resolver: error from ResolveIPAddresses when no nameserver answered (closes #190)

ResolveIPAddresses now returns an error, not no addresses, when no
nameserver of the name's zone answered. A nameserver with status
timeout or error is not an answer; one answer, even NXDOMAIN, is
enough for an empty result without an error.

When every server of a zone fails, FindAuthoritativeNameservers moves
on to the parent name, whose servers only refer the query onward. Such
a referral now has status error, so it is no answer either, and a
hostname's saved records show it as error. The only caller, the
nameserver address lookup, already keeps the previous addresses on an
error; its comment no longer says the resolver hides this case.

Model: opus-5-5

internal/resolver/iterative_test.go

@@ -5,10 +5,42 @@ import (
 
 	"github.com/miekg/dns"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 
 	"sneak.berlin/go/dnswatcher/internal/resolver"
 )
 
+// TestCollectIPs_OneAnswerIsEnough checks that one nameserver answering
+// NXDOMAIN says the name has no addresses, though the other timed out.
+func TestCollectIPs_OneAnswerIsEnough(t *testing.T) {
+	t.Parallel()
+
+	ips, _, err := resolver.CollectIPs(
+		map[string]*resolver.NameserverResponse{
+			"ns1.example.": {Status: resolver.StatusTimeout},
+			"ns2.example.": {Status: resolver.StatusNXDomain},
+		},
+	)
+	require.NoError(t, err)
+	assert.Empty(t, ips)
+}
+
+// TestCollectIPs_FailedIsNoAnswer checks that nameservers that all have
+// status error, from a refusal, a server failure, a network error or a
+// referral, are no answer rather than a name with no addresses.
+func TestCollectIPs_FailedIsNoAnswer(t *testing.T) {
+	t.Parallel()
+
+	ips, _, err := resolver.CollectIPs(
+		map[string]*resolver.NameserverResponse{
+			"ns1.example.": {Status: resolver.StatusError},
+			"ns2.example.": {Status: resolver.StatusError},
+		},
+	)
+	require.ErrorIs(t, err, resolver.ErrNoNameserverAnswered)
+	assert.Empty(t, ips)
+}
+
 func TestExtractRecordValue_LetterCase(t *testing.T) {
 	t.Parallel()