Faster room joins: Try other destinations when resyncing the state of a partial-state room

changelog.d/12812.misc

@@ -0,0 +1 @@
+Try other homeservers when re-syncing state for rooms with partial state.

synapse/federation/federation_client.py

@@ -416,7 +416,7 @@ async def get_room_state_ids(
         if not isinstance(state_event_ids, list) or not isinstance(
             auth_event_ids, list
         ):
-            raise Exception("invalid response from /state_ids")
+            raise InvalidResponseError("invalid response from /state_ids")
 
         return state_event_ids, auth_event_ids
 

synapse/handlers/federation.py

@@ -20,7 +20,16 @@
 import logging
 from enum import Enum
 from http import HTTPStatus
-from typing import TYPE_CHECKING, Dict, Iterable, List, Optional, Tuple, Union
+from typing import (
+    TYPE_CHECKING,
+    Collection,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    Tuple,
+    Union,
+)
 
 import attr
 from signedjson.key import decode_verify_key_bytes
@@ -34,6 +43,7 @@
     CodeMessageException,
     Codes,
     FederationDeniedError,
+    FederationError,
     HttpResponseException,
     NotFoundError,
     RequestSendFailed,
@@ -545,6 +555,7 @@ async def do_invite_join(
                     desc="sync_partial_state_room",
                     func=self._sync_partial_state_room,
                     destination=origin,
+                    destinations=ret.servers_in_room,
                     room_id=room_id,
                 )
 
@@ -1443,13 +1454,16 @@ async def get_room_complexity(
 
     async def _sync_partial_state_room(
         self,
-        destination: str,
+        destination: Optional[str],
+        destinations: Collection[str],
         room_id: str,
     ) -> None:
         """Background process to resync the state of a partial-state room
 
         Args:
-            destination: homeserver to pull the state from
+            destination: the initial homeserver to pull the state from
+            destinations: other homeservers to try to pull the state from, if
+                `destination` is unavailable
             room_id: room to be resynced
         """
 
@@ -1460,9 +1474,19 @@ async def _sync_partial_state_room(
         # TODO(faster_joins): what happens if we leave the room during a resync? if we
         #   really leave, that might mean we have difficulty getting the room state over
         #   federation.
-        #
-        # TODO(faster_joins): try other destinations if the one we have fails
 
+        # Make an infinite iterator of destinations to try. Once we find a working
+        # destination, we'll stick with it until it flakes.
+        if destination is not None:
+            # Move `destination` to the front of the list.
+            destinations = list(destinations)
+            if destination in destinations:
+                destinations.remove(destination)
+            destinations = [destination] + destinations
+        destination_iter = itertools.cycle(destinations)
+
+        # `destination` is now the current remote homeserver we're pulling from.
+        destination = next(destination_iter)
         logger.info("Syncing state for room %s via %s", room_id, destination)
 
         # we work through the queue in order of increasing stream ordering.
@@ -1498,6 +1522,36 @@ async def _sync_partial_state_room(
                 allow_rejected=True,
             )
             for event in events:
-                await self._federation_event_handler.update_state_for_partial_state_event(
-                    destination, event
-                )
+                for attempt in range(len(destinations)):
+                    try:
+                        await self._federation_event_handler.update_state_for_partial_state_event(
+                            destination, event
+                        )
+                        break
+                    except FederationError as e:
+                        if attempt == len(destinations) - 1:
+                            # We have tried every remote server for this event. Give up.
+                            logger.error(
+                                "Failed to get state for %s at %s from %s because %s, "
+                                "giving up!",
+                                room_id,
+                                event,
+                                destination,
+                                e,
+                            )
+                            raise
+
+                        # Try the next remote server.
+                        logger.info(
+                            "Failed to get state for %s at %s from %s because %s",
+                            room_id,
+                            event,
+                            destination,
+                            e,
+                        )
+                        destination = next(destination_iter)
+                        logger.info(
+                            "Syncing state for room %s via %s instead",
+                            room_id,
+                            destination,
+                        )

synapse/handlers/federation_event.py

@@ -485,6 +485,9 @@ async def update_state_for_partial_state_event(
         Args:
             destination: server to request full state from
             event: partial-state event to be de-partial-stated
+
+        Raises:
+            FederationError if we fail to request state from the remote server.
         """
         logger.info("Updating state for %s", event.event_id)
         with nested_logging_context(suffix=event.event_id):
@@ -792,6 +795,10 @@ async def _resolve_state_at_missing_prevs(
         Returns:
             if we already had all the prev events, `None`. Otherwise, returns a list of
             the events in the state at `event`.
+
+        Raises:
+            FederationError if we fail to get the state from the remote server after any
+                missing `prev_event`s.
         """
         room_id = event.room_id
         event_id = event.event_id