resolver, watcher: a record type whose query fails keeps its previous records (closes #231)

The resolver lists in FailedTypes each record type whose query to a
nameserver got no usable reply (none after two tries, an error reply, a
referral, or a truncated reply whose TCP retry failed), and logs it with
the reason unless shutdown cut it short. A nameserver that answered no
type has failed, as before.
The watcher saves such a type in failedTypes, keeping the previous
check's records, leaves it out of the comparison with other nameservers
on that check, and compares the kept records with the next answer. With
nothing to keep, it is also in unknownTypes and not compared until it
answers. Change messages leave out what was not compared. A nameserver
whose A, AAAA or CNAME query failed is no answer when following a CNAME
or resolving addresses.

Model: opus-5-5

internal/watcher/cname_test.go

@@ -202,6 +202,42 @@ func TestCNAMEWhoseNameserversAllFailedKeepsPrevious(t *testing.T) {
 	}
 }
 
+// TestCNAMEWhoseAddressQueryFailedKeepsPrevious checks a name whose
+// nameserver answered, but whose query for A, AAAA or CNAME failed with
+// nothing kept for it. That is not an answer with no address: the
+// addresses the previous check saved from following its CNAME are kept,
+// and nothing is looked up, the watcher having no resolver.
+func TestCNAMEWhoseAddressQueryFailedKeepsPrevious(t *testing.T) {
+	t.Parallel()
+
+	for _, rtype := range []string{"A", "AAAA", "CNAME"} {
+		t.Run(rtype, func(t *testing.T) {
+			t.Parallel()
+
+			w := watcher.NewForTest(nil, nil, nil, nil, nil, nil)
+
+			current := saved(map[string]*state.NameserverRecordState{
+				nsA: {
+					Records:      map[string][]string{},
+					FailedTypes:  []string{rtype},
+					UnknownTypes: []string{rtype},
+					Status:       "ok",
+				},
+			})
+			prev := cnameState(oldIP)
+
+			w.ResolveCNAMEAddresses(t.Context(), host, current, prev)
+
+			if !slices.Equal(current.CNAMEAddresses, prev.CNAMEAddresses) {
+				t.Errorf(
+					"saved %v, want %v",
+					current.CNAMEAddresses, prev.CNAMEAddresses,
+				)
+			}
+		})
+	}
+}
+
 // cnameTo builds the records of a nameserver that answered with a CNAME
 // to target and no address.
 func cnameTo(target string) map[string][]string {