Recursively fetch the thread for receipts & notifications. (#13824)

Consider an event to be part of a thread if you can follow a
chain of relations up to a thread root.

Part of MSC3773 & MSC3771.

1 files changed, 5 insertions, 0 deletions

diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py

index 61d952742d..f8c4dd74f0 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -286,8 +286,13 @@ class BulkPushRuleEvaluator:
                 relation.parent_id,
                 itertools.chain(*(r.rules() for r in rules_by_user.values())),
             )
+            # Recursively attempt to find the thread this event relates to.
             if relation.rel_type == RelationTypes.THREAD:
                 thread_id = relation.parent_id
+            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"
 
         evaluator = PushRuleEvaluator(
             _flatten_dict(event),