• Issue:shutil.copytree infinite recursion crash on Windows Directory Junctions (WinError 206) #142155

Summary

This PR fixes a critical crash (WinError 206 / WinError 1921) inshutil.copytree on Windows when encountering directory junctions that point to a parent directory (recursive cycles).

The Problem

On Windows,shutil.copytree contains specific logic to force traversal into Directory Junctions by treating them as standard directories (is_symlink is forced toFalse).

However, unlikeos.walk orshutil.rmtree (after recent fixes),copytree lacked a cycle detection mechanism for these traversed junctions. This caused infinite recursion until the path exceeded the OS limit, resulting in an unhandledOSError (Crash) rather than a PythonRecursionError.

The Fix

I implemented cycle detection using file system identity (st_dev,st_ino), similar to how deepcopy handles recursion:

1. Added a hidden_seen set parameter tocopytree and its helper_copytree.
2. Before entering a directory, the code now checks if(st.st_dev, st.st_ino) is already in_seen.
3. If a cycle is detected, it raises ashutil.Error("Infinite recursion detected") immediately, preventing the crash.

Verification

1. Regression Test: Addedtest_copytree_recursive_junction toLib/test/test_shutil.py.
   • Status:PASSED on local Windows build.
2. Manual Verification: Verified with a reproduction script that previously caused aWinError 206 crash.
   • Before fix: Script crashed after creating a path > 4000 chars.
   • After fix: Script correctly raisesshutil.Error: Infinite recursion detected.