Golang OpenFile O_APPEND不尊重Seek

When I open file in mode like this:

file, _ := os.OpenFile("/path/to/my/file", os.O_RDWR|os.O_APPEND, os.FileMode(0666))
file.Seek(start, os.SEEK_SET)
io.CopyN(file, resp.Body, length)

io.CopyN does not respect the position where I sought. It seems it just append to the tail of the file. Instead if I open the file like this:

file, _ := os.OpenFile("/path/to/my/file", os.O_RDWR, os.FileMode(0666))
file.Seek(start, os.SEEK_SET)
io.CopyN(file, resp.Body, length)

It works as I expected. io.CopyN writes to the file from the "start" point I sought. Not sure if this is a feature or a bug?

It's definitely a feature (http://man7.org/linux/man-pages/man2/open.2.html) and it's controlled by underlying OS, not golang runtime.

O_APPEND
          The file is opened in append mode.  Before each write(2), the
          file offset is positioned at the end of the file, as if with
          lseek(2).  O_APPEND may lead to corrupted files on NFS
          filesystems if more than one process appends data to a file at
          once.  This is because NFS does not support appending to a
          file, so the client kernel has to simulate it, which can't be
          done without a race condition.