Allow ConnectionNotEstablished for DB rake tasks using validate_config

If we can't connect to one of the databases to grab the identifier,
ActiveRecord::ConnectionNotEstablished is also one of the errors
that can be raised.

This rescues it so that the task won't fail even if the DB is down.

Changelog: fixed

lib/tasks/gitlab/db/validate_config.rake

@@ -20,7 +20,7 @@ namespace :gitlab do
           begin
             ActiveRecord::Base.establish_connection(db_config) # rubocop: disable Database/EstablishConnection
             ActiveRecord::Base.connection.select_one("SELECT system_identifier, current_database() FROM pg_control_system()")
-          rescue ActiveRecord::NoDatabaseError, PG::ConnectionBad
+          rescue ActiveRecord::NoDatabaseError, ActiveRecord::ConnectionNotEstablished, PG::ConnectionBad
           end
 
         {

spec/tasks/gitlab/db/validate_config_rake_spec.rb

@@ -53,6 +53,18 @@ RSpec.describe 'gitlab:db:validate_config', :silence_stdout do
         expect { run_rake_task('gitlab:db:validate_config') }.not_to output(/Database config validation failure/).to_stderr
         expect { run_rake_task('gitlab:db:validate_config') }.not_to raise_error
       end
+
+      context 'when finding the initializer fails' do
+        where(:raised_error) { [ActiveRecord::NoDatabaseError, ActiveRecord::ConnectionNotEstablished, PG::ConnectionBad] }
+        with_them do
+          it "does not raise an error for #{params[:raised_error]}" do
+            allow(ActiveRecord::Base.connection).to receive(:select_one).and_raise(raised_error) # rubocop: disable Database/MultipleDatabases
+
+            expect { run_rake_task('gitlab:db:validate_config') }.not_to output(/Database config validation failure/).to_stderr
+            expect { run_rake_task('gitlab:db:validate_config') }.not_to raise_error
+          end
+        end
+      end
     end
 
     shared_examples 'raises an error' do |match|