gh-117394: Reduce syscalls for posixpath.ismount

[nineteendo]

Benchmark:

python -m timeit -s "import before.posixpath" "before.posixpath.ismount('/Volumes/2GB_001')" && python -m timeit -s "import after.posixpath" "after.posixpath.ismount('/Volumes/2GB_001')"
10000 loops, best of 5: 21.3 usec per loop # before
50000 loops, best of 5: 8.5 usec per loop # after
# -> 2.51x faster

• Issue: Reduce syscalls for posixpath.ismount #117394

[barneygale]

What happens if /mnt/storage is a mount point, /mnt/storage/link is a symlink to /mnt, and you call ismount('/mnt/storage/link/storage')? I think the new implementation gives a different answer.

[nineteendo]

Good thinking! The old implementation returns True in that case, the new implementation False.
But that can easily fixed by replacing os.lstat() with os.stat().

[barneygale]

Serhiy suggested a similar patch previously: #46718 (comment)

Compared to Serhiy's patch, this PR adds a abspath() call, which internally calls normpath(). I think that eliminates the remaining edge cases (e.g. trailing slashes, dots in paths, etc).

[nineteendo]

What's going on here? https://github.com/python/cpython/actions/runs/8504085005/job/23290509561?pr=117395#step:16:481
I don't know the value of ABSTFN.

[nineteendo]

Oh, I know why. This test needs to be updated: it monkey patches os.lstat instead of os.stat.

[serhiy-storchaka]

Sorry, but abspath() is unsuitable for this. See details in #117394 (comment).