Fix bug where a new writer advances their token too quickly (#16473)

* Fix bug where a new writer advances their token too quickly

When starting a new writer (for e.g. persisting events), the
`MultiWriterIdGenerator` doesn't have a minimum token for it as there
are no rows matching that new writer in the DB.

This results in the the first stream ID it acquired being announced as
persisted *before* it actually finishes persisting, if another writer
gets and persists a subsequent stream ID. This is due to the logic of
setting the minimum persisted position to the minimum known position of
across all writers, and the new writer starts off not being considered.

* Fix sending out POSITIONs when our token advances without update

Broke in #14820

* For replication HTTP requests, only wait for minimal position

1 files changed, 5 insertions, 5 deletions

diff --git a/synapse/replication/tcp/streams/partial_state.py b/synapse/replication/tcp/streams/partial_state.py
index a8ce5ffd72..ad181d7e93 100644
--- a/synapse/replication/tcp/streams/partial_state.py
+++ b/synapse/replication/tcp/streams/partial_state.py
@@ -15,7 +15,7 @@ from typing import TYPE_CHECKING
 
 import attr
 
-from synapse.replication.tcp.streams import Stream
+from synapse.replication.tcp.streams._base import _StreamFromIdGen
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -27,7 +27,7 @@ class UnPartialStatedRoomStreamRow:
     room_id: str
 
 
-class UnPartialStatedRoomStream(Stream):
+class UnPartialStatedRoomStream(_StreamFromIdGen):
     """
     Stream to notify about rooms becoming un-partial-stated;
     that is, when the background sync finishes such that we now have full state for
@@ -41,8 +41,8 @@ class UnPartialStatedRoomStream(Stream):
         store = hs.get_datastores().main
         super().__init__(
             hs.get_instance_name(),
-            store.get_un_partial_stated_rooms_token,
             store.get_un_partial_stated_rooms_from_stream,
+            store._un_partial_stated_rooms_stream_id_gen,
         )
 
 
@@ -56,7 +56,7 @@ class UnPartialStatedEventStreamRow:
     rejection_status_changed: bool
 
 
-class UnPartialStatedEventStream(Stream):
+class UnPartialStatedEventStream(_StreamFromIdGen):
     """
     Stream to notify about events becoming un-partial-stated.
     """
@@ -68,6 +68,6 @@ class UnPartialStatedEventStream(Stream):
         store = hs.get_datastores().main
         super().__init__(
             hs.get_instance_name(),
-            store.get_un_partial_stated_events_token,
             store.get_un_partial_stated_events_from_stream,
+            store._un_partial_stated_events_stream_id_gen,
         )