appview,knotserver: use ETag based caching for blobs

this avoids stale raw content from being sent to clients.

Signed-off-by: oppiliappan <[e-mail]>

appview/repo/repo.go

···
 	if !rp.config.Core.Dev {
 		protocol = "https"
 	}
+
 	blobURL := fmt.Sprintf("%s://%s/%s/%s/raw/%s/%s", protocol, f.Knot, f.OwnerDid(), f.Repo.Name, ref, filePath)
-	resp, err := http.Get(blobURL)
+
+	req, err := http.NewRequest("GET", blobURL, nil)
 	if err != nil {
-		log.Println("failed to reach knotserver:", err)
+		log.Println("failed to create request", err)
+		return
+	}
+
+	// forward the If-None-Match header
+	if clientETag := r.Header.Get("If-None-Match"); clientETag != "" {
+		req.Header.Set("If-None-Match", clientETag)
+	}
+
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	if err != nil {
+		log.Println("failed to reach knotserver", err)
 		rp.pages.Error503(w)
 		return
 	}
 	defer resp.Body.Close()
+
+	// forward 304 not modified
+	if resp.StatusCode == http.StatusNotModified {
+		w.WriteHeader(http.StatusNotModified)
+		return
+	}
 
 	if resp.StatusCode != http.StatusOK {
 		log.Printf("knotserver returned non-OK status for raw blob %s: %d", blobURL, resp.StatusCode)

knotserver/routes.go

···
 		mimeType = "image/svg+xml"
 	}
 
+	contentHash := sha256.Sum256(contents)
+	eTag := fmt.Sprintf("\"%x\"", contentHash)
+
 	// allow image, video, and text/plain files to be served directly
 	switch {
-	case strings.HasPrefix(mimeType, "image/"):
-		// allowed
-	case strings.HasPrefix(mimeType, "video/"):
-		// allowed
+	case strings.HasPrefix(mimeType, "image/"), strings.HasPrefix(mimeType, "video/"):
+		if clientETag := r.Header.Get("If-None-Match"); clientETag == eTag {
+			w.WriteHeader(http.StatusNotModified)
+			return
+		}
+		w.Header().Set("ETag", eTag)
+
 	case strings.HasPrefix(mimeType, "text/plain"):
-		// allowed
+		w.Header().Set("Cache-Control", "public, no-cache")
+
 	default:
 		l.Error("attempted to serve disallowed file type", "mimetype", mimeType)
 		writeError(w, "only image, video, and text files can be accessed directly", http.StatusForbidden)
 		return
 	}
 
-	w.Header().Set("Cache-Control", "public, max-age=86400") // cache for 24 hours
-	w.Header().Set("ETag", fmt.Sprintf("%x", sha256.Sum256(contents)))
 	w.Header().Set("Content-Type", mimeType)
 	w.Write(contents)
 }