Add test for cache_path race condition

This mocks out the isdir call so that the directory is created
immediately after you determine whether or not it exists, thus
simulating a race condition between two threads or processes
creating the same directory.

pkg_resources/tests/test_pkg_resources.py

@@ -12,6 +12,11 @@ import stat
 import distutils.dist
 import distutils.command.install_egg_info
 
+try:
+    from unittest import mock
+except ImportError:
+    import mock
+
 from pkg_resources.extern.six.moves import map
 from pkg_resources.extern.six import text_type, string_types
 
@@ -138,8 +143,34 @@ class TestResourceManager:
         message = "Unexpected type from get_cache_path: " + type_
         assert isinstance(path, string_types), message
 
+    def test_get_cache_path_race(self, tmpdir):
+        # Patch to os.path.isdir to create a race condition
+        def patched_isdir(dirname, unpatched_isdir=pkg_resources.isdir):
+            patched_isdir.dirnames.append(dirname)
+
+            was_dir = unpatched_isdir(dirname)
+            if not was_dir:
+                os.makedirs(dirname)
+            return was_dir
+
+        patched_isdir.dirnames = []
+
+        # Get a cache path with a "race condition"
+        mgr = pkg_resources.ResourceManager()
+        mgr.set_extraction_path(str(tmpdir))
+
+        archive_name = os.sep.join(('foo', 'bar', 'baz'))
+        with mock.patch.object(pkg_resources, 'isdir', new=patched_isdir):
+            mgr.get_cache_path(archive_name)
+
+        # Because this test relies on the implementation details of this
+        # function, these assertions are a sentinel to ensure that the
+        # test suite will not fail silently if the implementation changes.
+        called_dirnames = patched_isdir.dirnames
+        assert len(called_dirnames) == 2
+        assert called_dirnames[0].split(os.sep)[-2:] == ['foo', 'bar']
+        assert called_dirnames[1].split(os.sep)[-1:] == ['foo']
 
-class TestIndependence:
     """
     Tests to ensure that pkg_resources runs independently from setuptools.
     """