resolved: try to reduce number or DnsResourceKeys we keep around by merging them

Quite often we read the same RR key multiple times from the same message. Try to replace them by a single object when
we notice this. Do so again when we add things to the cache.

This should reduce memory consumption a tiny bit.

4 files changed, 61 insertions, 1 deletions

diff --git a/src/resolve/resolved-dns-cache.c b/src/resolve/resolved-dns-cache.c
index 301f383809..fdb34d11df 100644
--- a/src/resolve/resolved-dns-cache.c
+++ b/src/resolve/resolved-dns-cache.c
@@ -247,6 +247,19 @@ static int dns_cache_link_item(DnsCache *c, DnsCacheItem *i) {
 
         first = hashmap_get(c->by_key, i->key);
         if (first) {
+                _cleanup_(dns_resource_key_unrefp) DnsResourceKey *k = NULL;
+
+                /* Keep a reference to the original key, while we manipulate the list. */
+                k = dns_resource_key_ref(first->key);
+
+                /* Now, try to reduce the number of keys we keep */
+                dns_resource_key_reduce(&first->key, &i->key);
+
+                if (first->rr)
+                        dns_resource_key_reduce(&first->rr->key, &i->key);
+                if (i->rr)
+                        dns_resource_key_reduce(&i->rr->key, &i->key);
+
                 LIST_PREPEND(by_key, first, i);
                 assert_se(hashmap_replace(c->by_key, first->key, first) >= 0);
         } else {
diff --git a/src/resolve/resolved-dns-packet.c b/src/resolve/resolved-dns-packet.c
index 57cfc1ab9b..0acbcfe261 100644
--- a/src/resolve/resolved-dns-packet.c
+++ b/src/resolve/resolved-dns-packet.c
@@ -2106,6 +2106,7 @@ int dns_packet_extract(DnsPacket *p) {
 
         n = DNS_PACKET_RRCOUNT(p);
         if (n > 0) {
+                DnsResourceRecord *previous = NULL;
                 bool bad_opt = false;
 
                 answer = dns_answer_new(n);
@@ -2122,6 +2123,10 @@ int dns_packet_extract(DnsPacket *p) {
                         if (r < 0)
                                 goto finish;
 
+                        /* Try to reduce memory usage a bit */
+                        if (previous)
+                                dns_resource_key_reduce(&rr->key, &previous->key);
+
                         if (rr->key->type == DNS_TYPE_OPT) {
                                 bool has_rfc6975;
 
diff --git a/src/resolve/resolved-dns-rr.c b/src/resolve/resolved-dns-rr.c
index 02c6b239d5..7273ef3825 100644
--- a/src/resolve/resolved-dns-rr.c
+++ b/src/resolve/resolved-dns-rr.c
@@ -334,6 +334,46 @@ int dns_resource_key_to_string(const DnsResourceKey *key, char **ret) {
         return 0;
 }
 
+bool dns_resource_key_reduce(DnsResourceKey **a, DnsResourceKey **b) {
+        assert(a);
+        assert(b);
+
+        /* Try to replace one RR key by another if they are identical, thus saving a bit of memory. Note that we do
+         * this only for RR keys, not for RRs themselves, as they carry a lot of additional metadata (where they come
+         * from, validity data, and suchlike), and cannot be replaced so easily by other RRs that have the same
+         * superficial data. */
+
+        if (!*a)
+                return false;
+        if (!*b)
+                return false;
+
+        /* We refuse merging const keys */
+        if ((*a)->n_ref == (unsigned) -1)
+                return false;
+        if ((*b)->n_ref == (unsigned) -1)
+                return false;
+
+        /* Already the same? */
+        if (*a == *b)
+                return true;
+
+        /* Are they really identical? */
+        if (dns_resource_key_equal(*a, *b) <= 0)
+                return false;
+
+        /* Keep the one which already has more references. */
+        if ((*a)->n_ref > (*b)->n_ref) {
+                dns_resource_key_unref(*b);
+                *b = dns_resource_key_ref(*a);
+        } else {
+                dns_resource_key_unref(*a);
+                *a = dns_resource_key_ref(*b);
+        }
+
+        return true;
+}
+
 DnsResourceRecord* dns_resource_record_new(DnsResourceKey *key) {
         DnsResourceRecord *rr;
 
diff --git a/src/resolve/resolved-dns-rr.h b/src/resolve/resolved-dns-rr.h
index 8ab53211a9..d9c31e81c5 100644
--- a/src/resolve/resolved-dns-rr.h
+++ b/src/resolve/resolved-dns-rr.h
@@ -81,7 +81,7 @@ enum {
 };
 
 struct DnsResourceKey {
-        unsigned n_ref;
+        unsigned n_ref; /* (unsigned -1) for const keys, see below */
         uint16_t class, type;
         char *_name; /* don't access directy, use DNS_RESOURCE_KEY_NAME()! */
 };
@@ -294,6 +294,8 @@ static inline bool dns_key_is_shared(const DnsResourceKey *key) {
         return IN_SET(key->type, DNS_TYPE_PTR);
 }
 
+bool dns_resource_key_reduce(DnsResourceKey **a, DnsResourceKey **b);
+
 DnsResourceRecord* dns_resource_record_new(DnsResourceKey *key);
 DnsResourceRecord* dns_resource_record_new_full(uint16_t class, uint16_t type, const char *name);
 DnsResourceRecord* dns_resource_record_ref(DnsResourceRecord *rr);