plugin/cache: Add refresh mode setting to serve_stale (#5131)

This PR adds an optional REFRESH_MODE parameter on the serve_stale configuration directive of the
cache plugin, which verifies that the upstream is still unavailable before returning stale entries.

Signed-off-by: Antoine Tollenaere <atollena@gmail.com>

plugin/cache/cache.go

@@ -38,7 +38,9 @@ type Cache struct {
 	duration   time.Duration
 	percentage int
 
-	staleUpTo time.Duration
+	// Stale serve
+	staleUpTo   time.Duration
+	verifyStale bool
 
 	// Testing.
 	now func() time.Time
@@ -227,6 +229,33 @@ func (w *ResponseWriter) Write(buf []byte) (int, error) {
 	return n, err
 }
 
+// verifyStaleResponseWriter is a response writer that only writes messages if they should replace a
+// stale cache entry, and otherwise discards them.
+type verifyStaleResponseWriter struct {
+	*ResponseWriter
+	refreshed bool // set to true if the last WriteMsg wrote to ResponseWriter, false otherwise.
+}
+
+// newVerifyStaleResponseWriter returns a ResponseWriter to be used when verifying stale cache
+// entries. It only forward writes if an entry was successfully refreshed according to RFC8767,
+// section 4 (response is NoError or NXDomain), and ignores any other response.
+func newVerifyStaleResponseWriter(w *ResponseWriter) *verifyStaleResponseWriter {
+	return &verifyStaleResponseWriter{
+		w,
+		false,
+	}
+}
+
+// WriteMsg implements the dns.ResponseWriter interface.
+func (w *verifyStaleResponseWriter) WriteMsg(res *dns.Msg) error {
+	w.refreshed = false
+	if res.Rcode == dns.RcodeSuccess || res.Rcode == dns.RcodeNameError {
+		w.refreshed = true
+		return w.ResponseWriter.WriteMsg(res) // stores to the cache and send to client
+	}
+	return nil // else discard
+}
+
 const (
 	maxTTL  = dnsutil.MaximumDefaulTTL
 	minTTL  = dnsutil.MinimalDefaultTTL