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-rw-r--r--synapse/api/auth.py83
-rw-r--r--synapse/api/auth_blocking.py104
-rw-r--r--synapse/events/snapshot.py7
-rw-r--r--synapse/federation/send_queue.py84
-rw-r--r--synapse/federation/sender/__init__.py12
-rw-r--r--synapse/federation/sender/per_destination_queue.py6
-rw-r--r--synapse/federation/sender/transaction_manager.py6
-rw-r--r--synapse/handlers/room.py42
-rw-r--r--synapse/handlers/sync.py12
-rw-r--r--synapse/logging/context.py41
-rw-r--r--synapse/replication/tcp/handler.py53
-rw-r--r--synapse/replication/tcp/redis.py55
-rw-r--r--synapse/replication/tcp/resource.py2
-rw-r--r--synapse/replication/tcp/streams/_base.py3
-rw-r--r--synapse/replication/tcp/streams/federation.py30
-rw-r--r--synapse/storage/data_stores/main/devices.py64
-rw-r--r--synapse/storage/data_stores/main/end_to_end_keys.py98
-rw-r--r--synapse/storage/data_stores/main/schema/delta/58/04device_lists_outbound_last_success_unique_idx.sql28
-rw-r--r--synapse/storage/database.py74
-rw-r--r--synapse/storage/util/id_generators.py169
-rw-r--r--synapse/util/caches/stream_change_cache.py19
21 files changed, 671 insertions, 321 deletions
diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index c5d1eb952b..1ad5ff9410 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -26,16 +26,15 @@ from twisted.internet import defer
 import synapse.logging.opentracing as opentracing
 import synapse.types
 from synapse import event_auth
-from synapse.api.constants import EventTypes, LimitBlockingTypes, Membership, UserTypes
+from synapse.api.auth_blocking import AuthBlocking
+from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import (
     AuthError,
     Codes,
     InvalidClientTokenError,
     MissingClientTokenError,
-    ResourceLimitError,
 )
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
-from synapse.config.server import is_threepid_reserved
 from synapse.events import EventBase
 from synapse.types import StateMap, UserID
 from synapse.util.caches import CACHE_SIZE_FACTOR, register_cache
@@ -77,7 +76,11 @@ class Auth(object):
         self.token_cache = LruCache(CACHE_SIZE_FACTOR * 10000)
         register_cache("cache", "token_cache", self.token_cache)
 
+        self._auth_blocking = AuthBlocking(self.hs)
+
         self._account_validity = hs.config.account_validity
+        self._track_appservice_user_ips = hs.config.track_appservice_user_ips
+        self._macaroon_secret_key = hs.config.macaroon_secret_key
 
     @defer.inlineCallbacks
     def check_from_context(self, room_version: str, event, context, do_sig_check=True):
@@ -191,7 +194,7 @@ class Auth(object):
                 opentracing.set_tag("authenticated_entity", user_id)
                 opentracing.set_tag("appservice_id", app_service.id)
 
-                if ip_addr and self.hs.config.track_appservice_user_ips:
+                if ip_addr and self._track_appservice_user_ips:
                     yield self.store.insert_client_ip(
                         user_id=user_id,
                         access_token=access_token,
@@ -454,7 +457,7 @@ class Auth(object):
         # access_tokens include a nonce for uniqueness: any value is acceptable
         v.satisfy_general(lambda c: c.startswith("nonce = "))
 
-        v.verify(macaroon, self.hs.config.macaroon_secret_key)
+        v.verify(macaroon, self._macaroon_secret_key)
 
     def _verify_expiry(self, caveat):
         prefix = "time < "
@@ -663,71 +666,5 @@ class Auth(object):
                 % (user_id, room_id),
             )
 
-    @defer.inlineCallbacks
-    def check_auth_blocking(self, user_id=None, threepid=None, user_type=None):
-        """Checks if the user should be rejected for some external reason,
-        such as monthly active user limiting or global disable flag
-
-        Args:
-            user_id(str|None): If present, checks for presence against existing
-                MAU cohort
-
-            threepid(dict|None): If present, checks for presence against configured
-                reserved threepid. Used in cases where the user is trying register
-                with a MAU blocked server, normally they would be rejected but their
-                threepid is on the reserved list. user_id and
-                threepid should never be set at the same time.
-
-            user_type(str|None): If present, is used to decide whether to check against
-                certain blocking reasons like MAU.
-        """
-
-        # Never fail an auth check for the server notices users or support user
-        # This can be a problem where event creation is prohibited due to blocking
-        if user_id is not None:
-            if user_id == self.hs.config.server_notices_mxid:
-                return
-            if (yield self.store.is_support_user(user_id)):
-                return
-
-        if self.hs.config.hs_disabled:
-            raise ResourceLimitError(
-                403,
-                self.hs.config.hs_disabled_message,
-                errcode=Codes.RESOURCE_LIMIT_EXCEEDED,
-                admin_contact=self.hs.config.admin_contact,
-                limit_type=LimitBlockingTypes.HS_DISABLED,
-            )
-        if self.hs.config.limit_usage_by_mau is True:
-            assert not (user_id and threepid)
-
-            # If the user is already part of the MAU cohort or a trial user
-            if user_id:
-                timestamp = yield self.store.user_last_seen_monthly_active(user_id)
-                if timestamp:
-                    return
-
-                is_trial = yield self.store.is_trial_user(user_id)
-                if is_trial:
-                    return
-            elif threepid:
-                # If the user does not exist yet, but is signing up with a
-                # reserved threepid then pass auth check
-                if is_threepid_reserved(
-                    self.hs.config.mau_limits_reserved_threepids, threepid
-                ):
-                    return
-            elif user_type == UserTypes.SUPPORT:
-                # If the user does not exist yet and is of type "support",
-                # allow registration. Support users are excluded from MAU checks.
-                return
-            # Else if there is no room in the MAU bucket, bail
-            current_mau = yield self.store.get_monthly_active_count()
-            if current_mau >= self.hs.config.max_mau_value:
-                raise ResourceLimitError(
-                    403,
-                    "Monthly Active User Limit Exceeded",
-                    admin_contact=self.hs.config.admin_contact,
-                    errcode=Codes.RESOURCE_LIMIT_EXCEEDED,
-                    limit_type=LimitBlockingTypes.MONTHLY_ACTIVE_USER,
-                )
+    def check_auth_blocking(self, *args, **kwargs):
+        return self._auth_blocking.check_auth_blocking(*args, **kwargs)
diff --git a/synapse/api/auth_blocking.py b/synapse/api/auth_blocking.py
new file mode 100644
index 0000000000..5c499b6b4e
--- /dev/null
+++ b/synapse/api/auth_blocking.py
@@ -0,0 +1,104 @@
+# -*- coding: utf-8 -*-
+# 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.
+
+import logging
+
+from twisted.internet import defer
+
+from synapse.api.constants import LimitBlockingTypes, UserTypes
+from synapse.api.errors import Codes, ResourceLimitError
+from synapse.config.server import is_threepid_reserved
+
+logger = logging.getLogger(__name__)
+
+
+class AuthBlocking(object):
+    def __init__(self, hs):
+        self.store = hs.get_datastore()
+
+        self._server_notices_mxid = hs.config.server_notices_mxid
+        self._hs_disabled = hs.config.hs_disabled
+        self._hs_disabled_message = hs.config.hs_disabled_message
+        self._admin_contact = hs.config.admin_contact
+        self._max_mau_value = hs.config.max_mau_value
+        self._limit_usage_by_mau = hs.config.limit_usage_by_mau
+        self._mau_limits_reserved_threepids = hs.config.mau_limits_reserved_threepids
+
+    @defer.inlineCallbacks
+    def check_auth_blocking(self, user_id=None, threepid=None, user_type=None):
+        """Checks if the user should be rejected for some external reason,
+        such as monthly active user limiting or global disable flag
+
+        Args:
+            user_id(str|None): If present, checks for presence against existing
+                MAU cohort
+
+            threepid(dict|None): If present, checks for presence against configured
+                reserved threepid. Used in cases where the user is trying register
+                with a MAU blocked server, normally they would be rejected but their
+                threepid is on the reserved list. user_id and
+                threepid should never be set at the same time.
+
+            user_type(str|None): If present, is used to decide whether to check against
+                certain blocking reasons like MAU.
+        """
+
+        # Never fail an auth check for the server notices users or support user
+        # This can be a problem where event creation is prohibited due to blocking
+        if user_id is not None:
+            if user_id == self._server_notices_mxid:
+                return
+            if (yield self.store.is_support_user(user_id)):
+                return
+
+        if self._hs_disabled:
+            raise ResourceLimitError(
+                403,
+                self._hs_disabled_message,
+                errcode=Codes.RESOURCE_LIMIT_EXCEEDED,
+                admin_contact=self._admin_contact,
+                limit_type=LimitBlockingTypes.HS_DISABLED,
+            )
+        if self._limit_usage_by_mau is True:
+            assert not (user_id and threepid)
+
+            # If the user is already part of the MAU cohort or a trial user
+            if user_id:
+                timestamp = yield self.store.user_last_seen_monthly_active(user_id)
+                if timestamp:
+                    return
+
+                is_trial = yield self.store.is_trial_user(user_id)
+                if is_trial:
+                    return
+            elif threepid:
+                # If the user does not exist yet, but is signing up with a
+                # reserved threepid then pass auth check
+                if is_threepid_reserved(self._mau_limits_reserved_threepids, threepid):
+                    return
+            elif user_type == UserTypes.SUPPORT:
+                # If the user does not exist yet and is of type "support",
+                # allow registration. Support users are excluded from MAU checks.
+                return
+            # Else if there is no room in the MAU bucket, bail
+            current_mau = yield self.store.get_monthly_active_count()
+            if current_mau >= self._max_mau_value:
+                raise ResourceLimitError(
+                    403,
+                    "Monthly Active User Limit Exceeded",
+                    admin_contact=self._admin_contact,
+                    errcode=Codes.RESOURCE_LIMIT_EXCEEDED,
+                    limit_type=LimitBlockingTypes.MONTHLY_ACTIVE_USER,
+                )
diff --git a/synapse/events/snapshot.py b/synapse/events/snapshot.py
index 9ea85e93e6..7c5f620d09 100644
--- a/synapse/events/snapshot.py
+++ b/synapse/events/snapshot.py
@@ -322,11 +322,14 @@ class _AsyncEventContextImpl(EventContext):
         self._current_state_ids = yield self._storage.state.get_state_ids_for_group(
             self.state_group
         )
-        if self._prev_state_id and self._event_state_key is not None:
+        if self._event_state_key is not None:
             self._prev_state_ids = dict(self._current_state_ids)
 
             key = (self._event_type, self._event_state_key)
-            self._prev_state_ids[key] = self._prev_state_id
+            if self._prev_state_id:
+                self._prev_state_ids[key] = self._prev_state_id
+            else:
+                self._prev_state_ids.pop(key, None)
         else:
             self._prev_state_ids = self._current_state_ids
 
diff --git a/synapse/federation/send_queue.py b/synapse/federation/send_queue.py
index e1700ca8aa..52f4f54215 100644
--- a/synapse/federation/send_queue.py
+++ b/synapse/federation/send_queue.py
@@ -31,6 +31,7 @@ Events are replicated via a separate events stream.
 
 import logging
 from collections import namedtuple
+from typing import Dict, List, Tuple, Type
 
 from six import iteritems
 
@@ -56,21 +57,35 @@ class FederationRemoteSendQueue(object):
         self.notifier = hs.get_notifier()
         self.is_mine_id = hs.is_mine_id
 
-        self.presence_map = {}  # Pending presence map user_id -> UserPresenceState
-        self.presence_changed = SortedDict()  # Stream position -> list[user_id]
+        # Pending presence map user_id -> UserPresenceState
+        self.presence_map = {}  # type: Dict[str, UserPresenceState]
+
+        # Stream position -> list[user_id]
+        self.presence_changed = SortedDict()  # type: SortedDict[int, List[str]]
 
         # Stores the destinations we need to explicitly send presence to about a
         # given user.
         # Stream position -> (user_id, destinations)
-        self.presence_destinations = SortedDict()
+        self.presence_destinations = (
+            SortedDict()
+        )  # type: SortedDict[int, Tuple[str, List[str]]]
+
+        # (destination, key) -> EDU
+        self.keyed_edu = {}  # type: Dict[Tuple[str, tuple], Edu]
 
-        self.keyed_edu = {}  # (destination, key) -> EDU
-        self.keyed_edu_changed = SortedDict()  # stream position -> (destination, key)
+        # stream position -> (destination, key)
+        self.keyed_edu_changed = (
+            SortedDict()
+        )  # type: SortedDict[int, Tuple[str, tuple]]
 
-        self.edus = SortedDict()  # stream position -> Edu
+        self.edus = SortedDict()  # type: SortedDict[int, Edu]
 
+        # stream ID for the next entry into presence_changed/keyed_edu_changed/edus.
         self.pos = 1
-        self.pos_time = SortedDict()
+
+        # map from stream ID to the time that stream entry was generated, so that we
+        # can clear out entries after a while
+        self.pos_time = SortedDict()  # type: SortedDict[int, int]
 
         # EVERYTHING IS SAD. In particular, python only makes new scopes when
         # we make a new function, so we need to make a new function so the inner
@@ -158,8 +173,10 @@ class FederationRemoteSendQueue(object):
             for edu_key in self.keyed_edu_changed.values():
                 live_keys.add(edu_key)
 
-            to_del = [edu_key for edu_key in self.keyed_edu if edu_key not in live_keys]
-            for edu_key in to_del:
+            keys_to_del = [
+                edu_key for edu_key in self.keyed_edu if edu_key not in live_keys
+            ]
+            for edu_key in keys_to_del:
                 del self.keyed_edu[edu_key]
 
             # Delete things out of edu map
@@ -250,19 +267,23 @@ class FederationRemoteSendQueue(object):
         self._clear_queue_before_pos(token)
 
     async def get_replication_rows(
-        self, from_token, to_token, limit, federation_ack=None
-    ):
+        self, instance_name: str, from_token: int, to_token: int, target_row_count: int
+    ) -> Tuple[List[Tuple[int, Tuple]], int, bool]:
         """Get rows to be sent over federation between the two tokens
 
         Args:
-            from_token (int)
-            to_token(int)
-            limit (int)
-            federation_ack (int): Optional. The position where the worker is
-                explicitly acknowledged it has handled. Allows us to drop
-                data from before that point
+            instance_name: the name of the current process
+            from_token: the previous stream token: the starting point for fetching the
+                updates
+            to_token: the new stream token: the point to get updates up to
+            target_row_count: a target for the number of rows to be returned.
+
+        Returns: a triplet `(updates, new_last_token, limited)`, where:
+           * `updates` is a list of `(token, row)` entries.
+           * `new_last_token` is the new position in stream.
+           * `limited` is whether there are more updates to fetch.
         """
-        # TODO: Handle limit.
+        # TODO: Handle target_row_count.
 
         # To handle restarts where we wrap around
         if from_token > self.pos:
@@ -270,12 +291,7 @@ class FederationRemoteSendQueue(object):
 
         # list of tuple(int, BaseFederationRow), where the first is the position
         # of the federation stream.
-        rows = []
-
-        # There should be only one reader, so lets delete everything its
-        # acknowledged its seen.
-        if federation_ack:
-            self._clear_queue_before_pos(federation_ack)
+        rows = []  # type: List[Tuple[int, BaseFederationRow]]
 
         # Fetch changed presence
         i = self.presence_changed.bisect_right(from_token)
@@ -332,7 +348,11 @@ class FederationRemoteSendQueue(object):
         # Sort rows based on pos
         rows.sort()
 
-        return [(pos, row.TypeId, row.to_data()) for pos, row in rows]
+        return (
+            [(pos, (row.TypeId, row.to_data())) for pos, row in rows],
+            to_token,
+            False,
+        )
 
 
 class BaseFederationRow(object):
@@ -341,7 +361,7 @@ class BaseFederationRow(object):
     Specifies how to identify, serialize and deserialize the different types.
     """
 
-    TypeId = None  # Unique string that ids the type. Must be overriden in sub classes.
+    TypeId = ""  # Unique string that ids the type. Must be overriden in sub classes.
 
     @staticmethod
     def from_data(data):
@@ -454,10 +474,14 @@ class EduRow(BaseFederationRow, namedtuple("EduRow", ("edu",))):  # Edu
         buff.edus.setdefault(self.edu.destination, []).append(self.edu)
 
 
-TypeToRow = {
-    Row.TypeId: Row
-    for Row in (PresenceRow, PresenceDestinationsRow, KeyedEduRow, EduRow,)
-}
+_rowtypes = (
+    PresenceRow,
+    PresenceDestinationsRow,
+    KeyedEduRow,
+    EduRow,
+)  # type: Tuple[Type[BaseFederationRow], ...]
+
+TypeToRow = {Row.TypeId: Row for Row in _rowtypes}
 
 
 ParsedFederationStreamData = namedtuple(
diff --git a/synapse/federation/sender/__init__.py b/synapse/federation/sender/__init__.py
index a477578e44..d473576902 100644
--- a/synapse/federation/sender/__init__.py
+++ b/synapse/federation/sender/__init__.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 
 import logging
-from typing import Dict, Hashable, Iterable, List, Optional, Set
+from typing import Dict, Hashable, Iterable, List, Optional, Set, Tuple
 
 from six import itervalues
 
@@ -498,14 +498,16 @@ class FederationSender(object):
 
         self._get_per_destination_queue(destination).attempt_new_transaction()
 
-    def get_current_token(self) -> int:
+    @staticmethod
+    def get_current_token() -> int:
         # Dummy implementation for case where federation sender isn't offloaded
         # to a worker.
         return 0
 
+    @staticmethod
     async def get_replication_rows(
-        self, from_token, to_token, limit, federation_ack=None
-    ):
+        instance_name: str, from_token: int, to_token: int, target_row_count: int
+    ) -> Tuple[List[Tuple[int, Tuple]], int, bool]:
         # Dummy implementation for case where federation sender isn't offloaded
         # to a worker.
-        return []
+        return [], 0, False
diff --git a/synapse/federation/sender/per_destination_queue.py b/synapse/federation/sender/per_destination_queue.py
index e13cd20ffa..276a2b596f 100644
--- a/synapse/federation/sender/per_destination_queue.py
+++ b/synapse/federation/sender/per_destination_queue.py
@@ -15,11 +15,10 @@
 # limitations under the License.
 import datetime
 import logging
-from typing import Dict, Hashable, Iterable, List, Tuple
+from typing import TYPE_CHECKING, Dict, Hashable, Iterable, List, Tuple
 
 from prometheus_client import Counter
 
-import synapse.server
 from synapse.api.errors import (
     FederationDeniedError,
     HttpResponseException,
@@ -34,6 +33,9 @@ from synapse.storage.presence import UserPresenceState
 from synapse.types import ReadReceipt
 from synapse.util.retryutils import NotRetryingDestination, get_retry_limiter
 
+if TYPE_CHECKING:
+    import synapse.server
+
 # This is defined in the Matrix spec and enforced by the receiver.
 MAX_EDUS_PER_TRANSACTION = 100
 
diff --git a/synapse/federation/sender/transaction_manager.py b/synapse/federation/sender/transaction_manager.py
index 3c2a02a3b3..a2752a54a5 100644
--- a/synapse/federation/sender/transaction_manager.py
+++ b/synapse/federation/sender/transaction_manager.py
@@ -13,11 +13,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import List
+from typing import TYPE_CHECKING, List
 
 from canonicaljson import json
 
-import synapse.server
 from synapse.api.errors import HttpResponseException
 from synapse.events import EventBase
 from synapse.federation.persistence import TransactionActions
@@ -31,6 +30,9 @@ from synapse.logging.opentracing import (
 )
 from synapse.util.metrics import measure_func
 
+if TYPE_CHECKING:
+    import synapse.server
+
 logger = logging.getLogger(__name__)
 
 
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index da12df7f53..73f9eeb399 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -25,8 +25,6 @@ from collections import OrderedDict
 
 from six import iteritems, string_types
 
-from twisted.internet import defer
-
 from synapse.api.constants import EventTypes, JoinRules, RoomCreationPreset
 from synapse.api.errors import AuthError, Codes, NotFoundError, StoreError, SynapseError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
@@ -103,8 +101,7 @@ class RoomCreationHandler(BaseHandler):
 
         self.third_party_event_rules = hs.get_third_party_event_rules()
 
-    @defer.inlineCallbacks
-    def upgrade_room(
+    async def upgrade_room(
         self, requester: Requester, old_room_id: str, new_version: RoomVersion
     ):
         """Replace a room with a new room with a different version
@@ -117,7 +114,7 @@ class RoomCreationHandler(BaseHandler):
         Returns:
             Deferred[unicode]: the new room id
         """
-        yield self.ratelimit(requester)
+        await self.ratelimit(requester)
 
         user_id = requester.user.to_string()
 
@@ -138,7 +135,7 @@ class RoomCreationHandler(BaseHandler):
         # If this user has sent multiple upgrade requests for the same room
         # and one of them is not complete yet, cache the response and
         # return it to all subsequent requests
-        ret = yield self._upgrade_response_cache.wrap(
+        ret = await self._upgrade_response_cache.wrap(
             (old_room_id, user_id),
             self._upgrade_room,
             requester,
@@ -856,8 +853,7 @@ class RoomCreationHandler(BaseHandler):
         for (etype, state_key), content in initial_state.items():
             await send(etype=etype, state_key=state_key, content=content)
 
-    @defer.inlineCallbacks
-    def _generate_room_id(
+    async def _generate_room_id(
         self, creator_id: str, is_public: str, room_version: RoomVersion,
     ):
         # autogen room IDs and try to create it. We may clash, so just
@@ -869,7 +865,7 @@ class RoomCreationHandler(BaseHandler):
                 gen_room_id = RoomID(random_string, self.hs.hostname).to_string()
                 if isinstance(gen_room_id, bytes):
                     gen_room_id = gen_room_id.decode("utf-8")
-                yield self.store.store_room(
+                await self.store.store_room(
                     room_id=gen_room_id,
                     room_creator_user_id=creator_id,
                     is_public=is_public,
@@ -888,8 +884,7 @@ class RoomContextHandler(object):
         self.storage = hs.get_storage()
         self.state_store = self.storage.state
 
-    @defer.inlineCallbacks
-    def get_event_context(self, user, room_id, event_id, limit, event_filter):
+    async def get_event_context(self, user, room_id, event_id, limit, event_filter):
         """Retrieves events, pagination tokens and state around a given event
         in a room.
 
@@ -908,7 +903,7 @@ class RoomContextHandler(object):
         before_limit = math.floor(limit / 2.0)
         after_limit = limit - before_limit
 
-        users = yield self.store.get_users_in_room(room_id)
+        users = await self.store.get_users_in_room(room_id)
         is_peeking = user.to_string() not in users
 
         def filter_evts(events):
@@ -916,17 +911,17 @@ class RoomContextHandler(object):
                 self.storage, user.to_string(), events, is_peeking=is_peeking
             )
 
-        event = yield self.store.get_event(
+        event = await self.store.get_event(
             event_id, get_prev_content=True, allow_none=True
         )
         if not event:
             return None
 
-        filtered = yield (filter_evts([event]))
+        filtered = await filter_evts([event])
         if not filtered:
             raise AuthError(403, "You don't have permission to access that event.")
 
-        results = yield self.store.get_events_around(
+        results = await self.store.get_events_around(
             room_id, event_id, before_limit, after_limit, event_filter
         )
 
@@ -934,8 +929,8 @@ class RoomContextHandler(object):
             results["events_before"] = event_filter.filter(results["events_before"])
             results["events_after"] = event_filter.filter(results["events_after"])
 
-        results["events_before"] = yield filter_evts(results["events_before"])
-        results["events_after"] = yield filter_evts(results["events_after"])
+        results["events_before"] = await filter_evts(results["events_before"])
+        results["events_after"] = await filter_evts(results["events_after"])
         # filter_evts can return a pruned event in case the user is allowed to see that
         # there's something there but not see the content, so use the event that's in
         # `filtered` rather than the event we retrieved from the datastore.
@@ -962,7 +957,7 @@ class RoomContextHandler(object):
         # first? Shouldn't we be consistent with /sync?
         # https://github.com/matrix-org/matrix-doc/issues/687
 
-        state = yield self.state_store.get_state_for_events(
+        state = await self.state_store.get_state_for_events(
             [last_event_id], state_filter=state_filter
         )
 
@@ -970,7 +965,7 @@ class RoomContextHandler(object):
         if event_filter:
             state_events = event_filter.filter(state_events)
 
-        results["state"] = yield filter_evts(state_events)
+        results["state"] = await filter_evts(state_events)
 
         # We use a dummy token here as we only care about the room portion of
         # the token, which we replace.
@@ -989,13 +984,12 @@ class RoomEventSource(object):
     def __init__(self, hs):
         self.store = hs.get_datastore()
 
-    @defer.inlineCallbacks
-    def get_new_events(
+    async def get_new_events(
         self, user, from_key, limit, room_ids, is_guest, explicit_room_id=None
     ):
         # We just ignore the key for now.
 
-        to_key = yield self.get_current_key()
+        to_key = await self.get_current_key()
 
         from_token = RoomStreamToken.parse(from_key)
         if from_token.topological:
@@ -1008,11 +1002,11 @@ class RoomEventSource(object):
             # See https://github.com/matrix-org/matrix-doc/issues/1144
             raise NotImplementedError()
         else:
-            room_events = yield self.store.get_membership_changes_for_user(
+            room_events = await self.store.get_membership_changes_for_user(
                 user.to_string(), from_key, to_key
             )
 
-            room_to_events = yield self.store.get_room_events_stream_for_rooms(
+            room_to_events = await self.store.get_room_events_stream_for_rooms(
                 room_ids=room_ids,
                 from_key=from_key,
                 to_key=to_key,
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 4f76b7a743..00718d7f2d 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1143,10 +1143,14 @@ class SyncHandler(object):
                 user_id
             )
 
-            tracked_users = set(users_who_share_room)
-
-            # Always tell the user about their own devices
-            tracked_users.add(user_id)
+            # Always tell the user about their own devices. We check as the user
+            # ID is almost certainly already included (unless they're not in any
+            # rooms) and taking a copy of the set is relatively expensive.
+            if user_id not in users_who_share_room:
+                users_who_share_room = set(users_who_share_room)
+                users_who_share_room.add(user_id)
+
+            tracked_users = users_who_share_room
 
             # Step 1a, check for changes in devices of users we share a room with
             users_that_have_changed = await self.store.get_users_whose_devices_changed(
diff --git a/synapse/logging/context.py b/synapse/logging/context.py
index a8f674d13d..856534e91a 100644
--- a/synapse/logging/context.py
+++ b/synapse/logging/context.py
@@ -27,6 +27,7 @@ import inspect
 import logging
 import threading
 import types
+import warnings
 from typing import TYPE_CHECKING, Optional, Tuple, TypeVar, Union
 
 from typing_extensions import Literal
@@ -287,6 +288,46 @@ class LoggingContext(object):
             return str(self.request)
         return "%s@%x" % (self.name, id(self))
 
+    @classmethod
+    def current_context(cls) -> LoggingContextOrSentinel:
+        """Get the current logging context from thread local storage
+
+        This exists for backwards compatibility. ``current_context()`` should be
+        called directly.
+
+        Returns:
+            LoggingContext: the current logging context
+        """
+        warnings.warn(
+            "synapse.logging.context.LoggingContext.current_context() is deprecated "
+            "in favor of synapse.logging.context.current_context().",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        return current_context()
+
+    @classmethod
+    def set_current_context(
+        cls, context: LoggingContextOrSentinel
+    ) -> LoggingContextOrSentinel:
+        """Set the current logging context in thread local storage
+
+        This exists for backwards compatibility. ``set_current_context()`` should be
+        called directly.
+
+        Args:
+            context(LoggingContext): The context to activate.
+        Returns:
+            The context that was previously active
+        """
+        warnings.warn(
+            "synapse.logging.context.LoggingContext.set_current_context() is deprecated "
+            "in favor of synapse.logging.context.set_current_context().",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        return set_current_context(context)
+
     def __enter__(self) -> "LoggingContext":
         """Enters this logging context into thread local storage"""
         old_context = set_current_context(self)
diff --git a/synapse/replication/tcp/handler.py b/synapse/replication/tcp/handler.py
index 2d1d119c7c..b14a3d9fca 100644
--- a/synapse/replication/tcp/handler.py
+++ b/synapse/replication/tcp/handler.py
@@ -81,9 +81,6 @@ class ReplicationCommandHandler:
         self._instance_id = hs.get_instance_id()
         self._instance_name = hs.get_instance_name()
 
-        # Set of streams that we've caught up with.
-        self._streams_connected = set()  # type: Set[str]
-
         self._streams = {
             stream.NAME: stream(hs) for stream in STREAMS_MAP.values()
         }  # type: Dict[str, Stream]
@@ -99,9 +96,13 @@ class ReplicationCommandHandler:
         # The factory used to create connections.
         self._factory = None  # type: Optional[ReconnectingClientFactory]
 
-        # The currently connected connections.
+        # The currently connected connections. (The list of places we need to send
+        # outgoing replication commands to.)
         self._connections = []  # type: List[AbstractConnection]
 
+        # For each connection, the incoming streams that are coming from that connection
+        self._streams_by_connection = {}  # type: Dict[AbstractConnection, Set[str]]
+
         LaterGauge(
             "synapse_replication_tcp_resource_total_connections",
             "",
@@ -257,12 +258,14 @@ class ReplicationCommandHandler:
         #   2. so we don't race with getting a POSITION command and fetching
         #      missing RDATA.
         with await self._position_linearizer.queue(cmd.stream_name):
-            if stream_name not in self._streams_connected:
-                # If the stream isn't marked as connected then we haven't seen a
-                # `POSITION` command yet, and so we may have missed some rows.
+            # make sure that we've processed a POSITION for this stream *on this
+            # connection*. (A POSITION on another connection is no good, as there
+            # is no guarantee that we have seen all the intermediate updates.)
+            sbc = self._streams_by_connection.get(conn)
+            if not sbc or stream_name not in sbc:
                 # Let's drop the row for now, on the assumption we'll receive a
                 # `POSITION` soon and we'll catch up correctly then.
-                logger.warning(
+                logger.debug(
                     "Discarding RDATA for unconnected stream %s -> %s",
                     stream_name,
                     cmd.token,
@@ -302,21 +305,25 @@ class ReplicationCommandHandler:
             # Ignore POSITION that are just our own echoes
             return
 
-        stream = self._streams.get(cmd.stream_name)
+        logger.info("Handling '%s %s'", cmd.NAME, cmd.to_line())
+
+        stream_name = cmd.stream_name
+        stream = self._streams.get(stream_name)
         if not stream:
-            logger.error("Got POSITION for unknown stream: %s", cmd.stream_name)
+            logger.error("Got POSITION for unknown stream: %s", stream_name)
             return
 
         # We protect catching up with a linearizer in case the replication
         # connection reconnects under us.
-        with await self._position_linearizer.queue(cmd.stream_name):
+        with await self._position_linearizer.queue(stream_name):
             # We're about to go and catch up with the stream, so remove from set
             # of connected streams.
-            self._streams_connected.discard(cmd.stream_name)
+            for streams in self._streams_by_connection.values():
+                streams.discard(stream_name)
 
             # We clear the pending batches for the stream as the fetching of the
             # missing updates below will fetch all rows in the batch.
-            self._pending_batches.pop(cmd.stream_name, [])
+            self._pending_batches.pop(stream_name, [])
 
             # Find where we previously streamed up to.
             current_token = stream.current_token()
@@ -326,6 +333,12 @@ class ReplicationCommandHandler:
             # between then and now.
             missing_updates = cmd.token != current_token
             while missing_updates:
+                logger.info(
+                    "Fetching replication rows for '%s' between %i and %i",
+                    stream_name,
+                    current_token,
+                    cmd.token,
+                )
                 (
                     updates,
                     current_token,
@@ -341,16 +354,18 @@ class ReplicationCommandHandler:
 
                 for token, rows in _batch_updates(updates):
                     await self.on_rdata(
-                        cmd.stream_name,
+                        stream_name,
                         cmd.instance_name,
                         token,
                         [stream.parse_row(row) for row in rows],
                     )
 
+            logger.info("Caught up with stream '%s' to %i", stream_name, cmd.token)
+
             # We've now caught up to position sent to us, notify handler.
-            await self._replication_data_handler.on_position(cmd.stream_name, cmd.token)
+            await self._replication_data_handler.on_position(stream_name, cmd.token)
 
-            self._streams_connected.add(cmd.stream_name)
+            self._streams_by_connection.setdefault(conn, set()).add(stream_name)
 
     async def on_REMOTE_SERVER_UP(
         self, conn: AbstractConnection, cmd: RemoteServerUpCommand
@@ -408,6 +423,12 @@ class ReplicationCommandHandler:
     def lost_connection(self, connection: AbstractConnection):
         """Called when a connection is closed/lost.
         """
+        # we no longer need _streams_by_connection for this connection.
+        streams = self._streams_by_connection.pop(connection, None)
+        if streams:
+            logger.info(
+                "Lost replication connection; streams now disconnected: %s", streams
+            )
         try:
             self._connections.remove(connection)
         except ValueError:
diff --git a/synapse/replication/tcp/redis.py b/synapse/replication/tcp/redis.py
index 617e860f95..db69f92557 100644
--- a/synapse/replication/tcp/redis.py
+++ b/synapse/replication/tcp/redis.py
@@ -18,7 +18,7 @@ from typing import TYPE_CHECKING
 
 import txredisapi
 
-from synapse.logging.context import PreserveLoggingContext
+from synapse.logging.context import make_deferred_yieldable
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.replication.tcp.commands import (
     Command,
@@ -41,8 +41,14 @@ logger = logging.getLogger(__name__)
 class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
     """Connection to redis subscribed to replication stream.
 
-    Parses incoming messages from redis into replication commands, and passes
-    them to `ReplicationCommandHandler`
+    This class fulfils two functions:
+
+    (a) it implements the twisted Protocol API, where it handles the SUBSCRIBEd redis
+    connection, parsing *incoming* messages into replication commands, and passing them
+    to `ReplicationCommandHandler`
+
+    (b) it implements the AbstractConnection API, where it sends *outgoing* commands
+    onto outbound_redis_connection.
 
     Due to the vagaries of `txredisapi` we don't want to have a custom
     constructor, so instead we expect the defined attributes below to be set
@@ -50,8 +56,8 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
 
     Attributes:
         handler: The command handler to handle incoming commands.
-        stream_name: The *redis* stream name to subscribe to (not anything to
-            do with Synapse replication streams).
+        stream_name: The *redis* stream name to subscribe to and publish from
+            (not anything to do with Synapse replication streams).
         outbound_redis_connection: The connection to redis to use to send
             commands.
     """
@@ -61,12 +67,23 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
     outbound_redis_connection = None  # type: txredisapi.RedisProtocol
 
     def connectionMade(self):
-        logger.info("Connected to redis instance")
-        self.subscribe(self.stream_name)
-        self.send_command(ReplicateCommand())
-
+        logger.info("Connected to redis")
+        super().connectionMade()
+        run_as_background_process("subscribe-replication", self._send_subscribe)
         self.handler.new_connection(self)
 
+    async def _send_subscribe(self):
+        # it's important to make sure that we only send the REPLICATE command once we
+        # have successfully subscribed to the stream - otherwise we might miss the
+        # POSITION response sent back by the other end.
+        logger.info("Sending redis SUBSCRIBE for %s", self.stream_name)
+        await make_deferred_yieldable(self.subscribe(self.stream_name))
+        logger.info(
+            "Successfully subscribed to redis stream, sending REPLICATE command"
+        )
+        await self._async_send_command(ReplicateCommand())
+        logger.info("REPLICATE successfully sent")
+
     def messageReceived(self, pattern: str, channel: str, message: str):
         """Received a message from redis.
         """
@@ -119,7 +136,8 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
             logger.warning("Unhandled command: %r", cmd)
 
     def connectionLost(self, reason):
-        logger.info("Lost connection to redis instance")
+        logger.info("Lost connection to redis")
+        super().connectionLost(reason)
         self.handler.lost_connection(self)
 
     def send_command(self, cmd: Command):
@@ -128,6 +146,10 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
         Args:
             cmd (Command)
         """
+        run_as_background_process("send-cmd", self._async_send_command, cmd)
+
+    async def _async_send_command(self, cmd: Command):
+        """Encode a replication command and send it over our outbound connection"""
         string = "%s %s" % (cmd.NAME, cmd.to_line())
         if "\n" in string:
             raise Exception("Unexpected newline in command: %r", string)
@@ -138,15 +160,9 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
         # remote instances.
         tcp_outbound_commands_counter.labels(cmd.NAME, "redis").inc()
 
-        async def _send():
-            with PreserveLoggingContext():
-                # Note that we use the other connection as we can't send
-                # commands using the subscription connection.
-                await self.outbound_redis_connection.publish(
-                    self.stream_name, encoded_string
-                )
-
-        run_as_background_process("send-cmd", _send)
+        await make_deferred_yieldable(
+            self.outbound_redis_connection.publish(self.stream_name, encoded_string)
+        )
 
 
 class RedisDirectTcpReplicationClientFactory(txredisapi.SubscriberFactory):
@@ -189,5 +205,6 @@ class RedisDirectTcpReplicationClientFactory(txredisapi.SubscriberFactory):
         p.handler = self.handler
         p.outbound_redis_connection = self.outbound_redis_connection
         p.stream_name = self.stream_name
+        p.password = self.password
 
         return p
diff --git a/synapse/replication/tcp/resource.py b/synapse/replication/tcp/resource.py
index 33d2f589ac..b690abedad 100644
--- a/synapse/replication/tcp/resource.py
+++ b/synapse/replication/tcp/resource.py
@@ -80,7 +80,7 @@ class ReplicationStreamer(object):
             for stream in STREAMS_MAP.values():
                 if stream == FederationStream and hs.config.send_federation:
                     # We only support federation stream if federation sending
-                    # hase been disabled on the master.
+                    # has been disabled on the master.
                     continue
 
                 self.streams.append(stream(hs))
diff --git a/synapse/replication/tcp/streams/_base.py b/synapse/replication/tcp/streams/_base.py
index b0f87c365b..084604e8b0 100644
--- a/synapse/replication/tcp/streams/_base.py
+++ b/synapse/replication/tcp/streams/_base.py
@@ -104,7 +104,8 @@ class Stream(object):
         implemented by subclasses.
 
         current_token_function is called to get the current token of the underlying
-        stream.
+        stream. It is only meaningful on the process that is the source of the
+        replication stream (ie, usually the master).
 
         update_function is called to get updates for this stream between a pair of
         stream tokens. See the UpdateFunction type definition for more info.
diff --git a/synapse/replication/tcp/streams/federation.py b/synapse/replication/tcp/streams/federation.py
index e8bd52e389..b0505b8a2c 100644
--- a/synapse/replication/tcp/streams/federation.py
+++ b/synapse/replication/tcp/streams/federation.py
@@ -15,7 +15,7 @@
 # limitations under the License.
 from collections import namedtuple
 
-from synapse.replication.tcp.streams._base import Stream, db_query_to_update_function
+from synapse.replication.tcp.streams._base import Stream, make_http_update_function
 
 
 class FederationStream(Stream):
@@ -35,21 +35,33 @@ class FederationStream(Stream):
     ROW_TYPE = FederationStreamRow
 
     def __init__(self, hs):
-        # Not all synapse instances will have a federation sender instance,
-        # whether that's a `FederationSender` or a `FederationRemoteSendQueue`,
-        # so we stub the stream out when that is the case.
-        if hs.config.worker_app is None or hs.should_send_federation():
+        if hs.config.worker_app is None:
+            # master process: get updates from the FederationRemoteSendQueue.
+            # (if the master is configured to send federation itself, federation_sender
+            # will be a real FederationSender, which has stubs for current_token and
+            # get_replication_rows.)
             federation_sender = hs.get_federation_sender()
             current_token = federation_sender.get_current_token
-            update_function = db_query_to_update_function(
-                federation_sender.get_replication_rows
-            )
+            update_function = federation_sender.get_replication_rows
+
+        elif hs.should_send_federation():
+            # federation sender: Query master process
+            update_function = make_http_update_function(hs, self.NAME)
+            current_token = self._stub_current_token
+
         else:
-            current_token = lambda: 0
+            # other worker: stub out the update function (we're not interested in
+            # any updates so when we get a POSITION we do nothing)
             update_function = self._stub_update_function
+            current_token = self._stub_current_token
 
         super().__init__(hs.get_instance_name(), current_token, update_function)
 
     @staticmethod
+    def _stub_current_token():
+        # dummy current-token method for use on workers
+        return 0
+
+    @staticmethod
     async def _stub_update_function(instance_name, from_token, upto_token, limit):
         return [], upto_token, False
diff --git a/synapse/storage/data_stores/main/devices.py b/synapse/storage/data_stores/main/devices.py
index ee3a2ab031..fe6d6ecfe0 100644
--- a/synapse/storage/data_stores/main/devices.py
+++ b/synapse/storage/data_stores/main/devices.py
@@ -55,6 +55,10 @@ DROP_DEVICE_LIST_STREAMS_NON_UNIQUE_INDEXES = (
 
 BG_UPDATE_REMOVE_DUP_OUTBOUND_POKES = "remove_dup_outbound_pokes"
 
+BG_UPDATE_DROP_DEVICE_LISTS_OUTBOUND_LAST_SUCCESS_NON_UNIQUE_IDX = (
+    "drop_device_lists_outbound_last_success_non_unique_idx"
+)
+
 
 class DeviceWorkerStore(SQLBaseStore):
     def get_device(self, user_id, device_id):
@@ -342,32 +346,23 @@ class DeviceWorkerStore(SQLBaseStore):
 
     def _mark_as_sent_devices_by_remote_txn(self, txn, destination, stream_id):
         # We update the device_lists_outbound_last_success with the successfully
-        # poked users. We do the join to see which users need to be inserted and
-        # which updated.
+        # poked users.
         sql = """
-            SELECT user_id, coalesce(max(o.stream_id), 0), (max(s.stream_id) IS NOT NULL)
+            SELECT user_id, coalesce(max(o.stream_id), 0)
             FROM device_lists_outbound_pokes as o
-            LEFT JOIN device_lists_outbound_last_success as s
-                USING (destination, user_id)
             WHERE destination = ? AND o.stream_id <= ?
             GROUP BY user_id
         """
         txn.execute(sql, (destination, stream_id))
         rows = txn.fetchall()
 
-        sql = """
-            UPDATE device_lists_outbound_last_success
-            SET stream_id = ?
-            WHERE destination = ? AND user_id = ?
-        """
-        txn.executemany(sql, ((row[1], destination, row[0]) for row in rows if row[2]))
-
-        sql = """
-            INSERT INTO device_lists_outbound_last_success
-            (destination, user_id, stream_id) VALUES (?, ?, ?)
-        """
-        txn.executemany(
-            sql, ((destination, row[0], row[1]) for row in rows if not row[2])
+        self.db.simple_upsert_many_txn(
+            txn=txn,
+            table="device_lists_outbound_last_success",
+            key_names=("destination", "user_id"),
+            key_values=((destination, user_id) for user_id, _ in rows),
+            value_names=("stream_id",),
+            value_values=((stream_id,) for _, stream_id in rows),
         )
 
         # Delete all sent outbound pokes
@@ -541,8 +536,8 @@ class DeviceWorkerStore(SQLBaseStore):
 
         # Get set of users who *may* have changed. Users not in the returned
         # list have definitely not changed.
-        to_check = list(
-            self._device_list_stream_cache.get_entities_changed(user_ids, from_key)
+        to_check = self._device_list_stream_cache.get_entities_changed(
+            user_ids, from_key
         )
 
         if not to_check:
@@ -725,6 +720,21 @@ class DeviceBackgroundUpdateStore(SQLBaseStore):
             BG_UPDATE_REMOVE_DUP_OUTBOUND_POKES, self._remove_duplicate_outbound_pokes,
         )
 
+        # create a unique index on device_lists_outbound_last_success
+        self.db.updates.register_background_index_update(
+            "device_lists_outbound_last_success_unique_idx",
+            index_name="device_lists_outbound_last_success_unique_idx",
+            table="device_lists_outbound_last_success",
+            columns=["destination", "user_id"],
+            unique=True,
+        )
+
+        # once that completes, we can remove the old non-unique index.
+        self.db.updates.register_background_update_handler(
+            BG_UPDATE_DROP_DEVICE_LISTS_OUTBOUND_LAST_SUCCESS_NON_UNIQUE_IDX,
+            self._drop_device_lists_outbound_last_success_non_unique_idx,
+        )
+
     @defer.inlineCallbacks
     def _drop_device_list_streams_non_unique_indexes(self, progress, batch_size):
         def f(conn):
@@ -799,6 +809,20 @@ class DeviceBackgroundUpdateStore(SQLBaseStore):
 
         return rows
 
+    async def _drop_device_lists_outbound_last_success_non_unique_idx(
+        self, progress, batch_size
+    ):
+        def f(txn):
+            txn.execute("DROP INDEX IF EXISTS device_lists_outbound_last_success_idx")
+
+        await self.db.runInteraction(
+            "drop_device_lists_outbound_last_success_non_unique_idx", f,
+        )
+        await self.db.updates._end_background_update(
+            BG_UPDATE_DROP_DEVICE_LISTS_OUTBOUND_LAST_SUCCESS_NON_UNIQUE_IDX
+        )
+        return 1
+
 
 class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
     def __init__(self, database: Database, db_conn, hs):
diff --git a/synapse/storage/data_stores/main/end_to_end_keys.py b/synapse/storage/data_stores/main/end_to_end_keys.py
index bcf746b7ef..20698bfd16 100644
--- a/synapse/storage/data_stores/main/end_to_end_keys.py
+++ b/synapse/storage/data_stores/main/end_to_end_keys.py
@@ -25,7 +25,9 @@ from twisted.internet import defer
 
 from synapse.logging.opentracing import log_kv, set_tag, trace
 from synapse.storage._base import SQLBaseStore, db_to_json
+from synapse.storage.database import make_in_list_sql_clause
 from synapse.util.caches.descriptors import cached, cachedList
+from synapse.util.iterutils import batch_iter
 
 
 class EndToEndKeyWorkerStore(SQLBaseStore):
@@ -268,53 +270,7 @@ class EndToEndKeyWorkerStore(SQLBaseStore):
             "count_e2e_one_time_keys", _count_e2e_one_time_keys
         )
 
-    def _get_e2e_cross_signing_key_txn(self, txn, user_id, key_type, from_user_id=None):
-        """Returns a user's cross-signing key.
-
-        Args:
-            txn (twisted.enterprise.adbapi.Connection): db connection
-            user_id (str): the user whose key is being requested
-            key_type (str): the type of key that is being requested: either 'master'
-                for a master key, 'self_signing' for a self-signing key, or
-                'user_signing' for a user-signing key
-            from_user_id (str): if specified, signatures made by this user on
-                the key will be included in the result
-
-        Returns:
-            dict of the key data or None if not found
-        """
-        sql = (
-            "SELECT keydata "
-            "  FROM e2e_cross_signing_keys "
-            " WHERE user_id = ? AND keytype = ? ORDER BY stream_id DESC LIMIT 1"
-        )
-        txn.execute(sql, (user_id, key_type))
-        row = txn.fetchone()
-        if not row:
-            return None
-        key = json.loads(row[0])
-
-        device_id = None
-        for k in key["keys"].values():
-            device_id = k
-
-        if from_user_id is not None:
-            sql = (
-                "SELECT key_id, signature "
-                "  FROM e2e_cross_signing_signatures "
-                " WHERE user_id = ? "
-                "   AND target_user_id = ? "
-                "   AND target_device_id = ? "
-            )
-            txn.execute(sql, (from_user_id, user_id, device_id))
-            row = txn.fetchone()
-            if row:
-                key.setdefault("signatures", {}).setdefault(from_user_id, {})[
-                    row[0]
-                ] = row[1]
-
-        return key
-
+    @defer.inlineCallbacks
     def get_e2e_cross_signing_key(self, user_id, key_type, from_user_id=None):
         """Returns a user's cross-signing key.
 
@@ -329,13 +285,11 @@ class EndToEndKeyWorkerStore(SQLBaseStore):
         Returns:
             dict of the key data or None if not found
         """
-        return self.db.runInteraction(
-            "get_e2e_cross_signing_key",
-            self._get_e2e_cross_signing_key_txn,
-            user_id,
-            key_type,
-            from_user_id,
-        )
+        res = yield self.get_e2e_cross_signing_keys_bulk([user_id], from_user_id)
+        user_keys = res.get(user_id)
+        if not user_keys:
+            return None
+        return user_keys.get(key_type)
 
     @cached(num_args=1)
     def _get_bare_e2e_cross_signing_keys(self, user_id):
@@ -391,26 +345,24 @@ class EndToEndKeyWorkerStore(SQLBaseStore):
         """
         result = {}
 
-        batch_size = 100
-        chunks = [
-            user_ids[i : i + batch_size] for i in range(0, len(user_ids), batch_size)
-        ]
-        for user_chunk in chunks:
-            sql = """
+        for user_chunk in batch_iter(user_ids, 100):
+            clause, params = make_in_list_sql_clause(
+                txn.database_engine, "k.user_id", user_chunk
+            )
+            sql = (
+                """
                 SELECT k.user_id, k.keytype, k.keydata, k.stream_id
                   FROM e2e_cross_signing_keys k
                   INNER JOIN (SELECT user_id, keytype, MAX(stream_id) AS stream_id
                                 FROM e2e_cross_signing_keys
                                GROUP BY user_id, keytype) s
                  USING (user_id, stream_id, keytype)
-                 WHERE k.user_id IN (%s)
-            """ % (
-                ",".join("?" for u in user_chunk),
+                 WHERE
+            """
+                + clause
             )
-            query_params = []
-            query_params.extend(user_chunk)
 
-            txn.execute(sql, query_params)
+            txn.execute(sql, params)
             rows = self.db.cursor_to_dict(txn)
 
             for row in rows:
@@ -453,15 +405,7 @@ class EndToEndKeyWorkerStore(SQLBaseStore):
                     device_id = k
                 devices[(user_id, device_id)] = key_type
 
-        device_list = list(devices)
-
-        # split into batches
-        batch_size = 100
-        chunks = [
-            device_list[i : i + batch_size]
-            for i in range(0, len(device_list), batch_size)
-        ]
-        for user_chunk in chunks:
+        for batch in batch_iter(devices.keys(), size=100):
             sql = """
                 SELECT target_user_id, target_device_id, key_id, signature
                   FROM e2e_cross_signing_signatures
@@ -469,11 +413,11 @@ class EndToEndKeyWorkerStore(SQLBaseStore):
                    AND (%s)
             """ % (
                 " OR ".join(
-                    "(target_user_id = ? AND target_device_id = ?)" for d in devices
+                    "(target_user_id = ? AND target_device_id = ?)" for _ in batch
                 )
             )
             query_params = [from_user_id]
-            for item in devices:
+            for item in batch:
                 # item is a (user_id, device_id) tuple
                 query_params.extend(item)
 
diff --git a/synapse/storage/data_stores/main/schema/delta/58/04device_lists_outbound_last_success_unique_idx.sql b/synapse/storage/data_stores/main/schema/delta/58/04device_lists_outbound_last_success_unique_idx.sql
new file mode 100644
index 0000000000..d5e6deb878
--- /dev/null
+++ b/synapse/storage/data_stores/main/schema/delta/58/04device_lists_outbound_last_success_unique_idx.sql
@@ -0,0 +1,28 @@
+/* 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.
+ */
+
+-- register a background update which will create a unique index on
+-- device_lists_outbound_last_success
+INSERT into background_updates (ordering, update_name, progress_json)
+    VALUES (5804, 'device_lists_outbound_last_success_unique_idx', '{}');
+
+-- once that completes, we can drop the old index.
+INSERT into background_updates (ordering, update_name, progress_json, depends_on)
+    VALUES (
+        5804,
+        'drop_device_lists_outbound_last_success_non_unique_idx',
+        '{}',
+        'device_lists_outbound_last_success_unique_idx'
+    );
diff --git a/synapse/storage/database.py b/synapse/storage/database.py
index a7cd97b0b0..2b635d6ca0 100644
--- a/synapse/storage/database.py
+++ b/synapse/storage/database.py
@@ -49,6 +49,7 @@ from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage.background_updates import BackgroundUpdater
 from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine, Sqlite3Engine
 from synapse.storage.types import Connection, Cursor
+from synapse.types import Collection
 from synapse.util.stringutils import exception_to_unicode
 
 logger = logging.getLogger(__name__)
@@ -78,6 +79,7 @@ UNIQUE_INDEX_BACKGROUND_UPDATES = {
     "device_lists_remote_extremeties": "device_lists_remote_extremeties_unique_idx",
     "device_lists_remote_cache": "device_lists_remote_cache_unique_idx",
     "event_search": "event_search_event_id_idx",
+    "device_lists_outbound_last_success": "device_lists_outbound_last_success_unique_idx",
 }
 
 
@@ -889,20 +891,24 @@ class Database(object):
         txn.execute(sql, list(allvalues.values()))
 
     def simple_upsert_many_txn(
-        self, txn, table, key_names, key_values, value_names, value_values
-    ):
+        self,
+        txn: LoggingTransaction,
+        table: str,
+        key_names: Collection[str],
+        key_values: Collection[Iterable[Any]],
+        value_names: Collection[str],
+        value_values: Iterable[Iterable[str]],
+    ) -> None:
         """
         Upsert, many times.
 
         Args:
-            table (str): The table to upsert into
-            key_names (list[str]): The key column names.
-            key_values (list[list]): A list of each row's key column values.
-            value_names (list[str]): The value column names. If empty, no
-                values will be used, even if value_values is provided.
-            value_values (list[list]): A list of each row's value column values.
-        Returns:
-            None
+            table: The table to upsert into
+            key_names: The key column names.
+            key_values: A list of each row's key column values.
+            value_names: The value column names
+            value_values: A list of each row's value column values.
+                Ignored if value_names is empty.
         """
         if self.engine.can_native_upsert and table not in self._unsafe_to_upsert_tables:
             return self.simple_upsert_many_txn_native_upsert(
@@ -914,20 +920,24 @@ class Database(object):
             )
 
     def simple_upsert_many_txn_emulated(
-        self, txn, table, key_names, key_values, value_names, value_values
-    ):
+        self,
+        txn: LoggingTransaction,
+        table: str,
+        key_names: Iterable[str],
+        key_values: Collection[Iterable[Any]],
+        value_names: Collection[str],
+        value_values: Iterable[Iterable[str]],
+    ) -> None:
         """
         Upsert, many times, but without native UPSERT support or batching.
 
         Args:
-            table (str): The table to upsert into
-            key_names (list[str]): The key column names.
-            key_values (list[list]): A list of each row's key column values.
-            value_names (list[str]): The value column names. If empty, no
-                values will be used, even if value_values is provided.
-            value_values (list[list]): A list of each row's value column values.
-        Returns:
-            None
+            table: The table to upsert into
+            key_names: The key column names.
+            key_values: A list of each row's key column values.
+            value_names: The value column names
+            value_values: A list of each row's value column values.
+                Ignored if value_names is empty.
         """
         # No value columns, therefore make a blank list so that the following
         # zip() works correctly.
@@ -941,20 +951,24 @@ class Database(object):
             self.simple_upsert_txn_emulated(txn, table, _keys, _vals)
 
     def simple_upsert_many_txn_native_upsert(
-        self, txn, table, key_names, key_values, value_names, value_values
-    ):
+        self,
+        txn: LoggingTransaction,
+        table: str,
+        key_names: Collection[str],
+        key_values: Collection[Iterable[Any]],
+        value_names: Collection[str],
+        value_values: Iterable[Iterable[Any]],
+    ) -> None:
         """
         Upsert, many times, using batching where possible.
 
         Args:
-            table (str): The table to upsert into
-            key_names (list[str]): The key column names.
-            key_values (list[list]): A list of each row's key column values.
-            value_names (list[str]): The value column names. If empty, no
-                values will be used, even if value_values is provided.
-            value_values (list[list]): A list of each row's value column values.
-        Returns:
-            None
+            table: The table to upsert into
+            key_names: The key column names.
+            key_values: A list of each row's key column values.
+            value_names: The value column names
+            value_values: A list of each row's value column values.
+                Ignored if value_names is empty.
         """
         allnames = []  # type: List[str]
         allnames.extend(key_names)
diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 9d851beaa5..86d04ea9ac 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -16,6 +16,11 @@
 import contextlib
 import threading
 from collections import deque
+from typing import Dict, Set, Tuple
+
+from typing_extensions import Deque
+
+from synapse.storage.database import Database, LoggingTransaction
 
 
 class IdGenerator(object):
@@ -87,7 +92,7 @@ class StreamIdGenerator(object):
             self._current = (max if step > 0 else min)(
                 self._current, _load_current_id(db_conn, table, column, step)
             )
-        self._unfinished_ids = deque()
+        self._unfinished_ids = deque()  # type: Deque[int]
 
     def get_next(self):
         """
@@ -163,7 +168,7 @@ class ChainedIdGenerator(object):
         self.chained_generator = chained_generator
         self._lock = threading.Lock()
         self._current_max = _load_current_id(db_conn, table, column)
-        self._unfinished_ids = deque()
+        self._unfinished_ids = deque()  # type: Deque[Tuple[int, int]]
 
     def get_next(self):
         """
@@ -198,3 +203,163 @@ class ChainedIdGenerator(object):
                 return stream_id - 1, chained_id
 
             return self._current_max, self.chained_generator.get_current_token()
+
+
+class MultiWriterIdGenerator:
+    """An ID generator that tracks a stream that can have multiple writers.
+
+    Uses a Postgres sequence to coordinate ID assignment, but positions of other
+    writers will only get updated when `advance` is called (by replication).
+
+    Note: Only works with Postgres.
+
+    Args:
+        db_conn
+        db
+        instance_name: The name of this instance.
+        table: Database table associated with stream.
+        instance_column: Column that stores the row's writer's instance name
+        id_column: Column that stores the stream ID.
+        sequence_name: The name of the postgres sequence used to generate new
+            IDs.
+    """
+
+    def __init__(
+        self,
+        db_conn,
+        db: Database,
+        instance_name: str,
+        table: str,
+        instance_column: str,
+        id_column: str,
+        sequence_name: str,
+    ):
+        self._db = db
+        self._instance_name = instance_name
+        self._sequence_name = sequence_name
+
+        # We lock as some functions may be called from DB threads.
+        self._lock = threading.Lock()
+
+        self._current_positions = self._load_current_ids(
+            db_conn, table, instance_column, id_column
+        )
+
+        # Set of local IDs that we're still processing. The current position
+        # should be less than the minimum of this set (if not empty).
+        self._unfinished_ids = set()  # type: Set[int]
+
+    def _load_current_ids(
+        self, db_conn, table: str, instance_column: str, id_column: str
+    ) -> Dict[str, int]:
+        sql = """
+            SELECT %(instance)s, MAX(%(id)s) FROM %(table)s
+            GROUP BY %(instance)s
+        """ % {
+            "instance": instance_column,
+            "id": id_column,
+            "table": table,
+        }
+
+        cur = db_conn.cursor()
+        cur.execute(sql)
+
+        # `cur` is an iterable over returned rows, which are 2-tuples.
+        current_positions = dict(cur)
+
+        cur.close()
+
+        return current_positions
+
+    def _load_next_id_txn(self, txn):
+        txn.execute("SELECT nextval(?)", (self._sequence_name,))
+        (next_id,) = txn.fetchone()
+        return next_id
+
+    async def get_next(self):
+        """
+        Usage:
+            with await stream_id_gen.get_next() as stream_id:
+                # ... persist event ...
+        """
+        next_id = await self._db.runInteraction("_load_next_id", self._load_next_id_txn)
+
+        # 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
+
+        with self._lock:
+            self._unfinished_ids.add(next_id)
+
+        @contextlib.contextmanager
+        def manager():
+            try:
+                yield next_id
+            finally:
+                self._mark_id_as_finished(next_id)
+
+        return manager()
+
+    def get_next_txn(self, txn: LoggingTransaction):
+        """
+        Usage:
+
+            stream_id = stream_id_gen.get_next(txn)
+            # ... persist event ...
+        """
+
+        next_id = self._load_next_id_txn(txn)
+
+        with self._lock:
+            self._unfinished_ids.add(next_id)
+
+        txn.call_after(self._mark_id_as_finished, next_id)
+        txn.call_on_exception(self._mark_id_as_finished, next_id)
+
+        return next_id
+
+    def _mark_id_as_finished(self, next_id: int):
+        """The ID has finished being processed so we should advance the
+        current poistion if possible.
+        """
+
+        with self._lock:
+            self._unfinished_ids.discard(next_id)
+
+            # Figure out if its safe to advance the position by checking there
+            # aren't any lower allocated IDs that are yet to finish.
+            if all(c > next_id for c in self._unfinished_ids):
+                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).
+        """
+
+        if instance_name is None:
+            instance_name = self._instance_name
+
+        with self._lock:
+            return self._current_positions.get(instance_name, 0)
+
+    def get_positions(self) -> Dict[str, int]:
+        """Get a copy of the current positon map.
+        """
+
+        with self._lock:
+            return dict(self._current_positions)
+
+    def advance(self, instance_name: str, new_id: int):
+        """Advance the postion of the named writer to the given ID, if greater
+        than existing entry.
+        """
+
+        with self._lock:
+            self._current_positions[instance_name] = max(
+                new_id, self._current_positions.get(instance_name, 0)
+            )
diff --git a/synapse/util/caches/stream_change_cache.py b/synapse/util/caches/stream_change_cache.py
index 38dc3f501e..e54f80d76e 100644
--- a/synapse/util/caches/stream_change_cache.py
+++ b/synapse/util/caches/stream_change_cache.py
@@ -14,12 +14,13 @@
 # limitations under the License.
 
 import logging
-from typing import Dict, Iterable, List, Mapping, Optional, Set
+from typing import Dict, FrozenSet, List, Mapping, Optional, Set, Union
 
 from six import integer_types
 
 from sortedcontainers import SortedDict
 
+from synapse.types import Collection
 from synapse.util import caches
 
 logger = logging.getLogger(__name__)
@@ -85,8 +86,8 @@ class StreamChangeCache:
         return False
 
     def get_entities_changed(
-        self, entities: Iterable[EntityType], stream_pos: int
-    ) -> Set[EntityType]:
+        self, entities: Collection[EntityType], stream_pos: int
+    ) -> Union[Set[EntityType], FrozenSet[EntityType]]:
         """
         Returns subset of entities that have had new things since the given
         position.  Entities unknown to the cache will be returned.  If the
@@ -94,7 +95,17 @@ class StreamChangeCache:
         """
         changed_entities = self.get_all_entities_changed(stream_pos)
         if changed_entities is not None:
-            result = set(changed_entities).intersection(entities)
+            # We now do an intersection, trying to do so in the most efficient
+            # way possible (some of these sets are *large*). First check in the
+            # given iterable is already set that we can reuse, otherwise we
+            # create a set of the *smallest* of the two iterables and call
+            # `intersection(..)` on it (this can be twice as fast as the reverse).
+            if isinstance(entities, (set, frozenset)):
+                result = entities.intersection(changed_entities)
+            elif len(changed_entities) < len(entities):
+                result = set(changed_entities).intersection(entities)
+            else:
+                result = set(entities).intersection(changed_entities)
             self.metrics.inc_hits()
         else:
             result = set(entities)