plugin/cache: cap TTL on first answer (#1092)

Cache would let the first response through and would then cap subsequent
ones to whatever the cache duration was. This would lead to huge drops
in TTL values: 3600 -> 20 for instance, which is not only bad, but can
mess up your careful TTL planning business.

This PR fixes that and applies the cache duration to all replies. As a
bonus I could remove a time.Sleep() from the cache test and just check
for the cache duration as the TTL on the reply.

Fixes #1038

plugin/cache/cache.go

@@ -117,6 +117,19 @@ func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
 		return nil
 	}
 
+	// Apply capped TTL to this reply to avoid jarring TTL experience 1799 -> 8 (e.g.)
+	ttl := uint32(duration.Seconds())
+	for i := range res.Answer {
+		res.Answer[i].Header().Ttl = ttl
+	}
+	for i := range res.Ns {
+		res.Ns[i].Header().Ttl = ttl
+	}
+	for i := range res.Extra {
+		if res.Extra[i].Header().Rrtype != dns.TypeOPT {
+			res.Extra[i].Header().Ttl = ttl
+		}
+	}
 	return w.ResponseWriter.WriteMsg(res)
 }