fix(gateway): resend thread member lists on resume and leave (#3372)

fluxer_gateway/src/guild/guild_state_threads.erl

@@ -293,12 +293,16 @@ after_store_event(channel_delete, Data, Tab, State) ->
     end,
     State;
 after_store_event(guild_member_remove, Data, Tab, State) ->
-    _ =
     case guild_state_member:extract_user_id(Data) of
-            UserId when is_integer(UserId) -> guild_thread_store:remove_user(Tab, UserId);
-            _ -> []
-        end,
-    State;
+        UserId when is_integer(UserId) ->
+            lists:foldl(
+                fun guild_thread_subscriptions:member_lists_changed/2,
+                State,
+                guild_thread_store:remove_user(Tab, UserId)
+            );
+        _ ->
+            State
+    end;
 after_store_event(guild_member_update, Data, _Tab, State) ->
     case guild_state_member:extract_user_id(Data) of
         UserId when is_integer(UserId) ->

fluxer_gateway/src/guild/guild_thread_subscriptions.erl

@@ -75,7 +75,6 @@ apply_threads_flag(_Other, Session) ->
 
 -spec apply_member_lists(term(), map(), guild_state()) -> {map(), [integer()]}.
 apply_member_lists(Ids, Session, State) when is_list(Ids) ->
-    Previous = maps:get(thread_member_lists, Session, []),
     Viewable = lists:sublist(
         [Id || Id <- lists:usort(Ids), viewable_thread(Session, Id, State)], ?MAX_MEMBER_LISTS
     ),
@@ -84,7 +83,7 @@ apply_member_lists(Ids, Session, State) when is_list(Ids) ->
             [] -> maps:remove(thread_member_lists, Session);
             _ -> Session#{thread_member_lists => Viewable}
         end,
-    {Updated, Viewable -- Previous};
+    {Updated, Viewable};
 apply_member_lists(_Other, Session, _State) ->
     {Session, []}.
 

fluxer_gateway/test/guild_threads_tests.erl

@@ -1515,6 +1515,45 @@ thread_member_list_follows_member_updates_test() ->
         ?assertNot(maps:is_key(thread_list_dirty, Unlisted))
     end).
 
+thread_member_list_resubscribe_resends_snapshot_test() ->
+    with_guild(fun(State) ->
+        Subscribed = guild_thread_subscriptions:update(
+            <<"a">>, #{member_lists => [?T1]}, State
+        ),
+        [_] = received(a, thread_member_list_update, drain()),
+        Again = guild_thread_subscriptions:update(
+            <<"a">>, #{member_lists => [?T1]}, Subscribed
+        ),
+        [_] = received(a, thread_member_list_update, drain()),
+        _ = guild_thread_subscriptions:update(<<"a">>, #{threads => true}, Again),
+        ?assertEqual([], received(a, thread_member_list_update, drain()))
+    end).
+
+thread_member_list_drops_removed_guild_members_test() ->
+    with_guild(fun(State0) ->
+        Added = #{
+            <<"id">> => integer_to_binary(?T1),
+            <<"member_count">> => 2,
+            <<"added_members">> => [thread_member(?T1, ?V)]
+        },
+        State1 = dispatch(thread_members_update, Added, State0),
+        Subscribed = guild_thread_subscriptions:update(
+            <<"a">>, #{member_lists => [?T1]}, State1
+        ),
+        _ = drain(),
+        Removed = dispatch(
+            guild_member_remove,
+            #{<<"user">> => #{<<"id">> => integer_to_binary(?V)}},
+            Subscribed
+        ),
+        ?assertEqual(#{?T1 => true}, maps:get(thread_list_dirty, Removed)),
+        _ = guild_thread_subscriptions:handle_list_flush(Removed),
+        [#{<<"members">> := Members}] = received(a, thread_member_list_update, drain()),
+        ?assertEqual(
+            [integer_to_binary(?A)], [maps:get(<<"user_id">>, M) || M <- Members]
+        )
+    end).
+
 handoff_through_an_old_node_round_trips_test() ->
     with_guild(fun(State) ->
         Exported = guild_handoff:export_handoff_state(State),