Bug fix: Memcached does not allow negative numbers passed to incr. Adding/using decr.

swift/common/memcached.py

@@ -190,6 +190,32 @@ class MemcacheRing(object):
             except Exception, e:
                 self._exception_occurred(server, e)
 
+    def decr(self, key, delta=1, timeout=0):
+        """
+        Decrements a key which has a numeric value by delta.
+        If the key can't be found, it's added as 0.  Memcached
+        will treat data values below 0 as 0 with incr/decr.
+
+        :param key: key
+        :param delta: amount to subtract to the value of key (or set
+                      as the value if the key is not found)
+        :param timeout: ttl in memcache
+        """
+        key = md5hash(key)
+        for (server, fp, sock) in self._get_conns(key):
+            try:
+                sock.sendall('decr %s %s\r\n' % (key, delta))
+                line = fp.readline().strip().split()
+                if line[0].upper() == 'NOT_FOUND':
+                    line[0] = '0'
+                    sock.sendall('add %s %d %d %s noreply\r\n%s\r\n' %
+                                 (key, 0, timeout, len(line[0]), line[0]))
+                ret = int(line[0].strip())
+                self._return_conn(server, fp, sock)
+                return ret
+            except Exception, e:
+                self._exception_occurred(server, e)
+
     def delete(self, key):
         """
         Deletes a key/value pair from memcache.

swift/common/middleware/ratelimit.py

@@ -148,7 +148,7 @@ class RateLimitMiddleware(object):
         max_sleep_m = self.max_sleep_time_seconds * self.clock_accuracy
         if max_sleep_m - need_to_sleep_m <= self.clock_accuracy * 0.01:
             # treat as no-op decrement time
-            self.memcache_client.incr(key, delta=-time_per_request_m)
+            self.memcache_client.decr(key, delta=time_per_request_m)
             raise MaxSleepTimeHit("Max Sleep Time Exceeded: %s" %
                                   need_to_sleep_m)
 

test/unit/common/middleware/test_ratelimit.py

@@ -36,9 +36,19 @@ class FakeMemcache(object):
         return True
 
     def incr(self, key, delta=1, timeout=0):
+        if delta < 0:
+            raise "Cannot incr by a negative number"
         self.store[key] = int(self.store.setdefault(key, 0)) + delta
         return int(self.store[key])
 
+    def decr(self, key, delta=1, timeout=0):
+        if delta < 0:
+            raise "Cannot decr by a negative number"
+        self.store[key] = int(self.store.setdefault(key, 0)) - delta
+        if self.store[key] < 0:
+            self.store[key] = 0
+        return int(self.store[key])
+
     @contextmanager
     def soft_lock(self, key, timeout=0, retries=5):
         yield True