race in `realpath`

realpath_test.py

# If TEST_RACE = False, no exception is thrown. If it's True, it is. The only difference is
# a sleep in _joinrealpath.
TEST_RACE = False

import os
import time
import threading
import pathlib

def realpath(filename):
    """Return the canonical path of the specified filename, eliminating any
symbolic links encountered in the path."""
    filename = os.fspath(filename)
    path, ok = _joinrealpath(filename[:0], filename, {})
    return os.path.abspath(path)

# Join two paths, normalizing and eliminating any symbolic links
# encountered in the second path.
def _joinrealpath(path, rest, seen):
    if isinstance(path, bytes):
        sep = b'/'
        curdir = b'.'
        pardir = b'..'
    else:
        sep = '/'
        curdir = '.'
        pardir = '..'

    if os.path.isabs(rest):
        rest = rest[1:]
        path = sep

    while rest:
        name, _, rest = rest.partition(sep)
        if not name or name == curdir:
            # current dir
            continue
        if name == pardir:
            # parent dir
            if path:
                path, name = os.path.split(path)
                if name == pardir:
                    path = os.path.join(path, pardir, pardir)
            else:
                path = pardir
            continue
        newpath = os.path.join(path, name)
        if not os.path.islink(newpath):
            path = newpath
            continue
        if TEST_RACE:
            time.sleep(5)
        # Resolve the symbolic link
        if newpath in seen:
            # Already seen this path
            path = seen[newpath]
            if path is not None:
                # use cached value
                continue
            # The symlink is not resolved, so we must have a symlink loop.
            # Return already resolved part + rest of the path unchanged.
            return os.path.join(newpath, rest), False
        seen[newpath] = None # not resolved symlink
        path, ok = _joinrealpath(path, os.readlink(newpath), seen)
        if not ok:
            return os.path.join(path, rest), False
        seen[newpath] = path # resolved symlink

    return path, True

link_dst = 'test-link-dst'
link_src = 'test-link-src'

try:
    os.remove(link_dst)
    os.remove(link_src)
except OSError:
    pass

pathlib.Path(link_src).touch()
os.symlink(link_src, link_dst)

t = threading.Thread(target=realpath, args=(link_dst,))
t.start()
time.sleep(1)
os.remove(link_dst)
t.join()