request.Scrub: test for rl==size case (#1631)

* request.Scrub: test for rl==size case

Make a test case for the new break statement in Scrub and also
account for the OPT record that may get re-added in SizeAndDo() -
otherwise we may break clients that expect this.

* Fix comment

request/request_test.go

@@ -109,6 +109,56 @@ func TestRequestScrubExtra(t *testing.T) {
 	}
 }
 
+func TestRequestScrubExtraEdns0(t *testing.T) {
+	m := new(dns.Msg)
+	m.SetQuestion("large.example.com.", dns.TypeSRV)
+	m.SetEdns0(4096, true)
+	req := Request{W: &test.ResponseWriter{}, Req: m}
+
+	reply := new(dns.Msg)
+	reply.SetReply(m)
+	for i := 1; i < 200; i++ {
+		reply.Extra = append(reply.Extra, test.SRV(
+			fmt.Sprintf("large.example.com. 10 IN SRV 0 0 80 10-0-0-%d.default.pod.k8s.example.com.", i)))
+	}
+
+	_, got := req.Scrub(reply)
+	if want := ScrubExtra; want != got {
+		t.Errorf("want scrub result %d, got %d", want, got)
+	}
+	if want, got := req.Size(), reply.Len(); want < got {
+		t.Errorf("want scrub to reduce message length below %d bytes, got %d bytes", want, got)
+	}
+	if reply.Truncated {
+		t.Errorf("want scrub to not set truncated bit")
+	}
+	opt := reply.Extra[len(reply.Extra)-1]
+	if opt.Header().Rrtype != dns.TypeOPT {
+		t.Errorf("Last RR must be OPT record")
+	}
+}
+
+func TestRequestScrubAnswerExact(t *testing.T) {
+	m := new(dns.Msg)
+	m.SetQuestion("large.example.com.", dns.TypeSRV)
+	m.SetEdns0(867, false) // Bit fiddly, but this hits the rl == size break clause in Scrub, 52 RRs should remain.
+	req := Request{W: &test.ResponseWriter{}, Req: m}
+
+	reply := new(dns.Msg)
+	reply.SetReply(m)
+	for i := 1; i < 200; i++ {
+		reply.Answer = append(reply.Answer, test.A(fmt.Sprintf("large.example.com. 10 IN A 127.0.0.%d", i)))
+	}
+
+	_, got := req.Scrub(reply)
+	if want := ScrubAnswer; want != got {
+		t.Errorf("want scrub result %d, got %d", want, got)
+	}
+	if want, got := req.Size(), reply.Len(); want < got {
+		t.Errorf("want scrub to reduce message length below %d bytes, got %d bytes", want, got)
+	}
+}
+
 func TestRequestMatch(t *testing.T) {
 	st := testRequest()
 	reply := new(dns.Msg)