Fixed #14315 -- Made memcached backend handle negative incr/decr values

Thanks Michael Manfre for the report and initial patch and
Tobias McNulty for the review.

django/core/cache/backends/memcached.py

@@ -90,6 +90,9 @@ class BaseMemcachedCache(BaseCache):
 
     def incr(self, key, delta=1, version=None):
         key = self.make_key(key, version=version)
+        # memcached doesn't support a negative delta
+        if delta < 0:
+            return self._cache.decr(key, -delta)
         try:
             val = self._cache.incr(key, delta)
 
@@ -105,6 +108,9 @@ class BaseMemcachedCache(BaseCache):
 
     def decr(self, key, delta=1, version=None):
         key = self.make_key(key, version=version)
+        # memcached doesn't support a negative delta
+        if delta < 0:
+            return self._cache.incr(key, -delta)
         try:
             val = self._cache.decr(key, delta)
 

tests/regressiontests/cache/tests.py

@@ -257,6 +257,7 @@ class BaseCacheTests(object):
         self.assertEqual(self.cache.get('answer'), 42)
         self.assertEqual(self.cache.incr('answer', 10), 52)
         self.assertEqual(self.cache.get('answer'), 52)
+        self.assertEqual(self.cache.incr('answer', -10), 42)
         self.assertRaises(ValueError, self.cache.incr, 'does_not_exist')
 
     def test_decr(self):
@@ -266,6 +267,7 @@ class BaseCacheTests(object):
         self.assertEqual(self.cache.get('answer'), 42)
         self.assertEqual(self.cache.decr('answer', 10), 32)
         self.assertEqual(self.cache.get('answer'), 32)
+        self.assertEqual(self.cache.decr('answer', -10), 42)
         self.assertRaises(ValueError, self.cache.decr, 'does_not_exist')
 
     def test_data_types(self):