Remove ChainedIdGenerator. (#8123)

It's just a thin wrapper around two ID gens to make `get_current_token`
and `get_next` return tuples. This can easily be replaced by calling the
appropriate methods on the underlying ID gens directly.

synapse/storage/util/id_generators.py

@@ -16,7 +16,7 @@
 import contextlib
 import threading
 from collections import deque
-from typing import Dict, Set, Tuple
+from typing import Dict, Set
 
 from typing_extensions import Deque
 
@@ -167,72 +167,6 @@ class StreamIdGenerator(object):
         return self.get_current_token()
 
 
-class ChainedIdGenerator(object):
-    """Used to generate new stream ids where the stream must be kept in sync
-    with another stream. It generates pairs of IDs, the first element is an
-    integer ID for this stream, the second element is the ID for the stream
-    that this stream needs to be kept in sync with."""
-
-    def __init__(self, chained_generator, db_conn, table, column):
-        self.chained_generator = chained_generator
-        self._table = table
-        self._lock = threading.Lock()
-        self._current_max = _load_current_id(db_conn, table, column)
-        self._unfinished_ids = deque()  # type: Deque[Tuple[int, int]]
-
-    def get_next(self):
-        """
-        Usage:
-            with stream_id_gen.get_next() as (stream_id, chained_id):
-                # ... persist event ...
-        """
-        with self._lock:
-            self._current_max += 1
-            next_id = self._current_max
-            chained_id = self.chained_generator.get_current_token()
-
-            self._unfinished_ids.append((next_id, chained_id))
-
-        @contextlib.contextmanager
-        def manager():
-            try:
-                yield (next_id, chained_id)
-            finally:
-                with self._lock:
-                    self._unfinished_ids.remove((next_id, chained_id))
-
-        return manager()
-
-    def get_current_token(self):
-        """Returns the maximum stream id such that all stream ids less than or
-        equal to it have been successfully persisted.
-        """
-        with self._lock:
-            if self._unfinished_ids:
-                stream_id, chained_id = self._unfinished_ids[0]
-                return stream_id - 1, chained_id
-
-            return self._current_max, self.chained_generator.get_current_token()
-
-    def advance(self, token: int):
-        """Stub implementation for advancing the token when receiving updates
-        over replication; raises an exception as this instance should be the
-        only source of updates.
-        """
-
-        raise Exception(
-            "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.