Allow events to be created with no prev_events (MSC2716) (#11243)

The event still needs to have `auth_events` defined to be valid.

Split out from https://github.com/matrix-org/synapse/pull/11114

synapse/handlers/message.py

@@ -496,6 +496,7 @@ class EventCreationHandler:
         require_consent: bool = True,
         outlier: bool = False,
         historical: bool = False,
+        allow_no_prev_events: bool = False,
         depth: Optional[int] = None,
     ) -> Tuple[EventBase, EventContext]:
         """
@@ -607,6 +608,7 @@ class EventCreationHandler:
             prev_event_ids=prev_event_ids,
             auth_event_ids=auth_event_ids,
             depth=depth,
+            allow_no_prev_events=allow_no_prev_events,
         )
 
         # In an ideal world we wouldn't need the second part of this condition. However,
@@ -882,6 +884,7 @@ class EventCreationHandler:
         prev_event_ids: Optional[List[str]] = None,
         auth_event_ids: Optional[List[str]] = None,
         depth: Optional[int] = None,
+        allow_no_prev_events: bool = False,
     ) -> Tuple[EventBase, EventContext]:
         """Create a new event for a local client
 
@@ -912,6 +915,7 @@ class EventCreationHandler:
         full_state_ids_at_event = None
         if auth_event_ids is not None:
             # If auth events are provided, prev events must be also.
+            # prev_event_ids could be an empty array though.
             assert prev_event_ids is not None
 
             # Copy the full auth state before it stripped down
@@ -943,14 +947,22 @@ class EventCreationHandler:
         else:
             prev_event_ids = await self.store.get_prev_events_for_room(builder.room_id)
 
-        # we now ought to have some prev_events (unless it's a create event).
-        #
-        # do a quick sanity check here, rather than waiting until we've created the
+        # Do a quick sanity check here, rather than waiting until we've created the
         # event and then try to auth it (which fails with a somewhat confusing "No
         # create event in auth events")
-        assert (
-            builder.type == EventTypes.Create or len(prev_event_ids) > 0
-        ), "Attempting to create an event with no prev_events"
+        if allow_no_prev_events:
+            # We allow events with no `prev_events` but it better have some `auth_events`
+            assert (
+                builder.type == EventTypes.Create
+                # Allow an event to have empty list of prev_event_ids
+                # only if it has auth_event_ids.
+                or auth_event_ids
+            ), "Attempting to create a non-m.room.create event with no prev_events or auth_event_ids"
+        else:
+            # we now ought to have some prev_events (unless it's a create event).
+            assert (
+                builder.type == EventTypes.Create or prev_event_ids
+            ), "Attempting to create a non-m.room.create event with no prev_events"
 
         event = await builder.build(
             prev_event_ids=prev_event_ids,