Enable real-time sidebar when ActionCable in-app

Enables the feature regardless of feature flag value when ActionCable
is configured to run in-app

app/controllers/projects/issues_controller.rb

@@ -51,7 +51,10 @@ class Projects::IssuesController < Projects::ApplicationController
   end
 
   before_action only: :show do
-    push_frontend_feature_flag(:real_time_issue_sidebar, @project)
+    real_time_feature_flag = :real_time_issue_sidebar
+    real_time_enabled = Gitlab::ActionCable::Config.in_app? || Feature.enabled?(real_time_feature_flag, @project)
+
+    gon.push({ features: { real_time_feature_flag.to_s.camelize(:lower) => real_time_enabled } }, true)
   end
 
   before_action only: :index do

app/services/issues/update_service.rb

@@ -22,7 +22,7 @@ module Issues
     end
 
     def after_update(issue)
-      IssuesChannel.broadcast_to(issue, event: 'updated') if Feature.enabled?(:broadcast_issue_updates, issue.project)
+      IssuesChannel.broadcast_to(issue, event: 'updated') if Gitlab::ActionCable::Config.in_app? || Feature.enabled?(:broadcast_issue_updates, issue.project)
     end
 
     def handle_changes(issue, options)

doc/user/project/issues/index.md

@@ -109,6 +109,15 @@ Key actions for Issues include:
 On an issue's page, you can view [all aspects of the issue](issue_data_and_actions.md),
 and modify them if you have the necessary [permissions](../../permissions.md).
 
+#### Real-time sidebar **(CORE ONLY)**
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/17589) in GitLab 13.3.
+> - It cannot be enabled or disabled per-project.
+> - It's not recommended for production use.
+
+Assignees in the sidebar are updated in real time. This feature is **disabled by default**.
+To enable, you need to enable [ActionCable in-app mode](https://docs.gitlab.com/omnibus/settings/actioncable.html).
+
 ### Issues list
 
 ![Project issues list view](img/project_issues_list_view.png)

spec/controllers/projects/issues_controller_spec.rb

@@ -964,6 +964,33 @@ RSpec.describe Projects::IssuesController do
         expect { issue.update(description: [issue.description, labels].join(' ')) }
           .not_to exceed_query_limit(control_count + 2 * labels.count)
       end
+
+      context 'real-time sidebar feature flag' do
+        using RSpec::Parameterized::TableSyntax
+
+        let_it_be(:project) { create(:project, :public) }
+        let_it_be(:issue) { create(:issue, project: project) }
+
+        where(:action_cable_in_app_enabled, :feature_flag_enabled, :gon_feature_flag) do
+          true  | true  | true
+          true  | false | true
+          false | true  | true
+          false | false | false
+        end
+
+        with_them do
+          before do
+            expect(Gitlab::ActionCable::Config).to receive(:in_app?).and_return(action_cable_in_app_enabled)
+            stub_feature_flags(real_time_issue_sidebar: feature_flag_enabled)
+          end
+
+          it 'broadcasts to the issues channel based on ActionCable and feature flag values' do
+            go(id: issue.to_param)
+
+            expect(Gon.features).to include('realTimeIssueSidebar' => gon_feature_flag)
+          end
+        end
+      end
     end
 
     describe 'GET #realtime_changes' do

spec/services/issues/update_service_spec.rb

@@ -840,27 +840,27 @@ RSpec.describe Issues::UpdateService, :mailer do
     end
 
     context 'real-time updates' do
-      let(:update_params) { { assignee_ids: [user2.id] } }
+      using RSpec::Parameterized::TableSyntax
 
-      context 'when broadcast_issue_updates is enabled' do
-        before do
-          stub_feature_flags(broadcast_issue_updates: true)
-        end
-
-        it 'broadcasts to the issues channel' do
-          expect(IssuesChannel).to receive(:broadcast_to).with(issue, event: 'updated')
+      let(:update_params) { { assignee_ids: [user2.id] } }
 
-          update_issue(update_params)
-        end
+      where(:action_cable_in_app_enabled, :feature_flag_enabled, :should_broadcast) do
+        true  | true  | true
+        true  | false | true
+        false | true  | true
+        false | false | false
       end
 
-      context 'when broadcast_issue_updates is disabled' do
-        before do
-          stub_feature_flags(broadcast_issue_updates: false)
-        end
+      with_them do
+        it 'broadcasts to the issues channel based on ActionCable and feature flag values' do
+          expect(Gitlab::ActionCable::Config).to receive(:in_app?).and_return(action_cable_in_app_enabled)
+          stub_feature_flags(broadcast_issue_updates: feature_flag_enabled)
 
-        it 'does not broadcast to the issues channel' do
-          expect(IssuesChannel).not_to receive(:broadcast_to)
+          if should_broadcast
+            expect(IssuesChannel).to receive(:broadcast_to).with(issue, event: 'updated')
+          else
+            expect(IssuesChannel).not_to receive(:broadcast_to)
+          end
 
           update_issue(update_params)
         end