admin: Make `Etag` a header, not a trailer (#6208)

* Making eTags a header not a trailer

* Checked the write

* Fixed typo

* Corrected comment

* Added sync Pool

* Changed control flow of buffer reset / putting and changed error code

* Switched from interface{} to any in bufferPool

admin.go

@@ -954,17 +954,28 @@ func makeEtag(path string, hash hash.Hash) string {
 	return fmt.Sprintf(`"%s %x"`, path, hash.Sum(nil))
 }
 
+// This buffer pool is used to keep buffers for
+// reading the config file during eTag header generation
+var bufferPool = sync.Pool{
+	New: func() any {
+		return new(bytes.Buffer)
+	},
+}
+
 func handleConfig(w http.ResponseWriter, r *http.Request) error {
 	switch r.Method {
 	case http.MethodGet:
 		w.Header().Set("Content-Type", "application/json")
-		// Set the ETag as a trailer header.
-		// The alternative is to write the config to a buffer, and
-		// then hash that.
-		w.Header().Set("Trailer", "ETag")
-
 		hash := etagHasher()
-		configWriter := io.MultiWriter(w, hash)
+
+		// Read the config into a buffer instead of writing directly to
+		// the response writer, as we want to set the ETag as the header,
+		// not the trailer.
+		buf := bufferPool.Get().(*bytes.Buffer)
+		buf.Reset()
+		defer bufferPool.Put(buf)
+
+		configWriter := io.MultiWriter(buf, hash)
 		err := readConfig(r.URL.Path, configWriter)
 		if err != nil {
 			return APIError{HTTPStatus: http.StatusBadRequest, Err: err}
@@ -973,6 +984,10 @@ func handleConfig(w http.ResponseWriter, r *http.Request) error {
 		// we could consider setting up a sync.Pool for the summed
 		// hashes to reduce GC pressure.
 		w.Header().Set("Etag", makeEtag(r.URL.Path, hash))
+		_, err = w.Write(buf.Bytes())
+		if err != nil {
+			return APIError{HTTPStatus: http.StatusInternalServerError, Err: err}
+		}
 
 		return nil