[1.10.x] 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().

Backport of 389a5318a06e7e4d8f8aba14af88c4cc4ea0db47 from master

django/urls/resolvers.py

@@ -9,6 +9,7 @@ from __future__ import unicode_literals
 
 import functools
 import re
+import threading
 from importlib import import_module
 
 from django.conf import settings
@@ -164,7 +165,7 @@ class RegexURLResolver(LocaleRegexProvider):
         # urlpatterns
         self._callback_strs = set()
         self._populated = False
-        self._populating = False
+        self._local = threading.local()
 
     def __repr__(self):
         if isinstance(self.urlconf_name, list) and len(self.urlconf_name):
@@ -178,9 +179,13 @@ class RegexURLResolver(LocaleRegexProvider):
         )
 
     def _populate(self):
-        if self._populating:
+        # Short-circuit if called recursively in this thread to prevent
+        # infinite recursion. Concurrent threads may call this at the same
+        # time and will need to continue, so set 'populating' on a
+        # thread-local variable.
+        if getattr(self._local, 'populating', False):
             return
-        self._populating = True
+        self._local.populating = True
         lookups = MultiValueDict()
         namespaces = {}
         apps = {}
@@ -213,7 +218,7 @@ class RegexURLResolver(LocaleRegexProvider):
                         namespaces[namespace] = (p_pattern + prefix, sub_pattern)
                     for app_name, namespace_list in pattern.app_dict.items():
                         apps.setdefault(app_name, []).extend(namespace_list)
-                if not pattern._populating:
+                if not getattr(pattern._local, 'populating', False):
                     pattern._populate()
                 self._callback_strs.update(pattern._callback_strs)
             else:
@@ -225,7 +230,7 @@ class RegexURLResolver(LocaleRegexProvider):
         self._namespace_dict[language_code] = namespaces
         self._app_dict[language_code] = apps
         self._populated = True
-        self._populating = False
+        self._local.populating = False
 
     @property
     def reverse_dict(self):

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):