Separate `get_current_token` into two. (#8113)

The function is used for two purposes: 1) for subscribers of streams to
get a token they can use to get further updates with, and 2) for
replication to track position of the writers of the stream.

For streams with a single writer the two scenarios produce the same
result, however the situation becomes complicated for streams with
multiple writers. The current `MultiWriterIdGenerator` does not
correctly handle the first case (which is not an issue as its only used
for the `caches` stream which nothing subscribes to outside of
replication).

1 files changed, 8 insertions, 0 deletions

diff --git a/synapse/replication/slave/storage/_slaved_id_tracker.py b/synapse/replication/slave/storage/_slaved_id_tracker.py
index 9d1d173b2f..d43eaf3a29 100644
--- a/synapse/replication/slave/storage/_slaved_id_tracker.py
+++ b/synapse/replication/slave/storage/_slaved_id_tracker.py
@@ -33,3 +33,11 @@ class SlavedIdTracker(object):
             int
         """
         return self._current
+
+    def get_current_token_for_writer(self, instance_name: str) -> int:
+        """Returns the position of the given writer.
+
+        For streams with single writers this is equivalent to
+        `get_current_token`.
+        """
+        return self.get_current_token()