Drop responses that fail plugin processing

Return nil from the resolver callback when ApplyResponsePlugins
errors, and stop suppressing unpack errors for truncated packets.

Author: jedisct1

dnscrypt-proxy/plugins.go

@@ -340,9 +340,6 @@ func (pluginsState *PluginsState) ApplyResponsePlugins(
 ) ([]byte, error) {
 	msg := dns.Msg{Data: packet}
 	if err := msg.Unpack(); err != nil {
-		if len(packet) >= MinDNSPacketSize && HasTCFlag(packet) {
-			err = nil
-		}
 		return packet, err
 	}
 	switch Rcode(packet) {

dnscrypt-proxy/proxy.go

@@ -876,7 +876,7 @@ func (proxy *Proxy) processIncomingQuery(
 			// Process the response through plugins
 			processedResponse, err := processPlugins(proxy, &pluginsState, query, serverInfo, response)
 			if err != nil {
-				return response
+				return nil
 			}
 
 			response = processedResponse