Renderer: Fix regression on callback URL in plugin mode (#103787)

* Renderer: Fix regression on callback URL in plugin mode

* cleanup

* refactor to not mutate cfg object

* apply review feedback

pkg/services/rendering/rendering.go

@@ -27,18 +27,19 @@ import (
 var _ Service = (*RenderingService)(nil)
 
 type RenderingService struct {
-	log               log.Logger
-	plugin            Plugin
-	renderAction      renderFunc
-	renderCSVAction   renderCSVFunc
-	sanitizeSVGAction sanitizeFunc
-	sanitizeURL       string
-	domain            string
-	inProgressCount   int32
-	version           string
-	versionMutex      sync.RWMutex
-	capabilities      []Capability
-	pluginAvailable   bool
+	log                 log.Logger
+	plugin              Plugin
+	renderAction        renderFunc
+	renderCSVAction     renderCSVFunc
+	sanitizeSVGAction   sanitizeFunc
+	sanitizeURL         string
+	domain              string
+	inProgressCount     int32
+	version             string
+	versionMutex        sync.RWMutex
+	capabilities        []Capability
+	pluginAvailable     bool
+	rendererCallbackURL string
 
 	perRequestRenderKeyProvider renderKeyProvider
 	Cfg                         *setting.Cfg
@@ -80,9 +81,24 @@ func ProvideService(cfg *setting.Cfg, features featuremgmt.FeatureToggles, remot
 	//  value used for domain attribute of renderKey cookie
 	var domain string
 
+	// value used by the image renderer to make requests to Grafana
+	rendererCallbackURL := cfg.RendererCallbackUrl
+	if cfg.RendererServerUrl != "" {
+		sanitizeURL = getSanitizerURL(cfg.RendererServerUrl)
+
+		// Default value for callback URL using a remote renderer should be AppURL
+		if rendererCallbackURL == "" {
+			rendererCallbackURL = cfg.AppURL
+		}
+	}
+
 	switch {
-	case cfg.RendererCallbackUrl != "":
-		u, err := url.Parse(cfg.RendererCallbackUrl)
+	case rendererCallbackURL != "":
+		if rendererCallbackURL[len(rendererCallbackURL)-1] != '/' {
+			rendererCallbackURL += "/"
+		}
+
+		u, err := url.Parse(rendererCallbackURL)
 		if err != nil {
 			logger.Warn("Image renderer callback url is not valid. " +
 				"Please provide a valid RendererCallbackUrl. " +
@@ -96,10 +112,6 @@ func ProvideService(cfg *setting.Cfg, features featuremgmt.FeatureToggles, remot
 		domain = "localhost"
 	}
 
-	if cfg.RendererUrl != "" {
-		sanitizeURL = getSanitizerURL(cfg.RendererUrl)
-	}
-
 	var renderKeyProvider renderKeyProvider
 	if features.IsEnabledGlobally(featuremgmt.FlagRenderAuthJWT) {
 		renderKeyProvider = &jwtRenderKeyProvider{
@@ -145,6 +157,7 @@ func ProvideService(cfg *setting.Cfg, features featuremgmt.FeatureToggles, remot
 		domain:                domain,
 		sanitizeURL:           sanitizeURL,
 		pluginAvailable:       exists,
+		rendererCallbackURL:   rendererCallbackURL,
 	}
 
 	gob.Register(&RenderUser{})
@@ -215,7 +228,7 @@ func (rs *RenderingService) Run(ctx context.Context) error {
 }
 
 func (rs *RenderingService) remoteAvailable() bool {
-	return rs.Cfg.RendererUrl != ""
+	return rs.Cfg.RendererServerUrl != ""
 }
 
 func (rs *RenderingService) IsAvailable(ctx context.Context) bool {
@@ -424,13 +437,14 @@ func (rs *RenderingService) getNewFilePath(rt RenderType) (string, error) {
 
 // getGrafanaCallbackURL creates a URL to send to the image rendering as callback for rendering a Grafana resource
 func (rs *RenderingService) getGrafanaCallbackURL(path string) string {
-	if rs.Cfg.RendererUrl != "" || rs.Cfg.RendererCallbackUrl != "" {
-		// The backend rendering service can potentially be remote.
-		// So we need to use the root_url to ensure the rendering service
-		// can reach this Grafana instance.
+	if rs.rendererCallbackURL != "" {
+		// rendererCallbackURL should be set if:
+		// - the backend rendering service is remote (default value is cfg.AppURL
+		// and set when initializing the service)
+		// - the service is a plugin and Grafana is running behind a proxy changing its domain
 
 		// &render=1 signals to the legacy redirect layer to
-		return fmt.Sprintf("%s%s&render=1", rs.Cfg.RendererCallbackUrl, path)
+		return fmt.Sprintf("%s%s&render=1", rs.rendererCallbackURL, path)
 	}
 
 	protocol := rs.Cfg.Protocol