api: Use exponential backoff in call_on_each_event.

Previously we paused 1s after each failure.

Author: neiljp

zulip/zulip/__init__.py

@@ -619,7 +619,13 @@ def do_register() -> Tuple[str, int]:
         # Make long-polling requests with `get_events`. Once a request
         # has received an answer, pass it to the callback and before
         # making a new long-polling request.
-        while True:
+        # NOTE: Back off exponentially to cover against potential bugs in this
+        #       library causing a DoS attack against a server when getting errors
+        #       (explicit values listed for clarity)
+        backoff = RandomExponentialBackoff(maximum_retries=10,
+                                           timeout_success_equivalent=300,
+                                           delay_cap=90)
+        while backoff.keep_going():
             if queue_id is None:
                 (queue_id, last_event_id) = do_register()
 
@@ -649,12 +655,11 @@ def do_register() -> Tuple[str, int]:
                         #
                         # Reset queue_id to register a new event queue.
                         queue_id = None
-                # Add a pause here to cover against potential bugs in this library
-                # causing a DoS attack against a server when getting errors.
-                # TODO: Make this back off exponentially.
-                time.sleep(1)
+
+                backoff.fail()
                 continue
 
+            backoff.succeed()
             for event in res['events']:
                 last_event_id = max(last_event_id, int(event['id']))
                 callback(event)