resolved: fix notification iteration logic when transactions are completed

When a transaction is complete, and we notify its owners, make sure we deal
correctly with the requesters removing themselves from the list of owners while
we continue iterating.

This was previously already dealt with with transactions that require other
transactions for DNSSEC purposes, fix this for other possibly transaction
owners too now.

Since iterating through "Set" objects is not safe regarding removal of entries
from it, rework the logic to use two Sets, and move each entry we notified from
one set to the other set before we dispatch the notification. This move operation
requires no additional memory, and enables us to ensure that we don't notify
any object twice.

Fixes: #2676

1 files changed, 5 insertions, 0 deletions

diff --git a/src/resolve/resolved-dns-query.c b/src/resolve/resolved-dns-query.c
index a7496aa586..dd133e1097 100644
--- a/src/resolve/resolved-dns-query.c
+++ b/src/resolve/resolved-dns-query.c
@@ -62,6 +62,7 @@ static void dns_query_candidate_stop(DnsQueryCandidate *c) {
 
         while ((t = set_steal_first(c->transactions))) {
                 set_remove(t->notify_query_candidates, c);
+                set_remove(t->notify_query_candidates_done, c);
                 dns_transaction_gc(t);
         }
 }
@@ -139,6 +140,10 @@ static int dns_query_candidate_add_transaction(DnsQueryCandidate *c, DnsResource
         if (r < 0)
                 goto gc;
 
+        r = set_ensure_allocated(&t->notify_query_candidates_done, NULL);
+        if (r < 0)
+                goto gc;
+
         r = set_put(t->notify_query_candidates, c);
         if (r < 0)
                 goto gc;