improve remote image rendering (#13102)

* improve remote image rendering

- determine "domain" during Init() so we are not re-parsing settings
  on every request
- if using http-mode via a rednererUrl, then use the AppUrl for the
  page that the renderer loads.  When in http-mode the renderer is likely
  running on another server so trying to use the localhost or even the
  specific IP:PORT grafana is listening on wont work.
- apply the request timeout via a context rather then directly on the http client.
- use a global http client so we can take advantage of connection re-use
- log and handle errors better.

* ensure imagesDir exists

* allow users to define callback_url for remote rendering

- allow users to define the url that a remote rendering service
  should use for connecting back to the grafana instance.
  By default the "root_url" is used.

* improve remote image rendering

- determine "domain" during Init() so we are not re-parsing settings
  on every request
- if using http-mode via a rednererUrl, then use the AppUrl for the
  page that the renderer loads.  When in http-mode the renderer is likely
  running on another server so trying to use the localhost or even the
  specific IP:PORT grafana is listening on wont work.
- apply the request timeout via a context rather then directly on the http client.
- use a global http client so we can take advantage of connection re-use
- log and handle errors better.

* ensure imagesDir exists

* allow users to define callback_url for remote rendering

- allow users to define the url that a remote rendering service
  should use for connecting back to the grafana instance.
  By default the "root_url" is used.

* rendering: fixed issue with renderKey where userId and orgId was in mixed up, added test for RenderCallbackUrl reading logic

pkg/services/rendering/rendering.go

@@ -3,6 +3,8 @@ package rendering
 import (
 	"context"
 	"fmt"
+	"net/url"
+	"os"
 	"path/filepath"
 
 	plugin "github.com/hashicorp/go-plugin"
@@ -27,12 +29,31 @@ type RenderingService struct {
 	grpcPlugin   pluginModel.RendererPlugin
 	pluginInfo   *plugins.RendererPlugin
 	renderAction renderFunc
+	domain       string
 
 	Cfg *setting.Cfg `inject:""`
 }
 
 func (rs *RenderingService) Init() error {
 	rs.log = log.New("rendering")
+
+	// ensure ImagesDir exists
+	err := os.MkdirAll(rs.Cfg.ImagesDir, 0700)
+	if err != nil {
+		return err
+	}
+
+	// set value used for domain attribute of renderKey cookie
+	if rs.Cfg.RendererUrl != "" {
+		// RendererCallbackUrl has already been passed, it wont generate an error.
+		u, _ := url.Parse(rs.Cfg.RendererCallbackUrl)
+		rs.domain = u.Hostname()
+	} else if setting.HttpAddr != setting.DEFAULT_HTTP_ADDR {
+		rs.domain = setting.HttpAddr
+	} else {
+		rs.domain = "localhost"
+	}
+
 	return nil
 }
 
@@ -82,16 +103,17 @@ func (rs *RenderingService) getFilePathForNewImage() string {
 }
 
 func (rs *RenderingService) getURL(path string) string {
-	// &render=1 signals to the legacy redirect layer to
-	return fmt.Sprintf("%s://%s:%s/%s&render=1", setting.Protocol, rs.getLocalDomain(), setting.HttpPort, path)
-}
+	if rs.Cfg.RendererUrl != "" {
+		// 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.
+
+		// &render=1 signals to the legacy redirect layer to
+		return fmt.Sprintf("%s%s&render=1", rs.Cfg.RendererCallbackUrl, path)
 
-func (rs *RenderingService) getLocalDomain() string {
-	if setting.HttpAddr != setting.DEFAULT_HTTP_ADDR {
-		return setting.HttpAddr
 	}
-
-	return "localhost"
+	// &render=1 signals to the legacy redirect layer to
+	return fmt.Sprintf("%s://%s:%s/%s&render=1", setting.Protocol, rs.domain, setting.HttpPort, path)
 }
 
 func (rs *RenderingService) getRenderKey(orgId, userId int64, orgRole models.RoleType) string {