Refactor get_user_ids_changed to pull less from DB

When a client asks for users whose devices have changed since a token we
used to pull *all* users from the database since the token, which could
easily be thousands of rows for old tokens.

This PR changes this to only check for changes for users the client is
actually interested in.

Fixes #5553

2 files changed, 16 insertions, 18 deletions

diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index f59d0479b5..2b6c2117f9 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -101,9 +101,13 @@ class DeviceWorkerHandler(BaseHandler):
 
         room_ids = yield self.store.get_rooms_for_user(user_id)
 
-        # First we check if any devices have changed
+        # First we check if any devices have changed for users that we share
+        # rooms with.
+        users_who_share_room = yield self.store.get_users_who_share_room_with_user(
+            user_id
+        )
         changed = yield self.store.get_user_whose_devices_changed(
-            from_token.device_list_key
+            from_token.device_list_key, users_who_share_room
         )
 
         # Then work out if any users have since joined
@@ -188,10 +192,6 @@ class DeviceWorkerHandler(BaseHandler):
                         break
 
         if possibly_changed or possibly_left:
-            users_who_share_room = yield self.store.get_users_who_share_room_with_user(
-                user_id
-            )
-
             # Take the intersection of the users whose devices may have changed
             # and those that actually still share a room with the user
             possibly_joined = possibly_changed & users_who_share_room
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index c5188a1f8e..8249e75ecd 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1062,10 +1062,6 @@ class SyncHandler(object):
         since_token = sync_result_builder.since_token
 
         if since_token and since_token.device_list_key:
-            changed = yield self.store.get_user_whose_devices_changed(
-                since_token.device_list_key
-            )
-
             # TODO: Be more clever than this, i.e. remove users who we already
             # share a room with?
             for room_id in newly_joined_rooms:
@@ -1076,21 +1072,23 @@ class SyncHandler(object):
                 left_users = yield self.state.get_current_users_in_room(room_id)
                 newly_left_users.update(left_users)
 
+            users_who_share_room = yield self.store.get_users_who_share_room_with_user(
+                user_id
+            )
+
             # TODO: Check that these users are actually new, i.e. either they
             # weren't in the previous sync *or* they left and rejoined.
-            changed.update(newly_joined_or_invited_users)
-
-            if not changed and not newly_left_users:
-                defer.returnValue(DeviceLists(changed=[], left=newly_left_users))
+            changed = users_who_share_room & set(newly_joined_or_invited_users)
 
-            users_who_share_room = yield self.store.get_users_who_share_room_with_user(
-                user_id
+            changed_users = yield self.store.get_user_whose_devices_changed(
+                since_token.device_list_key, users_who_share_room
             )
 
+            changed.update(changed_users)
+
             defer.returnValue(
                 DeviceLists(
-                    changed=users_who_share_room & changed,
-                    left=set(newly_left_users) - users_who_share_room,
+                    changed=changed, left=set(newly_left_users) - users_who_share_room
                 )
             )
         else: