Add base starting insertion event when no chunk ID is provided (MSC2716)

changelog.d/10250.bugfix

@@ -0,0 +1 @@
+Add base starting insertion event when no chunk ID is specified in the historical batch send API.

synapse/rest/client/v1/room.py

@@ -349,6 +349,30 @@ async def inherit_depth_from_prev_ids(self, prev_event_ids) -> int:
 
         return depth
 
+    def _create_insertion_event_dict(self, sender: str, origin_server_ts: int):
+        """
+        Creates an event dict for an "insertion" event with the proper fields
+        and a random chunk ID.
+        Args:
+            sender: The event author MXID
+            origin_server_ts: Timestamp when the event was sent
+        Returns:
+            Tuple of event ID and stream ordering position
+        """
+
+        next_chunk_id = random_string(64)
+        insertion_event = {
+            "type": EventTypes.MSC2716_INSERTION,
+            "sender": sender,
+            "content": {
+                EventContentFields.MSC2716_NEXT_CHUNK_ID: next_chunk_id,
+                EventContentFields.MSC2716_HISTORICAL: True,
+            },
+            "origin_server_ts": origin_server_ts,
+        }
+
+        return insertion_event
+
     async def on_POST(self, request, room_id):
         requester = await self.auth.get_user_by_req(request, allow_guest=False)
 
@@ -449,30 +473,40 @@ async def on_POST(self, request, room_id):
 
         events_to_create = body["events"]
 
-        # If provided, connect the chunk to the last insertion point
-        # The chunk ID passed in comes from the chunk_id in the
-        # "insertion" event from the previous chunk.
+        # Figure out which chunk to connect to. If they passed in
+        # chunk_id_from_query let's use it. The chunk ID passed in comes
+        # from the chunk_id in the "insertion" event from the previous chunk.
+        last_event_in_chunk = events_to_create[-1]
+        chunk_id_to_connect_to = chunk_id_from_query
         if chunk_id_from_query:
-            last_event_in_chunk = events_to_create[-1]
-            last_event_in_chunk["content"][
-                EventContentFields.MSC2716_CHUNK_ID
-            ] = chunk_id_from_query
+            # TODO: Verify the chunk_id_from_query corresponds to an insertion event
+            pass
+        # Otherwise, create an insertion event to be based off of and connect
+        # to as a starting point.
+        else:
+            base_insertion_event = self._create_insertion_event_dict(
+                sender=requester.user.to_string(),
+                origin_server_ts=last_event_in_chunk["origin_server_ts"],
+            )
+            events_to_create.append(base_insertion_event)
+            chunk_id_to_connect_to = base_insertion_event["content"][
+                EventContentFields.MSC2716_NEXT_CHUNK_ID
+            ]
+
+        # Connect this current chunk to the insertion event from the previous chunk
+        last_event_in_chunk["content"][
+            EventContentFields.MSC2716_CHUNK_ID
+        ] = chunk_id_to_connect_to
 
-        # Add an "insertion" event to the start of each chunk (next to the oldest
+        # Add an "insertion" event to the start of each chunk (next to the oldest-in-time
         # event in the chunk) so the next chunk can be connected to this one.
-        next_chunk_id = random_string(64)
-        insertion_event = {
-            "type": EventTypes.MSC2716_INSERTION,
-            "sender": requester.user.to_string(),
-            "content": {
-                EventContentFields.MSC2716_NEXT_CHUNK_ID: next_chunk_id,
-                EventContentFields.MSC2716_HISTORICAL: True,
-            },
+        insertion_event = self._create_insertion_event_dict(
+            sender=requester.user.to_string(),
             # Since the insertion event is put at the start of the chunk,
-            # where the oldest event is, copy the origin_server_ts from
+            # where the oldest-in-time event is, copy the origin_server_ts from
             # the first event we're inserting
-            "origin_server_ts": events_to_create[0]["origin_server_ts"],
-        }
+            origin_server_ts=events_to_create[0]["origin_server_ts"],
+        )
         # Prepend the insertion event to the start of the chunk
         events_to_create = [insertion_event] + events_to_create
 
@@ -536,7 +570,9 @@ async def on_POST(self, request, room_id):
         return 200, {
             "state_events": auth_event_ids,
             "events": event_ids,
-            "next_chunk_id": next_chunk_id,
+            "next_chunk_id": insertion_event["content"][
+                EventContentFields.MSC2716_NEXT_CHUNK_ID
+            ],
         }
 
     def on_GET(self, request, room_id):