Consider the root event when calculating if the current user has participated in a thread.

Author: clokep

changelog.d/12766.bugfix

@@ -0,0 +1,2 @@
+Implement [MSC3816](https://github.com/matrix-org/matrix-spec-proposals/pull/3816): sending the root event in a thread 
+should count as "participated" in it.

synapse/handlers/relations.py

@@ -12,16 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import (
-    TYPE_CHECKING,
-    Collection,
-    Dict,
-    FrozenSet,
-    Iterable,
-    List,
-    Optional,
-    Tuple,
-)
+from typing import TYPE_CHECKING, Dict, FrozenSet, Iterable, List, Optional, Tuple
 
 import attr
 
@@ -253,13 +244,19 @@ async def get_annotations_for_event(
 
         return filtered_results
 
-    async def get_threads_for_events(
-        self, event_ids: Collection[str], user_id: str, ignored_users: FrozenSet[str]
+    async def _get_threads_for_events(
+        self,
+        events_by_id: Dict[str, EventBase],
+        relations_by_id: Dict[str, str],
+        user_id: str,
+        ignored_users: FrozenSet[str],
     ) -> Dict[str, _ThreadAggregation]:
         """Get the bundled aggregations for threads for the requested events.
 
         Args:
-            event_ids: Events to get aggregations for threads.
+            events_by_id: A map of event_id to events to get aggregations for threads.
+            relations_by_id: A map of event_id to the relation type, if one exists
+                for that event.
             user_id: The user requesting the bundled aggregations.
             ignored_users: The users ignored by the requesting user.
 
@@ -270,6 +267,9 @@ async def get_threads_for_events(
         """
         user = UserID.from_string(user_id)
 
+        # It is not valid to start a thread on an event which itself relates to another event.
+        event_ids = [eid for eid in events_by_id.keys() if eid not in relations_by_id]
+
         # Fetch thread summaries.
         summaries = await self._main_store.get_thread_summaries(event_ids)
 
@@ -340,7 +340,8 @@ async def get_threads_for_events(
                     count=thread_count,
                     # If there's a thread summary it must also exist in the
                     # participated dictionary.
-                    current_user_participated=participated[event_id],
+                    current_user_participated=events_by_id[event_id].sender == user_id
+                    or participated[event_id],
                 )
 
         return results
@@ -398,9 +399,9 @@ async def get_bundled_aggregations(
         # events to be fetched. Thus, we check those first!
 
         # Fetch thread summaries (but only for the directly requested events).
-        threads = await self.get_threads_for_events(
-            # It is not valid to start a thread on an event which itself relates to another event.
-            [eid for eid in events_by_id.keys() if eid not in relations_by_id],
+        threads = await self._get_threads_for_events(
+            events_by_id,
+            relations_by_id,
             user_id,
             ignored_users,
         )

tests/rest/client/test_relations.py

@@ -1072,13 +1072,21 @@ def assert_thread(bundled_aggregations: JsonDict) -> None:
 
             return assert_thread
 
-        self._test_bundled_aggregations(RelationTypes.THREAD, _gen_assert(False), 10)
+        # A user which has sent the root event or replied has participated.
+        self._test_bundled_aggregations(RelationTypes.THREAD, _gen_assert(True), 10)
         # Note that this re-uses some cached values, so the total number of
         # queries is much smaller.
         self._test_bundled_aggregations(
             RelationTypes.THREAD, _gen_assert(True), 2, access_token=self.user2_token
         )
 
+        # A user with no interactions with the thread has not participated.
+        user3_id, user3_token = self._create_user("charlie")
+        self.helper.join(self.room, user=user3_id, tok=user3_token)
+        self._test_bundled_aggregations(
+            RelationTypes.THREAD, _gen_assert(False), 2, access_token=user3_token
+        )
+
     def test_thread_with_bundled_aggregations_for_latest(self) -> None:
         """
         Bundled aggregations should get applied to the latest thread event.