Add read-only banner to all pages

When the database is in a read-only state, display a banner on each
page informing the user they cannot write to that GitLab instance.

Closes gitlab-org/gitlab-ce#43937.

app/controllers/admin/application_controller.rb

@@ -3,23 +3,9 @@
 # Automatically sets the layout and ensures an administrator is logged in
 class Admin::ApplicationController < ApplicationController
   before_action :authenticate_admin!
-  before_action :display_read_only_information
   layout 'admin'
 
   def authenticate_admin!
     render_404 unless current_user.admin?
   end
-
-  def display_read_only_information
-    return unless Gitlab::Database.read_only?
-
-    flash.now[:notice] = read_only_message
-  end
-
-  private
-
-  # Overridden in EE
-  def read_only_message
-    _('You are on a read-only GitLab instance.')
-  end
 end

app/helpers/application_helper.rb

@@ -323,4 +323,11 @@ module ApplicationHelper
   def locale_path
     asset_path("locale/#{Gitlab::I18n.locale}/app.js")
   end
+
+  # Overridden in EE
+  def read_only_message
+    return unless Gitlab::Database.read_only?
+
+    _('You are on a read-only GitLab instance.')
+  end
 end

app/views/layouts/_page.html.haml

@@ -6,6 +6,7 @@
     .mobile-overlay
     .alert-wrapper
       = render "layouts/broadcast"
+      = render 'layouts/header/read_only_banner'
       = yield :flash_message
       - unless @hide_breadcrumbs
         = render "layouts/nav/breadcrumbs"

app/views/layouts/header/_read_only_banner.html.haml

+- message = read_only_message
+- if message
+  .flash-container.flash-container-page
+    .flash-notice
+      %div{ class: (container_class) }
+        %span
+          = message

changelogs/unreleased/tc-re-add-read-only-banner.yml

+---
+title: Add read-only banner to all pages
+merge_request: 17798
+author:
+type: fixed

spec/features/read_only_spec.rb

+require 'rails_helper'
+
+describe 'read-only message' do
+  set(:user) { create(:user) }
+
+  before do
+    sign_in(user)
+  end
+
+  it 'shows read-only banner when database is read-only' do
+    allow(Gitlab::Database).to receive(:read_only?).and_return(true)
+
+    visit root_dashboard_path
+
+    expect(page).to have_content('You are on a read-only GitLab instance.')
+  end
+
+  it 'does not show read-only banner when database is able to read-write' do
+    allow(Gitlab::Database).to receive(:read_only?).and_return(false)
+
+    visit root_dashboard_path
+
+    expect(page).not_to have_content('You are on a read-only GitLab instance.')
+  end
+end