Create idea of read-only databasetc-geo-read-only-idea

In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.

Closes gitlab-org/gitlab-ce#37534.

1 files changed, 14 insertions, 0 deletions

diff --git a/app/controllers/admin/application_controller.rb b/app/controllers/admin/application_controller.rb
index a4648b33cfa..c27f2ee3c09 100644
--- a/app/controllers/admin/application_controller.rb
+++ b/app/controllers/admin/application_controller.rb
@@ -3,9 +3,23 @@
 # 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