Fixed #26888 -- Fixed concurrency issue in URL resolver.

Fixed a regression in 625b8e9295d79650208bfb3fca8bf9e6aaf578e4:
improper short-circuiting could lead to a KeyError when threads
concurrently call RegexURLResolver._populate().

1 files changed, 13 insertions, 0 deletions

diff --git a/tests/urlpatterns_reverse/tests.py b/tests/urlpatterns_reverse/tests.py
index 0201e28bf8..67a4370e78 100644
--- a/tests/urlpatterns_reverse/tests.py
+++ b/tests/urlpatterns_reverse/tests.py
@@ -5,6 +5,7 @@ Unit tests for reverse URL lookups.
 from __future__ import unicode_literals
 
 import sys
+import threading
 
 from admin_scripts.tests import AdminScriptTestCase
 
@@ -429,6 +430,18 @@ class ResolverTests(SimpleTestCase):
         self.assertTrue(resolver._is_callback('urlpatterns_reverse.nested_urls.View3'))
         self.assertFalse(resolver._is_callback('urlpatterns_reverse.nested_urls.blub'))
 
+    def test_populate_concurrency(self):
+        """
+        RegexURLResolver._populate() can be called concurrently, but not more
+        than once per thread (#26888).
+        """
+        resolver = RegexURLResolver(r'^/', 'urlpatterns_reverse.urls')
+        resolver._local.populating = True
+        thread = threading.Thread(target=resolver._populate)
+        thread.start()
+        thread.join()
+        self.assertNotEqual(resolver._reverse_dict, {})
+
 
 @override_settings(ROOT_URLCONF='urlpatterns_reverse.reverse_lazy_urls')
 class ReverseLazyTest(TestCase):