refactor: Remove stream_data.update_subscribers_count.

This removes a bit of complexity.  If a piece of
settings code needs to render a stream with
subscribers, it just asks for it.

We no longer have the brittle, action-at-a-distance
mechanism of mutating the subscriber count on to
the stream_data version of a sub.

Stream subs are pretty small, so making copies of
them is cheap, and the blueslip timings from the
previous commit can help confirm that.

There is some discussion of putting `subscriber_count`
on the Stream model, which may eventually get us
away from tracking it in `peer_data.js`, but we will
cross that bridge when we get there.  See
https://github.com/zulip/zulip/issues/17101 for
more details.

frontend_tests/node_tests/stream_data.js

@@ -239,8 +239,7 @@ run_test("subscribers", () => {
     assert(ok);
     assert(stream_data.is_user_subscribed(sub.stream_id, brutus.user_id));
     sub = stream_data.get_sub("Rome");
-    stream_data.update_subscribers_count(sub);
-    assert.equal(sub.subscriber_count, 1);
+    assert.equal(peer_data.get_subscriber_count(sub.stream_id), 1);
     const sub_email = "Rome:214125235@zulipdev.com:9991";
     stream_data.update_stream_email_address(sub, sub_email);
     assert.equal(sub.email_address, sub_email);
@@ -249,16 +248,14 @@ run_test("subscribers", () => {
     peer_data.add_subscriber(sub.stream_id, brutus.user_id);
     assert(stream_data.is_user_subscribed(sub.stream_id, brutus.user_id));
     sub = stream_data.get_sub("Rome");
-    stream_data.update_subscribers_count(sub);
-    assert.equal(sub.subscriber_count, 1);
+    assert.equal(peer_data.get_subscriber_count(sub.stream_id), 1);
 
     // remove
     ok = peer_data.remove_subscriber(sub.stream_id, brutus.user_id);
     assert(ok);
     assert(!stream_data.is_user_subscribed(sub.stream_id, brutus.user_id));
     sub = stream_data.get_sub("Rome");
-    stream_data.update_subscribers_count(sub);
-    assert.equal(sub.subscriber_count, 0);
+    assert.equal(peer_data.get_subscriber_count(sub.stream_id), 0);
 
     // verify that checking subscription with undefined user id
 
@@ -280,8 +277,7 @@ run_test("subscribers", () => {
     assert(!ok);
     assert(!stream_data.is_user_subscribed(sub.stream_id, brutus.user_id));
     sub = stream_data.get_sub("Rome");
-    stream_data.update_subscribers_count(sub);
-    assert.equal(sub.subscriber_count, 0);
+    assert.equal(peer_data.get_subscriber_count(sub.stream_id), 0);
 
     // Verify defensive code in set_subscribers, where the second parameter
     // can be undefined.

static/js/stream_data.js

@@ -352,6 +352,25 @@ exports.get_unsorted_subs = function () {
     return Array.from(stream_info.values());
 };
 
+exports.get_sub_for_settings = (sub) => {
+    // Since we make a copy of the sub here, it may eventually
+    // make sense to get the other calculated fields here as
+    // well, instead of using update_calculated_fields everywhere.
+    const sub_count = peer_data.get_subscriber_count(sub.stream_id);
+    return {
+        ...sub,
+        subscriber_count: sub_count,
+    };
+};
+
+function get_subs_for_settings(subs) {
+    // We may eventually add subscribers to the subs here, rather than
+    // delegating, so that we can more efficiently compute subscriber counts
+    // (in bulk).  If that plan appears to have been aborted, feel free to
+    // inline this.
+    return subs.map((sub) => exports.get_sub_for_settings(sub));
+}
+
 exports.get_updated_unsorted_subs = function () {
     // This function is expensive in terms of calculating
     // some values (particularly stream counts) but avoids
@@ -368,7 +387,7 @@ exports.get_updated_unsorted_subs = function () {
         all_subs = all_subs.filter((sub) => sub.subscribed);
     }
 
-    return all_subs;
+    return get_subs_for_settings(all_subs);
 };
 
 exports.num_subscribed_subs = function () {
@@ -423,13 +442,6 @@ exports.get_colors = function () {
     return exports.subscribed_subs().map((sub) => sub.color);
 };
 
-exports.update_subscribers_count = function (sub) {
-    // This is part of an unfortunate legacy hack, where we
-    // put calculated fields onto the sub object instead of
-    // letting callers build their own objects.
-    sub.subscriber_count = peer_data.get_subscriber_count(sub.stream_id);
-};
-
 exports.update_stream_email_address = function (sub, email) {
     sub.email_address = email;
 };
@@ -462,6 +474,8 @@ exports.receives_notifications = function (stream_id, notification_name) {
 };
 
 exports.update_calculated_fields = function (sub) {
+    // Note that we don't calculate subscriber counts here.
+
     sub.is_realm_admin = page_params.is_admin;
     // Admin can change any stream's name & description either stream is public or
     // private, subscribed or unsubscribed.
@@ -485,7 +499,6 @@ exports.update_calculated_fields = function (sub) {
     if (sub.rendered_description !== undefined) {
         sub.rendered_description = sub.rendered_description.replace("<p>", "").replace("</p>", "");
     }
-    exports.update_subscribers_count(sub);
 
     // Apply the defaults for our notification settings for rendering.
     for (const setting of settings_config.stream_specific_notification_settings) {
@@ -786,7 +799,7 @@ exports.get_streams_for_settings_page = function () {
         exports.update_calculated_fields(sub);
     }
 
-    return all_subs;
+    return get_subs_for_settings(all_subs);
 };
 
 exports.sort_for_stream_settings = function (stream_ids, order) {

static/js/subs.js

@@ -213,7 +213,6 @@ exports.rerender_subscriptions_settings = function (sub) {
         blueslip.error("Undefined sub passed to function rerender_subscriptions_settings");
         return;
     }
-    stream_data.update_subscribers_count(sub);
     stream_ui_updates.update_subscribers_count(sub);
     stream_ui_updates.update_subscribers_list(sub);
 };
@@ -237,8 +236,10 @@ exports.add_sub_to_table = function (sub) {
         return;
     }
 
-    const html = render_subscription(sub);
+    const setting_sub = stream_data.get_sub_for_settings(sub);
+    const html = render_subscription(setting_sub);
     const settings_html = render_subscription_settings(sub);
+
     if (stream_create.get_name() === sub.name) {
         ui.get_content_element($(".streams-list")).prepend(html);
         ui.reset_scrollbar($(".streams-list"));
@@ -287,7 +288,6 @@ exports.update_settings_for_subscribed = function (sub) {
     ).show();
 
     if (exports.is_sub_already_present(sub)) {
-        stream_data.update_subscribers_count(sub);
         stream_ui_updates.update_stream_row_in_settings_tab(sub);
         stream_ui_updates.update_subscribers_count(sub, true);
         stream_ui_updates.update_check_button_for_sub(sub);