Issue #25583: Avoid incorrect errors raised by os.makedirs(exist_ok=True)

Lib/os.py

@@ -226,7 +226,7 @@ def makedirs(name, mode=0o777, exist_ok=False):
         try:
             makedirs(head, mode, exist_ok)
         except FileExistsError:
-            # be happy if someone already created the path
+            # Defeats race condition when another thread created the path
             pass
         cdir = curdir
         if isinstance(tail, bytes):
@@ -235,8 +235,10 @@ def makedirs(name, mode=0o777, exist_ok=False):
             return
     try:
         mkdir(name, mode)
-    except OSError as e:
-        if not exist_ok or e.errno != errno.EEXIST or not path.isdir(name):
+    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 path.isdir(name):
             raise
 
 def removedirs(name):

Lib/test/test_os.py

@@ -971,6 +971,9 @@ class MakedirTests(unittest.TestCase):
         os.makedirs(path, mode=mode, exist_ok=True)
         os.umask(old_mask)
 
+        # Issue #25583: A drive root could raise PermissionError on Windows
+        os.makedirs(os.path.abspath('/'), exist_ok=True)
+
     @unittest.skipUnless(hasattr(os, 'chown'), 'test needs os.chown')
     def test_chown_uid_gid_arguments_must_be_index(self):
         stat = os.stat(support.TESTFN)

Misc/NEWS

@@ -106,6 +106,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #25583: Avoid incorrect errors raised by os.makedirs(exist_ok=True)
+  when the OS gives priority to errors such as EACCES over EEXIST.
+
 - Issue #25593: Change semantics of EventLoop.stop() in asyncio.
 
 - Issue #6973: When we know a subprocess.Popen process has died, do