Feature or enhancement

Proposal:

I'd like to add support for file descriptors pointing to a symlink in os.readlink() . For example:

# setup
import os
open_flags = os.O_RDONLY
if hasattr(os, "O_SYMLINK"):  # MacOS
    open_flags |= os.O_SYMLINK
elif hasattr(os, "O_PATH") and hasattr(os, "O_NOFOLLOW"):  # Linux
    open_flags |= os.O_PATH | os.O_NOFOLLOW
else:
    raise NotImplementedError("can't get a file descriptor pointing to a symlink on this platform")
fd = os.open("path/to/some/symlink", open_flags)

# the actual new feature: passing a file descriptor for a symlink to os.readlink()
link_target = os.readlink(fd)

Of course, you can already read a symlink directly using the path with os.readlink(), and os.readlink() already supports the optional dir_fd parameter, but if all you have if a FD for the symlink itself, right now Python doesn't have a real way to read the symlink.
(As @Rosuav pointed out in the discourse, on Linux you can work around this with os.readlink("", fd), but that solution doesn't work on MacOS.)

Regarding implementation:

• On Linux, we simply call readlinkat(fd, "", ..., ...). That should work, according to the man page:

  Since Linux 2.6.39, pathname can be an empty string, in which case the
  call operates on the symbolic link referred to by dirfd (which should
  have been obtained using open(2) with the O_PATH and O_NOFOLLOW flags).

• On MacOS, we pass the FD to the freadlink function.
• On other OSs, I'd simply throw a NotImplementedError if os.readlink() gets called with an integer.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/support-file-descriptors-in-os-readlink/109101/6

Linked PRs

• gh-157899: Add support for fds pointing to symlinks in os.readlink(). #157915