UI Etag support for embedded content

pkg/api/etag_middleware.go

@@ -0,0 +1,78 @@
+package api
+
+import (
+	"crypto/md5" //nolint:gosec
+	"encoding/hex"
+	"io"
+	"io/fs"
+	"net/http"
+	"path"
+	"strings"
+)
+
+// EtagMiddleware returns a new Etag middleware handler.
+// It designs to work on embedded FS, where the content doesn't change.
+// It calculates the Etag for each file on startup and serves it on each request.
+func EtagMiddleware(root fs.FS, next http.Handler) http.Handler {
+	etags, err := scanFSEtags(root)
+	if err != nil {
+		panic(err)
+	}
+	return &etagHandler{
+		root:  root,
+		next:  next,
+		etags: etags,
+	}
+}
+
+type etagHandler struct {
+	root  fs.FS
+	next  http.Handler
+	etags map[string]string
+}
+
+func (e *etagHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	upath := r.URL.Path
+	if !strings.HasPrefix(upath, "/") {
+		upath = "/" + upath
+	}
+	upath = path.Clean(upath)
+	if strings.HasSuffix(upath, "/") {
+		upath += "index.html"
+	}
+	etag, ok := e.etags[upath]
+	if ok {
+		w.Header().Set("Etag", "\""+etag+"\"")
+	}
+	e.next.ServeHTTP(w, r)
+}
+
+func scanFSEtags(fSys fs.FS) (map[string]string, error) {
+	etags := make(map[string]string)
+	err := fs.WalkDir(fSys, ".", func(fpath string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			return nil
+		}
+
+		f, err := fSys.Open(fpath)
+		if err != nil {
+			return err
+		}
+		defer func() { _ = f.Close() }()
+
+		h := md5.New() //nolint:gosec
+		if _, err := io.Copy(h, f); err != nil {
+			return err
+		}
+		hashValue := hex.EncodeToString(h.Sum(nil))
+		etags["/"+fpath] = hashValue
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return etags, nil
+}

pkg/api/ui_handler.go

@@ -9,7 +9,6 @@ import (
 	"strings"
 
 	gomime "github.com/cubewise-code/go-mime"
-	"github.com/go-chi/chi/v5/middleware"
 	"github.com/treeverse/lakefs/pkg/api/params"
 	gwerrors "github.com/treeverse/lakefs/pkg/gateway/errors"
 	"github.com/treeverse/lakefs/pkg/gateway/operations"
@@ -34,8 +33,8 @@ func NewUIHandler(gatewayDomains []string, snippets []params.CodeSnippet) http.H
 		panic(err)
 	}
 	fileSystem := http.FS(injectedContent)
-	nocacheContent := middleware.NoCache(http.StripPrefix("/", http.FileServer(fileSystem)))
-	return NewHandlerWithDefault(fileSystem, nocacheContent, gatewayDomains)
+	etagHandler := EtagMiddleware(injectedContent, http.StripPrefix("/", http.FileServer(fileSystem)))
+	return NewHandlerWithDefault(fileSystem, etagHandler, gatewayDomains)
 }
 
 func NewS3GatewayEndpointErrorHandler(gatewayDomains []string) http.Handler {
@@ -45,7 +44,7 @@ func NewS3GatewayEndpointErrorHandler(gatewayDomains []string) http.Handler {
 			return
 		}
 
-		// For other requests, return generic not found error
+		// For other requests, return generic "not found" error
 		w.WriteHeader(http.StatusNotFound)
 	})
 }