Send something over the wire for responses

I accidentally made this change in the main repo, instead of here.

This ensures that the server can tell the difference between a response and a request. The old method (resp == nil) didn't work because we use interfaces here, and the interfaces would often have types, but not values.

handler_with_error.go

@@ -3,6 +3,7 @@ package jsonrpc2
 import (
 	"context"
 	"log"
+	"reflect"
 )
 
 // HandlerWithError implements Handler by calling the func for each
@@ -21,7 +22,7 @@ func (h HandlerWithError) Handle(ctx context.Context, conn *Conn, req *Request)
 
 	resp := &Response{ID: req.ID}
 	if err == nil {
-		if result == nil {
+		if isNilValue(result) {
 			result = struct{}{}
 		}
 		err = resp.SetResult(result)
@@ -40,3 +41,17 @@ func (h HandlerWithError) Handle(ctx context.Context, conn *Conn, req *Request)
 		}
 	}
 }
+
+// isNilValue tests if an interface is empty, because an empty interface does
+// not encode any information, we can't encode it in JSON so that the proxy
+// knows it's a response, not a request.
+func isNilValue(resp interface{}) bool {
+	if resp == nil {
+		return true
+	}
+	kind := reflect.TypeOf(resp).Kind()
+	value := reflect.ValueOf(resp)
+	nilPtr := kind == reflect.Ptr && value.IsNil()
+	nilSlice := kind == reflect.Slice && value.IsNil()
+	return nilPtr || nilSlice
+}