bpo-25803: Avoid incorrect errors raised by Path.mkdir(exist_ok=True)…

Lib/pathlib.py

@@ -1220,25 +1220,23 @@ def touch(self, mode=0o666, exist_ok=True):
         os.close(fd)
 
     def mkdir(self, mode=0o777, parents=False, exist_ok=False):
+        """
+        Create a new directory at this given path.
+        """
         if self._closed:
             self._raise_closed()
-        if not parents:
-            try:
-                self._accessor.mkdir(self, mode)
-            except FileExistsError:
-                if not exist_ok or not self.is_dir():
-                    raise
-        else:
-            try:
-                self._accessor.mkdir(self, mode)
-            except FileExistsError:
-                if not exist_ok or not self.is_dir():
-                    raise
-            except OSError as e:
-                if e.errno != ENOENT or self.parent == self:
-                    raise
-                self.parent.mkdir(parents=True)
-                self._accessor.mkdir(self, mode)
+        try:
+            self._accessor.mkdir(self, mode)
+        except FileNotFoundError:
+            if not parents or self.parent == self:
+                raise
+            self.parent.mkdir(parents=True)
+            self._accessor.mkdir(self, mode)
+        except OSError:
+            # Cannot rely on checking for EEXIST, since the operating system
+            # could give priority to other errors like EACCES or EROFS
+            if not exist_ok or not self.is_dir():
+                raise
 
     def chmod(self, mode):
         """

Lib/test/test_pathlib.py

@@ -1776,6 +1776,11 @@ def test_mkdir_exist_ok_with_parent(self):
         self.assertTrue(p.exists())
         self.assertEqual(p.stat().st_ctime, st_ctime_first)
 
+    def test_mkdir_exist_ok_root(self):
+        # Issue #25803: A drive root could raise PermissionError on Windows
+        self.cls('/').resolve().mkdir(exist_ok=True)
+        self.cls('/').resolve().mkdir(parents=True, exist_ok=True)
+
     @only_nt    # XXX: not sure how to test this on POSIX
     def test_mkdir_with_unknown_drive(self):
         for d in 'ZYXWVUTSRQPONMLKJIHGFEDCBA':

Misc/NEWS

@@ -27,6 +27,9 @@ Core and Builtins
 Library
 -------
 
+- bpo-25803: Avoid incorrect errors raised by Path.mkdir(exist_ok=True)
+  when the OS gives priority to errors such as EACCES over EEXIST.
+
 - bpo-29861: Release references to tasks, their arguments and their results
   as soon as they are finished in multiprocessing.Pool.