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).

synapse/storage/util/id_generators.py

@@ -158,6 +158,14 @@ class StreamIdGenerator(object):
 
             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()
+
 
 class ChainedIdGenerator(object):
     """Used to generate new stream ids where the stream must be kept in sync
@@ -216,6 +224,14 @@ class ChainedIdGenerator(object):
             "Attempted to advance token on source for table %r", self._table
         )
 
+    def get_current_token_for_writer(self, instance_name: str) -> Tuple[int, 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()
+
 
 class MultiWriterIdGenerator:
     """An ID generator that tracks a stream that can have multiple writers.
@@ -298,7 +314,7 @@ class MultiWriterIdGenerator:
         # Assert the fetched ID is actually greater than what we currently
         # believe the ID to be. If not, then the sequence and table have got
         # out of sync somehow.
-        assert self.get_current_token() < next_id
+        assert self.get_current_token_for_writer(self._instance_name) < next_id
 
         with self._lock:
             self._unfinished_ids.add(next_id)
@@ -344,16 +360,18 @@ class MultiWriterIdGenerator:
                 curr = self._current_positions.get(self._instance_name, 0)
                 self._current_positions[self._instance_name] = max(curr, next_id)
 
-    def get_current_token(self, instance_name: str = None) -> int:
-        """Gets the current position of a named writer (defaults to current
-        instance).
-
-        Returns 0 if we don't have a position for the named writer (likely due
-        to it being a new writer).
+    def get_current_token(self) -> int:
+        """Returns the maximum stream id such that all stream ids less than or
+        equal to it have been successfully persisted.
         """
 
-        if instance_name is None:
-            instance_name = self._instance_name
+        # Currently we don't support this operation, as it's not obvious how to
+        # condense the stream positions of multiple writers into a single int.
+        raise NotImplementedError()
+
+    def get_current_token_for_writer(self, instance_name: str) -> int:
+        """Returns the position of the given writer.
+        """
 
         with self._lock:
             return self._current_positions.get(instance_name, 0)