[1.10.x] Fixed #19914 -- Fixed test failures with pylibmc.

Backport of 674e3fe13e from master

tests/cache/tests.py

@@ -1204,7 +1204,15 @@ class BaseMemcachedTests(BaseCacheTests):
         self.assertEqual(cache.get('small_value'), 'a')
 
         large_value = 'a' * (max_value_length + 1)
-        cache.set('small_value', large_value)
+        try:
+            cache.set('small_value', large_value)
+        except Exception:
+            # Some clients (e.g. pylibmc) raise when the value is too large,
+            # while others (e.g. python-memcached) intentionally return True
+            # indicating success. This test is primarily checking that the key
+            # was deleted, so the return/exception behavior for the set()
+            # itself is not important.
+            pass
         # small_value should be deleted, or set if configured to accept larger values
         value = cache.get('small_value')
         self.assertTrue(value is None or value == large_value)
@@ -1232,6 +1240,15 @@ class MemcachedCacheTests(BaseMemcachedTests, TestCase):
 class PyLibMCCacheTests(BaseMemcachedTests, TestCase):
     base_params = PyLibMCCache_params
 
+    # By default, pylibmc/libmemcached don't verify keys client-side and so
+    # this test triggers a server-side bug that causes later tests to fail
+    # (#19914). The `verify_keys` behavior option could be set to True (which
+    # would avoid triggering the server-side bug), however this test would
+    # still fail due to https://github.com/lericson/pylibmc/issues/219.
+    @unittest.skip("triggers a memcached-server bug, causing subsequent tests to fail")
+    def test_invalid_key_characters(self):
+        pass
+
 
 @override_settings(CACHES=caches_setting_for_tests(
     BACKEND='django.core.cache.backends.filebased.FileBasedCache',