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, 10 insertions, 4 deletions

diff --git a/src/resolve/resolved-dns-server.h b/src/resolve/resolved-dns-server.h
index 49b550c1ff..19916794ab 100644
--- a/src/resolve/resolved-dns-server.h
+++ b/src/resolve/resolved-dns-server.h
@@ -51,6 +51,8 @@ struct DnsServer {
 
         bool marked:1;
 
+        /* If linked is set, then this server appears in the servers linked list */
+        bool linked:1;
         LIST_FIELDS(DnsServer, servers);
 };
 
@@ -65,18 +67,22 @@ int dns_server_new(
 DnsServer* dns_server_ref(DnsServer *s);
 DnsServer* dns_server_unref(DnsServer *s);
 
+void dns_server_unlink(DnsServer *s);
+
 void dns_server_packet_received(DnsServer *s, usec_t rtt);
 void dns_server_packet_lost(DnsServer *s, usec_t usec);
 
-DnsServer *manager_set_dns_server(Manager *m, DnsServer *s);
-DnsServer *manager_find_dns_server(Manager *m, int family, const union in_addr_union *in_addr);
-DnsServer *manager_get_dns_server(Manager *m);
-void manager_next_dns_server(Manager *m);
+DnsServer *manager_get_first_dns_server(Manager *m, DnsServerType t);
 
 void manager_flush_dns_servers(Manager *m, DnsServerType t);
 void manager_flush_marked_dns_servers(Manager *m, DnsServerType type);
 void manager_mark_dns_servers(Manager *m, DnsServerType type);
 
+DnsServer *manager_set_dns_server(Manager *m, DnsServer *s);
+DnsServer *manager_find_dns_server(Manager *m, DnsServerType t, int family, const union in_addr_union *in_addr);
+DnsServer *manager_get_dns_server(Manager *m);
+void manager_next_dns_server(Manager *m);
+
 DEFINE_TRIVIAL_CLEANUP_FUNC(DnsServer*, dns_server_unref);
 
 extern const struct hash_ops dns_server_hash_ops;