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/resolver/iterative_test.go

@@ -43,6 +43,25 @@ func TestCollectIPs_FailedIsNoAnswer(t *testing.T) {
 	assert.Empty(t, ips)
 }
 
+// TestCollectIPs_FailedTypeIsNoAnswer checks that a nameserver whose
+// query for one of the types failed is no answer: its addresses are
+// only part of them.
+func TestCollectIPs_FailedTypeIsNoAnswer(t *testing.T) {
+	t.Parallel()
+
+	ips, _, err := resolver.CollectIPs(
+		map[string]*resolver.NameserverResponse{
+			nsExample1: {
+				Records:     map[string][]string{"A": {"192.0.2.1"}},
+				FailedTypes: []string{"AAAA"},
+				Status:      resolver.StatusOK,
+			},
+		},
+	)
+	require.ErrorIs(t, err, resolver.ErrNoNameserverAnswered)
+	assert.Empty(t, ips)
+}
+
 const (
 	// exampleCom is the zone most cases of TestUsableReply and
 	// TestNSSetFrom are about, and wwwExampleCom a name in it.