resolver tests: retry an answer missing the record type read (closes #218)

QueryNameserver sends one query per record type. When only the AAAA or
MX query was lost, the answer still had status ok, liveQueryNameserver
did not retry it, and the test found no AAAA or MX records.
liveQueryNameserver now takes the record types a test reads and, through
livednstest, retries an answer that holds records of none of them. The
A, AAAA, MX and TXT tests name theirs. A resolver that loses a type for
good still fails, after the last attempt instead of the first.

Model: opus-5-5

internal/resolver/livedns_test.go

@@ -226,11 +226,17 @@ func liveLookupNS(
 // liveQueryNameserver queries one nameserver, retrying while that
 // nameserver fails to answer. NXDOMAIN and NODATA are answers and
 // are returned to the caller to assert on.
+//
+// QueryNameserver sends one query per record type, so one lost query
+// leaves its type out of an answer that is otherwise fine. A test names
+// in types the record types it reads; an answer holding records of none
+// of them is retried too.
 func liveQueryNameserver(
 	t *testing.T,
 	r *resolver.Resolver,
 	nameserver string,
 	hostname string,
+	types ...string,
 ) *resolver.NameserverResponse {
 	t.Helper()
 
@@ -260,6 +266,18 @@ func liveQueryNameserver(
 				)
 			}
 
+			hasRecords := func(recordType string) bool {
+				return len(resp.Records[recordType]) > 0
+			}
+
+			if len(types) > 0 && !slices.ContainsFunc(types, hasRecords) {
+				return fmt.Errorf(
+					"%w: %s returned no %s records",
+					livednstest.ErrNoAnswer, nameserver,
+					strings.Join(types, " or "),
+				)
+			}
+
 			out = resp
 
 			return nil

internal/resolver/resolver_test.go

@@ -171,7 +171,7 @@ func TestQueryNameserver_BasicA(t *testing.T) {
 
 	r := newTestResolver(t)
 	ns := findOneNSForDomain(t, r, "google.com")
-	resp := liveQueryNameserver(t, r, ns, "www.google.com")
+	resp := liveQueryNameserver(t, r, ns, "www.google.com", "A", "CNAME")
 
 	require.NotNil(t, resp)
 
@@ -190,7 +190,7 @@ func TestQueryNameserver_AAAA(t *testing.T) {
 
 	r := newTestResolver(t)
 	ns := findOneNSForDomain(t, r, "cloudflare.com")
-	resp := liveQueryNameserver(t, r, ns, "cloudflare.com")
+	resp := liveQueryNameserver(t, r, ns, "cloudflare.com", "AAAA")
 
 	aaaaRecords := resp.Records["AAAA"]
 	require.NotEmpty(t, aaaaRecords,
@@ -210,7 +210,7 @@ func TestQueryNameserver_MX(t *testing.T) {
 
 	r := newTestResolver(t)
 	ns := findOneNSForDomain(t, r, "google.com")
-	resp := liveQueryNameserver(t, r, ns, "google.com")
+	resp := liveQueryNameserver(t, r, ns, "google.com", "MX")
 
 	mxRecords := resp.Records["MX"]
 	require.NotEmpty(t, mxRecords,
@@ -223,7 +223,7 @@ func TestQueryNameserver_TXT(t *testing.T) {
 
 	r := newTestResolver(t)
 	ns := findOneNSForDomain(t, r, "google.com")
-	resp := liveQueryNameserver(t, r, ns, "google.com")
+	resp := liveQueryNameserver(t, r, ns, "google.com", "TXT")
 
 	txtRecords := resp.Records["TXT"]
 	require.NotEmpty(t, txtRecords,