[feature] Use maintenance router to serve 503 while server is starting/migrating (#3705)

* [feature] Use maintenance router to serve 503 while server is starting/migrating

* love you linter, kissies

cmd/gotosocial/action/server/server.go

@@ -69,6 +69,36 @@ import (
 	"go.uber.org/automaxprocs/maxprocs"
 )
 
+// Maintenance starts and creates a GoToSocial server
+// in maintenance mode (returns 503 for most requests).
+var Maintenance action.GTSAction = func(ctx context.Context) error {
+	route, err := router.New(ctx)
+	if err != nil {
+		return fmt.Errorf("error creating maintenance router: %w", err)
+	}
+
+	// Route maintenance handlers.
+	maintenance := web.NewMaintenance()
+	maintenance.Route(route)
+
+	// Start the maintenance router.
+	if err := route.Start(); err != nil {
+		return fmt.Errorf("error starting maintenance router: %w", err)
+	}
+
+	// Catch shutdown signals from the OS.
+	sigs := make(chan os.Signal, 1)
+	signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM)
+	sig := <-sigs // block until signal received
+	log.Infof(ctx, "received signal %s, shutting down", sig)
+
+	if err := route.Stop(); err != nil {
+		log.Errorf(ctx, "error stopping router: %v", err)
+	}
+
+	return nil
+}
+
 // Start creates and starts a gotosocial server
 var Start action.GTSAction = func(ctx context.Context) error {
 	// Set GOMAXPROCS / GOMEMLIMIT
@@ -148,6 +178,23 @@ var Start action.GTSAction = func(ctx context.Context) error {
 		log.Info(ctx, "done! exiting...")
 	}()
 
+	// Create maintenance router.
+	var err error
+	route, err = router.New(ctx)
+	if err != nil {
+		return fmt.Errorf("error creating maintenance router: %w", err)
+	}
+
+	// Route maintenance handlers.
+	maintenance := web.NewMaintenance()
+	maintenance.Route(route)
+
+	// Start the maintenance router to handle reqs
+	// while the instance is starting up / migrating.
+	if err := route.Start(); err != nil {
+		return fmt.Errorf("error starting maintenance router: %w", err)
+	}
+
 	// Initialize tracing (noop if not enabled).
 	if err := tracing.Initialize(); err != nil {
 		return fmt.Errorf("error initializing tracing: %w", err)
@@ -359,9 +406,15 @@ var Start action.GTSAction = func(ctx context.Context) error {
 		HTTP router initialization
 	*/
 
+	// Close down the maintenance router.
+	if err := route.Stop(); err != nil {
+		return fmt.Errorf("error stopping maintenance router: %w", err)
+	}
+
+	// Instantiate the main router.
 	route, err = router.New(ctx)
 	if err != nil {
-		return fmt.Errorf("error creating router: %s", err)
+		return fmt.Errorf("error creating main router: %s", err)
 	}
 
 	// Start preparing middleware stack.

cmd/gotosocial/server.go

@@ -41,5 +41,19 @@ func serverCommands() *cobra.Command {
 	}
 	config.AddServerFlags(serverStartCmd)
 	serverCmd.AddCommand(serverStartCmd)
+
+	serverMaintenanceCmd := &cobra.Command{
+		Use:   "maintenance",
+		Short: "start the gotosocial server in maintenance mode (returns 503 for almost all requests)",
+		PreRunE: func(cmd *cobra.Command, args []string) error {
+			return preRun(preRunArgs{cmd: cmd})
+		},
+		RunE: func(cmd *cobra.Command, args []string) error {
+			return run(cmd.Context(), server.Maintenance)
+		},
+	}
+	config.AddServerFlags(serverMaintenanceCmd)
+	serverCmd.AddCommand(serverMaintenanceCmd)
+
 	return serverCmd
 }