[1.7.x] Fixed #22495 -- Locmem cache.add() failed with infinite timeouts

cache.add() incorrectly succeeded when there was an existing key
with an infinite (None) timeout.
Backport of af5f688392 from master.

django/core/cache/backends/locmem.py

@@ -29,8 +29,8 @@ class LocMemCache(BaseCache):
         self.validate_key(key)
         pickled = pickle.dumps(value, pickle.HIGHEST_PROTOCOL)
         with self._lock.writer():
-            exp = self._expire_info.get(key)
-            if exp is None or exp <= time.time():
+            exp = self._expire_info.get(key, 0)
+            if exp is not None and exp <= time.time():
                 self._set(key, pickled, timeout)
                 return True
             return False

tests/cache/tests.py

@@ -491,6 +491,9 @@ class BaseCacheTests(object):
 
         cache.add('key2', 'ham', None)
         self.assertEqual(cache.get('key2'), 'ham')
+        added = cache.add('key1', 'new eggs', None)
+        self.assertEqual(added, False)
+        self.assertEqual(cache.get('key1'), 'eggs')
 
         cache.set_many({'key3': 'sausage', 'key4': 'lobster bisque'}, None)
         self.assertEqual(cache.get('key3'), 'sausage')