Reduce syscalls for `posixpath.ismount`

[nineteendo]

Feature or enhancement

Proposal:

Currently posixpath.ismount uses realpath(join(path, '..')) to get the parent of the mount. This is absolutely overkill and should only be used when join(path, '..') isn't sufficient.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

• gh-117349: Micro-optimize a few os.path functions #117350

Linked PRs

• gh-117394: Reduce syscalls for posixpath.ismount #117395
• gh-117394: Speed up os.path.ismount() on Posix #117447

[barneygale]

Related to #96328

[serhiy-storchaka]

abspath is broken by design.

Let /mnt/storage/link be a link to /mnt, /mnt/storage is a mount point, and the original path is /mnt/storage/link/../mnt/storage. abspath() will return /mnt/storage/mnt/storage. dirname() will then return /mnt/storage/mnt which is on the same device as the original path, therefore ismount() will return False. But the real path is /mnt/storage, and it is a mount point.

[nineteendo]

Are you sure? ntpath.realpath() doesn't handle symlinks that way:

>>> import ntpath
>>> ntpath.realpath(r"C:\Users\wanne\path-picker\link")
'C:\\Users\\wanne'
>>> ntpath.realpath(r"C:\Users\wanne\path-picker\link\..")
'C:\\Users\\wanne\\path-picker'

Does that work the same on Linux?

[barneygale]

It works differently on Linux - .. entries point to the real parent of a directory.

[serhiy-storchaka]

#117447 is an alternate PR which achieves the same or better speed up.

[nineteendo]

Let /mnt/storage/link be a link to /mnt, /mnt/storage is a mount point, and the original path is /mnt/storage/link/../mnt/storage.

Actually, you can work around this: ismount(realpath('/mnt/storage/link/../mnt/storage')).
You couldn't remove realpath from the old implemenation, because that would raise a PermissionError.
If documented, I feel like this is a cleaner solution, what do you think?

[serhiy-storchaka]

Currently this workaround is built in ismount() (see bpo-2466). Your solution replaces realpath() with abspath(), but the latter is not suitable for this. realpath() is relatively expensive, so my solution only calls it if it is needed.