Rewrite prune_old_outbound_device_pokes for efficiency (#7159)

make sure we clear out all but one update for the user

synapse/handlers/federation.py

@@ -49,6 +49,7 @@ from synapse.event_auth import auth_types_for_event
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
 from synapse.events.validator import EventValidator
+from synapse.handlers._base import BaseHandler
 from synapse.logging.context import (
     make_deferred_yieldable,
     nested_logging_context,
@@ -69,10 +70,9 @@ from synapse.types import JsonDict, StateMap, UserID, get_domain_from_id
 from synapse.util.async_helpers import Linearizer, concurrently_execute
 from synapse.util.distributor import user_joined_room
 from synapse.util.retryutils import NotRetryingDestination
+from synapse.util.stringutils import shortstr
 from synapse.visibility import filter_events_for_server
 
-from ._base import BaseHandler
-
 logger = logging.getLogger(__name__)
 
 
@@ -93,27 +93,6 @@ class _NewEventInfo:
     auth_events = attr.ib(type=Optional[StateMap[EventBase]], default=None)
 
 
-def shortstr(iterable, maxitems=5):
-    """If iterable has maxitems or fewer, return the stringification of a list
-    containing those items.
-
-    Otherwise, return the stringification of a a list with the first maxitems items,
-    followed by "...".
-
-    Args:
-        iterable (Iterable): iterable to truncate
-        maxitems (int): number of items to return before truncating
-
-    Returns:
-        unicode
-    """
-
-    items = list(itertools.islice(iterable, maxitems + 1))
-    if len(items) <= maxitems:
-        return str(items)
-    return "[" + ", ".join(repr(r) for r in items[:maxitems]) + ", ...]"
-
-
 class FederationHandler(BaseHandler):
     """Handles events that originated from federation.
         Responsible for: