etcdserver: swap priority of cert CN and username + password

Documentation/op-guide/authentication.md

@@ -161,4 +161,4 @@ Otherwise, all `etcdctl` commands remain the same. Users and roles can still be
 
 ## Using TLS Common Name
 
-If an etcd server is launched with the option `--client-cert-auth=true`, the field of Common Name (CN) in the client's TLS cert will be used as an etcd user. In this case, the common name authenticates the user and the client does not need a password.
+If an etcd server is launched with the option `--client-cert-auth=true`, the field of Common Name (CN) in the client's TLS cert will be used as an etcd user. In this case, the common name authenticates the user and the client does not need a password. Note that if both of 1. `--client-cert-auth=true` is passed and CN is provided by the client, and 2. username and password are provided by the client, the username and password based authentication is prioritized.

etcdserver/v3_server.go

@@ -686,12 +686,14 @@ func (s *EtcdServer) linearizableReadNotify(ctx context.Context) error {
 }
 
 func (s *EtcdServer) AuthInfoFromCtx(ctx context.Context) (*auth.AuthInfo, error) {
-	if s.Cfg.ClientCertAuthEnabled {
-		authInfo := s.AuthStore().AuthInfoFromTLS(ctx)
-		if authInfo != nil {
-			return authInfo, nil
-		}
+	authInfo, err := s.AuthStore().AuthInfoFromCtx(ctx)
+	if authInfo != nil || err != nil {
+		return authInfo, err
+	}
+	if !s.Cfg.ClientCertAuthEnabled {
+		return nil, nil
 	}
+	authInfo = s.AuthStore().AuthInfoFromTLS(ctx)
+	return authInfo, nil
 
-	return s.AuthStore().AuthInfoFromCtx(ctx)
 }