Issue #29416: Prevent infinite loop in pathlib.Path.mkdir

Lib/pathlib.py

@@ -1235,7 +1235,7 @@ class Path(PurePath):
                 if not exist_ok or not self.is_dir():
                     raise
             except OSError as e:
-                if e.errno != ENOENT:
+                if e.errno != ENOENT or self.parent == self:
                     raise
                 self.parent.mkdir(parents=True)
                 self._accessor.mkdir(self, mode)

Lib/test/test_pathlib.py

@@ -1776,6 +1776,17 @@ class _BasePathTest(object):
         self.assertTrue(p.exists())
         self.assertEqual(p.stat().st_ctime, st_ctime_first)
 
+    @only_nt    # XXX: not sure how to test this on POSIX
+    def test_mkdir_with_unknown_drive(self):
+        for d in 'ZYXWVUTSRQPONMLKJIHGFEDCBA':
+            p = self.cls(d + ':\\')
+            if not p.is_dir():
+                break
+        else:
+            self.skipTest("cannot find a drive that doesn't exist")
+        with self.assertRaises(OSError):
+            (p / 'child' / 'path').mkdir(parents=True)
+
     def test_mkdir_with_child_file(self):
         p = self.cls(BASE, 'dirB', 'fileB')
         self.assertTrue(p.exists())

Misc/NEWS

@@ -55,6 +55,8 @@ Extension Modules
 Library
 -------
 
+- Issue #29416: Prevent infinite loop in pathlib.Path.mkdir
+
 - Issue #29444: Fixed out-of-bounds buffer access in the group() method of
   the match object.  Based on patch by WGH.