How to modify the returned metadata

[baozi-2019]

I want to add error message to returned metadata,but it does not work.This is my code.

func ErrorServerInterceptor(ctx context.Context, req any, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (resp any, err error) {
	resp, err = handler(ctx, req)
	if err != nil {
		data := make(map[string]string)
		data["exceptionResult"] = err.Error()
		errorMessage := &grpc_message.ErrorMessage{
			ServiceName: ServiceName,
			Reason:      err.Error(),
			Metadata:    data,
		}
		//incomingContext, _ := metadata.FromIncomingContext(ctx)
		bytes, _ := json.Marshal(errorMessage)
		md, ok := metadata.FromOutgoingContext(ctx)
		if !ok {
			md = metadata.New(map[string]string{})
		}
		md.Append("custom-key", string(bytes))
		ctx = metadata.NewOutgoingContext(ctx, md)
		return nil, nil
	}
	return resp, err
}

[Purnesh Dixit]

@aranjans you can assign it to me

[Purnesh Dixit]

@baozi-2019 could you provide more context on why you are not returning the error (returning nil) from your interceptor?

[Purnesh Dixit]

@baozi-2019 you need to return a non-nil error otherwise the client will not receive any explicit indication of success or failure for the RPC call, and the associated context modifications made by the interceptor may not be observable by the client.

[Purnesh Dixit]

In order to add metadata to context from Unary RPC server, you can use grpc.SetTrailer function. Consider using the example in https://github.com/grpc/grpc-go/blob/master/examples/features/metadata/server/main.go#L55-L56

Having said that, metadata.FromOutgoingContext(ctx) doesn't work on server, you need to use metadata.FromIncomingContext(ctx) to read metadata by the client. Refer documentation for sending and receiving metadata https://github.com/grpc/grpc-go/blob/master/Documentation/grpc-metadata.md#sending-and-receiving-metadata---server-side