Fixed #22939 -- Delayed admin_static backend detection

Thanks generalov for the report.

django/contrib/admin/templatetags/admin_static.py

@@ -3,9 +3,14 @@ from django.template import Library
 
 register = Library()
 
-if apps.is_installed('django.contrib.staticfiles'):
-    from django.contrib.staticfiles.templatetags.staticfiles import static
-else:
-    from django.templatetags.static import static
+_static = None
 
-static = register.simple_tag(static)
+@register.simple_tag
+def static(path):
+    global _static
+    if _static is None:
+        if apps.is_installed('django.contrib.staticfiles'):
+            from django.contrib.staticfiles.templatetags.staticfiles import static as _static
+        else:
+            from django.templatetags.static import static as _static
+    return _static(path)

tests/admin_views/tests.py

@@ -18,6 +18,7 @@ from django.contrib.auth import get_permission_codename
 from django.contrib.admin import ModelAdmin
 from django.contrib.admin.helpers import ACTION_CHECKBOX_NAME
 from django.contrib.admin.models import LogEntry, DELETION
+from django.contrib.admin.templatetags.admin_static import static
 from django.contrib.admin.templatetags.admin_urls import add_preserved_filters
 from django.contrib.admin.tests import AdminSeleniumWebDriverTestCase
 from django.contrib.admin.utils import quote
@@ -26,6 +27,7 @@ from django.contrib.admin.views.main import IS_POPUP_VAR
 from django.contrib.auth import REDIRECT_FIELD_NAME
 from django.contrib.auth.models import Group, User, Permission
 from django.contrib.contenttypes.models import ContentType
+from django.contrib.staticfiles.storage import staticfiles_storage
 from django.forms.utils import ErrorList
 from django.template.response import TemplateResponse
 from django.test import TestCase, skipUnlessDBFeature
@@ -101,6 +103,18 @@ class AdminViewBasicTest(AdminViewBasicTestCase):
             '/test_admin/%s/admin_views/article/add/' % self.urlbit,
             status_code=301)
 
+    def test_admin_static_template_tag(self):
+        """
+        Test that admin_static.static is pointing to the collectstatic version
+        (as django.contrib.collectstatic is in installed apps).
+        """
+        old_url = staticfiles_storage.base_url
+        staticfiles_storage.base_url = '/test/'
+        try:
+            self.assertEqual(static('path'), '/test/path')
+        finally:
+            staticfiles_storage.base_url = old_url
+
     def test_basic_add_GET(self):
         """
         A smoke test to ensure GET on the add_view works.