Fix limit logic for EventsStream (#7358)

* Factor out functions for injecting events into database

I want to add some more flexibility to the tools for injecting events into the
database, and I don't want to clutter up HomeserverTestCase with them, so let's
factor them out to a new file.

* Rework TestReplicationDataHandler

This wasn't very easy to work with: the mock wrapping was largely superfluous,
and it's useful to be able to inspect the received rows, and clear out the
received list.

* Fix AssertionErrors being thrown by EventsStream

Part of the problem was that there was an off-by-one error in the assertion,
but also the limit logic was too simple. Fix it all up and add some tests.

14 files changed, 658 insertions, 67 deletions

diff --git a/changelog.d/7358.bugfix b/changelog.d/7358.bugfix
new file mode 100644
index 0000000000..f49c600173
--- /dev/null
+++ b/changelog.d/7358.bugfix
@@ -0,0 +1 @@
+Fix a bug where event updates might not be sent over replication to worker processes after the stream falls behind.
diff --git a/synapse/replication/tcp/handler.py b/synapse/replication/tcp/handler.py
index 0db5a3a24d..3a8c7c7e2d 100644
--- a/synapse/replication/tcp/handler.py
+++ b/synapse/replication/tcp/handler.py
@@ -87,7 +87,9 @@ class ReplicationCommandHandler:
             stream.NAME: stream(hs) for stream in STREAMS_MAP.values()
         }  # type: Dict[str, Stream]
 
-        self._position_linearizer = Linearizer("replication_position")
+        self._position_linearizer = Linearizer(
+            "replication_position", clock=self._clock
+        )
 
         # Map of stream to batched updates. See RdataCommand for info on how
         # batching works.
diff --git a/synapse/replication/tcp/streams/events.py b/synapse/replication/tcp/streams/events.py
index aa50492569..52df81b1bd 100644
--- a/synapse/replication/tcp/streams/events.py
+++ b/synapse/replication/tcp/streams/events.py
@@ -170,22 +170,16 @@ class EventsStream(Stream):
             limited = False
             upper_limit = current_token
 
-        # next up is the state delta table
-
-        state_rows = await self._store.get_all_updated_current_state_deltas(
+        # next up is the state delta table.
+        (
+            state_rows,
+            upper_limit,
+            state_rows_limited,
+        ) = await self._store.get_all_updated_current_state_deltas(
             from_token, upper_limit, target_row_count
-        )  # type: List[Tuple]
-
-        # again, if we've hit the limit there, we'll need to limit the other sources
-        assert len(state_rows) < target_row_count
-        if len(state_rows) == target_row_count:
-            assert state_rows[-1][0] <= upper_limit
-            upper_limit = state_rows[-1][0]
-            limited = True
+        )
 
-            # FIXME: is it a given that there is only one row per stream_id in the
-            # state_deltas table (so that we can be sure that we have got all of the
-            # rows for upper_limit)?
+        limited = limited or state_rows_limited
 
         # finally, fetch the ex-outliers rows. We assume there are few enough of these
         # not to bother with the limit.
diff --git a/synapse/server.pyi b/synapse/server.pyi
index f1a5717028..fc5886f762 100644
--- a/synapse/server.pyi
+++ b/synapse/server.pyi
@@ -25,6 +25,7 @@ import synapse.server_notices.server_notices_manager
 import synapse.server_notices.server_notices_sender
 import synapse.state
 import synapse.storage
+from synapse.events.builder import EventBuilderFactory
 
 class HomeServer(object):
     @property
@@ -121,3 +122,7 @@ class HomeServer(object):
         pass
     def get_instance_id(self) -> str:
         pass
+    def get_event_builder_factory(self) -> EventBuilderFactory:
+        pass
+    def get_storage(self) -> synapse.storage.Storage:
+        pass
diff --git a/synapse/storage/data_stores/main/events_worker.py b/synapse/storage/data_stores/main/events_worker.py
index ce8be72bfe..73df6b33ba 100644
--- a/synapse/storage/data_stores/main/events_worker.py
+++ b/synapse/storage/data_stores/main/events_worker.py
@@ -19,7 +19,7 @@ import itertools
 import logging
 import threading
 from collections import namedtuple
-from typing import List, Optional
+from typing import List, Optional, Tuple
 
 from canonicaljson import json
 from constantly import NamedConstant, Names
@@ -1084,7 +1084,28 @@ class EventsWorkerStore(SQLBaseStore):
             "get_all_new_backfill_event_rows", get_all_new_backfill_event_rows
         )
 
-    def get_all_updated_current_state_deltas(self, from_token, to_token, limit):
+    async def get_all_updated_current_state_deltas(
+        self, from_token: int, to_token: int, target_row_count: int
+    ) -> Tuple[List[Tuple], int, bool]:
+        """Fetch updates from current_state_delta_stream
+
+        Args:
+            from_token: The previous stream token. Updates from this stream id will
+                be excluded.
+
+            to_token: The current stream token (ie the upper limit). Updates up to this
+                stream id will be included (modulo the 'limit' param)
+
+            target_row_count: The number of rows to try to return. If more rows are
+                available, we will set 'limited' in the result. In the event of a large
+                batch, we may return more rows than this.
+        Returns:
+            A triplet `(updates, new_last_token, limited)`, where:
+               * `updates` is a list of database tuples.
+               * `new_last_token` is the new position in stream.
+               * `limited` is whether there are more updates to fetch.
+        """
+
         def get_all_updated_current_state_deltas_txn(txn):
             sql = """
                 SELECT stream_id, room_id, type, state_key, event_id
@@ -1092,10 +1113,45 @@ class EventsWorkerStore(SQLBaseStore):
                 WHERE ? < stream_id AND stream_id <= ?
                 ORDER BY stream_id ASC LIMIT ?
             """
-            txn.execute(sql, (from_token, to_token, limit))
+            txn.execute(sql, (from_token, to_token, target_row_count))
             return txn.fetchall()
 
-        return self.db.runInteraction(
+        def get_deltas_for_stream_id_txn(txn, stream_id):
+            sql = """
+                SELECT stream_id, room_id, type, state_key, event_id
+                FROM current_state_delta_stream
+                WHERE stream_id = ?
+            """
+            txn.execute(sql, [stream_id])
+            return txn.fetchall()
+
+        # we need to make sure that, for every stream id in the results, we get *all*
+        # the rows with that stream id.
+
+        rows = await self.db.runInteraction(
             "get_all_updated_current_state_deltas",
             get_all_updated_current_state_deltas_txn,
+        )  # type: List[Tuple]
+
+        # if we've got fewer rows than the limit, we're good
+        if len(rows) < target_row_count:
+            return rows, to_token, False
+
+        # we hit the limit, so reduce the upper limit so that we exclude the stream id
+        # of the last row in the result.
+        assert rows[-1][0] <= to_token
+        to_token = rows[-1][0] - 1
+
+        # search backwards through the list for the point to truncate
+        for idx in range(len(rows) - 1, 0, -1):
+            if rows[idx - 1][0] <= to_token:
+                return rows[:idx], to_token, True
+
+        # bother. We didn't get a full set of changes for even a single
+        # stream id. let's run the query again, without a row limit, but for
+        # just one stream id.
+        to_token += 1
+        rows = await self.db.runInteraction(
+            "get_deltas_for_stream_id", get_deltas_for_stream_id_txn, to_token
         )
+        return rows, to_token, True
diff --git a/tests/replication/tcp/streams/_base.py b/tests/replication/tcp/streams/_base.py
index 82f15c64e0..83e16cfe3d 100644
--- a/tests/replication/tcp/streams/_base.py
+++ b/tests/replication/tcp/streams/_base.py
@@ -12,10 +12,9 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import logging
-from typing import Optional
 
-from mock import Mock
+import logging
+from typing import Any, Dict, List, Optional, Tuple
 
 import attr
 
@@ -25,6 +24,7 @@ from twisted.web.http import HTTPChannel
 
 from synapse.app.generic_worker import GenericWorkerServer
 from synapse.http.site import SynapseRequest
+from synapse.replication.slave.storage._base import BaseSlavedStore
 from synapse.replication.tcp.client import ReplicationDataHandler
 from synapse.replication.tcp.handler import ReplicationCommandHandler
 from synapse.replication.tcp.protocol import ClientReplicationStreamProtocol
@@ -65,9 +65,7 @@ class BaseStreamTestCase(unittest.HomeserverTestCase):
         # databases objects are the same.
         self.worker_hs.get_datastore().db = hs.get_datastore().db
 
-        self.test_handler = Mock(
-            wraps=TestReplicationDataHandler(self.worker_hs.get_datastore())
-        )
+        self.test_handler = self._build_replication_data_handler()
         self.worker_hs.replication_data_handler = self.test_handler
 
         repl_handler = ReplicationCommandHandler(self.worker_hs)
@@ -78,6 +76,9 @@ class BaseStreamTestCase(unittest.HomeserverTestCase):
         self._client_transport = None
         self._server_transport = None
 
+    def _build_replication_data_handler(self):
+        return TestReplicationDataHandler(self.worker_hs.get_datastore())
+
     def reconnect(self):
         if self._client_transport:
             self.client.close()
@@ -174,22 +175,28 @@ class BaseStreamTestCase(unittest.HomeserverTestCase):
 class TestReplicationDataHandler(ReplicationDataHandler):
     """Drop-in for ReplicationDataHandler which just collects RDATA rows"""
 
-    def __init__(self, hs):
-        super().__init__(hs)
-        self.streams = set()
-        self._received_rdata_rows = []
+    def __init__(self, store: BaseSlavedStore):
+        super().__init__(store)
+
+        # streams to subscribe to: map from stream id to position
+        self.stream_positions = {}  # type: Dict[str, int]
+
+        # list of received (stream_name, token, row) tuples
+        self.received_rdata_rows = []  # type: List[Tuple[str, int, Any]]
 
     def get_streams_to_replicate(self):
-        positions = {s: 0 for s in self.streams}
-        for stream, token, _ in self._received_rdata_rows:
-            if stream in self.streams:
-                positions[stream] = max(token, positions.get(stream, 0))
-        return positions
+        return self.stream_positions
 
     async def on_rdata(self, stream_name, token, rows):
         await super().on_rdata(stream_name, token, rows)
         for r in rows:
-            self._received_rdata_rows.append((stream_name, token, r))
+            self.received_rdata_rows.append((stream_name, token, r))
+
+        if (
+            stream_name in self.stream_positions
+            and token > self.stream_positions[stream_name]
+        ):
+            self.stream_positions[stream_name] = token
 
 
 @attr.s()
@@ -221,7 +228,7 @@ class _PushHTTPChannel(HTTPChannel):
         super().__init__()
         self.reactor = reactor
 
-        self._pull_to_push_producer = None
+        self._pull_to_push_producer = None  # type: Optional[_PullToPushProducer]
 
     def registerProducer(self, producer, streaming):
         # Convert pull producers to push producer.
diff --git a/tests/replication/tcp/streams/test_events.py b/tests/replication/tcp/streams/test_events.py
new file mode 100644
index 0000000000..1fa28084f9
--- /dev/null
+++ b/tests/replication/tcp/streams/test_events.py
@@ -0,0 +1,417 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import List, Optional
+
+from synapse.api.constants import EventTypes, Membership
+from synapse.events import EventBase
+from synapse.replication.tcp.streams._base import _STREAM_UPDATE_TARGET_ROW_COUNT
+from synapse.replication.tcp.streams.events import (
+    EventsStreamCurrentStateRow,
+    EventsStreamEventRow,
+    EventsStreamRow,
+)
+from synapse.rest import admin
+from synapse.rest.client.v1 import login, room
+
+from tests.replication.tcp.streams._base import BaseStreamTestCase
+from tests.test_utils.event_injection import inject_event, inject_member_event
+
+
+class EventsStreamTestCase(BaseStreamTestCase):
+    servlets = [
+        admin.register_servlets,
+        login.register_servlets,
+        room.register_servlets,
+    ]
+
+    def prepare(self, reactor, clock, hs):
+        super().prepare(reactor, clock, hs)
+        self.user_id = self.register_user("u1", "pass")
+        self.user_tok = self.login("u1", "pass")
+
+        self.reconnect()
+        self.test_handler.stream_positions["events"] = 0
+
+        self.room_id = self.helper.create_room_as(tok=self.user_tok)
+        self.test_handler.received_rdata_rows.clear()
+
+    def test_update_function_event_row_limit(self):
+        """Test replication with many non-state events
+
+        Checks that all events are correctly replicated when there are lots of
+        event rows to be replicated.
+        """
+        # disconnect, so that we can stack up some changes
+        self.disconnect()
+
+        # generate lots of non-state events. We inject them using inject_event
+        # so that they are not send out over replication until we call self.replicate().
+        events = [
+            self._inject_test_event()
+            for _ in range(_STREAM_UPDATE_TARGET_ROW_COUNT + 1)
+        ]
+
+        # also one state event
+        state_event = self._inject_state_event()
+
+        # tell the notifier to catch up to avoid duplicate rows.
+        # workaround for https://github.com/matrix-org/synapse/issues/7360
+        # FIXME remove this when the above is fixed
+        self.replicate()
+
+        # check we're testing what we think we are: no rows should yet have been
+        # received
+        self.assertEqual([], self.test_handler.received_rdata_rows)
+
+        # now reconnect to pull the updates
+        self.reconnect()
+        self.replicate()
+
+        # we should have received all the expected rows in the right order
+        received_rows = self.test_handler.received_rdata_rows
+        for event in events:
+            stream_name, token, row = received_rows.pop(0)
+            self.assertEqual("events", stream_name)
+            self.assertIsInstance(row, EventsStreamRow)
+            self.assertEqual(row.type, "ev")
+            self.assertIsInstance(row.data, EventsStreamEventRow)
+            self.assertEqual(row.data.event_id, event.event_id)
+
+        stream_name, token, row = received_rows.pop(0)
+        self.assertIsInstance(row, EventsStreamRow)
+        self.assertIsInstance(row.data, EventsStreamEventRow)
+        self.assertEqual(row.data.event_id, state_event.event_id)
+
+        stream_name, token, row = received_rows.pop(0)
+        self.assertEqual("events", stream_name)
+        self.assertIsInstance(row, EventsStreamRow)
+        self.assertEqual(row.type, "state")
+        self.assertIsInstance(row.data, EventsStreamCurrentStateRow)
+        self.assertEqual(row.data.event_id, state_event.event_id)
+
+        self.assertEqual([], received_rows)
+
+    def test_update_function_huge_state_change(self):
+        """Test replication with many state events
+
+        Ensures that all events are correctly replicated when there are lots of
+        state change rows to be replicated.
+        """
+
+        # we want to generate lots of state changes at a single stream ID.
+        #
+        # We do this by having two branches in the DAG. On one, we have a moderator
+        # which that generates lots of state; on the other, we de-op the moderator,
+        # thus invalidating all the state.
+
+        OTHER_USER = "@other_user:localhost"
+
+        # have the user join
+        inject_member_event(self.hs, self.room_id, OTHER_USER, Membership.JOIN)
+
+        # Update existing power levels with mod at PL50
+        pls = self.helper.get_state(
+            self.room_id, EventTypes.PowerLevels, tok=self.user_tok
+        )
+        pls["users"][OTHER_USER] = 50
+        self.helper.send_state(
+            self.room_id, EventTypes.PowerLevels, pls, tok=self.user_tok,
+        )
+
+        # this is the point in the DAG where we make a fork
+        fork_point = self.get_success(
+            self.hs.get_datastore().get_latest_event_ids_in_room(self.room_id)
+        )  # type: List[str]
+
+        events = [
+            self._inject_state_event(sender=OTHER_USER)
+            for _ in range(_STREAM_UPDATE_TARGET_ROW_COUNT)
+        ]
+
+        self.replicate()
+        # all those events and state changes should have landed
+        self.assertGreaterEqual(
+            len(self.test_handler.received_rdata_rows), 2 * len(events)
+        )
+
+        # disconnect, so that we can stack up the changes
+        self.disconnect()
+        self.test_handler.received_rdata_rows.clear()
+
+        # a state event which doesn't get rolled back, to check that the state
+        # before the huge update comes through ok
+        state1 = self._inject_state_event()
+
+        # roll back all the state by de-modding the user
+        prev_events = fork_point
+        pls["users"][OTHER_USER] = 0
+        pl_event = inject_event(
+            self.hs,
+            prev_event_ids=prev_events,
+            type=EventTypes.PowerLevels,
+            state_key="",
+            sender=self.user_id,
+            room_id=self.room_id,
+            content=pls,
+        )
+
+        # one more bit of state that doesn't get rolled back
+        state2 = self._inject_state_event()
+
+        # tell the notifier to catch up to avoid duplicate rows.
+        # workaround for https://github.com/matrix-org/synapse/issues/7360
+        # FIXME remove this when the above is fixed
+        self.replicate()
+
+        # check we're testing what we think we are: no rows should yet have been
+        # received
+        self.assertEqual([], self.test_handler.received_rdata_rows)
+
+        # now reconnect to pull the updates
+        self.reconnect()
+        self.replicate()
+
+        # now we should have received all the expected rows in the right order.
+        #
+        # we expect:
+        #
+        # - two rows for state1
+        # - the PL event row, plus state rows for the PL event and each
+        #       of the states that got reverted.
+        # - two rows for state2
+
+        received_rows = self.test_handler.received_rdata_rows
+
+        # first check the first two rows, which should be state1
+
+        stream_name, token, row = received_rows.pop(0)
+        self.assertEqual("events", stream_name)
+        self.assertIsInstance(row, EventsStreamRow)
+        self.assertEqual(row.type, "ev")
+        self.assertIsInstance(row.data, EventsStreamEventRow)
+        self.assertEqual(row.data.event_id, state1.event_id)
+
+        stream_name, token, row = received_rows.pop(0)
+        self.assertIsInstance(row, EventsStreamRow)
+        self.assertEqual(row.type, "state")
+        self.assertIsInstance(row.data, EventsStreamCurrentStateRow)
+        self.assertEqual(row.data.event_id, state1.event_id)
+
+        # now the last two rows, which should be state2
+        stream_name, token, row = received_rows.pop(-2)
+        self.assertEqual("events", stream_name)
+        self.assertIsInstance(row, EventsStreamRow)
+        self.assertEqual(row.type, "ev")
+        self.assertIsInstance(row.data, EventsStreamEventRow)
+        self.assertEqual(row.data.event_id, state2.event_id)
+
+        stream_name, token, row = received_rows.pop(-1)
+        self.assertIsInstance(row, EventsStreamRow)
+        self.assertEqual(row.type, "state")
+        self.assertIsInstance(row.data, EventsStreamCurrentStateRow)
+        self.assertEqual(row.data.event_id, state2.event_id)
+
+        # that should leave us with the rows for the PL event
+        self.assertEqual(len(received_rows), len(events) + 2)
+
+        stream_name, token, row = received_rows.pop(0)
+        self.assertEqual("events", stream_name)
+        self.assertIsInstance(row, EventsStreamRow)
+        self.assertEqual(row.type, "ev")
+        self.assertIsInstance(row.data, EventsStreamEventRow)
+        self.assertEqual(row.data.event_id, pl_event.event_id)
+
+        # the state rows are unsorted
+        state_rows = []  # type: List[EventsStreamCurrentStateRow]
+        for stream_name, token, row in received_rows:
+            self.assertEqual("events", stream_name)
+            self.assertIsInstance(row, EventsStreamRow)
+            self.assertEqual(row.type, "state")
+            self.assertIsInstance(row.data, EventsStreamCurrentStateRow)
+            state_rows.append(row.data)
+
+        state_rows.sort(key=lambda r: r.state_key)
+
+        sr = state_rows.pop(0)
+        self.assertEqual(sr.type, EventTypes.PowerLevels)
+        self.assertEqual(sr.event_id, pl_event.event_id)
+        for sr in state_rows:
+            self.assertEqual(sr.type, "test_state_event")
+            # "None" indicates the state has been deleted
+            self.assertIsNone(sr.event_id)
+
+    def test_update_function_state_row_limit(self):
+        """Test replication with many state events over several stream ids.
+        """
+
+        # we want to generate lots of state changes, but for this test, we want to
+        # spread out the state changes over a few stream IDs.
+        #
+        # We do this by having two branches in the DAG. On one, we have four moderators,
+        # each of which that generates lots of state; on the other, we de-op the users,
+        # thus invalidating all the state.
+
+        NUM_USERS = 4
+        STATES_PER_USER = _STREAM_UPDATE_TARGET_ROW_COUNT // 4 + 1
+
+        user_ids = ["@user%i:localhost" % (i,) for i in range(NUM_USERS)]
+
+        # have the users join
+        for u in user_ids:
+            inject_member_event(self.hs, self.room_id, u, Membership.JOIN)
+
+        # Update existing power levels with mod at PL50
+        pls = self.helper.get_state(
+            self.room_id, EventTypes.PowerLevels, tok=self.user_tok
+        )
+        pls["users"].update({u: 50 for u in user_ids})
+        self.helper.send_state(
+            self.room_id, EventTypes.PowerLevels, pls, tok=self.user_tok,
+        )
+
+        # this is the point in the DAG where we make a fork
+        fork_point = self.get_success(
+            self.hs.get_datastore().get_latest_event_ids_in_room(self.room_id)
+        )  # type: List[str]
+
+        events = []  # type: List[EventBase]
+        for user in user_ids:
+            events.extend(
+                self._inject_state_event(sender=user) for _ in range(STATES_PER_USER)
+            )
+
+        self.replicate()
+
+        # all those events and state changes should have landed
+        self.assertGreaterEqual(
+            len(self.test_handler.received_rdata_rows), 2 * len(events)
+        )
+
+        # disconnect, so that we can stack up the changes
+        self.disconnect()
+        self.test_handler.received_rdata_rows.clear()
+
+        # now roll back all that state by de-modding the users
+        prev_events = fork_point
+        pl_events = []
+        for u in user_ids:
+            pls["users"][u] = 0
+            e = inject_event(
+                self.hs,
+                prev_event_ids=prev_events,
+                type=EventTypes.PowerLevels,
+                state_key="",
+                sender=self.user_id,
+                room_id=self.room_id,
+                content=pls,
+            )
+            prev_events = [e.event_id]
+            pl_events.append(e)
+
+        # tell the notifier to catch up to avoid duplicate rows.
+        # workaround for https://github.com/matrix-org/synapse/issues/7360
+        # FIXME remove this when the above is fixed
+        self.replicate()
+
+        # check we're testing what we think we are: no rows should yet have been
+        # received
+        self.assertEqual([], self.test_handler.received_rdata_rows)
+
+        # now reconnect to pull the updates
+        self.reconnect()
+        self.replicate()
+
+        # we should have received all the expected rows in the right order
+
+        received_rows = self.test_handler.received_rdata_rows
+        self.assertGreaterEqual(len(received_rows), len(events))
+        for i in range(NUM_USERS):
+            # for each user, we expect the PL event row, followed by state rows for
+            # the PL event and each of the states that got reverted.
+            stream_name, token, row = received_rows.pop(0)
+            self.assertEqual("events", stream_name)
+            self.assertIsInstance(row, EventsStreamRow)
+            self.assertEqual(row.type, "ev")
+            self.assertIsInstance(row.data, EventsStreamEventRow)
+            self.assertEqual(row.data.event_id, pl_events[i].event_id)
+
+            # the state rows are unsorted
+            state_rows = []  # type: List[EventsStreamCurrentStateRow]
+            for j in range(STATES_PER_USER + 1):
+                stream_name, token, row = received_rows.pop(0)
+                self.assertEqual("events", stream_name)
+                self.assertIsInstance(row, EventsStreamRow)
+                self.assertEqual(row.type, "state")
+                self.assertIsInstance(row.data, EventsStreamCurrentStateRow)
+                state_rows.append(row.data)
+
+            state_rows.sort(key=lambda r: r.state_key)
+
+            sr = state_rows.pop(0)
+            self.assertEqual(sr.type, EventTypes.PowerLevels)
+            self.assertEqual(sr.event_id, pl_events[i].event_id)
+            for sr in state_rows:
+                self.assertEqual(sr.type, "test_state_event")
+                # "None" indicates the state has been deleted
+                self.assertIsNone(sr.event_id)
+
+        self.assertEqual([], received_rows)
+
+    event_count = 0
+
+    def _inject_test_event(
+        self, body: Optional[str] = None, sender: Optional[str] = None, **kwargs
+    ) -> EventBase:
+        if sender is None:
+            sender = self.user_id
+
+        if body is None:
+            body = "event %i" % (self.event_count,)
+            self.event_count += 1
+
+        return inject_event(
+            self.hs,
+            room_id=self.room_id,
+            sender=sender,
+            type="test_event",
+            content={"body": body},
+            **kwargs
+        )
+
+    def _inject_state_event(
+        self,
+        body: Optional[str] = None,
+        state_key: Optional[str] = None,
+        sender: Optional[str] = None,
+    ) -> EventBase:
+        if sender is None:
+            sender = self.user_id
+
+        if state_key is None:
+            state_key = "state_%i" % (self.event_count,)
+            self.event_count += 1
+
+        if body is None:
+            body = "state event %s" % (state_key,)
+
+        return inject_event(
+            self.hs,
+            room_id=self.room_id,
+            sender=sender,
+            type="test_state_event",
+            state_key=state_key,
+            content={"body": body},
+        )
diff --git a/tests/replication/tcp/streams/test_receipts.py b/tests/replication/tcp/streams/test_receipts.py
index a0206f7363..c122b8589c 100644
--- a/tests/replication/tcp/streams/test_receipts.py
+++ b/tests/replication/tcp/streams/test_receipts.py
@@ -12,6 +12,11 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+# type: ignore
+
+from mock import Mock
+
 from synapse.replication.tcp.streams._base import ReceiptsStream
 
 from tests.replication.tcp.streams._base import BaseStreamTestCase
@@ -20,11 +25,14 @@ USER_ID = "@feeling:blue"
 
 
 class ReceiptsStreamTestCase(BaseStreamTestCase):
+    def _build_replication_data_handler(self):
+        return Mock(wraps=super()._build_replication_data_handler())
+
     def test_receipt(self):
         self.reconnect()
 
         # make the client subscribe to the receipts stream
-        self.test_handler.streams.add("receipts")
+        self.test_handler.stream_positions.update({"receipts": 0})
 
         # tell the master to send a new receipt
         self.get_success(
diff --git a/tests/replication/tcp/streams/test_typing.py b/tests/replication/tcp/streams/test_typing.py
index f0ad6402ae..4d354a9db8 100644
--- a/tests/replication/tcp/streams/test_typing.py
+++ b/tests/replication/tcp/streams/test_typing.py
@@ -12,6 +12,8 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from mock import Mock
+
 from synapse.handlers.typing import RoomMember
 from synapse.replication.http import streams
 from synapse.replication.tcp.streams import TypingStream
@@ -26,6 +28,9 @@ class TypingStreamTestCase(BaseStreamTestCase):
         streams.register_servlets,
     ]
 
+    def _build_replication_data_handler(self):
+        return Mock(wraps=super()._build_replication_data_handler())
+
     def test_typing(self):
         typing = self.hs.get_typing_handler()
 
@@ -33,8 +38,8 @@ class TypingStreamTestCase(BaseStreamTestCase):
 
         self.reconnect()
 
-        # make the client subscribe to the receipts stream
-        self.test_handler.streams.add("typing")
+        # make the client subscribe to the typing stream
+        self.test_handler.stream_positions.update({"typing": 0})
 
         typing._push_update(member=RoomMember(room_id, USER_ID), typing=True)
 
@@ -75,6 +80,6 @@ class TypingStreamTestCase(BaseStreamTestCase):
         stream_name, token, rdata_rows = self.test_handler.on_rdata.call_args[0]
         self.assertEqual(stream_name, "typing")
         self.assertEqual(1, len(rdata_rows))
-        row = rdata_rows[0]  # type: TypingStream.TypingStreamRow
+        row = rdata_rows[0]
         self.assertEqual(room_id, row.room_id)
         self.assertEqual([], row.user_ids)
diff --git a/tests/rest/client/v1/utils.py b/tests/rest/client/v1/utils.py
index 371637618d..22d734e763 100644
--- a/tests/rest/client/v1/utils.py
+++ b/tests/rest/client/v1/utils.py
@@ -39,7 +39,7 @@ class RestHelper(object):
     resource = attr.ib()
     auth_user_id = attr.ib()
 
-    def create_room_as(self, room_creator, is_public=True, tok=None):
+    def create_room_as(self, room_creator=None, is_public=True, tok=None):
         temp_id = self.auth_user_id
         self.auth_user_id = room_creator
         path = "/_matrix/client/r0/createRoom"
diff --git a/tests/test_utils/__init__.py b/tests/test_utils/__init__.py
index a7310cf12a..7b345b03bb 100644
--- a/tests/test_utils/__init__.py
+++ b/tests/test_utils/__init__.py
@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2019 New Vector Ltd
+# Copyright 2020 The Matrix.org Foundation C.I.C
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -16,3 +17,22 @@
 """
 Utilities for running the unit tests
 """
+from typing import Awaitable, TypeVar
+
+TV = TypeVar("TV")
+
+
+def get_awaitable_result(awaitable: Awaitable[TV]) -> TV:
+    """Get the result from an Awaitable which should have completed
+
+    Asserts that the given awaitable has a result ready, and returns its value
+    """
+    i = awaitable.__await__()
+    try:
+        next(i)
+    except StopIteration as e:
+        # awaitable returned a result
+        return e.value
+
+    # if next didn't raise, the awaitable hasn't completed.
+    raise Exception("awaitable has not yet completed")
diff --git a/tests/test_utils/event_injection.py b/tests/test_utils/event_injection.py
new file mode 100644
index 0000000000..8f6872761a
--- /dev/null
+++ b/tests/test_utils/event_injection.py
@@ -0,0 +1,96 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+# Copyright 2020 The Matrix.org Foundation C.I.C
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import Optional
+
+import synapse.server
+from synapse.api.constants import EventTypes
+from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
+from synapse.events import EventBase
+from synapse.types import Collection
+
+from tests.test_utils import get_awaitable_result
+
+
+"""
+Utility functions for poking events into the storage of the server under test.
+"""
+
+
+def inject_member_event(
+    hs: synapse.server.HomeServer,
+    room_id: str,
+    sender: str,
+    membership: str,
+    target: Optional[str] = None,
+    extra_content: Optional[dict] = None,
+    **kwargs
+) -> EventBase:
+    """Inject a membership event into a room."""
+    if target is None:
+        target = sender
+
+    content = {"membership": membership}
+    if extra_content:
+        content.update(extra_content)
+
+    return inject_event(
+        hs,
+        room_id=room_id,
+        type=EventTypes.Member,
+        sender=sender,
+        state_key=target,
+        content=content,
+        **kwargs
+    )
+
+
+def inject_event(
+    hs: synapse.server.HomeServer,
+    room_version: Optional[str] = None,
+    prev_event_ids: Optional[Collection[str]] = None,
+    **kwargs
+) -> EventBase:
+    """Inject a generic event into a room
+
+    Args:
+        hs: the homeserver under test
+        room_version: the version of the room we're inserting into.
+            if not specified, will be looked up
+        prev_event_ids: prev_events for the event. If not specified, will be looked up
+        kwargs: fields for the event to be created
+    """
+    test_reactor = hs.get_reactor()
+
+    if room_version is None:
+        d = hs.get_datastore().get_room_version_id(kwargs["room_id"])
+        test_reactor.advance(0)
+        room_version = get_awaitable_result(d)
+
+    builder = hs.get_event_builder_factory().for_room_version(
+        KNOWN_ROOM_VERSIONS[room_version], kwargs
+    )
+    d = hs.get_event_creation_handler().create_new_client_event(
+        builder, prev_event_ids=prev_event_ids
+    )
+    test_reactor.advance(0)
+    event, context = get_awaitable_result(d)
+
+    d = hs.get_storage().persistence.persist_event(event, context)
+    test_reactor.advance(0)
+    get_awaitable_result(d)
+
+    return event
diff --git a/tests/unittest.py b/tests/unittest.py
index 27af5228fe..6b6f224e9c 100644
--- a/tests/unittest.py
+++ b/tests/unittest.py
@@ -32,7 +32,6 @@ from twisted.python.threadpool import ThreadPool
 from twisted.trial import unittest
 
 from synapse.api.constants import EventTypes, Membership
-from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
 from synapse.config.homeserver import HomeServerConfig
 from synapse.config.ratelimiting import FederationRateLimitConfig
 from synapse.federation.transport import server as federation_server
@@ -55,6 +54,7 @@ from tests.server import (
     render,
     setup_test_homeserver,
 )
+from tests.test_utils import event_injection
 from tests.test_utils.logging_setup import setup_logging
 from tests.utils import default_config, setupdb
 
@@ -596,36 +596,14 @@ class HomeserverTestCase(TestCase):
         """
         Inject a membership event into a room.
 
+        Deprecated: use event_injection.inject_room_member directly
+
         Args:
             room: Room ID to inject the event into.
             user: MXID of the user to inject the membership for.
             membership: The membership type.
         """
-        event_builder_factory = self.hs.get_event_builder_factory()
-        event_creation_handler = self.hs.get_event_creation_handler()
-
-        room_version = self.get_success(
-            self.hs.get_datastore().get_room_version_id(room)
-        )
-
-        builder = event_builder_factory.for_room_version(
-            KNOWN_ROOM_VERSIONS[room_version],
-            {
-                "type": EventTypes.Member,
-                "sender": user,
-                "state_key": user,
-                "room_id": room,
-                "content": {"membership": membership},
-            },
-        )
-
-        event, context = self.get_success(
-            event_creation_handler.create_new_client_event(builder)
-        )
-
-        self.get_success(
-            self.hs.get_storage().persistence.persist_event(event, context)
-        )
+        event_injection.inject_member_event(self.hs, room, user, membership)
 
 
 class FederatingHomeserverTestCase(HomeserverTestCase):
diff --git a/tox.ini b/tox.ini
index 31011d7436..2630857436 100644
--- a/tox.ini
+++ b/tox.ini
@@ -204,6 +204,8 @@ commands = mypy \
             synapse/storage/database.py \
             synapse/streams \
             synapse/util/caches/stream_change_cache.py \
+            tests/replication/tcp/streams \
+            tests/test_utils \
             tests/util/test_stream_change_cache.py
 
 # To find all folders that pass mypy you run: