bpo-29694: race condition in pathlib mkdir with flags parents=True

Lib/pathlib.py

@@ -1217,8 +1217,8 @@ def mkdir(self, mode=0o777, parents=False, exist_ok=False):
         except FileNotFoundError:
             if not parents or self.parent == self:
                 raise
-            self.parent.mkdir(parents=True)
-            self._accessor.mkdir(self, mode)
+            self.parent.mkdir(parents=True, exist_ok=True)
+            self.mkdir(mode, parents=False, exist_ok=exist_ok)
         except OSError:
             # Cannot rely on checking for EEXIST, since the operating system
             # could give priority to other errors like EACCES or EROFS

Lib/test/test_pathlib.py

@@ -8,6 +8,7 @@
 import stat
 import tempfile
 import unittest
+from unittest import mock
 
 from test import support
 android_not_root = support.android_not_root
@@ -1801,6 +1802,35 @@ def test_mkdir_no_parents_file(self):
             p.mkdir(exist_ok=True)
         self.assertEqual(cm.exception.errno, errno.EEXIST)
 
+    def test_mkdir_concurrent_parent_creation(self):
+        for pattern_num in range(32):
+            p = self.cls(BASE, 'dirCPC%d' % pattern_num)
+            self.assertFalse(p.exists())
+
+            def my_mkdir(path, mode=0o777):
+                path = str(path)
+                # Emulate another process that would create the directory
+                # just before we try to create it ourselves.  We do it
+                # in all possible pattern combinations, assuming that this
+                # function is called at most 5 times (dirCPC/dir1/dir2,
+                # dirCPC/dir1, dirCPC, dirCPC/dir1, dirCPC/dir1/dir2).
+                if pattern.pop():
+                    os.mkdir(path, mode)      # from another process
+                    concurrently_created.add(path)
+                os.mkdir(path, mode)          # our real call
+
+            pattern = [bool(pattern_num & (1 << n)) for n in range(5)]
+            concurrently_created = set()
+            p12 = p / 'dir1' / 'dir2'
+            try:
+                with mock.patch("pathlib._normal_accessor.mkdir", my_mkdir):
+                    p12.mkdir(parents=True, exist_ok=False)
+            except FileExistsError:
+                self.assertIn(str(p12), concurrently_created)
+            else:
+                self.assertNotIn(str(p12), concurrently_created)
+            self.assertTrue(p.exists())
+
     @support.skip_unless_symlink
     def test_symlink_to(self):
         P = self.cls(BASE)

Misc/NEWS

@@ -306,6 +306,10 @@ Extension Modules
 
 Library
 -------
+
+- bpo-29694: Fixed race condition in pathlib mkdir with flags
+  parents=True.  Patch by Armin Rigo.
+
 - bpo-29692: Fixed arbitrary unchaining of RuntimeError exceptions in 
   contextlib.contextmanager.
   Patch by Siddharth Velankar.