Add a subclass of Retry providing linear backoff time

Signed-off-by: Wieland Hoffmann <themineo@gmail.com>

Author: mineo

musicbrainzngs/musicbrainz.py

@@ -23,9 +23,27 @@
 _version = "0.7dev"
 _log = logging.getLogger("musicbrainzngs")
 
-_retry = Retry(total=8,
-               status_forcelist=[500, 502, 503],
-               backoff_factor=0.1)
+
+class MBRetry(Retry):
+    """Retry class whose backoff time is::
+
+         { number of observed errors } * 2.0
+
+    """
+
+    def get_backoff_time(self):
+        """ Formula for computing the current backoff
+
+        :rtype: float
+        """
+        if self._observed_errors == 0:
+            return 0
+
+        return self._observed_errors * 2.0
+
+
+_retry = MBRetry(total=8,
+               status_forcelist=[500, 502, 503])
 
 LUCENE_SPECIAL = r'([+\-&|!(){}\[\]\^"~*?:\\\/])'