validate object implements the corresponding interface (#3724)

Signed-off-by: zouyee <zounengren@cmss.chinamobile.com>

core/dnsserver/server.go

@@ -20,6 +20,7 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/transport"
 	"github.com/coredns/coredns/request"
 
+	"github.com/caddyserver/caddy"
 	"github.com/miekg/dns"
 	ot "github.com/opentracing/opentracing-go"
 )
@@ -99,6 +100,9 @@ func NewServer(addr string, group []*Config) (*Server, error) {
 	return s, nil
 }
 
+// Compile-time check to ensure Server implements the caddy.GracefulServer interface
+var _ caddy.GracefulServer = &Server{}
+
 // Serve starts the server with an existing listener. It blocks until the server stops.
 // This implements caddy.TCPServer interface.
 func (s *Server) Serve(l net.Listener) error {