Geo: Add rake task to check DB replication

ee/changelogs/unreleased/mk-add-replication-working-rake-task.yml

+---
+title: 'Geo: Add rake task to check if DB replication is working'
+merge_request: 41618
+author:
+type: added

ee/lib/tasks/gitlab/geo.rake

@@ -32,5 +32,31 @@ namespace :gitlab do
         exit 1
       end
     end
+
+    desc 'GitLab | Geo | Check Geo database replication'
+    task check_database_replication_working: :gitlab_environment do
+      unless ::Gitlab::Geo.secondary?
+        abort 'This command is only available on a secondary node'.color(:red)
+      end
+
+      geo_health_check = Gitlab::Geo::HealthCheck.new
+
+      enabled = geo_health_check.replication_enabled?
+      success = enabled && geo_health_check.replication_working?
+
+      if success
+        puts 'SUCCESS - Database replication is working.'.color(:green)
+      elsif enabled
+        abort "ERROR - Database replication is enabled, but not working.\n"\
+              "This rake task is intended for programmatic use. Please run\n"\
+              "the full Geo check task for more information:\n"\
+              "  gitlab-rake gitlab:geo:check".color(:red)
+      else
+        abort "ERROR - Database replication is not enabled.\n"\
+              "This rake task is intended for programmatic use. Please run\n"\
+              "the full Geo check task for more information:\n"\
+              "  gitlab-rake gitlab:geo:check".color(:red)
+      end
+    end
   end
 end

ee/spec/tasks/gitlab/geo_rake_spec.rb

@@ -50,4 +50,55 @@ RSpec.describe 'gitlab:geo rake tasks', :geo do
       end
     end
   end
+
+  describe 'gitlab:geo:check_database_replication_working' do
+    let(:run_task) do
+      run_rake_task('gitlab:geo:check_database_replication_working')
+    end
+
+    before do
+      stub_secondary_node
+    end
+
+    context 'when DB replication is enabled' do
+      let(:enabled) { true }
+
+      before do
+        allow_next_instance_of(Gitlab::Geo::HealthCheck) do |health_check|
+          allow(health_check).to receive(:replication_enabled?).and_return(enabled)
+          allow(health_check).to receive(:replication_working?).and_return(working)
+        end
+      end
+
+      context 'when DB replication is working' do
+        let(:working) { true }
+
+        it 'prints a success message' do
+          expect { run_task }.to output(/SUCCESS - Database replication is working/).to_stdout
+        end
+      end
+
+      context 'when DB replication is not working' do
+        let(:working) { false }
+
+        it 'exits with non-success code' do
+          expect { run_task }.to abort_execution.with_message(/ERROR - Database replication is enabled, but not working/)
+        end
+      end
+    end
+
+    context 'when DB replication is not enabled' do
+      let(:enabled) { false }
+
+      before do
+        allow_next_instance_of(Gitlab::Geo::HealthCheck) do |health_check|
+          allow(health_check).to receive(:replication_enabled?).and_return(enabled)
+        end
+      end
+
+      it 'exits with non-success code' do
+        expect { run_task }.to abort_execution.with_message(/ERROR - Database replication is not enabled/)
+      end
+    end
+  end
 end