diff --git a/changelog.d/12256.misc b/changelog.d/12256.misc
new file mode 100644
index 0000000000..c5b6356799
--- /dev/null
+++ b/changelog.d/12256.misc
@@ -0,0 +1 @@
+Add type hints to tests files.
\ No newline at end of file
diff --git a/mypy.ini b/mypy.ini
index d8b3b3f9e5..24d4ba15d4 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -82,9 +82,7 @@ exclude = (?x)
    |tests/server.py
    |tests/server_notices/test_resource_limits_server_notices.py
    |tests/state/test_v2.py
-   |tests/storage/test_background_update.py
    |tests/storage/test_base.py
-   |tests/storage/test_id_generators.py
    |tests/storage/test_roommember.py
    |tests/test_metrics.py
    |tests/test_phone_home.py
diff --git a/tests/handlers/test_typing.py b/tests/handlers/test_typing.py
index f91a80b9fa..ffd5c4cb93 100644
--- a/tests/handlers/test_typing.py
+++ b/tests/handlers/test_typing.py
@@ -18,11 +18,14 @@ from typing import Dict
 from unittest.mock import ANY, Mock, call
 
 from twisted.internet import defer
+from twisted.test.proto_helpers import MemoryReactor
 from twisted.web.resource import Resource
 
 from synapse.api.errors import AuthError
 from synapse.federation.transport.server import TransportLayerServer
-from synapse.types import UserID, create_requester
+from synapse.server import HomeServer
+from synapse.types import JsonDict, UserID, create_requester
+from synapse.util import Clock
 
 from tests import unittest
 from tests.test_utils import make_awaitable
@@ -42,7 +45,9 @@ ROOM_ID = "a-room"
 OTHER_ROOM_ID = "another-room"
 
 
-def _expect_edu_transaction(edu_type, content, origin="test"):
+def _expect_edu_transaction(
+    edu_type: str, content: JsonDict, origin: str = "test"
+) -> JsonDict:
     return {
         "origin": origin,
         "origin_server_ts": 1000000,
@@ -51,12 +56,12 @@ def _expect_edu_transaction(edu_type, content, origin="test"):
     }
 
 
-def _make_edu_transaction_json(edu_type, content):
+def _make_edu_transaction_json(edu_type: str, content: JsonDict) -> bytes:
     return json.dumps(_expect_edu_transaction(edu_type, content)).encode("utf8")
 
 
 class TypingNotificationsTestCase(unittest.HomeserverTestCase):
-    def make_homeserver(self, reactor, clock):
+    def make_homeserver(self, reactor: MemoryReactor, clock: Clock) -> HomeServer:
         # we mock out the keyring so as to skip the authentication check on the
         # federation API call.
         mock_keyring = Mock(spec=["verify_json_for_server"])
@@ -83,7 +88,7 @@ class TypingNotificationsTestCase(unittest.HomeserverTestCase):
         d["/_matrix/federation"] = TransportLayerServer(self.hs)
         return d
 
-    def prepare(self, reactor, clock, hs):
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
         mock_notifier = hs.get_notifier()
         self.on_new_event = mock_notifier.on_new_event
 
@@ -111,24 +116,24 @@ class TypingNotificationsTestCase(unittest.HomeserverTestCase):
 
         self.room_members = []
 
-        async def check_user_in_room(room_id, user_id):
+        async def check_user_in_room(room_id: str, user_id: str) -> None:
             if user_id not in [u.to_string() for u in self.room_members]:
                 raise AuthError(401, "User is not in the room")
             return None
 
         hs.get_auth().check_user_in_room = check_user_in_room
 
-        async def check_host_in_room(room_id, server_name):
+        async def check_host_in_room(room_id: str, server_name: str) -> bool:
             return room_id == ROOM_ID
 
         hs.get_event_auth_handler().check_host_in_room = check_host_in_room
 
-        def get_joined_hosts_for_room(room_id):
+        def get_joined_hosts_for_room(room_id: str):
             return {member.domain for member in self.room_members}
 
         self.datastore.get_joined_hosts_for_room = get_joined_hosts_for_room
 
-        async def get_users_in_room(room_id):
+        async def get_users_in_room(room_id: str):
             return {str(u) for u in self.room_members}
 
         self.datastore.get_users_in_room = get_users_in_room
@@ -153,7 +158,7 @@ class TypingNotificationsTestCase(unittest.HomeserverTestCase):
             lambda *args, **kwargs: make_awaitable(None)
         )
 
-    def test_started_typing_local(self):
+    def test_started_typing_local(self) -> None:
         self.room_members = [U_APPLE, U_BANANA]
 
         self.assertEqual(self.event_source.get_current_key(), 0)
@@ -187,7 +192,7 @@ class TypingNotificationsTestCase(unittest.HomeserverTestCase):
         )
 
     @override_config({"send_federation": True})
-    def test_started_typing_remote_send(self):
+    def test_started_typing_remote_send(self) -> None:
         self.room_members = [U_APPLE, U_ONION]
 
         self.get_success(
@@ -217,7 +222,7 @@ class TypingNotificationsTestCase(unittest.HomeserverTestCase):
             try_trailing_slash_on_400=True,
         )
 
-    def test_started_typing_remote_recv(self):
+    def test_started_typing_remote_recv(self) -> None:
         self.room_members = [U_APPLE, U_ONION]
 
         self.assertEqual(self.event_source.get_current_key(), 0)
@@ -256,7 +261,7 @@ class TypingNotificationsTestCase(unittest.HomeserverTestCase):
             ],
         )
 
-    def test_started_typing_remote_recv_not_in_room(self):
+    def test_started_typing_remote_recv_not_in_room(self) -> None:
         self.room_members = [U_APPLE, U_ONION]
 
         self.assertEqual(self.event_source.get_current_key(), 0)
@@ -292,7 +297,7 @@ class TypingNotificationsTestCase(unittest.HomeserverTestCase):
         self.assertEqual(events[1], 0)
 
     @override_config({"send_federation": True})
-    def test_stopped_typing(self):
+    def test_stopped_typing(self) -> None:
         self.room_members = [U_APPLE, U_BANANA, U_ONION]
 
         # Gut-wrenching
@@ -343,7 +348,7 @@ class TypingNotificationsTestCase(unittest.HomeserverTestCase):
             [{"type": "m.typing", "room_id": ROOM_ID, "content": {"user_ids": []}}],
         )
 
-    def test_typing_timeout(self):
+    def test_typing_timeout(self) -> None:
         self.room_members = [U_APPLE, U_BANANA]
 
         self.assertEqual(self.event_source.get_current_key(), 0)
diff --git a/tests/push/test_push_rule_evaluator.py b/tests/push/test_push_rule_evaluator.py
index 3849beb9d6..5dba187076 100644
--- a/tests/push/test_push_rule_evaluator.py
+++ b/tests/push/test_push_rule_evaluator.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Any, Dict
+from typing import Dict, Optional, Union
 
 import frozendict
 
@@ -20,12 +20,13 @@ from synapse.api.room_versions import RoomVersions
 from synapse.events import FrozenEvent
 from synapse.push import push_rule_evaluator
 from synapse.push.push_rule_evaluator import PushRuleEvaluatorForEvent
+from synapse.types import JsonDict
 
 from tests import unittest
 
 
 class PushRuleEvaluatorTestCase(unittest.TestCase):
-    def _get_evaluator(self, content):
+    def _get_evaluator(self, content: JsonDict) -> PushRuleEvaluatorForEvent:
         event = FrozenEvent(
             {
                 "event_id": "$event_id",
@@ -39,12 +40,12 @@ class PushRuleEvaluatorTestCase(unittest.TestCase):
         )
         room_member_count = 0
         sender_power_level = 0
-        power_levels = {}
+        power_levels: Dict[str, Union[int, Dict[str, int]]] = {}
         return PushRuleEvaluatorForEvent(
             event, room_member_count, sender_power_level, power_levels
         )
 
-    def test_display_name(self):
+    def test_display_name(self) -> None:
         """Check for a matching display name in the body of the event."""
         evaluator = self._get_evaluator({"body": "foo bar baz"})
 
@@ -71,20 +72,20 @@ class PushRuleEvaluatorTestCase(unittest.TestCase):
         self.assertTrue(evaluator.matches(condition, "@user:test", "foo bar"))
 
     def _assert_matches(
-        self, condition: Dict[str, Any], content: Dict[str, Any], msg=None
+        self, condition: JsonDict, content: JsonDict, msg: Optional[str] = None
     ) -> None:
         evaluator = self._get_evaluator(content)
         self.assertTrue(evaluator.matches(condition, "@user:test", "display_name"), msg)
 
     def _assert_not_matches(
-        self, condition: Dict[str, Any], content: Dict[str, Any], msg=None
+        self, condition: JsonDict, content: JsonDict, msg: Optional[str] = None
     ) -> None:
         evaluator = self._get_evaluator(content)
         self.assertFalse(
             evaluator.matches(condition, "@user:test", "display_name"), msg
         )
 
-    def test_event_match_body(self):
+    def test_event_match_body(self) -> None:
         """Check that event_match conditions on content.body work as expected"""
 
         # if the key is `content.body`, the pattern matches substrings.
@@ -165,7 +166,7 @@ class PushRuleEvaluatorTestCase(unittest.TestCase):
             r"? after \ should match any character",
         )
 
-    def test_event_match_non_body(self):
+    def test_event_match_non_body(self) -> None:
         """Check that event_match conditions on other keys work as expected"""
 
         # if the key is anything other than 'content.body', the pattern must match the
@@ -241,7 +242,7 @@ class PushRuleEvaluatorTestCase(unittest.TestCase):
             "pattern should not match before a newline",
         )
 
-    def test_no_body(self):
+    def test_no_body(self) -> None:
         """Not having a body shouldn't break the evaluator."""
         evaluator = self._get_evaluator({})
 
@@ -250,7 +251,7 @@ class PushRuleEvaluatorTestCase(unittest.TestCase):
         }
         self.assertFalse(evaluator.matches(condition, "@user:test", "foo"))
 
-    def test_invalid_body(self):
+    def test_invalid_body(self) -> None:
         """A non-string body should not break the evaluator."""
         condition = {
             "kind": "contains_display_name",
@@ -260,7 +261,7 @@ class PushRuleEvaluatorTestCase(unittest.TestCase):
             evaluator = self._get_evaluator({"body": body})
             self.assertFalse(evaluator.matches(condition, "@user:test", "foo"))
 
-    def test_tweaks_for_actions(self):
+    def test_tweaks_for_actions(self) -> None:
         """
         This tests the behaviour of tweaks_for_actions.
         """
diff --git a/tests/storage/test_background_update.py b/tests/storage/test_background_update.py
index 5cf18b690e..fd619b64d4 100644
--- a/tests/storage/test_background_update.py
+++ b/tests/storage/test_background_update.py
@@ -17,8 +17,12 @@ from unittest.mock import Mock
 import yaml
 
 from twisted.internet.defer import Deferred, ensureDeferred
+from twisted.test.proto_helpers import MemoryReactor
 
+from synapse.server import HomeServer
 from synapse.storage.background_updates import BackgroundUpdater
+from synapse.types import JsonDict
+from synapse.util import Clock
 
 from tests import unittest
 from tests.test_utils import make_awaitable, simple_async_mock
@@ -26,7 +30,7 @@ from tests.unittest import override_config
 
 
 class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
-    def prepare(self, reactor, clock, homeserver):
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
         self.updates: BackgroundUpdater = self.hs.get_datastores().main.db_pool.updates
         # the base test class should have run the real bg updates for us
         self.assertTrue(
@@ -39,7 +43,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
         )
         self.store = self.hs.get_datastores().main
 
-    async def update(self, progress, count):
+    async def update(self, progress: JsonDict, count: int) -> int:
         duration_ms = 10
         await self.clock.sleep((count * duration_ms) / 1000)
         progress = {"my_key": progress["my_key"] + 1}
@@ -51,7 +55,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
         )
         return count
 
-    def test_do_background_update(self):
+    def test_do_background_update(self) -> None:
         # the time we claim it takes to update one item when running the update
         duration_ms = 10
 
@@ -80,7 +84,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
 
         # second step: complete the update
         # we should now get run with a much bigger number of items to update
-        async def update(progress, count):
+        async def update(progress: JsonDict, count: int) -> int:
             self.assertEqual(progress, {"my_key": 2})
             self.assertAlmostEqual(
                 count,
@@ -110,7 +114,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
             """
         )
     )
-    def test_background_update_default_batch_set_by_config(self):
+    def test_background_update_default_batch_set_by_config(self) -> None:
         """
         Test that the background update is run with the default_batch_size set by the config
         """
@@ -133,7 +137,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
         # on the first call, we should get run with the default background update size specified in the config
         self.update_handler.assert_called_once_with({"my_key": 1}, 20)
 
-    def test_background_update_default_sleep_behavior(self):
+    def test_background_update_default_sleep_behavior(self) -> None:
         """
         Test default background update behavior, which is to sleep
         """
@@ -147,7 +151,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
 
         self.update_handler.side_effect = self.update
         self.update_handler.reset_mock()
-        self.updates.start_doing_background_updates(),
+        self.updates.start_doing_background_updates()
 
         # 2: advance the reactor less than the default sleep duration (1000ms)
         self.reactor.pump([0.5])
@@ -167,7 +171,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
             """
         )
     )
-    def test_background_update_sleep_set_in_config(self):
+    def test_background_update_sleep_set_in_config(self) -> None:
         """
         Test that changing the sleep time in the config changes how long it sleeps
         """
@@ -181,7 +185,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
 
         self.update_handler.side_effect = self.update
         self.update_handler.reset_mock()
-        self.updates.start_doing_background_updates(),
+        self.updates.start_doing_background_updates()
 
         # 2: advance the reactor less than the configured sleep duration (500ms)
         self.reactor.pump([0.45])
@@ -201,7 +205,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
             """
         )
     )
-    def test_disabling_background_update_sleep(self):
+    def test_disabling_background_update_sleep(self) -> None:
         """
         Test that disabling sleep in the config results in bg update not sleeping
         """
@@ -215,7 +219,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
 
         self.update_handler.side_effect = self.update
         self.update_handler.reset_mock()
-        self.updates.start_doing_background_updates(),
+        self.updates.start_doing_background_updates()
 
         # 2: advance the reactor very little
         self.reactor.pump([0.025])
@@ -230,7 +234,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
             """
         )
     )
-    def test_background_update_duration_set_in_config(self):
+    def test_background_update_duration_set_in_config(self) -> None:
         """
         Test that the desired duration set in the config is used in determining batch size
         """
@@ -254,7 +258,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
 
         # the first update was run with the default batch size, this should be run with 500ms as the
         # desired duration
-        async def update(progress, count):
+        async def update(progress: JsonDict, count: int) -> int:
             self.assertEqual(progress, {"my_key": 2})
             self.assertAlmostEqual(
                 count,
@@ -275,7 +279,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
             """
         )
     )
-    def test_background_update_min_batch_set_in_config(self):
+    def test_background_update_min_batch_set_in_config(self) -> None:
         """
         Test that the minimum batch size set in the config is used
         """
@@ -290,7 +294,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
         )
 
         # Run the update with the long-running update item
-        async def update(progress, count):
+        async def update_long(progress: JsonDict, count: int) -> int:
             await self.clock.sleep((count * duration_ms) / 1000)
             progress = {"my_key": progress["my_key"] + 1}
             await self.store.db_pool.runInteraction(
@@ -301,7 +305,7 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
             )
             return count
 
-        self.update_handler.side_effect = update
+        self.update_handler.side_effect = update_long
         self.update_handler.reset_mock()
         res = self.get_success(
             self.updates.do_next_background_update(False),
@@ -311,25 +315,25 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
 
         # the first update was run with the default batch size, this should be run with minimum batch size
         # as the first items took a very long time
-        async def update(progress, count):
+        async def update_short(progress: JsonDict, count: int) -> int:
             self.assertEqual(progress, {"my_key": 2})
             self.assertEqual(count, 5)
             await self.updates._end_background_update("test_update")
             return count
 
-        self.update_handler.side_effect = update
+        self.update_handler.side_effect = update_short
         self.get_success(self.updates.do_next_background_update(False))
 
 
 class BackgroundUpdateControllerTestCase(unittest.HomeserverTestCase):
-    def prepare(self, reactor, clock, homeserver):
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
         self.updates: BackgroundUpdater = self.hs.get_datastores().main.db_pool.updates
         # the base test class should have run the real bg updates for us
         self.assertTrue(
             self.get_success(self.updates.has_completed_background_updates())
         )
 
-        self.update_deferred = Deferred()
+        self.update_deferred: Deferred[int] = Deferred()
         self.update_handler = Mock(return_value=self.update_deferred)
         self.updates.register_background_update_handler(
             "test_update", self.update_handler
@@ -358,7 +362,7 @@ class BackgroundUpdateControllerTestCase(unittest.HomeserverTestCase):
             ),
         )
 
-    def test_controller(self):
+    def test_controller(self) -> None:
         store = self.hs.get_datastores().main
         self.get_success(
             store.db_pool.simple_insert(
@@ -368,7 +372,7 @@ class BackgroundUpdateControllerTestCase(unittest.HomeserverTestCase):
         )
 
         # Set the return value for the context manager.
-        enter_defer = Deferred()
+        enter_defer: Deferred[int] = Deferred()
         self._update_ctx_manager.__aenter__ = Mock(return_value=enter_defer)
 
         # Start the background update.
diff --git a/tests/storage/test_id_generators.py b/tests/storage/test_id_generators.py
index 6ac4b93f98..395396340b 100644
--- a/tests/storage/test_id_generators.py
+++ b/tests/storage/test_id_generators.py
@@ -13,9 +13,13 @@
 # limitations under the License.
 from typing import List, Optional
 
-from synapse.storage.database import DatabasePool
+from twisted.test.proto_helpers import MemoryReactor
+
+from synapse.server import HomeServer
+from synapse.storage.database import DatabasePool, LoggingTransaction
 from synapse.storage.engines import IncorrectDatabaseSetup
 from synapse.storage.util.id_generators import MultiWriterIdGenerator
+from synapse.util import Clock
 
 from tests.unittest import HomeserverTestCase
 from tests.utils import USE_POSTGRES_FOR_TESTS
@@ -25,13 +29,13 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
     if not USE_POSTGRES_FOR_TESTS:
         skip = "Requires Postgres"
 
-    def prepare(self, reactor, clock, hs):
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
         self.store = hs.get_datastores().main
         self.db_pool: DatabasePool = self.store.db_pool
 
         self.get_success(self.db_pool.runInteraction("_setup_db", self._setup_db))
 
-    def _setup_db(self, txn):
+    def _setup_db(self, txn: LoggingTransaction) -> None:
         txn.execute("CREATE SEQUENCE foobar_seq")
         txn.execute(
             """
@@ -59,12 +63,12 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
 
         return self.get_success_or_raise(self.db_pool.runWithConnection(_create))
 
-    def _insert_rows(self, instance_name: str, number: int):
+    def _insert_rows(self, instance_name: str, number: int) -> None:
         """Insert N rows as the given instance, inserting with stream IDs pulled
         from the postgres sequence.
         """
 
-        def _insert(txn):
+        def _insert(txn: LoggingTransaction) -> None:
             for _ in range(number):
                 txn.execute(
                     "INSERT INTO foobar VALUES (nextval('foobar_seq'), ?)",
@@ -80,12 +84,12 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
 
         self.get_success(self.db_pool.runInteraction("_insert_rows", _insert))
 
-    def _insert_row_with_id(self, instance_name: str, stream_id: int):
+    def _insert_row_with_id(self, instance_name: str, stream_id: int) -> None:
         """Insert one row as the given instance with given stream_id, updating
         the postgres sequence position to match.
         """
 
-        def _insert(txn):
+        def _insert(txn: LoggingTransaction) -> None:
             txn.execute(
                 "INSERT INTO foobar VALUES (?, ?)",
                 (
@@ -104,7 +108,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
 
         self.get_success(self.db_pool.runInteraction("_insert_row_with_id", _insert))
 
-    def test_empty(self):
+    def test_empty(self) -> None:
         """Test an ID generator against an empty database gives sensible
         current positions.
         """
@@ -114,7 +118,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         # The table is empty so we expect an empty map for positions
         self.assertEqual(id_gen.get_positions(), {})
 
-    def test_single_instance(self):
+    def test_single_instance(self) -> None:
         """Test that reads and writes from a single process are handled
         correctly.
         """
@@ -130,7 +134,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         # Try allocating a new ID gen and check that we only see position
         # advanced after we leave the context manager.
 
-        async def _get_next_async():
+        async def _get_next_async() -> None:
             async with id_gen.get_next() as stream_id:
                 self.assertEqual(stream_id, 8)
 
@@ -142,7 +146,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         self.assertEqual(id_gen.get_positions(), {"master": 8})
         self.assertEqual(id_gen.get_current_token_for_writer("master"), 8)
 
-    def test_out_of_order_finish(self):
+    def test_out_of_order_finish(self) -> None:
         """Test that IDs persisted out of order are correctly handled"""
 
         # Prefill table with 7 rows written by 'master'
@@ -191,7 +195,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         self.assertEqual(id_gen.get_positions(), {"master": 11})
         self.assertEqual(id_gen.get_current_token_for_writer("master"), 11)
 
-    def test_multi_instance(self):
+    def test_multi_instance(self) -> None:
         """Test that reads and writes from multiple processes are handled
         correctly.
         """
@@ -215,7 +219,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         # Try allocating a new ID gen and check that we only see position
         # advanced after we leave the context manager.
 
-        async def _get_next_async():
+        async def _get_next_async() -> None:
             async with first_id_gen.get_next() as stream_id:
                 self.assertEqual(stream_id, 8)
 
@@ -233,7 +237,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         # ... but calling `get_next` on the second instance should give a unique
         # stream ID
 
-        async def _get_next_async():
+        async def _get_next_async2() -> None:
             async with second_id_gen.get_next() as stream_id:
                 self.assertEqual(stream_id, 9)
 
@@ -241,7 +245,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
                     second_id_gen.get_positions(), {"first": 3, "second": 7}
                 )
 
-        self.get_success(_get_next_async())
+        self.get_success(_get_next_async2())
 
         self.assertEqual(second_id_gen.get_positions(), {"first": 3, "second": 9})
 
@@ -249,7 +253,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         second_id_gen.advance("first", 8)
         self.assertEqual(second_id_gen.get_positions(), {"first": 8, "second": 9})
 
-    def test_get_next_txn(self):
+    def test_get_next_txn(self) -> None:
         """Test that the `get_next_txn` function works correctly."""
 
         # Prefill table with 7 rows written by 'master'
@@ -263,7 +267,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         # Try allocating a new ID gen and check that we only see position
         # advanced after we leave the context manager.
 
-        def _get_next_txn(txn):
+        def _get_next_txn(txn: LoggingTransaction) -> None:
             stream_id = id_gen.get_next_txn(txn)
             self.assertEqual(stream_id, 8)
 
@@ -275,7 +279,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         self.assertEqual(id_gen.get_positions(), {"master": 8})
         self.assertEqual(id_gen.get_current_token_for_writer("master"), 8)
 
-    def test_get_persisted_upto_position(self):
+    def test_get_persisted_upto_position(self) -> None:
         """Test that `get_persisted_upto_position` correctly tracks updates to
         positions.
         """
@@ -317,7 +321,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         id_gen.advance("second", 15)
         self.assertEqual(id_gen.get_persisted_upto_position(), 11)
 
-    def test_get_persisted_upto_position_get_next(self):
+    def test_get_persisted_upto_position_get_next(self) -> None:
         """Test that `get_persisted_upto_position` correctly tracks updates to
         positions when `get_next` is called.
         """
@@ -331,7 +335,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
 
         self.assertEqual(id_gen.get_persisted_upto_position(), 5)
 
-        async def _get_next_async():
+        async def _get_next_async() -> None:
             async with id_gen.get_next() as stream_id:
                 self.assertEqual(stream_id, 6)
                 self.assertEqual(id_gen.get_persisted_upto_position(), 5)
@@ -344,7 +348,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         # `persisted_upto_position` in this case, then it will be correct in the
         # other cases that are tested above (since they'll hit the same code).
 
-    def test_restart_during_out_of_order_persistence(self):
+    def test_restart_during_out_of_order_persistence(self) -> None:
         """Test that restarting a process while another process is writing out
         of order updates are handled correctly.
         """
@@ -388,7 +392,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         id_gen_worker.advance("master", 9)
         self.assertEqual(id_gen_worker.get_positions(), {"master": 9})
 
-    def test_writer_config_change(self):
+    def test_writer_config_change(self) -> None:
         """Test that changing the writer config correctly works."""
 
         self._insert_row_with_id("first", 3)
@@ -421,7 +425,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
 
         # Check that we get a sane next stream ID with this new config.
 
-        async def _get_next_async():
+        async def _get_next_async() -> None:
             async with id_gen_3.get_next() as stream_id:
                 self.assertEqual(stream_id, 6)
 
@@ -435,7 +439,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         self.assertEqual(id_gen_5.get_current_token_for_writer("first"), 6)
         self.assertEqual(id_gen_5.get_current_token_for_writer("third"), 6)
 
-    def test_sequence_consistency(self):
+    def test_sequence_consistency(self) -> None:
         """Test that we error out if the table and sequence diverges."""
 
         # Prefill with some rows
@@ -458,13 +462,13 @@ class BackwardsMultiWriterIdGeneratorTestCase(HomeserverTestCase):
     if not USE_POSTGRES_FOR_TESTS:
         skip = "Requires Postgres"
 
-    def prepare(self, reactor, clock, hs):
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
         self.store = hs.get_datastores().main
         self.db_pool: DatabasePool = self.store.db_pool
 
         self.get_success(self.db_pool.runInteraction("_setup_db", self._setup_db))
 
-    def _setup_db(self, txn):
+    def _setup_db(self, txn: LoggingTransaction) -> None:
         txn.execute("CREATE SEQUENCE foobar_seq")
         txn.execute(
             """
@@ -493,10 +497,10 @@ class BackwardsMultiWriterIdGeneratorTestCase(HomeserverTestCase):
 
         return self.get_success(self.db_pool.runWithConnection(_create))
 
-    def _insert_row(self, instance_name: str, stream_id: int):
+    def _insert_row(self, instance_name: str, stream_id: int) -> None:
         """Insert one row as the given instance with given stream_id."""
 
-        def _insert(txn):
+        def _insert(txn: LoggingTransaction) -> None:
             txn.execute(
                 "INSERT INTO foobar VALUES (?, ?)",
                 (
@@ -514,13 +518,13 @@ class BackwardsMultiWriterIdGeneratorTestCase(HomeserverTestCase):
 
         self.get_success(self.db_pool.runInteraction("_insert_row", _insert))
 
-    def test_single_instance(self):
+    def test_single_instance(self) -> None:
         """Test that reads and writes from a single process are handled
         correctly.
         """
         id_gen = self._create_id_generator()
 
-        async def _get_next_async():
+        async def _get_next_async() -> None:
             async with id_gen.get_next() as stream_id:
                 self._insert_row("master", stream_id)
 
@@ -530,7 +534,7 @@ class BackwardsMultiWriterIdGeneratorTestCase(HomeserverTestCase):
         self.assertEqual(id_gen.get_current_token_for_writer("master"), -1)
         self.assertEqual(id_gen.get_persisted_upto_position(), -1)
 
-        async def _get_next_async2():
+        async def _get_next_async2() -> None:
             async with id_gen.get_next_mult(3) as stream_ids:
                 for stream_id in stream_ids:
                     self._insert_row("master", stream_id)
@@ -548,14 +552,14 @@ class BackwardsMultiWriterIdGeneratorTestCase(HomeserverTestCase):
         self.assertEqual(second_id_gen.get_current_token_for_writer("master"), -4)
         self.assertEqual(second_id_gen.get_persisted_upto_position(), -4)
 
-    def test_multiple_instance(self):
+    def test_multiple_instance(self) -> None:
         """Tests that having multiple instances that get advanced over
         federation works corretly.
         """
         id_gen_1 = self._create_id_generator("first", writers=["first", "second"])
         id_gen_2 = self._create_id_generator("second", writers=["first", "second"])
 
-        async def _get_next_async():
+        async def _get_next_async() -> None:
             async with id_gen_1.get_next() as stream_id:
                 self._insert_row("first", stream_id)
                 id_gen_2.advance("first", stream_id)
@@ -567,7 +571,7 @@ class BackwardsMultiWriterIdGeneratorTestCase(HomeserverTestCase):
         self.assertEqual(id_gen_1.get_persisted_upto_position(), -1)
         self.assertEqual(id_gen_2.get_persisted_upto_position(), -1)
 
-        async def _get_next_async2():
+        async def _get_next_async2() -> None:
             async with id_gen_2.get_next() as stream_id:
                 self._insert_row("second", stream_id)
                 id_gen_1.advance("second", stream_id)
@@ -584,13 +588,13 @@ class MultiTableMultiWriterIdGeneratorTestCase(HomeserverTestCase):
     if not USE_POSTGRES_FOR_TESTS:
         skip = "Requires Postgres"
 
-    def prepare(self, reactor, clock, hs):
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
         self.store = hs.get_datastores().main
         self.db_pool: DatabasePool = self.store.db_pool
 
         self.get_success(self.db_pool.runInteraction("_setup_db", self._setup_db))
 
-    def _setup_db(self, txn):
+    def _setup_db(self, txn: LoggingTransaction) -> None:
         txn.execute("CREATE SEQUENCE foobar_seq")
         txn.execute(
             """
@@ -642,7 +646,7 @@ class MultiTableMultiWriterIdGeneratorTestCase(HomeserverTestCase):
         from the postgres sequence.
         """
 
-        def _insert(txn):
+        def _insert(txn: LoggingTransaction) -> None:
             for _ in range(number):
                 txn.execute(
                     "INSERT INTO %s VALUES (nextval('foobar_seq'), ?)" % (table,),
@@ -659,7 +663,7 @@ class MultiTableMultiWriterIdGeneratorTestCase(HomeserverTestCase):
 
         self.get_success(self.db_pool.runInteraction("_insert_rows", _insert))
 
-    def test_load_existing_stream(self):
+    def test_load_existing_stream(self) -> None:
         """Test creating ID gens with multiple tables that have rows from after
         the position in `stream_positions` table.
         """