Return the main timeline for events which are not part of a thread. (#14140)

Fixes a bug where threaded receipts could not be sent for the
main timeline.

changelog.d/14140.feature

+Support for thread-specific notifications & receipts ([MSC3771](https://github.com/matrix-org/matrix-spec-proposals/pull/3771) and [MSC3773](https://github.com/matrix-org/matrix-spec-proposals/pull/3773)).

synapse/push/bulk_push_rule_evaluator.py

@@ -236,7 +236,7 @@ class BulkPushRuleEvaluator:
             else:
                 # Since the event has not yet been persisted we check whether
                 # the parent is part of a thread.
-                thread_id = await self.store.get_thread_id(relation.parent_id) or "main"
+                thread_id = await self.store.get_thread_id(relation.parent_id)
 
         # It's possible that old room versions have non-integer power levels (floats or
         # strings). Workaround this by explicitly converting to int.

synapse/storage/databases/main/relations.py

@@ -28,7 +28,7 @@ from typing import (
 
 import attr
 
-from synapse.api.constants import RelationTypes
+from synapse.api.constants import MAIN_TIMELINE, RelationTypes
 from synapse.events import EventBase
 from synapse.storage._base import SQLBaseStore
 from synapse.storage.database import LoggingTransaction, make_in_list_sql_clause
@@ -777,7 +777,7 @@ class RelationsWorkerStore(SQLBaseStore):
         )
 
     @cached()
-    async def get_thread_id(self, event_id: str) -> Optional[str]:
+    async def get_thread_id(self, event_id: str) -> str:
         """
         Get the thread ID for an event. This considers multi-level relations,
         e.g. an annotation to an event which is part of a thread.
@@ -787,7 +787,7 @@ class RelationsWorkerStore(SQLBaseStore):
 
         Returns:
             The event ID of the root event in the thread, if this event is part
-            of a thread. None, otherwise.
+            of a thread. "main", otherwise.
         """
         # Since event relations form a tree, we should only ever find 0 or 1
         # results from the below query.
@@ -802,13 +802,15 @@ class RelationsWorkerStore(SQLBaseStore):
             ) SELECT relates_to_id FROM related_events WHERE relation_type = 'm.thread';
         """
 
-        def _get_thread_id(txn: LoggingTransaction) -> Optional[str]:
+        def _get_thread_id(txn: LoggingTransaction) -> str:
             txn.execute(sql, (event_id,))
             # TODO Should we ensure there's only a single result here?
             row = txn.fetchone()
             if row:
                 return row[0]
-            return None
+
+            # If no thread was found, it is part of the main timeline.
+            return MAIN_TIMELINE
 
         return await self.db_pool.runInteraction("get_thread_id", _get_thread_id)