Skip waiting for full state if a StateFilter does not require it (#12498)

If `StateFilter` specifies a state set which we will have regardless of
state-syncing, then we may as well return it immediately.

Author: richvdh

changelog.d/12498.misc

@@ -0,0 +1 @@
+Preparation for faster-room-join work: return subsets of room state which we already have, immediately.

synapse/storage/state.py

@@ -1,4 +1,5 @@
 # Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2022 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -15,6 +16,7 @@
 from typing import (
     TYPE_CHECKING,
     Awaitable,
+    Callable,
     Collection,
     Dict,
     Iterable,
@@ -532,6 +534,44 @@ def approx_difference(self, other: "StateFilter") -> "StateFilter":
             new_all, new_excludes, new_wildcards, new_concrete_keys
         )
 
+    def must_await_full_state(self, is_mine_id: Callable[[str], bool]) -> bool:
+        """Check if we need to wait for full state to complete to calculate this state
+
+        If we have a state filter which is completely satisfied even with partial
+        state, then we don't need to await_full_state before we can return it.
+
+        Args:
+            is_mine_id: a callable which confirms if a given state_key matches a mxid
+               of a local user
+        """
+
+        # TODO(faster_joins): it's not entirely clear that this is safe. In particular,
+        #  there may be circumstances in which we return a piece of state that, once we
+        #  resync the state, we discover is invalid. For example: if it turns out that
+        #  the sender of a piece of state wasn't actually in the room, then clearly that
+        #  state shouldn't have been returned.
+        #  We should at least add some tests around this to see what happens.
+
+        # if we haven't requested membership events, then it depends on the value of
+        # 'include_others'
+        if EventTypes.Member not in self.types:
+            return self.include_others
+
+        # if we're looking for *all* membership events, then we have to wait
+        member_state_keys = self.types[EventTypes.Member]
+        if member_state_keys is None:
+            return True
+
+        # otherwise, consider whose membership we are looking for. If it's entirely
+        # local users, then we don't need to wait.
+        for state_key in member_state_keys:
+            if not is_mine_id(state_key):
+                # remote user
+                return True
+
+        # local users only
+        return False
+
 
 _ALL_STATE_FILTER = StateFilter(types=frozendict(), include_others=True)
 _ALL_NON_MEMBER_STATE_FILTER = StateFilter(
@@ -544,6 +584,7 @@ class StateGroupStorage:
     """High level interface to fetching state for event."""
 
     def __init__(self, hs: "HomeServer", stores: "Databases"):
+        self._is_mine_id = hs.is_mine_id
         self.stores = stores
         self._partial_state_events_tracker = PartialStateEventsTracker(stores.main)
 
@@ -675,7 +716,13 @@ async def get_state_for_events(
             RuntimeError if we don't have a state group for one or more of the events
                (ie they are outliers or unknown)
         """
-        event_to_groups = await self.get_state_group_for_events(event_ids)
+        await_full_state = True
+        if state_filter and not state_filter.must_await_full_state(self._is_mine_id):
+            await_full_state = False
+
+        event_to_groups = await self.get_state_group_for_events(
+            event_ids, await_full_state=await_full_state
+        )
 
         groups = set(event_to_groups.values())
         group_to_state = await self.stores.state._get_state_for_groups(
@@ -699,7 +746,9 @@ async def get_state_for_events(
         return {event: event_to_state[event] for event in event_ids}
 
     async def get_state_ids_for_events(
-        self, event_ids: Collection[str], state_filter: Optional[StateFilter] = None
+        self,
+        event_ids: Collection[str],
+        state_filter: Optional[StateFilter] = None,
     ) -> Dict[str, StateMap[str]]:
         """
         Get the state dicts corresponding to a list of events, containing the event_ids
@@ -716,7 +765,13 @@ async def get_state_ids_for_events(
             RuntimeError if we don't have a state group for one or more of the events
                 (ie they are outliers or unknown)
         """
-        event_to_groups = await self.get_state_group_for_events(event_ids)
+        await_full_state = True
+        if state_filter and not state_filter.must_await_full_state(self._is_mine_id):
+            await_full_state = False
+
+        event_to_groups = await self.get_state_group_for_events(
+            event_ids, await_full_state=await_full_state
+        )
 
         groups = set(event_to_groups.values())
         group_to_state = await self.stores.state._get_state_for_groups(
@@ -802,7 +857,7 @@ async def get_state_group_for_events(
         Args:
             event_ids: events to get state groups for
             await_full_state: if true, will block if we do not yet have complete
-               state at this event.
+               state at these events.
         """
         if await_full_state:
             await self._partial_state_events_tracker.await_full_state(event_ids)