UI etag embedded content

Scan embedded content on startup and serve etag header to FileServer.
FileServer will use the Etag and provide no content change if needed.

This will allow cache all static resources, which allow better
performance after the first load, in the web ui.

pkg/api/etag_middleware.go

@@ -0,0 +1,77 @@
+package api
+
+import (
+	"crypto/md5"
+	"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()
+		if _, err := io.Copy(h, f); err != nil {
+			return err
+		}
+		etags["/"+fpath] = hex.EncodeToString(h.Sum(nil))
+		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"
@@ -34,7 +33,7 @@ func NewUIHandler(gatewayDomains []string, snippets []params.CodeSnippet) http.H
 		panic(err)
 	}
 	fileSystem := http.FS(injectedContent)
-	nocacheContent := middleware.NoCache(http.StripPrefix("/", http.FileServer(fileSystem)))
+	nocacheContent := EtagMiddleware(injectedContent, http.StripPrefix("/", http.FileServer(fileSystem)))
 	return NewHandlerWithDefault(fileSystem, nocacheContent, gatewayDomains)
 }
 
@@ -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)
 	})
 }