Add base starting insertion point when no chunk ID is provided

This is so we can have the marker event point to this initial
insertion event and be able to traverse the events in the first chunk.

Author: MadLittleMods

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):