Reduce device lists replication traffic. (#17333)

Reduce the replication traffic of device lists, by not sending every
destination that needs to be sent the device list update over
replication. Instead a "hosts to send to have been calculated"
notification over replication, and then federation senders read the
destinations from the DB.

For non federation senders this should heavily reduce the impact of a
user in many large rooms changing a device.

1 files changed, 8 insertions, 4 deletions

diff --git a/synapse/replication/tcp/streams/_base.py b/synapse/replication/tcp/streams/_base.py

index 661206c841..d021904de7 100644
--- a/synapse/replication/tcp/streams/_base.py
+++ b/synapse/replication/tcp/streams/_base.py
@@ -549,10 +549,14 @@ class DeviceListsStream(_StreamFromIdGen):
 
     @attr.s(slots=True, frozen=True, auto_attribs=True)
     class DeviceListsStreamRow:
-        entity: str
+        user_id: str
         # Indicates that a user has signed their own device with their user-signing key
         is_signature: bool
 
+        # Indicates if this is a notification that we've calculated the hosts we
+        # need to send the update to.
+        hosts_calculated: bool
+
     NAME = "device_lists"
     ROW_TYPE = DeviceListsStreamRow
 
@@ -594,13 +598,13 @@ class DeviceListsStream(_StreamFromIdGen):
             upper_limit_token = min(upper_limit_token, signatures_to_token)
 
         device_updates = [
-            (stream_id, (entity, False))
-            for stream_id, (entity,) in device_updates
+            (stream_id, (entity, False, hosts))
+            for stream_id, (entity, hosts) in device_updates
             if stream_id <= upper_limit_token
         ]
 
         signatures_updates = [
-            (stream_id, (entity, True))
+            (stream_id, (entity, True, False))
             for stream_id, (entity,) in signatures_updates
             if stream_id <= upper_limit_token
         ]