Add os.linkat() function to create a hard link with flags

[vstinner]

Feature or enhancement

Proposal:

os.link() can call linkat() but it only supports AT_SYMLINK_FOLLOW flag (follow_symlinks=True), it doesn't support AT_EMPTY_PATH. I propose adding the os.linkat() function to create a hard link with flags, and add the following constants:

• os.AT_FDCWD
• os.AT_SYMLINK_FOLLOW
• os.AT_EMPTY_PATH

AT_EMPTY_PATH allows creating a hard link from an unnamed name even if /proc is not mounted.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

#136156

Linked PRs

• gh-136413: Add os.linkat() function #136417

[serhiy-storchaka]

We already have os.link() for this. It does not implement Linux-specific AT_EMPTY_PATH yet, but there is an open issue #89341 for this.

Historical reference. It was the first issue from which I started my contribution to CPython. After seeing the addition of over a dozen new functions in the Python 3.3 preview, I suggested adding support for optional keyword arguments to existing functions. @larryhastings supported this idea, opened #58831 and implemented it.

[ronaldoussoren]

Would it be possible to fold the extra functionality into os.link, for example by mapping os.link(src_fd, ...) on to linkat(..., AT_EMPTY_PATH)?

[serhiy-storchaka]

This is what #136302 does.