Verify ?chunk_id actually corresponds to an insertion event that exists

Part of #10737

Part of [MSC2716](matrix-org/matrix-spec-proposals#2716)

Author: MadLittleMods

synapse/rest/client/room_batch.py

@@ -299,7 +299,18 @@ async def on_POST(
             #  event, which causes the HS to ask for the state at the start of
             #  the chunk later.
             prev_event_ids = [fake_prev_event_id]
-            # TODO: Verify the chunk_id_from_query corresponds to an insertion event
+
+            # Verify the chunk_id_from_query corresponds to an actual insertion event
+            # and have the chunk connected.
+            corresponding_insertion_event_id = (
+                await self.store.get_insertion_event_by_chunk_id(chunk_id_from_query)
+            )
+            if corresponding_insertion_event_id is None:
+                raise SynapseError(
+                    400,
+                    "No insertion event corresponds to the given ?chunk_id",
+                    errcode=Codes.INVALID_PARAM,
+                )
             pass
         # Otherwise, create an insertion event to act as a starting point.
         #

synapse/storage/databases/main/__init__.py

@@ -61,6 +61,7 @@
 from .rejections import RejectionsStore
 from .relations import RelationsStore
 from .room import RoomStore
+from .room_batch import RoomBatchStore
 from .roommember import RoomMemberStore
 from .search import SearchStore
 from .session import SessionStore
@@ -81,6 +82,7 @@ class DataStore(
     EventsBackgroundUpdatesStore,
     RoomMemberStore,
     RoomStore,
+    RoomBatchStore,
     RegistrationStore,
     StreamStore,
     ProfileStore,

synapse/storage/databases/main/room_batch.py

@@ -0,0 +1,40 @@
+# Copyright 2021 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.
+
+from typing import Optional
+
+from synapse.storage._base import SQLBaseStore
+from synapse.storage.database import DatabasePool
+
+
+class RoomBatchStore(SQLBaseStore):
+    def __init__(self, database: DatabasePool, db_conn, hs):
+        super().__init__(database, db_conn, hs)
+
+    async def get_insertion_event_by_chunk_id(self, chunk_id: str) -> Optional[str]:
+        """Retrieve a insertion event ID.
+
+        Args:
+            chunk_id: The chunk ID of the insertion event to retrieve.
+
+        Returns:
+            The event_id of an insertion event, or None if there is no known
+            insertion event for the given insertion event.
+        """
+        return await self.db_pool.simple_select_one_onecol(
+            table="insertion_events",
+            keyvalues={"next_chunk_id": chunk_id},
+            retcol="event_id",
+            allow_none=True,
+        )