`os.path.realpath('loop/../link')` does not resolve link

[barneygale]

If os.path.realpath() encounters a symlink loop on posix, it prematurely stops querying paths and resolving symlink targets. This differs from coreutils realpath:

$ cd /tmp
$ ln -s loop loop
$ ln -s target link
$ realpath -m loop/../link
/tmp/target
$ python -c 'import os.path; print(os.path.realpath("loop/../link"))'
/tmp/link

It also differs from how lstat() errors are handled:

$ realpath -m missing/../link
/tmp/target
$ python -c 'import os.path; print(os.path.realpath("missing/../link"))'
/tmp/target

Linked PRs

• GH-117546: Fix symlink resolution in os.path.realpath('loop/../link') #117568

[eryksun]

As Barney noted, this is only an issue on POSIX. The non-strict implementation on Windows ignores a symlink loop to resolve as much as possible. Also, on Windows, for the given examples, the ".." component gets resolved first, which is like the -L (i.e. --logical) option of coreutils realpath. That makes os.path.realpath() on Windows partially non-strict even with strict=True. Nowadays the system implements this logical path handling in both the user-mode runtime library and the kernel. (In older versions of Windows, the kernel implemented POSIX-like physical path parsing when resolving the target of a relative symlink. For example, for "loop\..\target", it used to resolve "loop" before it resolved "..".)

[barneygale]

Thanks Eryk!

I suppose users could do realpath(normpath(path)) to get realpath --logical behaviour on POSIX.

[encukou]

This behaviour goes back to Python 2. IMO, we should be very careful when changing it. The status quo seems fine.
OTOH, it would be nice if realpath was idempotent.

I don't feel qualified to review this.

[barneygale]

Appreciate the feedback.

Serhiy has separately approved the change, but given your reservations, I won't merge yet. Hopefully another core dev comes along to share their views and perhaps cast the deciding vote (if not I'll summon one in due course).

This bug is pretty rare. To hit it you need to supply a path that:

1. Includes a symlink loop, and
2. Has subsequent .. segment(s) that walk up past the loop (e.g. loop/.., or loop/blah/../../)
3. Has further segments that point to a different, non-looping symlink

The unlikelihood of hitting all three might explain why I couldn't find any existing bug reports, despite the bug going way back :)

[encukou]

Serhiy is the os.path expert, he should get the deciding vote. I am careful because I'm not familiar with os.path maintenance :)

I agree that this is an edge case you hopefully won't see in production.

[barneygale]

Thanks! Fix merged into main. I'm not planning to backport because the fix would need to be different (due to #114848) and because the bug is so rare. If anyone would like it backported, please leave a comment and I'll re-open.