resolved: replace transaction list by hashmap

Right now we keep track of ongoing transactions in a linked listed for
each scope. Replace this by a hashmap that is indexed by the RR key.
Given that all ongoing transactions will be placed in pretty much the
same scopes usually this should optimize behaviour.

We used to require a list here, since we wanted to do "superset" query
checks, but this became obsolete since transactions are now single-key
instead of multi-key.

3 files changed, 30 insertions, 23 deletions

diff --git a/src/resolve/resolved-dns-scope.c b/src/resolve/resolved-dns-scope.c
index 57a2c7d6c1..336269ca8a 100644
--- a/src/resolve/resolved-dns-scope.c
+++ b/src/resolve/resolved-dns-scope.c
@@ -78,8 +78,7 @@ DnsScope* dns_scope_free(DnsScope *s) {
 
         dns_scope_llmnr_membership(s, false);
 
-        while ((t = s->transactions)) {
-
+        while ((t = hashmap_steal_first(s->transactions))) {
                 /* Abort the transaction, but make sure it is not
                  * freed while we still look at it */
 
@@ -90,6 +89,8 @@ DnsScope* dns_scope_free(DnsScope *s) {
                 dns_transaction_free(t);
         }
 
+        hashmap_free(s->transactions);
+
         while ((rr = ordered_hashmap_steal_first(s->conflict_queue)))
                 dns_resource_record_unref(rr);
 
@@ -623,24 +624,20 @@ DnsTransaction *dns_scope_find_transaction(DnsScope *scope, DnsResourceKey *key,
         assert(scope);
         assert(key);
 
-        /* Try to find an ongoing transaction that is a equal or a
-         * superset of the specified question */
-
-        LIST_FOREACH(transactions_by_scope, t, scope->transactions) {
-
-                /* Refuse reusing transactions that completed based on
-                 * cached data instead of a real packet, if that's
-                 * requested. */
-                if (!cache_ok &&
-                    IN_SET(t->state, DNS_TRANSACTION_SUCCESS, DNS_TRANSACTION_FAILURE) &&
-                    !t->received)
-                        continue;
+        /* Try to find an ongoing transaction that is a equal to the
+         * specified question */
+        t = hashmap_get(scope->transactions, key);
+        if (!t)
+                return NULL;
 
-                if (dns_resource_key_equal(t->key, key) > 0)
-                        return t;
-        }
+        /* Refuse reusing transactions that completed based on cached
+         * data instead of a real packet, if that's requested. */
+        if (!cache_ok &&
+            IN_SET(t->state, DNS_TRANSACTION_SUCCESS, DNS_TRANSACTION_FAILURE) &&
+            !t->received)
+                return NULL;
 
-        return NULL;
+        return t;
 }
 
 static int dns_scope_make_conflict_packet(
diff --git a/src/resolve/resolved-dns-scope.h b/src/resolve/resolved-dns-scope.h
index 47f285db47..88fb8224e2 100644
--- a/src/resolve/resolved-dns-scope.h
+++ b/src/resolve/resolved-dns-scope.h
@@ -60,7 +60,7 @@ struct DnsScope {
         usec_t resend_timeout;
         usec_t max_rtt;
 
-        LIST_HEAD(DnsTransaction, transactions);
+        Hashmap *transactions;
 
         LIST_FIELDS(DnsScope, scopes);
 };
diff --git a/src/resolve/resolved-dns-transaction.c b/src/resolve/resolved-dns-transaction.c
index 81156dfa45..608decd3ac 100644
--- a/src/resolve/resolved-dns-transaction.c
+++ b/src/resolve/resolved-dns-transaction.c
@@ -35,7 +35,6 @@ DnsTransaction* dns_transaction_free(DnsTransaction *t) {
 
         sd_event_source_unref(t->timeout_event_source);
 
-        dns_resource_key_unref(t->key);
         dns_packet_unref(t->sent);
         dns_packet_unref(t->received);
         dns_answer_unref(t->cached);
@@ -47,12 +46,14 @@ DnsTransaction* dns_transaction_free(DnsTransaction *t) {
         dns_stream_free(t->stream);
 
         if (t->scope) {
-                LIST_REMOVE(transactions_by_scope, t->scope->transactions, t);
+                hashmap_remove(t->scope->transactions, t->key);
 
                 if (t->id != 0)
                         hashmap_remove(t->scope->manager->dns_transactions, UINT_TO_PTR(t->id));
         }
 
+        dns_resource_key_unref(t->key);
+
         while ((q = set_steal_first(t->queries)))
                 set_remove(q->transactions, t);
         set_free(t->queries);
@@ -89,14 +90,18 @@ int dns_transaction_new(DnsTransaction **ret, DnsScope *s, DnsResourceKey *key)
         if (r < 0)
                 return r;
 
+        r = hashmap_ensure_allocated(&s->transactions, &dns_resource_key_hash_ops);
+        if (r < 0)
+                return r;
+
         t = new0(DnsTransaction, 1);
         if (!t)
                 return -ENOMEM;
 
         t->dns_fd = -1;
-
         t->key = dns_resource_key_ref(key);
 
+        /* Find a fresh, unused transaction id */
         do
                 random_bytes(&t->id, sizeof(t->id));
         while (t->id == 0 ||
@@ -108,7 +113,12 @@ int dns_transaction_new(DnsTransaction **ret, DnsScope *s, DnsResourceKey *key)
                 return r;
         }
 
-        LIST_PREPEND(transactions_by_scope, s->transactions, t);
+        r = hashmap_put(s->transactions, t->key, t);
+        if (r < 0) {
+                hashmap_remove(s->manager->dns_transactions, UINT_TO_PTR(t->id));
+                return r;
+        }
+
         t->scope = s;
 
         if (ret)