Integrate knock rooms with the public rooms directory (#9359)

This PR implements the ["Changes regarding the Public Rooms Directory"](https://github.com/Sorunome/matrix-doc/blob/soru/knock/proposals/2403-knock.md#changes-regarding-the-public-rooms-directory) section of knocking MSC2403.

Specifically, it:

* Allows rooms with `join_rule` "knock" to be returned by the query behind the public rooms directory
* Adds the field `join_rule` to each room entry returned by a public rooms directory query, so clients can know whether to attempt a join or knock on a room

Based on #6739. Complement tests for this change: matrix-org/complement#72

changelog.d/9359.feature

@@ -0,0 +1 @@
+Implement "room knocking" as per [MSC2403](https://github.com/matrix-org/matrix-doc/pull/2403). Contributed by Sorunome and anoa.

synapse/handlers/room_list.py

@@ -169,6 +169,7 @@ def build_room_entry(room):
                 "world_readable": room["history_visibility"]
                 == HistoryVisibility.WORLD_READABLE,
                 "guest_can_join": room["guest_access"] == "can_join",
+                "join_rule": room["join_rules"],
             }
 
             # Filter out Nones – rather omit the field altogether

synapse/storage/databases/main/room.py

@@ -19,7 +19,7 @@
 from enum import Enum
 from typing import Any, Dict, List, Optional, Tuple
 
-from synapse.api.constants import EventTypes
+from synapse.api.constants import EventTypes, JoinRules
 from synapse.api.errors import StoreError
 from synapse.api.room_versions import RoomVersion, RoomVersions
 from synapse.storage._base import SQLBaseStore, db_to_json
@@ -177,11 +177,13 @@ def _count_public_rooms_txn(txn):
                 INNER JOIN room_stats_current USING (room_id)
                 WHERE
                     (
-                        join_rules = 'public' OR history_visibility = 'world_readable'
+                        join_rules = 'public' OR join_rules = '%(knock_join_rule)s'
+                        OR history_visibility = 'world_readable'
                     )
                     AND joined_members > 0
             """ % {
-                "published_sql": published_sql
+                "published_sql": published_sql,
+                "knock_join_rule": JoinRules.KNOCK,
             }
 
             txn.execute(sql, query_args)
@@ -303,15 +305,16 @@ async def get_largest_public_rooms(
         sql = """
             SELECT
                 room_id, name, topic, canonical_alias, joined_members,
-                avatar, history_visibility, joined_members, guest_access
+                avatar, history_visibility, guest_access, join_rules
             FROM (
                 %(published_sql)s
             ) published
             INNER JOIN room_stats_state USING (room_id)
             INNER JOIN room_stats_current USING (room_id)
             WHERE
                 (
-                    join_rules = 'public' OR history_visibility = 'world_readable'
+                    join_rules = 'public' OR join_rules = '%(knock_join_rule)s'
+                    OR history_visibility = 'world_readable'
                 )
                 AND joined_members > 0
                 %(where_clause)s
@@ -320,6 +323,7 @@ async def get_largest_public_rooms(
             "published_sql": published_sql,
             "where_clause": where_clause,
             "dir": "DESC" if forwards else "ASC",
+            "knock_join_rule": JoinRules.KNOCK,
         }
 
         if limit is not None: