first)
-                if (s->family == family && in_addr_equal(family, &s->address, &address))
-                        break;
-
+        s = manager_find_dns_server(m, type, family, &address);
         if (s) {
                 /*
                  * Drop the marker. This is used to find the servers