Fixed #29867 -- Added support for storing None value in caches.

Many of the cache operations make use of the default argument to the
.get() operation to determine whether the key was found in the cache.
The default value of the default argument is None, so this results in
these operations assuming that None is not stored in the cache when it
actually is. Adding a sentinel object solves this issue.

Unfortunately the unmaintained python-memcached library does not support
a default argument to .get(), so the previous behavior is preserved for
the deprecated MemcachedCache backend.

tests/cache/tests.py

@@ -278,6 +278,14 @@ class BaseCacheTests:
     # A common set of tests to apply to all cache backends
     factory = RequestFactory()
 
+    # RemovedInDjango41Warning: python-memcached doesn't support .get() with
+    # default.
+    supports_get_with_default = True
+
+    # Some clients raise custom exceptions when .incr() or .decr() are called
+    # with a non-integer value.
+    incr_decr_type_error = TypeError
+
     def tearDown(self):
         cache.clear()
 
@@ -320,6 +328,8 @@ class BaseCacheTests:
         self.assertEqual(cache.get_many(['a', 'c', 'd']), {'a': 'a', 'c': 'c', 'd': 'd'})
         self.assertEqual(cache.get_many(['a', 'b', 'e']), {'a': 'a', 'b': 'b'})
         self.assertEqual(cache.get_many(iter(['a', 'b', 'e'])), {'a': 'a', 'b': 'b'})
+        cache.set_many({'x': None, 'y': 1})
+        self.assertEqual(cache.get_many(['x', 'y']), {'x': None, 'y': 1})
 
     def test_delete(self):
         # Cache keys can be deleted
@@ -339,12 +349,22 @@ class BaseCacheTests:
         self.assertIs(cache.has_key("goodbye1"), False)
         cache.set("no_expiry", "here", None)
         self.assertIs(cache.has_key("no_expiry"), True)
+        cache.set('null', None)
+        self.assertIs(
+            cache.has_key('null'),
+            True if self.supports_get_with_default else False,
+        )
 
     def test_in(self):
         # The in operator can be used to inspect cache contents
         cache.set("hello2", "goodbye2")
         self.assertIn("hello2", cache)
         self.assertNotIn("goodbye2", cache)
+        cache.set('null', None)
+        if self.supports_get_with_default:
+            self.assertIn('null', cache)
+        else:
+            self.assertNotIn('null', cache)
 
     def test_incr(self):
         # Cache values can be incremented
@@ -356,6 +376,9 @@ class BaseCacheTests:
         self.assertEqual(cache.incr('answer', -10), 42)
         with self.assertRaises(ValueError):
             cache.incr('does_not_exist')
+        cache.set('null', None)
+        with self.assertRaises(self.incr_decr_type_error):
+            cache.incr('null')
 
     def test_decr(self):
         # Cache values can be decremented
@@ -367,6 +390,9 @@ class BaseCacheTests:
         self.assertEqual(cache.decr('answer', -10), 42)
         with self.assertRaises(ValueError):
             cache.decr('does_not_exist')
+        cache.set('null', None)
+        with self.assertRaises(self.incr_decr_type_error):
+            cache.decr('null')
 
     def test_close(self):
         self.assertTrue(hasattr(cache, 'close'))
@@ -914,6 +940,13 @@ class BaseCacheTests:
         with self.assertRaises(ValueError):
             cache.incr_version('does_not_exist')
 
+        cache.set('null', None)
+        if self.supports_get_with_default:
+            self.assertEqual(cache.incr_version('null'), 2)
+        else:
+            with self.assertRaises(self.incr_decr_type_error):
+                cache.incr_version('null')
+
     def test_decr_version(self):
         cache.set('answer', 42, version=2)
         self.assertIsNone(cache.get('answer'))
@@ -938,6 +971,13 @@ class BaseCacheTests:
         with self.assertRaises(ValueError):
             cache.decr_version('does_not_exist', version=2)
 
+        cache.set('null', None, version=2)
+        if self.supports_get_with_default:
+            self.assertEqual(cache.decr_version('null', version=2), 1)
+        else:
+            with self.assertRaises(self.incr_decr_type_error):
+                cache.decr_version('null', version=2)
+
     def test_custom_key_func(self):
         # Two caches with different key functions aren't visible to each other
         cache.set('answer1', 42)
@@ -995,6 +1035,11 @@ class BaseCacheTests:
         self.assertEqual(cache.get_or_set('projector', 42), 42)
         self.assertEqual(cache.get('projector'), 42)
         self.assertIsNone(cache.get_or_set('null', None))
+        if self.supports_get_with_default:
+            # Previous get_or_set() stores None in the cache.
+            self.assertIsNone(cache.get('null', 'default'))
+        else:
+            self.assertEqual(cache.get('null', 'default'), 'default')
 
     def test_get_or_set_callable(self):
         def my_callable():
@@ -1003,10 +1048,12 @@ class BaseCacheTests:
         self.assertEqual(cache.get_or_set('mykey', my_callable), 'value')
         self.assertEqual(cache.get_or_set('mykey', my_callable()), 'value')
 
-    def test_get_or_set_callable_returning_none(self):
-        self.assertIsNone(cache.get_or_set('mykey', lambda: None))
-        # Previous get_or_set() doesn't store None in the cache.
-        self.assertEqual(cache.get('mykey', 'default'), 'default')
+        self.assertIsNone(cache.get_or_set('null', lambda: None))
+        if self.supports_get_with_default:
+            # Previous get_or_set() stores None in the cache.
+            self.assertIsNone(cache.get('null', 'default'))
+        else:
+            self.assertEqual(cache.get('null', 'default'), 'default')
 
     def test_get_or_set_version(self):
         msg = "get_or_set() missing 1 required positional argument: 'default'"
@@ -1399,6 +1446,8 @@ MemcachedCache_params = configured_caches.get('django.core.cache.backends.memcac
 ))
 class MemcachedCacheTests(BaseMemcachedTests, TestCase):
     base_params = MemcachedCache_params
+    supports_get_with_default = False
+    incr_decr_type_error = ValueError
 
     def test_memcached_uses_highest_pickle_version(self):
         # Regression test for #19810
@@ -1459,6 +1508,10 @@ class PyLibMCCacheTests(BaseMemcachedTests, TestCase):
     # libmemcached manages its own connections.
     should_disconnect_on_close = False
 
+    @property
+    def incr_decr_type_error(self):
+        return cache._lib.ClientError
+
     @override_settings(CACHES=caches_setting_for_tests(
         base=PyLibMCCache_params,
         exclude=memcached_excluded_caches,
@@ -1497,6 +1550,10 @@ class PyLibMCCacheTests(BaseMemcachedTests, TestCase):
 class PyMemcacheCacheTests(BaseMemcachedTests, TestCase):
     base_params = PyMemcacheCache_params
 
+    @property
+    def incr_decr_type_error(self):
+        return cache._lib.exceptions.MemcacheClientError
+
     def test_pymemcache_highest_pickle_version(self):
         self.assertEqual(
             cache._cache.default_kwargs['serde']._serialize_func.keywords['pickle_version'],