reverseproxy: handle buffered data during hijack (#6274)

modules/caddyhttp/responsewriter.go

@@ -263,6 +263,8 @@ func (rr *responseRecorder) Hijack() (net.Conn, *bufio.ReadWriter, error) {
 		conn.(*hijackedConn).updateReadSize(buffered)
 		data, _ := brw.Peek(buffered)
 		brw.Reader.Reset(io.MultiReader(bytes.NewReader(data), conn))
+		// peek to make buffered data appear, as Reset will make it 0
+		_, _ = brw.Peek(buffered)
 	} else {
 		brw.Reader.Reset(conn)
 	}

modules/caddyhttp/reverseproxy/streaming.go

@@ -101,6 +101,17 @@ func (h *Handler) handleUpgradeResponse(logger *zap.Logger, wg *sync.WaitGroup,
 		return
 	}
 
+	// There may be buffered data in the *bufio.Reader
+	// see: https://github.com/caddyserver/caddy/issues/6273
+	if buffered := brw.Reader.Buffered(); buffered > 0 {
+		data, _ := brw.Peek(buffered)
+		_, err := backConn.Write(data)
+		if err != nil {
+			logger.Debug("backConn write failed", zap.Error(err))
+			return
+		}
+	}
+
 	// Ensure the hijacked client connection, and the new connection established
 	// with the backend, are both closed in the event of a server shutdown. This
 	// is done by registering them. We also try to gracefully close connections