From 47ffc7e5482cc8d7bc376f362f8db5baddbcf4b3 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erikj@matrix.org>
Date: Tue, 26 Sep 2023 13:49:44 +0300
Subject: [PATCH] Reduce calls to `send_presence_to_destinations` (#16385)

---
 changelog.d/16385.misc       |  1 +
 synapse/handlers/presence.py | 33 ++++++++++++++++++---------------
 2 files changed, 19 insertions(+), 15 deletions(-)
 create mode 100644 changelog.d/16385.misc

diff --git a/changelog.d/16385.misc b/changelog.d/16385.misc
new file mode 100644
index 0000000000..d439a931d6
--- /dev/null
+++ b/changelog.d/16385.misc
@@ -0,0 +1 @@
+Minor performance improvement when sending presence to federated servers.
diff --git a/synapse/handlers/presence.py b/synapse/handlers/presence.py
index 375c7d0901..7c7cda3e95 100644
--- a/synapse/handlers/presence.py
+++ b/synapse/handlers/presence.py
@@ -401,9 +401,9 @@ class BasePresenceHandler(abc.ABC):
             states,
         )
 
-        for destination, host_states in hosts_to_states.items():
+        for destinations, host_states in hosts_to_states:
             await self._federation.send_presence_to_destinations(
-                host_states, [destination]
+                host_states, destinations
             )
 
     async def send_full_presence_to_users(self, user_ids: StrCollection) -> None:
@@ -1000,9 +1000,9 @@ class PresenceHandler(BasePresenceHandler):
                     list(to_federation_ping.values()),
                 )
 
-                for destination, states in hosts_to_states.items():
+                for destinations, states in hosts_to_states:
                     await self._federation_queue.send_presence_to_destinations(
-                        states, [destination]
+                        states, destinations
                     )
 
     @wrap_as_background_process("handle_presence_timeouts")
@@ -2276,7 +2276,7 @@ async def get_interested_remotes(
     store: DataStore,
     presence_router: PresenceRouter,
     states: List[UserPresenceState],
-) -> Dict[str, Set[UserPresenceState]]:
+) -> List[Tuple[StrCollection, Collection[UserPresenceState]]]:
     """Given a list of presence states figure out which remote servers
     should be sent which.
 
@@ -2290,23 +2290,26 @@ async def get_interested_remotes(
     Returns:
         A map from destinations to presence states to send to that destination.
     """
-    hosts_and_states: Dict[str, Set[UserPresenceState]] = {}
+    hosts_and_states: List[Tuple[StrCollection, Collection[UserPresenceState]]] = []
 
     # First we look up the rooms each user is in (as well as any explicit
     # subscriptions), then for each distinct room we look up the remote
     # hosts in those rooms.
-    room_ids_to_states, users_to_states = await get_interested_parties(
-        store, presence_router, states
-    )
+    for state in states:
+        room_ids = await store.get_rooms_for_user(state.user_id)
+        hosts: Set[str] = set()
+        for room_id in room_ids:
+            room_hosts = await store.get_current_hosts_in_room(room_id)
+            hosts.update(room_hosts)
+        hosts_and_states.append((hosts, [state]))
 
-    for room_id, states in room_ids_to_states.items():
-        hosts = await store.get_current_hosts_in_room(room_id)
-        for host in hosts:
-            hosts_and_states.setdefault(host, set()).update(states)
+    # Ask a presence routing module for any additional parties if one
+    # is loaded.
+    router_users_to_states = await presence_router.get_users_for_states(states)
 
-    for user_id, states in users_to_states.items():
+    for user_id, user_states in router_users_to_states.items():
         host = get_domain_from_id(user_id)
-        hosts_and_states.setdefault(host, set()).update(states)
+        hosts_and_states.append(([host], user_states))
 
     return hosts_and_states