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, 7 insertions, 7 deletions

diff --git a/plugin/cache/handler.go b/plugin/cache/handler.go
index 987dd61b2..406ece8e6 100644
--- a/plugin/cache/handler.go
+++ b/plugin/cache/handler.go
@@ -14,12 +14,13 @@ import (
 
 // ServeDNS implements the plugin.Handler interface.
 func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
-	state := request.Request{W: w, Req: r}
+	rc := r.Copy() // We potentially modify r, to prevent other plugins from seeing this (r is a pointer), copy r into rc.
+	state := request.Request{W: w, Req: rc}
 	do := state.Do()
 
 	zone := plugin.Zones(c.Zones).Matches(state.Name())
 	if zone == "" {
-		return plugin.NextOrFailure(c.Name(), c.Next, ctx, w, r)
+		return plugin.NextOrFailure(c.Name(), c.Next, ctx, w, rc)
 	}
 
 	now := c.now().UTC()
@@ -39,22 +40,21 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	}
 	if i == nil {
 		if !do {
-			setDo(r)
+			setDo(rc)
 		}
 		crr := &ResponseWriter{ResponseWriter: w, Cache: c, state: state, server: server, do: do}
-		return plugin.NextOrFailure(c.Name(), c.Next, ctx, crr, r)
+		return plugin.NextOrFailure(c.Name(), c.Next, ctx, crr, rc)
 	}
 	if ttl < 0 {
 		servedStale.WithLabelValues(server).Inc()
 		// Adjust the time to get a 0 TTL in the reply built from a stale item.
 		now = now.Add(time.Duration(ttl) * time.Second)
 		go func() {
-			r := r.Copy()
 			if !do {
-				setDo(r)
+				setDo(rc)
 			}
 			crr := &ResponseWriter{Cache: c, state: state, server: server, prefetch: true, remoteAddr: w.LocalAddr(), do: do}
-			plugin.NextOrFailure(c.Name(), c.Next, ctx, crr, r)
+			plugin.NextOrFailure(c.Name(), c.Next, ctx, crr, rc)
 		}()
 	}
 	resp := i.toMsg(r, now, do)