Avoid race condition in ensure_directory. Ref #1083.

CHANGES.rst

 v36.1.0
 -------
 
+* #1083: Avoid race condition on directory creation in
+  ``pkg_resources.ensure_directory``.
+
 * Removed deprecation of and restored support for
   ``upload_docs`` command for sites other than PyPI.
   Only warehouse is dropping support, but services like

pkg_resources/__init__.py

@@ -67,6 +67,7 @@ try:
 except ImportError:
     importlib_machinery = None
 
+from . import py31compat
 from pkg_resources.extern import appdirs
 from pkg_resources.extern import packaging
 __import__('pkg_resources.extern.packaging.version')
@@ -74,6 +75,7 @@ __import__('pkg_resources.extern.packaging.specifiers')
 __import__('pkg_resources.extern.packaging.requirements')
 __import__('pkg_resources.extern.packaging.markers')
 
+
 if (3, 0) < sys.version_info < (3, 3):
     raise RuntimeError("Python 3.3 or later is required")
 
@@ -2958,8 +2960,7 @@ def _find_adapter(registry, ob):
 def ensure_directory(path):
     """Ensure that the parent directory of `path` exists"""
     dirname = os.path.dirname(path)
-    if not os.path.isdir(dirname):
-        os.makedirs(dirname)
+    py31compat.makedirs(dirname, exist_ok=True)
 
 
 def _bypass_ensure_directory(path):

pkg_resources/py31compat.py

+import os
+import errno
+import sys
+
+
+PY32 = sys.version_info >= (3, 2)
+
+
+def _makedirs_31(path, exist_ok=False):
+    try:
+        os.makedirs(path)
+    except OSError as exc:
+        if exc.errno != errno.EEXIST:
+            raise
+
+
+makedirs = os.makedirs if PY32 else _makedirs_31