Improve handling of old/slow-to-start remote renderer (#40492)

* Assume the remote renderer is old if it returns 404 to the version endpoint

* Retry fetch of remote image renderer version on failure

Co-authored-by: Agnès Toulet <35176601+AgnesToulet@users.noreply.github.com>

pkg/services/rendering/http_mode.go

@@ -28,6 +28,11 @@ var netClient = &http.Client{
 	Transport: netTransport,
 }
 
+var (
+	remoteVersionFetchInterval time.Duration = time.Second * 15
+	remoteVersionFetchRetries  uint          = 4
+)
+
 func (rs *RenderingService) renderViaHTTP(ctx context.Context, renderKey string, opts Opts) (*RenderResult, error) {
 	filePath, err := rs.getNewFilePath(RenderPNG)
 	if err != nil {
@@ -194,6 +199,24 @@ func (rs *RenderingService) readFileResponse(ctx context.Context, resp *http.Res
 	return nil
 }
 
+func (rs *RenderingService) getRemotePluginVersionWithRetry(callback func(string, error)) {
+	go func() {
+		var err error
+		for try := uint(0); try < remoteVersionFetchRetries; try++ {
+			version, err := rs.getRemotePluginVersion()
+			if err == nil {
+				callback(version, err)
+				return
+			}
+			rs.log.Info("Couldn't get remote renderer version, retrying", "err", err, "try", try)
+
+			time.Sleep(remoteVersionFetchInterval)
+		}
+
+		callback("", err)
+	}()
+}
+
 func (rs *RenderingService) getRemotePluginVersion() (string, error) {
 	rendererURL, err := url.Parse(rs.Cfg.RendererUrl + "/version")
 	if err != nil {
@@ -212,7 +235,10 @@ func (rs *RenderingService) getRemotePluginVersion() (string, error) {
 		}
 	}()
 
-	if resp.StatusCode != http.StatusOK {
+	if resp.StatusCode == http.StatusNotFound {
+		// Old versions of the renderer lacked the version endpoint
+		return "1.0.0", nil
+	} else if resp.StatusCode != http.StatusOK {
 		return "", fmt.Errorf("remote rendering request to get version failed, status code: %d, status: %s", resp.StatusCode,
 			resp.Status)
 	}