cache: do the msg copy right (#4207)

Not sure why this is proving so difficult.. pointers are hard? [Was
tempted to rollback all tweaks here, but the original issue we're fixing
it too important to not have a proper fix].

But we need to make a copy of the message at the earliest point in the
handler because we are changing it (adding an opt rr). If we do this on
the original message (which is a pointer) we change it (obvs). When
undoing those changes we do work on a copy.

Re: testing. There isn't a explicit test for this, so I've added on to
the top-level test/ directory, which indeed makes the issue visible:

master:

~~~
go test -v -run=TestLookupCacheWithoutEdns
=== RUN   TestLookupCacheWithoutEdns
    cache_test.go:154: Expected no OPT RR, but got:
        ;; OPT PSEUDOSECTION:
        ; EDNS: version 0; flags: do; udp: 2048
--- FAIL: TestLookupCacheWithoutEdns (0.01s)
FAIL
~~~

This branch:

~~~
% go test -v -run=TestLookupCacheWithoutEdns
=== RUN   TestLookupCacheWithoutEdns
--- PASS: TestLookupCacheWithoutEdns (0.01s)
PASS
ok  	github.com/coredns/coredns/test	0.109s
~~~

Signed-off-by: Miek Gieben <miek@miek.nl>

test/cache_test.go

@@ -110,3 +110,48 @@ func testCaseDNSSEC(t *testing.T, name, addr string, bufsize int) {
 		}
 	}
 }
+
+func TestLookupCacheWithoutEdns(t *testing.T) {
+	name, rm, err := test.TempFile(".", exampleOrg)
+	if err != nil {
+		t.Fatalf("Failed to create zone: %s", err)
+	}
+	defer rm()
+
+	corefile := `example.org:0 {
+		file ` + name + `
+	}`
+
+	i, udp, _, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	// Start caching forward CoreDNS that we want to test.
+	corefile = `example.org:0 {
+		forward . ` + udp + `
+		cache 10
+	}`
+
+	i, udp, _, err = CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	m := new(dns.Msg)
+	m.SetQuestion("example.org.", dns.TypeA)
+	resp, err := dns.Exchange(m, udp)
+	if err != nil {
+		t.Fatalf("Expected to receive reply, but didn't: %s", err)
+	}
+	if len(resp.Extra) == 0 {
+		return
+	}
+
+	if resp.Extra[0].Header().Rrtype == dns.TypeOPT {
+		t.Fatalf("Expected no OPT RR, but got: %s", resp.Extra[0])
+	}
+	t.Fatalf("Expected empty additional section, got %v", resp.Extra)
+}