Skip unused calculations in sync handler. (#14908)

If a sync request does not need to calculate per-room entries &
is not generating presence & is not generating device list data
(e.g. during initial sync) avoid the expensive calculation of room
specific data.

This is a micro-optimisation for clients syncing simply to receive
to-device information.

3 files changed, 137 insertions, 125 deletions

diff --git a/changelog.d/14908.misc b/changelog.d/14908.misc
new file mode 100644
index 0000000000..3657623602
--- /dev/null
+++ b/changelog.d/14908.misc
@@ -0,0 +1 @@
+Improve performance of `/sync` in a few situations.
diff --git a/synapse/api/filtering.py b/synapse/api/filtering.py
index 4cf8f0cc8e..2b5af264b4 100644
--- a/synapse/api/filtering.py
+++ b/synapse/api/filtering.py
@@ -283,6 +283,9 @@ class FilterCollection:
             await self._room_filter.filter(events)
         )
 
+    def blocks_all_rooms(self) -> bool:
+        return self._room_filter.filters_all_rooms()
+
     def blocks_all_presence(self) -> bool:
         return (
             self._presence_filter.filters_all_types()
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 0cb8d5ef4b..3566537894 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1448,41 +1448,67 @@ class SyncHandler:
             sync_result_builder
         )
 
-        logger.debug("Fetching room data")
-
-        (
-            newly_joined_rooms,
-            newly_joined_or_invited_or_knocked_users,
-            newly_left_rooms,
-            newly_left_users,
-        ) = await self._generate_sync_entry_for_rooms(
-            sync_result_builder, account_data_by_room
-        )
-
         # Presence data is included if the server has it enabled and not filtered out.
-        include_presence_data = (
+        include_presence_data = bool(
             self.hs_config.server.use_presence
             and not sync_config.filter_collection.blocks_all_presence()
         )
-        if include_presence_data:
-            logger.debug("Fetching presence data")
-            await self._generate_sync_entry_for_presence(
-                sync_result_builder,
+        # Device list updates are sent if a since token is provided.
+        include_device_list_updates = bool(since_token and since_token.device_list_key)
+
+        # If we do not care about the rooms or things which depend on the room
+        # data (namely presence and device list updates), then we can skip
+        # this process completely.
+        device_lists = DeviceListUpdates()
+        if (
+            not sync_result_builder.sync_config.filter_collection.blocks_all_rooms()
+            or include_presence_data
+            or include_device_list_updates
+        ):
+            logger.debug("Fetching room data")
+
+            # Note that _generate_sync_entry_for_rooms sets sync_result_builder.joined, which
+            # is used in calculate_user_changes below.
+            (
                 newly_joined_rooms,
-                newly_joined_or_invited_or_knocked_users,
+                newly_left_rooms,
+            ) = await self._generate_sync_entry_for_rooms(
+                sync_result_builder, account_data_by_room
             )
 
+            # Work out which users have joined or left rooms we're in. We use this
+            # to build the presence and device_list parts of the sync response in
+            # `_generate_sync_entry_for_presence` and
+            # `_generate_sync_entry_for_device_list` respectively.
+            if include_presence_data or include_device_list_updates:
+                # This uses the sync_result_builder.joined which is set in
+                # `_generate_sync_entry_for_rooms`, if that didn't find any joined
+                # rooms for some reason it is a no-op.
+                (
+                    newly_joined_or_invited_or_knocked_users,
+                    newly_left_users,
+                ) = sync_result_builder.calculate_user_changes()
+
+                if include_presence_data:
+                    logger.debug("Fetching presence data")
+                    await self._generate_sync_entry_for_presence(
+                        sync_result_builder,
+                        newly_joined_rooms,
+                        newly_joined_or_invited_or_knocked_users,
+                    )
+
+                if include_device_list_updates:
+                    device_lists = await self._generate_sync_entry_for_device_list(
+                        sync_result_builder,
+                        newly_joined_rooms=newly_joined_rooms,
+                        newly_joined_or_invited_or_knocked_users=newly_joined_or_invited_or_knocked_users,
+                        newly_left_rooms=newly_left_rooms,
+                        newly_left_users=newly_left_users,
+                    )
+
         logger.debug("Fetching to-device data")
         await self._generate_sync_entry_for_to_device(sync_result_builder)
 
-        device_lists = await self._generate_sync_entry_for_device_list(
-            sync_result_builder,
-            newly_joined_rooms=newly_joined_rooms,
-            newly_joined_or_invited_or_knocked_users=newly_joined_or_invited_or_knocked_users,
-            newly_left_rooms=newly_left_rooms,
-            newly_left_users=newly_left_users,
-        )
-
         logger.debug("Fetching OTK data")
         device_id = sync_config.device_id
         one_time_keys_count: JsonDict = {}
@@ -1551,6 +1577,7 @@ class SyncHandler:
 
         user_id = sync_result_builder.sync_config.user.to_string()
         since_token = sync_result_builder.since_token
+        assert since_token is not None
 
         # Take a copy since these fields will be mutated later.
         newly_joined_or_invited_or_knocked_users = set(
@@ -1558,92 +1585,85 @@ class SyncHandler:
         )
         newly_left_users = set(newly_left_users)
 
-        if since_token and since_token.device_list_key:
-            # We want to figure out what user IDs the client should refetch
-            # device keys for, and which users we aren't going to track changes
-            # for anymore.
-            #
-            # For the first step we check:
-            #   a. if any users we share a room with have updated their devices,
-            #      and
-            #   b. we also check if we've joined any new rooms, or if a user has
-            #      joined a room we're in.
-            #
-            # For the second step we just find any users we no longer share a
-            # room with by looking at all users that have left a room plus users
-            # that were in a room we've left.
+        # We want to figure out what user IDs the client should refetch
+        # device keys for, and which users we aren't going to track changes
+        # for anymore.
+        #
+        # For the first step we check:
+        #   a. if any users we share a room with have updated their devices,
+        #      and
+        #   b. we also check if we've joined any new rooms, or if a user has
+        #      joined a room we're in.
+        #
+        # For the second step we just find any users we no longer share a
+        # room with by looking at all users that have left a room plus users
+        # that were in a room we've left.
 
-            users_that_have_changed = set()
+        users_that_have_changed = set()
 
-            joined_rooms = sync_result_builder.joined_room_ids
+        joined_rooms = sync_result_builder.joined_room_ids
 
-            # Step 1a, check for changes in devices of users we share a room
-            # with
-            #
-            # We do this in two different ways depending on what we have cached.
-            # If we already have a list of all the user that have changed since
-            # the last sync then it's likely more efficient to compare the rooms
-            # they're in with the rooms the syncing user is in.
-            #
-            # If we don't have that info cached then we get all the users that
-            # share a room with our user and check if those users have changed.
-            cache_result = self.store.get_cached_device_list_changes(
-                since_token.device_list_key
-            )
-            if cache_result.hit:
-                changed_users = cache_result.entities
-
-                result = await self.store.get_rooms_for_users(changed_users)
-
-                for changed_user_id, entries in result.items():
-                    # Check if the changed user shares any rooms with the user,
-                    # or if the changed user is the syncing user (as we always
-                    # want to include device list updates of their own devices).
-                    if user_id == changed_user_id or any(
-                        rid in joined_rooms for rid in entries
-                    ):
-                        users_that_have_changed.add(changed_user_id)
-            else:
-                users_that_have_changed = (
-                    await self._device_handler.get_device_changes_in_shared_rooms(
-                        user_id,
-                        sync_result_builder.joined_room_ids,
-                        from_token=since_token,
-                    )
-                )
-
-            # Step 1b, check for newly joined rooms
-            for room_id in newly_joined_rooms:
-                joined_users = await self.store.get_users_in_room(room_id)
-                newly_joined_or_invited_or_knocked_users.update(joined_users)
+        # Step 1a, check for changes in devices of users we share a room
+        # with
+        #
+        # We do this in two different ways depending on what we have cached.
+        # If we already have a list of all the user that have changed since
+        # the last sync then it's likely more efficient to compare the rooms
+        # they're in with the rooms the syncing user is in.
+        #
+        # If we don't have that info cached then we get all the users that
+        # share a room with our user and check if those users have changed.
+        cache_result = self.store.get_cached_device_list_changes(
+            since_token.device_list_key
+        )
+        if cache_result.hit:
+            changed_users = cache_result.entities
 
-            # TODO: Check that these users are actually new, i.e. either they
-            # weren't in the previous sync *or* they left and rejoined.
-            users_that_have_changed.update(newly_joined_or_invited_or_knocked_users)
+            result = await self.store.get_rooms_for_users(changed_users)
 
-            user_signatures_changed = (
-                await self.store.get_users_whose_signatures_changed(
-                    user_id, since_token.device_list_key
+            for changed_user_id, entries in result.items():
+                # Check if the changed user shares any rooms with the user,
+                # or if the changed user is the syncing user (as we always
+                # want to include device list updates of their own devices).
+                if user_id == changed_user_id or any(
+                    rid in joined_rooms for rid in entries
+                ):
+                    users_that_have_changed.add(changed_user_id)
+        else:
+            users_that_have_changed = (
+                await self._device_handler.get_device_changes_in_shared_rooms(
+                    user_id,
+                    sync_result_builder.joined_room_ids,
+                    from_token=since_token,
                 )
             )
-            users_that_have_changed.update(user_signatures_changed)
 
-            # Now find users that we no longer track
-            for room_id in newly_left_rooms:
-                left_users = await self.store.get_users_in_room(room_id)
-                newly_left_users.update(left_users)
+        # Step 1b, check for newly joined rooms
+        for room_id in newly_joined_rooms:
+            joined_users = await self.store.get_users_in_room(room_id)
+            newly_joined_or_invited_or_knocked_users.update(joined_users)
 
-            # Remove any users that we still share a room with.
-            left_users_rooms = await self.store.get_rooms_for_users(newly_left_users)
-            for user_id, entries in left_users_rooms.items():
-                if any(rid in joined_rooms for rid in entries):
-                    newly_left_users.discard(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.
+        users_that_have_changed.update(newly_joined_or_invited_or_knocked_users)
 
-            return DeviceListUpdates(
-                changed=users_that_have_changed, left=newly_left_users
-            )
-        else:
-            return DeviceListUpdates()
+        user_signatures_changed = await self.store.get_users_whose_signatures_changed(
+            user_id, since_token.device_list_key
+        )
+        users_that_have_changed.update(user_signatures_changed)
+
+        # Now find users that we no longer track
+        for room_id in newly_left_rooms:
+            left_users = await self.store.get_users_in_room(room_id)
+            newly_left_users.update(left_users)
+
+        # Remove any users that we still share a room with.
+        left_users_rooms = await self.store.get_rooms_for_users(newly_left_users)
+        for user_id, entries in left_users_rooms.items():
+            if any(rid in joined_rooms for rid in entries):
+                newly_left_users.discard(user_id)
+
+        return DeviceListUpdates(changed=users_that_have_changed, left=newly_left_users)
 
     @trace
     async def _generate_sync_entry_for_to_device(
@@ -1720,6 +1740,7 @@ class SyncHandler:
         since_token = sync_result_builder.since_token
 
         if since_token and not sync_result_builder.full_state:
+            # TODO Do not fetch room account data if it will be unused.
             (
                 global_account_data,
                 account_data_by_room,
@@ -1736,6 +1757,7 @@ class SyncHandler:
                     sync_config.user
                 )
         else:
+            # TODO Do not fetch room account data if it will be unused.
             (
                 global_account_data,
                 account_data_by_room,
@@ -1818,7 +1840,7 @@ class SyncHandler:
         self,
         sync_result_builder: "SyncResultBuilder",
         account_data_by_room: Dict[str, Dict[str, JsonDict]],
-    ) -> Tuple[AbstractSet[str], AbstractSet[str], AbstractSet[str], AbstractSet[str]]:
+    ) -> Tuple[AbstractSet[str], AbstractSet[str]]:
         """Generates the rooms portion of the sync response. Populates the
         `sync_result_builder` with the result.
 
@@ -1831,24 +1853,22 @@ class SyncHandler:
             account_data_by_room: Dictionary of per room account data
 
         Returns:
-            Returns a 4-tuple describing rooms the user has joined or left, and users who've
-            joined or left rooms any rooms the user is in. This gets used later in
-            `_generate_sync_entry_for_device_list`.
+            Returns a 2-tuple describing rooms the user has joined or left.
 
             Its entries are:
             - newly_joined_rooms
-            - newly_joined_or_invited_or_knocked_users
             - newly_left_rooms
-            - newly_left_users
         """
 
         since_token = sync_result_builder.since_token
         user_id = sync_result_builder.sync_config.user.to_string()
 
         # 1. Start by fetching all ephemeral events in rooms we've joined (if required).
-        if (
-            sync_result_builder.sync_config.filter_collection.blocks_all_room_ephemeral()
-        ):
+        block_all_room_ephemeral = (
+            sync_result_builder.sync_config.filter_collection.blocks_all_rooms()
+            or sync_result_builder.sync_config.filter_collection.blocks_all_room_ephemeral()
+        )
+        if block_all_room_ephemeral:
             ephemeral_by_room: Dict[str, List[JsonDict]] = {}
         else:
             now_token, ephemeral_by_room = await self.ephemeral_by_room(
@@ -1870,7 +1890,7 @@ class SyncHandler:
                     )
                     if not tags_by_room:
                         logger.debug("no-oping sync")
-                        return set(), set(), set(), set()
+                        return set(), set()
 
         # 3. Work out which rooms need reporting in the sync response.
         ignored_users = await self.store.ignored_users(user_id)
@@ -1899,6 +1919,7 @@ class SyncHandler:
         # joined or archived).
         async def handle_room_entries(room_entry: "RoomSyncResultBuilder") -> None:
             logger.debug("Generating room entry for %s", room_entry.room_id)
+            # Note that this mutates sync_result_builder.{joined,archived}.
             await self._generate_room_entry(
                 sync_result_builder,
                 room_entry,
@@ -1915,20 +1936,7 @@ class SyncHandler:
         sync_result_builder.invited.extend(invited)
         sync_result_builder.knocked.extend(knocked)
 
-        # 5. Work out which users have joined or left rooms we're in. We use this
-        # to build the device_list part of the sync response in
-        # `_generate_sync_entry_for_device_list`.
-        (
-            newly_joined_or_invited_or_knocked_users,
-            newly_left_users,
-        ) = sync_result_builder.calculate_user_changes()
-
-        return (
-            set(newly_joined_rooms),
-            newly_joined_or_invited_or_knocked_users,
-            set(newly_left_rooms),
-            newly_left_users,
-        )
+        return set(newly_joined_rooms), set(newly_left_rooms)
 
     async def _have_rooms_changed(
         self, sync_result_builder: "SyncResultBuilder"