Add ability to wait for replication streams (#7542)

The idea here is that if an instance persists an event via the replication HTTP API it can return before we receive that event over replication, which can lead to races where code assumes that persisting an event immediately updates various caches (e.g. current state of the room).

Most of Synapse doesn't hit such races, so we don't do the waiting automagically, instead we do so where necessary to avoid unnecessary delays. We may decide to change our minds here if it turns out there are a lot of subtle races going on.

People probably want to look at this commit by commit.

synapse/handlers/message.py

@@ -15,7 +15,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import Optional
+from typing import Optional, Tuple
 
 from six import iteritems, itervalues, string_types
 
@@ -42,6 +42,7 @@ from synapse.api.errors import (
 )
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersions
 from synapse.api.urls import ConsentURIBuilder
+from synapse.events import EventBase
 from synapse.events.validator import EventValidator
 from synapse.logging.context import run_in_background
 from synapse.metrics.background_process_metrics import run_as_background_process
@@ -630,7 +631,9 @@ class EventCreationHandler(object):
         msg = self._block_events_without_consent_error % {"consent_uri": consent_uri}
         raise ConsentNotGivenError(msg=msg, consent_uri=consent_uri)
 
-    async def send_nonmember_event(self, requester, event, context, ratelimit=True):
+    async def send_nonmember_event(
+        self, requester, event, context, ratelimit=True
+    ) -> int:
         """
         Persists and notifies local clients and federation of an event.
 
@@ -639,6 +642,9 @@ class EventCreationHandler(object):
             context (Context) the context of the event.
             ratelimit (bool): Whether to rate limit this send.
             is_guest (bool): Whether the sender is a guest.
+
+        Return:
+            The stream_id of the persisted event.
         """
         if event.type == EventTypes.Member:
             raise SynapseError(
@@ -659,7 +665,7 @@ class EventCreationHandler(object):
                 )
                 return prev_state
 
-        await self.handle_new_client_event(
+        return await self.handle_new_client_event(
             requester=requester, event=event, context=context, ratelimit=ratelimit
         )
 
@@ -688,7 +694,7 @@ class EventCreationHandler(object):
 
     async def create_and_send_nonmember_event(
         self, requester, event_dict, ratelimit=True, txn_id=None
-    ):
+    ) -> Tuple[EventBase, int]:
         """
         Creates an event, then sends it.
 
@@ -711,10 +717,10 @@ class EventCreationHandler(object):
                     spam_error = "Spam is not permitted here"
                 raise SynapseError(403, spam_error, Codes.FORBIDDEN)
 
-            await self.send_nonmember_event(
+            stream_id = await self.send_nonmember_event(
                 requester, event, context, ratelimit=ratelimit
             )
-        return event
+        return event, stream_id
 
     @measure_func("create_new_client_event")
     @defer.inlineCallbacks
@@ -774,7 +780,7 @@ class EventCreationHandler(object):
     @measure_func("handle_new_client_event")
     async def handle_new_client_event(
         self, requester, event, context, ratelimit=True, extra_users=[]
-    ):
+    ) -> int:
         """Processes a new event. This includes checking auth, persisting it,
         notifying users, sending to remote servers, etc.
 
@@ -787,6 +793,9 @@ class EventCreationHandler(object):
             context (EventContext)
             ratelimit (bool)
             extra_users (list(UserID)): Any extra users to notify about event
+
+        Return:
+            The stream_id of the persisted event.
         """
 
         if event.is_state() and (event.type, event.state_key) == (
@@ -827,7 +836,7 @@ class EventCreationHandler(object):
         try:
             # If we're a worker we need to hit out to the master.
             if self.config.worker_app:
-                await self.send_event_to_master(
+                result = await self.send_event_to_master(
                     event_id=event.event_id,
                     store=self.store,
                     requester=requester,
@@ -836,14 +845,17 @@ class EventCreationHandler(object):
                     ratelimit=ratelimit,
                     extra_users=extra_users,
                 )
+                stream_id = result["stream_id"]
+                event.internal_metadata.stream_ordering = stream_id
                 success = True
-                return
+                return stream_id
 
-            await self.persist_and_notify_client_event(
+            stream_id = await self.persist_and_notify_client_event(
                 requester, event, context, ratelimit=ratelimit, extra_users=extra_users
             )
 
             success = True
+            return stream_id
         finally:
             if not success:
                 # Ensure that we actually remove the entries in the push actions
@@ -886,7 +898,7 @@ class EventCreationHandler(object):
 
     async def persist_and_notify_client_event(
         self, requester, event, context, ratelimit=True, extra_users=[]
-    ):
+    ) -> int:
         """Called when we have fully built the event, have already
         calculated the push actions for the event, and checked auth.
 
@@ -1076,6 +1088,8 @@ class EventCreationHandler(object):
             # matters as sometimes presence code can take a while.
             run_in_background(self._bump_active_time, requester.user)
 
+        return event_stream_id
+
     async def _bump_active_time(self, user):
         try:
             presence = self.hs.get_presence_handler()