Store information in the database about partial-state joins

When we get a partial_state response from send_join, store information in the
database about it:
 * store a record about the room as a whole having partial state, and stash the
   list of member servers too.
 * flag the join event itself as having partial state
 * also, for any new events whose prev-events are partial-stated, note that
   they will *also* be partial-stated.

We don't yet make any attempt to interpret this data, so API calls (and a bunch
of other things) are just going to get incorrect datat.

synapse/events/snapshot.py

@@ -101,6 +101,9 @@ class EventContext:
 
             As with _current_state_ids, this is a private attribute. It should be
             accessed via get_prev_state_ids.
+
+        partial_state: if True, we may be storing this event with a temporary,
+            incomplete state.
     """
 
     rejected: Union[bool, str] = False
@@ -113,12 +116,15 @@ class EventContext:
     _current_state_ids: Optional[StateMap[str]] = None
     _prev_state_ids: Optional[StateMap[str]] = None
 
+    partial_state: bool = False
+
     @staticmethod
     def with_state(
         state_group: Optional[int],
         state_group_before_event: Optional[int],
         current_state_ids: Optional[StateMap[str]],
         prev_state_ids: Optional[StateMap[str]],
+        partial_state: bool,
         prev_group: Optional[int] = None,
         delta_ids: Optional[StateMap[str]] = None,
     ) -> "EventContext":
@@ -129,6 +135,7 @@ def with_state(
             state_group_before_event=state_group_before_event,
             prev_group=prev_group,
             delta_ids=delta_ids,
+            partial_state=partial_state,
         )
 
     @staticmethod
@@ -170,6 +177,7 @@ async def serialize(self, event: EventBase, store: "DataStore") -> JsonDict:
             "prev_group": self.prev_group,
             "delta_ids": _encode_state_dict(self.delta_ids),
             "app_service_id": self.app_service.id if self.app_service else None,
+            "partial_state": self.partial_state,
         }
 
     @staticmethod
@@ -196,6 +204,7 @@ def deserialize(storage: "Storage", input: JsonDict) -> "EventContext":
             prev_group=input["prev_group"],
             delta_ids=_decode_state_dict(input["delta_ids"]),
             rejected=input["rejected"],
+            partial_state=input.get("partial_state", False),
         )
 
         app_service_id = input["app_service_id"]

synapse/handlers/federation.py

@@ -519,8 +519,17 @@ async def do_invite_join(
                 auth_events=auth_chain,
             )
 
+            if ret.partial_state:
+                await self.store.store_partial_state_room(room_id, ret.servers_in_room)
+
             max_stream_id = await self._federation_event_handler.process_remote_join(
-                origin, room_id, auth_chain, state, event, room_version_obj
+                origin,
+                room_id,
+                auth_chain,
+                state,
+                event,
+                room_version_obj,
+                partial_state=ret.partial_state,
             )
 
             # We wait here until this instance has seen the events come down

synapse/handlers/federation_event.py

@@ -397,6 +397,7 @@ async def process_remote_join(
         state: List[EventBase],
         event: EventBase,
         room_version: RoomVersion,
+        partial_state: bool,
     ) -> int:
         """Persists the events returned by a send_join
 
@@ -412,6 +413,7 @@ async def process_remote_join(
             event
             room_version: The room version we expect this room to have, and
                 will raise if it doesn't match the version in the create event.
+            partial_state: True if the state omits non-critical membership events
 
         Returns:
             The stream ID after which all events have been persisted.
@@ -453,10 +455,14 @@ async def process_remote_join(
         )
 
         # and now persist the join event itself.
-        logger.info("Peristing join-via-remote %s", event)
+        logger.info(
+            "Peristing join-via-remote %s (partial_state: %s)", event, partial_state
+        )
         with nested_logging_context(suffix=event.event_id):
             context = await self._state_handler.compute_event_context(
-                event, old_state=state
+                event,
+                old_state=state,
+                partial_state=partial_state,
             )
 
             context = await self._check_event_auth(origin, event, context)
@@ -1791,6 +1797,7 @@ async def _update_context_for_auth_events(
             prev_state_ids=prev_state_ids,
             prev_group=prev_group,
             delta_ids=state_updates,
+            partial_state=context.partial_state,
         )
 
     async def _run_push_actions_and_persist_event(

synapse/state/__init__.py

@@ -258,7 +258,10 @@ async def get_hosts_in_room_at_events(
         return await self.store.get_joined_hosts(room_id, entry)
 
     async def compute_event_context(
-        self, event: EventBase, old_state: Optional[Iterable[EventBase]] = None
+        self,
+        event: EventBase,
+        old_state: Optional[Iterable[EventBase]] = None,
+        partial_state: bool = False,
     ) -> EventContext:
         """Build an EventContext structure for a non-outlier event.
 
@@ -273,6 +276,8 @@ async def compute_event_context(
                 calculated from existing events. This is normally only specified
                 when receiving an event from federation where we don't have the
                 prev events for, e.g. when backfilling.
+            partial_state: True if `old_state` is partial and omits non-critical
+                membership events
         Returns:
             The event context.
         """
@@ -295,8 +300,24 @@ async def compute_event_context(
 
         else:
             # otherwise, we'll need to resolve the state across the prev_events.
-            logger.debug("calling resolve_state_groups from compute_event_context")
 
+            # partial_state should not be set explicitly in this case:
+            # we work it out dynamically
+            assert not partial_state
+
+            # if any of the prev-events have partial state, so do we.
+            # (This is slightly racy - the prev-events might get fixed up before we use
+            # their states - but I don't think that really matters; it just means we
+            # might redundantly recalculate the state for this event later.)
+            prev_event_ids = event.prev_event_ids()
+            incomplete_prev_events = await self.store.get_partial_state_events(
+                prev_event_ids
+            )
+            if any(incomplete_prev_events):
+                logger.debug("Incoming event refers to prev-events with partial state")
+                partial_state = True
+
+            logger.debug("calling resolve_state_groups from compute_event_context")
             entry = await self.resolve_state_groups_for_events(
                 event.room_id, event.prev_event_ids()
             )
@@ -342,6 +363,7 @@ async def compute_event_context(
                 prev_state_ids=state_ids_before_event,
                 prev_group=state_group_before_event_prev_group,
                 delta_ids=deltas_to_state_group_before_event,
+                partial_state=partial_state,
             )
 
         #
@@ -373,6 +395,7 @@ async def compute_event_context(
             prev_state_ids=state_ids_before_event,
             prev_group=state_group_before_event,
             delta_ids=delta_ids,
+            partial_state=partial_state,
         )
 
     @measure_func()

synapse/storage/databases/main/events.py

@@ -2152,6 +2152,23 @@ def _store_event_state_mappings_txn(
 
             state_groups[event.event_id] = context.state_group
 
+        # if we have partial state for these events, record the fact. (This happens
+        # here rather than in _store_event_txn because it also needs to happen when
+        # we de-outlier an event.)
+        self.db_pool.simple_insert_many_txn(
+            txn,
+            table="partial_state_events",
+            keys=("room_id", "event_id"),
+            values=[
+                (
+                    event.room_id,
+                    event.event_id,
+                )
+                for event, ctx in events_and_contexts
+                if ctx.partial_state
+            ],
+        )
+
         self.db_pool.simple_upsert_many_txn(
             txn,
             table="event_to_state_groups",

synapse/storage/databases/main/events_worker.py

@@ -1953,3 +1953,31 @@ def get_event_id_for_timestamp_txn(txn: LoggingTransaction) -> Optional[str]:
             "get_event_id_for_timestamp_txn",
             get_event_id_for_timestamp_txn,
         )
+
+    @cachedList("is_partial_state_event", list_name="event_ids")
+    async def get_partial_state_events(
+        self, event_ids: Collection[str]
+    ) -> Dict[str, bool]:
+        """Checks which of the given events have partial state"""
+        result = await self.db_pool.simple_select_many_batch(
+            table="partial_state_events",
+            column="event_id",
+            iterable=event_ids,
+            retcols=["event_id"],
+            desc="get_partial_state_events",
+        )
+        # convert the result to a dict, to make @cachedList work
+        partial = {r["event_id"] for r in result}
+        return {e_id: e_id in partial for e_id in event_ids}
+
+    @cached()
+    async def is_partial_state_event(self, event_id: str) -> bool:
+        """Checks if the given event has partial state"""
+        result = await self.db_pool.simple_select_one_onecol(
+            table="partial_state_events",
+            keyvalues={"event_id": event_id},
+            retcol="1",
+            allow_none=True,
+            desc="is_partial_state_event",
+        )
+        return result is not None

synapse/storage/databases/main/room.py

@@ -20,6 +20,7 @@
     TYPE_CHECKING,
     Any,
     Awaitable,
+    Collection,
     Dict,
     List,
     Optional,
@@ -1543,6 +1544,42 @@ async def upsert_room_on_join(
             lock=False,
         )
 
+    async def store_partial_state_room(
+        self,
+        room_id: str,
+        servers: Collection[str],
+    ) -> None:
+        """Mark the given room as containing events with partial state
+
+        Args:
+            room_id: the ID of the room
+            servers: other servers known to be in the room
+        """
+        await self.db_pool.runInteraction(
+            "store_partial_state_room",
+            self._store_partial_state_room_txn,
+            room_id,
+            servers,
+        )
+
+    @staticmethod
+    def _store_partial_state_room_txn(
+        txn: LoggingTransaction, room_id: str, servers: Collection[str]
+    ):
+        DatabasePool.simple_insert_txn(
+            txn,
+            table="partial_state_rooms",
+            values={
+                "room_id": room_id,
+            },
+        )
+        DatabasePool.simple_insert_many_txn(
+            txn,
+            table="partial_state_rooms_servers",
+            keys=("room_id", "server_name"),
+            values=((room_id, s) for s in servers),
+        )
+
     async def maybe_store_room_on_outlier_membership(
         self, room_id: str, room_version: RoomVersion
     ) -> None: