From 54cda5546abd693b30d1978189fcca347a6ae056 Mon Sep 17 00:00:00 2001
From: Evsyukov Denis <denis@evsyukov.org>
Date: Thu, 11 Apr 2024 15:12:37 +0300
Subject: [PATCH] feat: add ETag header to static assets

we are adding an ETag tag based on calculating the md5 hash of the file contents
---
 misc/handlers.go | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/misc/handlers.go b/misc/handlers.go
index d097763..3e4d98d 100644
--- a/misc/handlers.go
+++ b/misc/handlers.go
@@ -2,12 +2,15 @@
 package misc
 
 import (
+	"crypto/md5"
+	"encoding/hex"
 	"io"
 	"log"
 	"math/rand"
 	"mime"
 	"net/http"
 	"path/filepath"
+	"strings"
 
 	"github.com/gorilla/mux"
 
@@ -141,29 +144,42 @@ var stylesheets = []string{"default.css", "custom.css"}
 
 func handlerStyle(w http.ResponseWriter, rq *http.Request) {
 	w.Header().Set("Content-Type", mime.TypeByExtension(".css"))
+	hash := md5.New()
+	var buf strings.Builder
 	for _, name := range stylesheets {
 		file, err := static.FS.Open(name)
 		if err != nil {
 			continue
 		}
-		_, err = io.Copy(w, file)
+		_, err = io.Copy(io.MultiWriter(&buf, hash), file)
 		if err != nil {
 			log.Println(err)
 		}
 		_ = file.Close()
 	}
+	w.Header().Set("ETag", hex.EncodeToString(hash.Sum(nil)))
+
+	_, err := io.Copy(w, strings.NewReader(buf.String()))
+	if err != nil {
+		log.Println(err)
+	}
 }
 
 func handlerRobotsTxt(w http.ResponseWriter, rq *http.Request) {
 	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
-
+	hash := md5.New()
 	file, err := static.FS.Open("robots.txt")
 	if err != nil {
 		return
 	}
+	_, err = io.Copy(hash, file)
+	if err != nil {
+		log.Println(err)
+	}
+	w.Header().Set("ETag", hex.EncodeToString(hash.Sum(nil)))
 	_, err = io.Copy(w, file)
 	if err != nil {
-		log.Println()
+		log.Println(err)
 	}
 	_ = file.Close()
 }