From 8c2825276fec6e03434f1924482788ea3281a9fc Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Fri, 1 Jul 2022 10:19:27 +0100
Subject: [PATCH] Skip waiting for full state for incoming events (#13144)

When we receive an event over federation during a faster join, there is no need
to wait for full state, since we have a whole reconciliation process designed
to take the partial state into account.
---
 changelog.d/13144.misc    |  1 +
 synapse/state/__init__.py | 12 +++++++++---
 tests/test_state.py       |  4 +++-
 3 files changed, 13 insertions(+), 4 deletions(-)
 create mode 100644 changelog.d/13144.misc

diff --git a/changelog.d/13144.misc b/changelog.d/13144.misc
new file mode 100644
index 0000000000..34762e2fcd
--- /dev/null
+++ b/changelog.d/13144.misc
@@ -0,0 +1 @@
+Faster joins: skip waiting for full state when processing incoming events over federation.
diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py
index 9d3fe66100..d5cbdb3eef 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -249,8 +249,12 @@ class StateHandler:
                 partial_state = True
 
             logger.debug("calling resolve_state_groups from compute_event_context")
+            # we've already taken into account partial state, so no need to wait for
+            # complete state here.
             entry = await self.resolve_state_groups_for_events(
-                event.room_id, event.prev_event_ids()
+                event.room_id,
+                event.prev_event_ids(),
+                await_full_state=False,
             )
 
             state_ids_before_event = entry.state
@@ -335,7 +339,7 @@ class StateHandler:
 
     @measure_func()
     async def resolve_state_groups_for_events(
-        self, room_id: str, event_ids: Collection[str]
+        self, room_id: str, event_ids: Collection[str], await_full_state: bool = True
     ) -> _StateCacheEntry:
         """Given a list of event_ids this method fetches the state at each
         event, resolves conflicts between them and returns them.
@@ -343,6 +347,8 @@ class StateHandler:
         Args:
             room_id
             event_ids
+            await_full_state: if true, will block if we do not yet have complete
+               state at these events.
 
         Returns:
             The resolved state
@@ -350,7 +356,7 @@ class StateHandler:
         logger.debug("resolve_state_groups event_ids %s", event_ids)
 
         state_groups = await self._state_storage_controller.get_state_group_for_events(
-            event_ids
+            event_ids, await_full_state=await_full_state
         )
 
         state_group_ids = state_groups.values()
diff --git a/tests/test_state.py b/tests/test_state.py
index b005dd8d0f..7b3f52f68e 100644
--- a/tests/test_state.py
+++ b/tests/test_state.py
@@ -131,7 +131,9 @@ class _DummyStore:
     async def get_room_version_id(self, room_id):
         return RoomVersions.V1.identifier
 
-    async def get_state_group_for_events(self, event_ids):
+    async def get_state_group_for_events(
+        self, event_ids, await_full_state: bool = True
+    ):
         res = {}
         for event in event_ids:
             res[event] = self._event_to_state_group[event]