plugin/cache: add minttl test (#1141)

See https://github.com/kubernetes/kubernetes/pull/53604, explicitaly add
test to make sure we do the right thing.

plugin/cache/minttl_test.go

+package cache
+
+import (
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/response"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+// See https://github.com/kubernetes/dns/issues/121, add some specific tests for those use cases.
+
+func TestMinMsgTTL(t *testing.T) {
+	m := new(dns.Msg)
+	m.SetQuestion("z.alm.im.", dns.TypeA)
+	m.Ns = []dns.RR{
+		test.SOA("alm.im.	1800	IN	SOA	ivan.ns.cloudflare.com. dns.cloudflare.com. 2025042470 10000 2400 604800 3600"),
+	}
+
+	utc := time.Now().UTC()
+
+	mt, _ := response.Typify(m, utc)
+	if mt != response.NoData {
+		t.Fatalf("Expected type to be response.NoData, got %s", mt)
+	}
+	dur := minMsgTTL(m, mt) // minTTL on msg is 3600 (neg. ttl on SOA)
+	if dur != time.Duration(3600*time.Second) {
+		t.Fatalf("Expected minttl duration to be %d, got %d", 3600, dur)
+	}
+
+	m.Rcode = dns.RcodeNameError
+	mt, _ = response.Typify(m, utc)
+	if mt != response.NameError {
+		t.Fatalf("Expected type to be response.NameError, got %s", mt)
+	}
+	dur = minMsgTTL(m, mt) // minTTL on msg is 3600 (neg. ttl on SOA)
+	if dur != time.Duration(3600*time.Second) {
+		t.Fatalf("Expected minttl duration to be %d, got %d", 3600, dur)
+	}
+}

plugin/pkg/response/typify.go

@@ -99,8 +99,6 @@ func Typify(m *dns.Msg, t time.Time) (Type, *dns.OPT) {
 		}
 	}
 
-	// Check length of different sections, and drop stuff that is just to large? TODO(miek).
-
 	if soa && m.Rcode == dns.RcodeSuccess {
 		return NoData, opt
 	}