From 3ff7b15bb79f2ee5b7af245c55ae14546243bb77 Mon Sep 17 00:00:00 2001
From: Iuri de Silvio <iurisilvio@gmail.com>
Date: Sun, 31 Oct 2021 21:54:13 -0300
Subject: [PATCH] Fixed #33252 -- Made cache middlewares thread-safe.

---
 django/middleware/cache.py | 11 ++++++++---
 tests/cache/tests.py       | 15 +++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/django/middleware/cache.py b/django/middleware/cache.py
index 85de0e91c5..c191118183 100644
--- a/django/middleware/cache.py
+++ b/django/middleware/cache.py
@@ -67,7 +67,10 @@ class UpdateCacheMiddleware(MiddlewareMixin):
         self.page_timeout = None
         self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
         self.cache_alias = settings.CACHE_MIDDLEWARE_ALIAS
-        self.cache = caches[self.cache_alias]
+
+    @property
+    def cache(self):
+        return caches[self.cache_alias]
 
     def _should_update_cache(self, request, response):
         return hasattr(request, '_cache_update_cache') and request._cache_update_cache
@@ -126,7 +129,10 @@ class FetchFromCacheMiddleware(MiddlewareMixin):
         super().__init__(get_response)
         self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
         self.cache_alias = settings.CACHE_MIDDLEWARE_ALIAS
-        self.cache = caches[self.cache_alias]
+
+    @property
+    def cache(self):
+        return caches[self.cache_alias]
 
     def process_request(self, request):
         """
@@ -183,7 +189,6 @@ class CacheMiddleware(UpdateCacheMiddleware, FetchFromCacheMiddleware):
             if cache_alias is None:
                 cache_alias = DEFAULT_CACHE_ALIAS
             self.cache_alias = cache_alias
-            self.cache = caches[self.cache_alias]
         except KeyError:
             pass
 
diff --git a/tests/cache/tests.py b/tests/cache/tests.py
index 13ff0a45aa..ddfe9ddfe6 100644
--- a/tests/cache/tests.py
+++ b/tests/cache/tests.py
@@ -2488,6 +2488,21 @@ class CacheMiddlewareTest(SimpleTestCase):
         self.assertIn('Cache-Control', response)
         self.assertIn('Expires', response)
 
+    def test_per_thread(self):
+        """The cache instance is different for each thread."""
+        thread_caches = []
+        middleware = CacheMiddleware(empty_response)
+
+        def runner():
+            thread_caches.append(middleware.cache)
+
+        for _ in range(2):
+            thread = threading.Thread(target=runner)
+            thread.start()
+            thread.join()
+
+        self.assertIsNot(thread_caches[0], thread_caches[1])
+
 
 @override_settings(
     CACHE_MIDDLEWARE_KEY_PREFIX='settingsprefix',