server: add read only mode

In read only mode the endpoint to save new effects or add new versions
is deactivated. Also in the gallery the create button is deactivated and
there is a notice that says:

    The server is in maintenance mode. You can not create or modify effects.

The button to save effects in the editor is not deactivated as it will
take a bit more time and wanted to have the page up as soon as possible.

server/assets/gallery.html

@@ -123,7 +123,13 @@ <h1><a href="/" style="text-transform:uppercase">GLSL Sandbox</a> <a href="https
 <a href="https://twitter.com/mrdoob">@mrdoob</a><br/>
 editor by <a href="https://twitter.com/mrdoob">@mrdoob</a>, <a href="https://twitter.com/mrkishi">@mrkishi</a>, <a href="https://twitter.com/p01">@p01</a>, <a href="https://twitter.com/alteredq">@alteredq</a>, <a href="https://twitter.com/kusmabite">@kusmabite</a> and <a href="https://twitter.com/emackey">@emackey</a>
 <br/><br/>
+
+{{ if .ReadOnly }}
+<h2 style="color:#ff6961">The server is in maintenance mode. You can not create or modify effects.</h2>
+{{ else }}
 <a href="/e"><button>new shader</button></a>
+{{ end }}
+
 <div id="gallery">
 
 {{ if .Admin }}

server/cmd/glslsandbox/main.go

@@ -25,6 +25,7 @@ type Config struct {
 	TLSAddr    string `envconfig:"TLS_ADDR"`
 	Domains    string `envconfig:"DOMAINS" default:"www.glslsandbox.com,glslsandbox.com"`
 	Dev        bool   `envconfig:"DEV" default:"true"`
+	ReadOnly   bool   `envconfig:"READ_ONLY" default:"false"`
 }
 
 func main() {
@@ -82,6 +83,7 @@ func start() error {
 		auth,
 		cfg.DataPath,
 		cfg.Dev,
+		cfg.ReadOnly,
 	)
 	if err != nil {
 		return fmt.Errorf("could not create server: %w", err)

server/server.go

@@ -27,9 +27,7 @@ const (
 	perPage     = 50
 )
 
-var (
-	ErrInvalidData = fmt.Errorf("invalid data")
-)
+var ErrInvalidData = fmt.Errorf("invalid data")
 
 type Template struct {
 	templates *template.Template
@@ -80,6 +78,7 @@ type Server struct {
 	effects  *store.Effects
 	auth     *Auth
 	dataPath string
+	readOnly bool
 }
 
 func New(
@@ -90,6 +89,7 @@ func New(
 	auth *Auth,
 	dataPath string,
 	dev bool,
+	readOnly bool,
 ) (*Server, error) {
 	var tpl *template.Template
 	if !dev {
@@ -115,6 +115,7 @@ func New(
 		effects:  e,
 		auth:     auth,
 		dataPath: dataPath,
+		readOnly: readOnly,
 	}, nil
 }
 
@@ -163,7 +164,10 @@ func (s *Server) routes() {
 	s.echo.GET("/", s.indexHandler)
 	s.echo.GET("/e", s.effectHandler)
 	s.echo.GET("/e_", s.effectHandler_)
-	s.echo.POST("/e", s.saveHandler)
+
+	if !s.readOnly {
+		s.echo.POST("/e", s.saveHandler)
+	}
 
 	cors := middleware.CORSWithConfig(middleware.CORSConfig{
 		Skipper:      middleware.DefaultSkipper,
@@ -228,6 +232,8 @@ type galleryData struct {
 	NextPage string
 	// Admin is true when accessing "/admin" path.
 	Admin bool
+	// ReadOnly tells the server is in read only mode.
+	ReadOnly bool
 }
 
 func (s *Server) indexRender(c echo.Context, admin bool) error {
@@ -290,6 +296,7 @@ func (s *Server) indexRender(c echo.Context, admin bool) error {
 		IsPrevious:   page > 0,
 		PreviousPage: previousPage,
 		Admin:        admin,
+		ReadOnly:     s.readOnly,
 	}
 
 	return c.Render(http.StatusOK, "gallery", d)