I am looking at this.

Based on the comments from a historical perspective - copyfile() needs to be calling the copy_mode function before any copying actually occurs.

As the dest is already open for writing it does not matter (on posix)
what mode it has later on.

Question: in copystat() there is a block that talks about chown() in the comments but in the code it only seems to be accessing chmod(). Should there (also) be a call to chown "if _SUPER"?

_SUPER = _POSIX and os.geteuid() == 0

basically - mode becomes:

    # remove setuid, setgid and sticky bits if _POSIX and not _SUPER
    mode = stat.S_IMODE(st.st_mode) & _REMOVE_HARMFUL_MASK if _POSIX and not _SUPER else stat.S_IMODE(st.st_mode)

Comments?