shutil.copytree infinite recursion crash on Windows Directory Junctions (WinError 206)

Bug report

Bug description:

Bug Description

On Windows, shutil.copytree fails to detect recursive cycles when processing Directory Junctions.

In Lib/shutil.py, the code explicitly forces traversal into Windows Junctions by treating them as standard directories (is_symlink is forced to False). However, there is no cycle detection mechanism (e.g., checking st_ino / nFileIndex) for these traversed directories.

If a Junction points to a parent directory, shutil.copytree enters an infinite recursion loop until it hits the OS path length limit, causing the process to crash with WinError 206 (Path too long) or WinError 1921.

Reproduction Steps

Run the following script on Windows:

import os
import shutil
import subprocess
import tempfile

def test_junction_crash():
    base_temp = tempfile.mkdtemp()
    src = os.path.join(base_temp, "Source")
    dst = os.path.join(base_temp, "Dest")
    junction = os.path.join(src, "Loop")
    
    try:
        os.makedirs(src)
        # Create a recursive Junction: Source/Loop -> Source
        subprocess.run(f'mklink /J "{junction}" "{src}"', shell=True, check=True)
        
        print("Starting copytree (expecting crash)...")
        shutil.copytree(src, dst)
        
    except Exception as e:
        print(e)

if __name__ == "__main__":
    test_junction_crash()

Actual Behavior

The script crashes with an unhandled shutil.Error wrapping a WinError 206, showing a deeply nested path.

Traceback:

Traceback (most recent call last):
  ...
  File "C:\...\lib\shutil.py", line 559, in copytree
    return _copytree(entries=entries, src=src, dst=dst, symlinks=symlinks,
  File "C:\...\lib\shutil.py", line 513, in _copytree
    raise Error(errors)
shutil.Error: [('...\\Source\\Infinite_Loop\\Infinite_Loop\\Infinite_Loop...', '...', "[WinError 206] The filename or extension is too long")]

Expected Behavior

shutil.copytree should detect that the Junction points to a directory that has already been visited (cycle detection) and either raise a specific RecursionError / FileExistsError or skip it.

System Details

OS: Windows 10/11
Python Version: Verified on main branch

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-142156

Read Lib/shutil.py around copytree and reproduce the reported junction cycle on Windows using the script in the issue. Done means recursive junctions are handled without reaching the path-length limit, with behavior matching the selected error or skip outcome; gh-142156 is already linked to this issue.