Include heroes in partial join responses' state (#14442)

* Pull out hero selection logic

* Include heroes in partial join response's state

* Changelog

* Fixup trial test

* Remove TODO

1 files changed, 30 insertions, 0 deletions

diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index e56a13f21e..f02c1d7ea7 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -1517,6 +1517,36 @@ class RoomMemberStore(
         await self.db_pool.runInteraction("forget_membership", f)
 
 
+def extract_heroes_from_room_summary(
+    details: Mapping[str, MemberSummary], me: str
+) -> List[str]:
+    """Determine the users that represent a room, from the perspective of the `me` user.
+
+    The rules which say which users we select are specified in the "Room Summary"
+    section of
+    https://spec.matrix.org/v1.4/client-server-api/#get_matrixclientv3sync
+
+    Returns a list (possibly empty) of heroes' mxids.
+    """
+    empty_ms = MemberSummary([], 0)
+
+    joined_user_ids = [
+        r[0] for r in details.get(Membership.JOIN, empty_ms).members if r[0] != me
+    ]
+    invited_user_ids = [
+        r[0] for r in details.get(Membership.INVITE, empty_ms).members if r[0] != me
+    ]
+    gone_user_ids = [
+        r[0] for r in details.get(Membership.LEAVE, empty_ms).members if r[0] != me
+    ] + [r[0] for r in details.get(Membership.BAN, empty_ms).members if r[0] != me]
+
+    # FIXME: order by stream ordering rather than as returned by SQL
+    if joined_user_ids or invited_user_ids:
+        return sorted(joined_user_ids + invited_user_ids)[0:5]
+    else:
+        return sorted(gone_user_ids)[0:5]
+
+
 @attr.s(slots=True, auto_attribs=True)
 class _JoinedHostsCache:
     """The cached data used by the `_get_joined_hosts_cache`."""