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>

1 files changed, 9 insertions, 12 deletions

diff --git a/plugin/cache/cache.go b/plugin/cache/cache.go
index 8dae9a42b..2a56500a3 100644
--- a/plugin/cache/cache.go
+++ b/plugin/cache/cache.go
@@ -143,15 +143,12 @@ func (w *ResponseWriter) RemoteAddr() net.Addr {
 
 // WriteMsg implements the dns.ResponseWriter interface.
 func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
-	// res needs to be copied otherwise we will be modifying the underlaying arrays which are now cached.
-	resc := res.Copy()
-
-	mt, _ := response.Typify(resc, w.now().UTC())
+	mt, _ := response.Typify(res, w.now().UTC())
 
 	// key returns empty string for anything we don't want to cache.
-	hasKey, key := key(w.state.Name(), resc, mt)
+	hasKey, key := key(w.state.Name(), res, mt)
 
-	msgTTL := dnsutil.MinimalTTL(resc, mt)
+	msgTTL := dnsutil.MinimalTTL(res, mt)
 	var duration time.Duration
 	if mt == response.NameError || mt == response.NoData {
 		duration = computeTTL(msgTTL, w.minnttl, w.nttl)
@@ -163,8 +160,8 @@ func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
 	}
 
 	if hasKey && duration > 0 {
-		if w.state.Match(resc) {
-			w.set(resc, key, mt, duration)
+		if w.state.Match(res) {
+			w.set(res, key, mt, duration)
 			cacheSize.WithLabelValues(w.server, Success).Set(float64(w.pcache.Len()))
 			cacheSize.WithLabelValues(w.server, Denial).Set(float64(w.ncache.Len()))
 		} else {
@@ -180,11 +177,11 @@ func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
 	// Apply capped TTL to this reply to avoid jarring TTL experience 1799 -> 8 (e.g.)
 	// We also may need to filter out DNSSEC records, see toMsg() for similar code.
 	ttl := uint32(duration.Seconds())
-	resc.Answer = filterRRSlice(resc.Answer, ttl, w.do, false)
-	resc.Ns = filterRRSlice(resc.Ns, ttl, w.do, false)
-	resc.Extra = filterRRSlice(resc.Extra, ttl, w.do, false)
+	res.Answer = filterRRSlice(res.Answer, ttl, w.do, false)
+	res.Ns = filterRRSlice(res.Ns, ttl, w.do, false)
+	res.Extra = filterRRSlice(res.Extra, ttl, w.do, false)
 
-	return w.ResponseWriter.WriteMsg(resc)
+	return w.ResponseWriter.WriteMsg(res)
 }
 
 func (w *ResponseWriter) set(m *dns.Msg, key uint64, mt response.Type, duration time.Duration) {