etcdserver: return error of apply result without touching response

Current etcdserver tries to return result.resp even if result.err is
not nil. A situation of result.resp == nil and result.err != nil can
happen and it results an error like below:

18:49:57 etcd1 | interface conversion: proto.Message is nil, not *etcdserverpb.PutResponse

This commit lets the functions return result.err if it is not nil.

etcdserver/v3_server.go

@@ -75,7 +75,10 @@ func (s *EtcdServer) Range(ctx context.Context, r *pb.RangeRequest) (*pb.RangeRe
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.RangeResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.RangeResponse), nil
 }
 
 func (s *EtcdServer) Put(ctx context.Context, r *pb.PutRequest) (*pb.PutResponse, error) {
@@ -83,7 +86,10 @@ func (s *EtcdServer) Put(ctx context.Context, r *pb.PutRequest) (*pb.PutResponse
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.PutResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.PutResponse), nil
 }
 
 func (s *EtcdServer) DeleteRange(ctx context.Context, r *pb.DeleteRangeRequest) (*pb.DeleteRangeResponse, error) {
@@ -91,7 +97,10 @@ func (s *EtcdServer) DeleteRange(ctx context.Context, r *pb.DeleteRangeRequest)
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.DeleteRangeResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.DeleteRangeResponse), nil
 }
 
 func (s *EtcdServer) Txn(ctx context.Context, r *pb.TxnRequest) (*pb.TxnResponse, error) {
@@ -103,7 +112,10 @@ func (s *EtcdServer) Txn(ctx context.Context, r *pb.TxnRequest) (*pb.TxnResponse
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.TxnResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.TxnResponse), nil
 }
 
 func isTxnSerializable(r *pb.TxnRequest) bool {
@@ -134,6 +146,9 @@ func (s *EtcdServer) Compact(ctx context.Context, r *pb.CompactionRequest) (*pb.
 	if err != nil {
 		return nil, err
 	}
+	if result.err != nil {
+		return nil, result.err
+	}
 	resp := result.resp.(*pb.CompactionResponse)
 	if resp == nil {
 		resp = &pb.CompactionResponse{}
@@ -142,7 +157,7 @@ func (s *EtcdServer) Compact(ctx context.Context, r *pb.CompactionRequest) (*pb.
 		resp.Header = &pb.ResponseHeader{}
 	}
 	resp.Header.Revision = s.kv.Rev()
-	return resp, result.err
+	return resp, nil
 }
 
 func (s *EtcdServer) LeaseGrant(ctx context.Context, r *pb.LeaseGrantRequest) (*pb.LeaseGrantResponse, error) {
@@ -155,7 +170,10 @@ func (s *EtcdServer) LeaseGrant(ctx context.Context, r *pb.LeaseGrantRequest) (*
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.LeaseGrantResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.LeaseGrantResponse), nil
 }
 
 func (s *EtcdServer) LeaseRevoke(ctx context.Context, r *pb.LeaseRevokeRequest) (*pb.LeaseRevokeResponse, error) {
@@ -163,7 +181,10 @@ func (s *EtcdServer) LeaseRevoke(ctx context.Context, r *pb.LeaseRevokeRequest)
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.LeaseRevokeResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.LeaseRevokeResponse), nil
 }
 
 func (s *EtcdServer) LeaseRenew(id lease.LeaseID) (int64, error) {
@@ -206,7 +227,10 @@ func (s *EtcdServer) Alarm(ctx context.Context, r *pb.AlarmRequest) (*pb.AlarmRe
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AlarmResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AlarmResponse), nil
 }
 
 func (s *EtcdServer) AuthEnable(ctx context.Context, r *pb.AuthEnableRequest) (*pb.AuthEnableResponse, error) {
@@ -214,7 +238,10 @@ func (s *EtcdServer) AuthEnable(ctx context.Context, r *pb.AuthEnableRequest) (*
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AuthEnableResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AuthEnableResponse), nil
 }
 
 func (s *EtcdServer) AuthDisable(ctx context.Context, r *pb.AuthDisableRequest) (*pb.AuthDisableResponse, error) {
@@ -222,7 +249,10 @@ func (s *EtcdServer) AuthDisable(ctx context.Context, r *pb.AuthDisableRequest)
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AuthDisableResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AuthDisableResponse), nil
 }
 
 func (s *EtcdServer) Authenticate(ctx context.Context, r *pb.AuthenticateRequest) (*pb.AuthenticateResponse, error) {
@@ -230,7 +260,10 @@ func (s *EtcdServer) Authenticate(ctx context.Context, r *pb.AuthenticateRequest
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AuthenticateResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AuthenticateResponse), nil
 }
 
 func (s *EtcdServer) UserAdd(ctx context.Context, r *pb.AuthUserAddRequest) (*pb.AuthUserAddResponse, error) {
@@ -238,7 +271,10 @@ func (s *EtcdServer) UserAdd(ctx context.Context, r *pb.AuthUserAddRequest) (*pb
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AuthUserAddResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AuthUserAddResponse), nil
 }
 
 func (s *EtcdServer) UserDelete(ctx context.Context, r *pb.AuthUserDeleteRequest) (*pb.AuthUserDeleteResponse, error) {
@@ -246,7 +282,10 @@ func (s *EtcdServer) UserDelete(ctx context.Context, r *pb.AuthUserDeleteRequest
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AuthUserDeleteResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AuthUserDeleteResponse), nil
 }
 
 func (s *EtcdServer) UserChangePassword(ctx context.Context, r *pb.AuthUserChangePasswordRequest) (*pb.AuthUserChangePasswordResponse, error) {
@@ -254,7 +293,10 @@ func (s *EtcdServer) UserChangePassword(ctx context.Context, r *pb.AuthUserChang
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AuthUserChangePasswordResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AuthUserChangePasswordResponse), nil
 }
 
 func (s *EtcdServer) UserGrant(ctx context.Context, r *pb.AuthUserGrantRequest) (*pb.AuthUserGrantResponse, error) {
@@ -262,7 +304,10 @@ func (s *EtcdServer) UserGrant(ctx context.Context, r *pb.AuthUserGrantRequest)
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AuthUserGrantResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AuthUserGrantResponse), nil
 }
 
 func (s *EtcdServer) RoleAdd(ctx context.Context, r *pb.AuthRoleAddRequest) (*pb.AuthRoleAddResponse, error) {
@@ -270,7 +315,10 @@ func (s *EtcdServer) RoleAdd(ctx context.Context, r *pb.AuthRoleAddRequest) (*pb
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AuthRoleAddResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AuthRoleAddResponse), nil
 }
 
 func (s *EtcdServer) RoleGrant(ctx context.Context, r *pb.AuthRoleGrantRequest) (*pb.AuthRoleGrantResponse, error) {
@@ -278,7 +326,10 @@ func (s *EtcdServer) RoleGrant(ctx context.Context, r *pb.AuthRoleGrantRequest)
 	if err != nil {
 		return nil, err
 	}
-	return result.resp.(*pb.AuthRoleGrantResponse), result.err
+	if result.err != nil {
+		return nil, result.err
+	}
+	return result.resp.(*pb.AuthRoleGrantResponse), nil
 }
 
 func (s *EtcdServer) processInternalRaftRequest(ctx context.Context, r pb.InternalRaftRequest) (*applyResult, error) {