Previews: use ETag header (#51008) (#51041)

* use etag header for previews

* use unix millis rather than tz-dependant string

(cherry picked from commit 644503f5e6)

Co-authored-by: Artur Wierzbicki <wierzbicki.artur.94@gmail.com>

pkg/services/thumbs/service.go

@@ -227,12 +227,21 @@ func (hs *thumbService) GetImage(c *models.ReqContext) {
 		return
 	}
 
-	if err != nil {
+	if err != nil || res == nil {
 		hs.log.Error("Error when retrieving thumbnail", "dashboardUid", req.UID, "err", err.Error())
 		c.JSON(500, map[string]string{"dashboardUID": req.UID, "error": "unknown"})
 		return
 	}
 
+	currentEtag := fmt.Sprintf("%d", res.Updated.Unix())
+	c.Resp.Header().Set("ETag", currentEtag)
+
+	previousEtag := c.Req.Header.Get("If-None-Match")
+	if previousEtag == currentEtag {
+		c.Resp.WriteHeader(http.StatusNotModified)
+		return
+	}
+
 	c.Resp.Header().Set("Content-Type", res.MimeType)
 	if _, err := c.Resp.Write(res.Image); err != nil {
 		hs.log.Error("Error writing to response", "dashboardUid", req.UID, "err", err)