Fixed #12397 -- allow safe_join to work with the root file system path, which means you can have your root template or file upload path at this location. You almost certainly don't want to do this, except in *very* limited sandboxed situations.

tests/regressiontests/utils/os_utils.py

@@ -0,0 +1,21 @@
+from django.utils import unittest
+from django.utils._os import safe_join
+
+
+class SafeJoinTests(unittest.TestCase):
+    def test_base_path_ends_with_sep(self):
+        self.assertEqual(
+            safe_join("/abc/", "abc"),
+            "/abc/abc",
+        )
+
+    def test_root_path(self):
+        self.assertEqual(
+            safe_join("/", "path"),
+            "/path",
+        )
+
+        self.assertEqual(
+            safe_join("/", ""),
+            "/",
+        )