Fixed #6464 -- Added incr() and decr() operations on cache backends. Atomic on Memcache; implemented as a 2 stage retrieve/update on other backends. Includes refactor of the cache tests to ensure all the backends are actually tested, and a fix to the DB cache backend that was discovered as a result. Thanks to Michael Malone for the original patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10031 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/core/cache/backends/base.py

@@ -65,6 +65,24 @@ class BaseCache(object):
         """
         return self.get(key) is not None
 
+    def incr(self, key, delta=1):
+        """
+        Add delta to value in the cache. If the key does not exist, raise a
+        ValueError exception.
+        """
+        if key not in self:
+            raise ValueError, "Key '%s' not found" % key
+        new_value = self.get(key) + delta
+        self.set(key, new_value)
+        return new_value
+
+    def decr(self, key, delta=1):
+        """
+        Subtract delta from value in the cache. If the key does not exist, raise
+        a ValueError exception.
+        """
+        return self.incr(key, -delta)
+
     def __contains__(self, key):
         """
         Returns True if the key is in the cache and has not expired.

django/core/cache/backends/db.py

@@ -64,6 +64,7 @@ class CacheClass(BaseCache):
                 cursor.execute("INSERT INTO %s (cache_key, value, expires) VALUES (%%s, %%s, %%s)" % self._table, [key, encoded, str(exp)])
         except DatabaseError:
             # To be threadsafe, updates/inserts are allowed to fail silently
+            transaction.rollback()
             return False
         else:
             transaction.commit_unless_managed()

django/core/cache/backends/memcached.py

@@ -45,3 +45,8 @@ class CacheClass(BaseCache):
     def close(self, **kwargs):
         self._cache.disconnect_all()
 
+    def incr(self, key, delta=1):
+        return self._cache.incr(key, delta)
+
+    def decr(self, key, delta=1):
+        return self._cache.decr(key, delta)