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

閱讀 Lib/shutil.py 中 copytree 附近的程式碼，並使用 issue 中的腳本在 Windows 上重現回報的 junction cycle。完成標準是處理遞迴 junctions 時不會達到 path-length limit，且行為與所選的錯誤或跳過結果一致；gh-142156 已經連結到此 issue。