[3.0.x] Fixed #31253 -- Fixed data loss possibility when using caching from async code.

Case missed in a415ce70be.

Backport of e3f6e18513 from master

django/core/cache/__init__.py

@@ -12,7 +12,7 @@ object.
 
 See docs/topics/cache.txt for information on the public API.
 """
-from threading import local
+from asgiref.local import Local
 
 from django.conf import settings
 from django.core import signals
@@ -61,7 +61,7 @@ class CacheHandler:
     Ensure only one instance of each alias exists per thread.
     """
     def __init__(self):
-        self._caches = local()
+        self._caches = Local()
 
     def __getitem__(self, alias):
         try:

docs/releases/3.0.4.txt

@@ -9,4 +9,5 @@ Django 3.0.4 fixes several bugs in 3.0.3.
 Bugfixes
 ========
 
-* ...
+* Fixed a data loss possibility when using caching from async code
+  (:ticket:`31253`).

tests/async/tests.py

@@ -4,6 +4,7 @@ from unittest import mock, skipIf
 
 from asgiref.sync import async_to_sync
 
+from django.core.cache import DEFAULT_CACHE_ALIAS, caches
 from django.core.exceptions import SynchronousOnlyOperation
 from django.test import SimpleTestCase
 from django.utils.asyncio import async_unsafe
@@ -11,6 +12,18 @@ from django.utils.asyncio import async_unsafe
 from .models import SimpleModel
 
 
+@skipIf(sys.platform == 'win32' and (3, 8, 0) < sys.version_info < (3, 8, 1), 'https://bugs.python.org/issue38563')
+class CacheTest(SimpleTestCase):
+    def test_caches_local(self):
+        @async_to_sync
+        async def async_cache():
+            return caches[DEFAULT_CACHE_ALIAS]
+
+        cache_1 = async_cache()
+        cache_2 = async_cache()
+        self.assertIs(cache_1, cache_2)
+
+
 @skipIf(sys.platform == 'win32' and (3, 8, 0) < sys.version_info < (3, 8, 1), 'https://bugs.python.org/issue38563')
 class DatabaseConnectionTest(SimpleTestCase):
     """A database connection cannot be used in an async context."""