user_groups: Add `is_empty_group` function to check for nobody group.

Removes hardcoded checks for "role:nobody" by checking if the given group
is empty.

web/src/compose_recipient.ts

@@ -259,8 +259,7 @@ function get_options_for_recipient_widget(): Option[] {
         name: $t({defaultMessage: "Direct message"}),
     };
 
-    const {name} = user_groups.get_user_group_from_id(realm.realm_direct_message_permission_group);
-    if (name !== "role:nobody") {
+    if (!user_groups.is_empty_group(realm.realm_direct_message_permission_group)) {
         options.unshift(direct_messages_option);
     } else {
         options.push(direct_messages_option);

web/src/compose_validate.ts

@@ -114,10 +114,7 @@ export function needs_subscribe_warning(user_id: number, stream_id: number): boo
 
 export function check_dm_permissions_and_get_error_string(user_ids_string: string): string {
     if (!people.user_can_direct_message(user_ids_string)) {
-        const {name} = user_groups.get_user_group_from_id(
-            realm.realm_direct_message_permission_group,
-        );
-        if (name === "role:nobody") {
+        if (user_groups.is_empty_group(realm.realm_direct_message_permission_group)) {
             return $t({
                 defaultMessage: "Direct messages are disabled in this organization.",
             });

web/src/settings_org.js

@@ -370,7 +370,7 @@ function set_create_web_public_stream_dropdown_visibility() {
 }
 
 export function check_disable_direct_message_initiator_group_dropdown(current_value) {
-    if (current_value === user_groups.get_user_group_from_name("role:nobody").id) {
+    if (user_groups.is_empty_group(current_value)) {
         $("#realm_direct_message_initiator_group_widget").prop("disabled", true);
     } else {
         $("#realm_direct_message_initiator_group_widget").prop("disabled", false);

web/src/user_groups.ts

@@ -178,6 +178,37 @@ export function is_user_group(
     return item.members !== undefined;
 }
 
+export function is_empty_group(user_group_id: number): boolean {
+    const user_group = user_group_by_id_dict.get(user_group_id);
+    if (user_group === undefined) {
+        blueslip.error("Could not find user group", {user_group_id});
+        return false;
+    }
+    if (user_group.members.size > 0) {
+        return false;
+    }
+
+    // Check if all the recursive subgroups are empty.
+    // Correctness of this algorithm relying on the ES6 Set
+    // implementation having the property that a `for of` loop will
+    // visit all items that are added to the set during the loop.
+    const subgroup_ids = new Set(user_group.direct_subgroup_ids);
+    for (const subgroup_id of subgroup_ids) {
+        const subgroup = user_group_by_id_dict.get(subgroup_id);
+        if (subgroup === undefined) {
+            blueslip.error("Could not find subgroup", {subgroup_id});
+            return false;
+        }
+        if (subgroup.members.size > 0) {
+            return false;
+        }
+        for (const direct_subgroup_id of subgroup.direct_subgroup_ids) {
+            subgroup_ids.add(direct_subgroup_id);
+        }
+    }
+    return true;
+}
+
 export function get_user_groups_of_user(user_id: number): UserGroup[] {
     const user_groups_realm = get_realm_user_groups();
     const groups_of_user = user_groups_realm.filter((group) =>