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 (no reply, a code other than NOERROR or
NXDOMAIN, a referral, or a truncated reply whose TCP retry failed) and
logs it unless shutdown cut it short. A nameserver that answered no
type has failed.
The watcher saves such a type in failedTypes with the previous check's
records, leaves it out of the comparison with other nameservers on that
check, and compares it with the next answer. When the previous check
did not know its records either, it is also in unknownTypes and not
compared until it answers. 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 {