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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"shutil.copytree infinite recursion crash on Windows Directory Junctions (WinError 206)","articleBody":"# Bug report\n\n### Bug description:\n\n# Bug Description\n\nOn Windows, `shutil.copytree` fails to detect recursive cycles when processing **Directory Junctions**.\n\nIn `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.\n\nIf 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`.\n\n# Reproduction Steps\n\nRun the following script on Windows:\n\n```python\nimport os\nimport shutil\nimport subprocess\nimport tempfile\n\ndef test_junction_crash():\n    base_temp = tempfile.mkdtemp()\n    src = os.path.join(base_temp, \"Source\")\n    dst = os.path.join(base_temp, \"Dest\")\n    junction = os.path.join(src, \"Loop\")\n    \n    try:\n        os.makedirs(src)\n        # Create a recursive Junction: Source/Loop -\u003e Source\n        subprocess.run(f'mklink /J \"{junction}\" \"{src}\"', shell=True, check=True)\n        \n        print(\"Starting copytree (expecting crash)...\")\n        shutil.copytree(src, dst)\n        \n    except Exception as e:\n        print(e)\n\nif __name__ == \"__main__\":\n    test_junction_crash()\n```\n\n# Actual Behavior\nThe script crashes with an unhandled `shutil.Error` wrapping a `WinError 206`, showing a deeply nested path.\n\n# Traceback:\n```\nTraceback (most recent call last):\n  ...\n  File \"C:\\...\\lib\\shutil.py\", line 559, in copytree\n    return _copytree(entries=entries, src=src, dst=dst, symlinks=symlinks,\n  File \"C:\\...\\lib\\shutil.py\", line 513, in _copytree\n    raise Error(errors)\nshutil.Error: [('...\\\\Source\\\\Infinite_Loop\\\\Infinite_Loop\\\\Infinite_Loop...', '...', \"[WinError 206] The filename or extension is too long\")]\n```\n\n# Expected Behavior\n\n`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.\n\n# System Details\n\nOS: Windows 10/11\nPython Version: Verified on main branch\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nWindows\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-142156\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ChuheLin","@type":"Person","name":"ChuheLin"},"datePublished":"2025-12-01T17:08:38.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/142155/cpython/issues/142155"}