Fixed #25501 -- Made the file-based cache backend use the highest pickling protocol.

django/core/cache/backends/filebased.py

@@ -54,8 +54,8 @@ class FileBasedCache(BaseCache):
         try:
             with io.open(fd, 'wb') as f:
                 expiry = self.get_backend_timeout(timeout)
-                f.write(pickle.dumps(expiry, -1))
-                f.write(zlib.compress(pickle.dumps(value), -1))
+                f.write(pickle.dumps(expiry, pickle.HIGHEST_PROTOCOL))
+                f.write(zlib.compress(pickle.dumps(value, pickle.HIGHEST_PROTOCOL), -1))
             file_move_safe(tmp_path, fname, allow_overwrite=True)
             renamed = True
         finally:

docs/releases/1.10.txt

@@ -106,7 +106,7 @@ Minor features
 Cache
 ^^^^^
 
-* ...
+* The file-based cache backend now uses the highest pickling protocol.
 
 CSRF
 ^^^^

tests/cache/tests.py

@@ -65,6 +65,11 @@ class Unpickable(object):
         raise pickle.PickleError()
 
 
+class UnpicklableType(object):
+    # Unpicklable using the default pickling protocol on Python 2.
+    __slots__ = 'a',
+
+
 @override_settings(CACHES={
     'default': {
         'BACKEND': 'django.core.cache.backends.dummy.DummyCache',
@@ -1221,6 +1226,10 @@ class FileBasedCacheTests(BaseCacheTests, TestCase):
         cache.set('foo', 'bar')
         os.path.exists(self.dirname)
 
+    def test_cache_write_unpickable_type(self):
+        # This fails if not using the highest pickling protocol on Python 2.
+        cache.set('unpickable', UnpicklableType())
+
 
 @override_settings(CACHES={
     'default': {