GH-114847: Speed up posixpath.realpath()

[barneygale]

Apply the following optimizations to posixpath.realpath():

• Remove use of recursion
• Construct child paths directly rather than using join()
• Use os.getcwd[b]() rather than abspath()
• Use startswith(sep) rather than isabs()
• Use slicing rather than split()

---

$ ./python -m timeit -s "from os.path import realpath" "realpath('.')"
100000 loops, best of 5: 2.74 usec per loop  # before
200000 loops, best of 5: 1.53 usec per loop  # after
# --> 1.79x faster

$ ./python -m timeit -s "from os.path import realpath" "realpath('..')"
100000 loops, best of 5: 2.9  usec per loop  # before
200000 loops, best of 5: 1.71 usec per loop  # after
# --> 1.7x faster

$ ./python -m timeit -s "from os.path import realpath" "realpath('../..')"
 50000 loops, best of 5: 4.1  usec per loop  # before
200000 loops, best of 5: 1.99 usec per loop  # after
# --> 2.06x faster

$ ./python -m timeit -s "from os.path import realpath" "realpath('Lib/test')"
50000 loops, best of 5: 8.06 usec per loop  # before
50000 loops, best of 5: 6.19 usec per loop  # after
# --> 1.3x faster

$ ./python -m timeit -s "from os.path import realpath" "realpath('Lib/test/nonexistent/..')"
20000 loops, best of 5: 12.6 usec per loop  # before
50000 loops, best of 5:  9.4 usec per loop  # after
# --> 1.34x faster

$ ./python -m timeit -s "from os.path import realpath" "realpath('/home/barney')"
50000 loops, best of 5: 6.41 usec per loop  # before
50000 loops, best of 5: 4.62 usec per loop  # after
# --> 1.39x faster

---

• Issue: Speed up posixpath.realpath() #114847

[barneygale]

I've added a few wordy comments because:

1. When I first read the code, it took me some time to really understand what seen is doing
2. Applying tail call optimisation (this PR) further obscures the purpose of seen

Hopefully it's a bit easier for the next person to grok with the comments in place.

[gpshead]

I like that this gets rid of recursion.

[barneygale]

Given that symlinks are rare, perhaps I've actually slowed things down by checking in seen prior to running lstat(). I'll see if reverting that change is faster.

[encukou]

The introduction of a querying flag that's reset once changes behaviour, since rest can return back into an existing path.

Given a directory and a link to it:

$ mkdir a_dir
$ ln -s a_dir a_link

with Python 3.12 I get:

>>> os.path.realpath('foo/../a_link')
'/tmp/links/a_dir'

With this PR:

>>> os.path.realpath('foo/../a_link')
'/tmp/links/a_link'

The previous behaviour matches coreutils:

$ realpath -m foo/../a_link
/tmp/links/a_dir

[encukou]

Thanks. I see the other use of querying -- a symlink loop -- retains previous behaviour (which is tested), even though it doesn't match coreutils' realpath -m :/

$ mkdir a_dir
$ ln -s a_dir a_link
$ ln -s loop1 loop2
$ ln -s loop2 loop1
$ realpath -m loop1/../a_link/
/tmp/links/a_dir

>>> os.path.realpath('foo/../loop1/../a_link/')
'/tmp/links/a_link'

Guess that's one bug we need to let live.

[encukou]

LGTM, thank you!

[barneygale]

Thank you for the reviews!

[barneygale]

Thanks. I see the other use of querying -- a symlink loop -- retains previous behaviour (which is tested), even though it doesn't match coreutils' realpath -m :/

(For posterity:) I opened an issue about this: #117546