transparently simplify control flow

conn.go

@@ -187,15 +187,17 @@ func (c *Conn) close(cause error) error {
 }
 
 func (c *Conn) readMessages(ctx context.Context) {
-	var err error
-	for err == nil {
+	for {
 		var m anyMessage
-		err = c.stream.ReadObject(&m)
+		err := c.stream.ReadObject(&m)
 		if err != nil {
-			break
+			c.close(err)
+			return
 		}
 
 		switch {
+		// TODO: handle the case where both request and response are nil.
+
 		case m.request != nil:
 			for _, onRecv := range c.onRecv {
 				onRecv(m.request, nil)
@@ -204,43 +206,36 @@ func (c *Conn) readMessages(ctx context.Context) {
 
 		case m.response != nil:
 			resp := m.response
-			if resp != nil {
-				id := resp.ID
-				c.mu.Lock()
-				call := c.pending[id]
-				delete(c.pending, id)
-				c.mu.Unlock()
+			id := resp.ID
+			c.mu.Lock()
+			call := c.pending[id]
+			delete(c.pending, id)
+			c.mu.Unlock()
 
-				if call != nil {
-					call.response = resp
-				}
-
-				if len(c.onRecv) > 0 {
-					var req *Request
-					if call != nil {
-						req = call.request
-					}
-					for _, onRecv := range c.onRecv {
-						onRecv(req, resp)
-					}
-				}
-
-				switch {
-				case call == nil:
-					c.logger.Printf("jsonrpc2: ignoring response #%s with no corresponding request\n", id)
-
-				case resp.Error != nil:
-					call.done <- resp.Error
-					close(call.done)
-
-				default:
-					call.done <- nil
-					close(call.done)
-				}
+			var req *Request
+			if call != nil {
+				call.response = resp
+				req = call.request
 			}
+
+			for _, onRecv := range c.onRecv {
+				onRecv(req, resp)
+			}
+
+			if call == nil {
+				c.logger.Printf("jsonrpc2: ignoring response #%s with no corresponding request\n", id)
+				continue
+			}
+
+			var err error
+			if resp.Error != nil {
+				err = resp.Error
+			}
+
+			call.done <- err
+			close(call.done)
 		}
 	}
-	c.close(err)
 }
 
 func (c *Conn) send(_ context.Context, m *anyMessage, wait bool) (cc *call, err error) {
@@ -339,25 +334,20 @@ type Waiter struct {
 // error is returned.
 func (w Waiter) Wait(ctx context.Context, result interface{}) error {
 	select {
-	case err, ok := <-w.call.done:
-		if !ok {
-			err = ErrClosed
-		}
-		if err != nil {
-			return err
-		}
-		if result != nil {
-			if w.call.response.Result == nil {
-				w.call.response.Result = &jsonNull
-			}
-			if err := json.Unmarshal(*w.call.response.Result, result); err != nil {
-				return err
-			}
-		}
-		return nil
-
 	case <-ctx.Done():
 		return ctx.Err()
+
+	case err, ok := <-w.call.done:
+		if !ok {
+			return ErrClosed
+		}
+		if err != nil || result == nil {
+			return err
+		}
+		if w.call.response.Result == nil {
+			w.call.response.Result = &jsonNull
+		}
+		return json.Unmarshal(*w.call.response.Result, result)
 	}
 }
 
@@ -423,12 +413,7 @@ func (m *anyMessage) UnmarshalJSON(data []byte) error {
 			return errors.New("jsonrpc2: invalid empty batch")
 		}
 		for i := range msgs {
-			if err := checkType(&msg{
-				ID:     msgs[i].ID,
-				Method: msgs[i].Method,
-				Result: msgs[i].Result,
-				Error:  msgs[i].Error,
-			}); err != nil {
+			if err := checkType(&msgs[i]); err != nil {
 				return err
 			}
 		}

conn_opt.go

@@ -44,11 +44,9 @@ func LogMessages(logger Logger) ConnOpt {
 		OnRecv(func(req *Request, resp *Response) {
 			switch {
 			case resp != nil:
-				var method string
+				method := "(no matching request)"
 				if req != nil {
 					method = req.Method
-				} else {
-					method = "(no matching request)"
 				}
 				switch {
 				case resp.Result != nil:

handler_with_error.go

@@ -30,20 +30,16 @@ func (h *HandlerWithErrorConfigurer) Handle(ctx context.Context, conn *Conn, req
 	if err == nil {
 		err = resp.SetResult(result)
 	}
-	if err != nil {
-		if e, ok := err.(*Error); ok {
-			resp.Error = e
-		} else {
-			resp.Error = &Error{Message: err.Error()}
-		}
+
+	if e, ok := err.(*Error); ok {
+		resp.Error = e
+	} else if err != nil {
+		resp.Error = &Error{Message: err.Error()}
 	}
 
-	if !req.Notif {
-		if err := conn.SendResponse(ctx, resp); err != nil {
-			if err != ErrClosed || !h.suppressErrClosed {
-				conn.logger.Printf("jsonrpc2 handler: sending response %s: %v\n", resp.ID, err)
-			}
-		}
+	err = conn.SendResponse(ctx, resp)
+	if err != nil && (err != ErrClosed || !h.suppressErrClosed) {
+		conn.logger.Printf("jsonrpc2 handler: sending response %s: %v\n", resp.ID, err)
 	}
 }
 

request.go

@@ -55,6 +55,10 @@ func (r Request) MarshalJSON() ([]byte, error) {
 // UnmarshalJSON implements json.Unmarshaler.
 func (r *Request) UnmarshalJSON(data []byte) error {
 	r2 := make(map[string]interface{})
+	pop := func(key string) interface{} {
+		defer delete(r2, key)
+		return r2[key]
+	}
 
 	// Detect if the "params" or "meta" fields are JSON "null" or just not
 	// present by seeing if the field gets overwritten to nil.
@@ -68,36 +72,37 @@ func (r *Request) UnmarshalJSON(data []byte) error {
 	if err := decoder.Decode(&r2); err != nil {
 		return err
 	}
+
 	var ok bool
-	r.Method, ok = r2["method"].(string)
+	r.Method, ok = pop("method").(string)
 	if !ok {
 		return errors.New("missing method field")
 	}
-	switch {
-	case r2["params"] == nil:
+	switch params := pop("params"); params {
+	case nil:
 		r.Params = &jsonNull
-	case r2["params"] == emptyParams:
+	case emptyParams:
 		r.Params = nil
 	default:
-		b, err := json.Marshal(r2["params"])
+		b, err := json.Marshal(params)
 		if err != nil {
 			return fmt.Errorf("failed to marshal params: %w", err)
 		}
 		r.Params = (*json.RawMessage)(&b)
 	}
-	switch {
-	case r2["meta"] == nil:
+	switch meta := pop("meta"); meta {
+	case nil:
 		r.Meta = &jsonNull
-	case r2["meta"] == emptyMeta:
+	case emptyMeta:
 		r.Meta = nil
 	default:
-		b, err := json.Marshal(r2["meta"])
+		b, err := json.Marshal(meta)
 		if err != nil {
 			return fmt.Errorf("failed to marshal Meta: %w", err)
 		}
 		r.Meta = (*json.RawMessage)(&b)
 	}
-	switch rawID := r2["id"].(type) {
+	switch rawID := pop("id").(type) {
 	case nil:
 		r.ID = ID{}
 		r.Notif = true
@@ -115,13 +120,12 @@ func (r *Request) UnmarshalJSON(data []byte) error {
 		return fmt.Errorf("unexpected ID type: %T", rawID)
 	}
 
+	// The jsonrpc field should not be added to ExtraFields.
+	delete(r2, "jsonrpc")
+
 	// Clear the extra fields before populating them again.
 	r.ExtraFields = nil
 	for name, value := range r2 {
-		switch name {
-		case "id", "jsonrpc", "meta", "method", "params":
-			continue
-		}
 		r.ExtraFields = append(r.ExtraFields, RequestField{
 			Name:  name,
 			Value: value,