如何实现对BFS上非空文件的写操作？

怎么才能实现以附加写（append write)的方式打开BFS file?
eg. 第一次写入的内容为 "hello world! "，第二次写入的内容为 "hello apady".
文件最终的内容为"hello world! hello apady"

Start by locating the BFS file-open and write interfaces and checking whether they expose append semantics for non-empty files. Compare the current behavior with the requested two-write example and determine whether the expected result requires a documented usage pattern or a new feature. Done means append behavior is clearly supported and verified, or the limitation is documented.

Written by the indexing model from the issue text.