message_events: Discard message list that cannot be live updated.

Since we cannot cheaply live update `is-followed` narrow, we
discard any message list or data with that narrow if we
received a message event that changes topic visibility
from or to `FOLLOWED`.

web/src/message_events.js

@@ -47,6 +47,34 @@ function filter_has_term_type(filter, term_type) {
     );
 }
 
+export function discard_cached_lists_with_term_type(term_type) {
+    // Discards cached MessageList and MessageListData which have
+    // `term_type` and `not-term_type`.
+    assert(!term_type.includes("not-"));
+
+    // We loop over rendered message lists and cached message data separately since
+    // they are separately maintained and can have different items.
+    for (const msg_list of message_lists.all_rendered_message_lists()) {
+        // We never want to discard the current message list.
+        if (msg_list === message_lists.current) {
+            continue;
+        }
+
+        const filter = msg_list.data.filter;
+        if (filter_has_term_type(filter, term_type)) {
+            message_lists.delete_message_list(msg_list);
+            message_list_data_cache.remove(filter);
+        }
+    }
+
+    for (const msg_list_data of message_lists.non_rendered_data()) {
+        const filter = msg_list_data.filter;
+        if (filter_has_term_type(filter, term_type)) {
+            message_list_data_cache.remove(filter);
+        }
+    }
+}
+
 export function update_current_view_for_topic_visibility() {
     // If we have rendered message list / cached data based on topic
     // visibility policy, we need to rerender it to reflect the changes. It

web/src/message_lists.ts

@@ -15,7 +15,7 @@ export function set_current(msg_list: MessageList | undefined): void {
     current = msg_list;
 }
 
-function delete_message_list(message_list: MessageList): void {
+export function delete_message_list(message_list: MessageList): void {
     message_list.view.$list.remove();
     rendered_message_lists.delete(message_list.id);
     message_list.data.set_rendered_message_list_id(undefined);

web/src/server_events_dispatch.js

@@ -90,6 +90,7 @@ import * as user_group_edit from "./user_group_edit";
 import * as user_groups from "./user_groups";
 import {user_settings} from "./user_settings";
 import * as user_status from "./user_status";
+import * as user_topics from "./user_topics";
 import * as user_topics_ui from "./user_topics_ui";
 
 export function dispatch_normal_event(event) {
@@ -1017,11 +1018,23 @@ export function dispatch_normal_event(event) {
             }
             break;
 
-        case "user_topic":
+        case "user_topic": {
+            const previous_topic_visibility = user_topics.get_topic_visibility_policy(
+                event.stream_id,
+                event.topic_name,
+            );
             user_topics_ui.handle_topic_updates(
                 event,
                 message_events.update_current_view_for_topic_visibility(),
             );
+            // Discard cached message lists if `event` topic was / is followed.
+            if (
+                event.visibility_policy === user_topics.all_visibility_policies.FOLLOWED ||
+                previous_topic_visibility === user_topics.all_visibility_policies.FOLLOWED
+            ) {
+                message_events.discard_cached_lists_with_term_type("is-followed");
+            }
             break;
+        }
     }
 }

web/tests/dispatch.test.js

@@ -379,6 +379,20 @@ run_test("muted_topics", ({override}) => {
     assert_same(args.user_topic, event);
 });
 
+run_test("followed_topic", ({override}) => {
+    const event = event_fixtures.user_topic_with_followed_policy_change;
+
+    const stub = make_stub();
+    const discard_msg_list_stub = make_stub();
+    override(user_topics_ui, "handle_topic_updates", stub.f);
+    override(message_events, "discard_cached_lists_with_term_type", discard_msg_list_stub.f);
+    dispatch(event);
+    assert.equal(stub.num_calls, 1);
+    assert.equal(discard_msg_list_stub.num_calls, 1);
+    const args = stub.get_args("user_topic");
+    assert_same(args.user_topic, event);
+});
+
 run_test("muted_users", ({override}) => {
     const event = event_fixtures.muted_users;
 

web/tests/lib/events.js

@@ -1164,6 +1164,14 @@ exports.fixtures = {
         visibility_policy: 1,
     },
 
+    user_topic_with_followed_policy_change: {
+        type: "user_topic",
+        stream_id: 101,
+        topic_name: "js",
+        last_updated: fake_now,
+        visibility_policy: 3,
+    },
+
     web_reload_client: {
         type: "web_reload_client",
         immediate: true,