gh-142155: Fix infinite recursion in shutil.copytree on Windows junctions

Lib/shutil.py

@@ -543,12 +543,22 @@ def _ignore_patterns(path, names):
     return _ignore_patterns
 
 def _copytree(entries, src, dst, symlinks, ignore, copy_function,
-              ignore_dangling_symlinks, dirs_exist_ok=False):
+              ignore_dangling_symlinks, dirs_exist_ok=False, _seen=None):
     if ignore is not None:
         ignored_names = ignore(os.fspath(src), [x.name for x in entries])
     else:
         ignored_names = ()
 
+    # Track visited directories to detect cycles (e.g., Windows junctions)
+    if _seen is None:
+        _seen = frozenset()
+    src_st = os.stat(src)
+    src_id = (src_st.st_dev, src_st.st_ino)
+    if src_id in _seen:
+        raise Error([(src, dst, "Infinite recursion detected")])
+
+    _seen = _seen | {src_id}
+
     os.makedirs(dst, exist_ok=dirs_exist_ok)
     errors = []
     use_srcentry = copy_function is copy2 or copy_function is copy
@@ -583,12 +593,12 @@ def _copytree(entries, src, dst, symlinks, ignore, copy_function,
                     if srcentry.is_dir():
                         copytree(srcobj, dstname, symlinks, ignore,
                                  copy_function, ignore_dangling_symlinks,
-                                 dirs_exist_ok)
+                                 dirs_exist_ok, _seen=_seen)
                     else:
                         copy_function(srcobj, dstname)
             elif srcentry.is_dir():
                 copytree(srcobj, dstname, symlinks, ignore, copy_function,
-                         ignore_dangling_symlinks, dirs_exist_ok)
+                         ignore_dangling_symlinks, dirs_exist_ok, _seen=_seen)
             else:
                 # Will raise a SpecialFileError for unsupported file types
                 copy_function(srcobj, dstname)
@@ -609,7 +619,7 @@ def _copytree(entries, src, dst, symlinks, ignore, copy_function,
     return dst
 
 def copytree(src, dst, symlinks=False, ignore=None, copy_function=copy2,
-             ignore_dangling_symlinks=False, dirs_exist_ok=False):
+             ignore_dangling_symlinks=False, dirs_exist_ok=False, _seen=None):
     """Recursively copy a directory tree and return the destination directory.
 
     If exception(s) occur, an Error is raised with a list of reasons.
@@ -654,7 +664,7 @@ def copytree(src, dst, symlinks=False, ignore=None, copy_function=copy2,
     return _copytree(entries=entries, src=src, dst=dst, symlinks=symlinks,
                      ignore=ignore, copy_function=copy_function,
                      ignore_dangling_symlinks=ignore_dangling_symlinks,
-                     dirs_exist_ok=dirs_exist_ok)
+                     dirs_exist_ok=dirs_exist_ok, _seen=_seen)
 
 if hasattr(os.stat_result, 'st_file_attributes'):
     def _rmtree_islink(st):

Lib/test/test_shutil.py

@@ -1099,6 +1099,42 @@ def test_copytree_subdirectory(self):
         rv = shutil.copytree(src_dir, dst_dir)
         self.assertEqual(['pol'], os.listdir(rv))
 
+    @unittest.skipUnless(sys.platform == "win32", "Windows-specific test")
+    def test_copytree_recursive_junction(self):
+        # Test that copytree raises Error for recursive junctions (Windows)
+        base_dir = self.mkdtemp()
+        self.addCleanup(shutil.rmtree, base_dir, ignore_errors=True)
+
+        # Create source directory structure
+        src_dir = os.path.join(base_dir, "source")
+        junction_dir = os.path.join(src_dir, "junction")
+        os.makedirs(junction_dir)
+
+        # Create a junction pointing to its parent, creating a cycle
+        junction_target = os.path.dirname(junction_dir)  # Points to Source
+        try:
+            result = subprocess.run(
+                ["mklink", "/J", junction_dir, junction_target],
+                shell=True, check=False, capture_output=True, text=True
+            )
+            if result.returncode != 0:
+                # Skip if we don't have permission to create junctions
+                self.skipTest(f"Failed to create junction: {result.stderr.strip()}")
+        except Exception as e:
+            # Skip if mklink is not available or fails for any reason
+            self.skipTest(f"Failed to create junction: {e}")
+
+        # Create destination directory
+        dst_dir = os.path.join(base_dir, "Dest")
+
+        # Test that copytree raises Error with infinite recursion message
+        with self.assertRaises(shutil.Error) as cm:
+            shutil.copytree(src_dir, dst_dir)
+
+        # Verify the error message contains "Infinite recursion detected"
+        self.assertIn("Infinite recursion detected", str(cm.exception))
+
+
 class TestCopy(BaseTest, unittest.TestCase):
 
     ### shutil.copymode

Misc/NEWS.d/next/Library/2025-12-01-17-17-12.gh-issue-142155.wiUAfL.rst

@@ -0,0 +1 @@
+Fix infinite recursion crash in :func:`shutil.copytree` when a cycle involving a directory junction is present on Windows.