From 315770a1c03ef165424f55ede6456f7d5879b172 Mon Sep 17 00:00:00 2001
From: Barak Amar <barak.amar@treeverse.io>
Date: Sat, 9 Dec 2023 16:38:50 +0200
Subject: [PATCH] UI Etag support for embedded content (#7132)

* 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.

* apply code review changes
---
 pkg/api/etag_middleware.go | 78 ++++++++++++++++++++++++++++++++++++++
 pkg/api/ui_handler.go      |  7 ++--
 2 files changed, 81 insertions(+), 4 deletions(-)
 create mode 100644 pkg/api/etag_middleware.go

diff --git a/pkg/api/etag_middleware.go b/pkg/api/etag_middleware.go
new file mode 100644
index 00000000000..32d54895782
--- /dev/null
+++ b/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
+}
diff --git a/pkg/api/ui_handler.go b/pkg/api/ui_handler.go
index 899a7a18867..8db6a18aafd 100644
--- a/pkg/api/ui_handler.go
+++ b/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)
 	})
 }