handle ETag + If-None-Match

Author: alancolant

headers.go

@@ -0,0 +1,51 @@
+package CADDY_FILE_SERVER
+
+import (
+	"bytes"
+	"fmt"
+	"github.com/cespare/xxhash/v2"
+	"net/url"
+	"regexp"
+	"sort"
+)
+
+func getProcessedImageEtag(initialEtag string, form *url.Values) string {
+	// Return early if the initial ETag is empty
+	if initialEtag == "" {
+		return ""
+	}
+
+	re := regexp.MustCompile(`^(W/"|")(.*?)(")$`)
+	matches := re.FindStringSubmatch(initialEtag)
+	if len(matches) != 4 {
+		return initialEtag
+	}
+
+	// Create a slice to store the sorted parameters
+	var params []string
+	for key, values := range *form {
+		// Add only the first value for each key (to avoid duplicates)
+		params = append(params, key+"="+values[0])
+	}
+
+	// Sort the parameters to ensure consistent order
+	sort.Strings(params)
+
+	// Use a bytes.Buffer to join parameters efficiently
+	var buffer bytes.Buffer
+	for _, param := range params {
+		if buffer.Len() > 0 {
+			buffer.WriteString("&")
+		}
+		buffer.WriteString(param)
+	}
+
+	// Generate the SHA1 hash of the concatenated parameters
+	hash := xxhash.New()
+	_, err := hash.Write(buffer.Bytes())
+	if err != nil {
+		return ""
+	}
+	hashString := fmt.Sprintf("%x", hash.Sum(nil))
+	return matches[1] + matches[2] + "-" + hashString + matches[3]
+}

module.go

@@ -138,6 +138,20 @@ func (m *Middleware) ServeHTTP(w http.ResponseWriter, r *http.Request, next cadd
 		}
 	}
 
+	// Generate specific ETag ig necessary
+	processedEtag := getProcessedImageEtag(responseRecorder.Header().Get("ETag"), &r.Form)
+	if processedEtag != "" {
+		responseRecorder.Header().Del("ETag") // Remove initial ETag
+		w.Header().Set("ETag", processedEtag)
+
+		// Check If-None-Match header to avoid reprocessing
+		ifNoneMatchHeader := r.Header.Get("If-None-Match")
+		if ifNoneMatchHeader != "" && ifNoneMatchHeader == processedEtag {
+			w.WriteHeader(http.StatusNotModified)
+			return nil
+		}
+	}
+
 	// Parse options
 	options, err := getOptions(&r.Form)
 	if err != nil {
@@ -164,7 +178,7 @@ func (m *Middleware) ServeHTTP(w http.ResponseWriter, r *http.Request, next cadd
 	w.Header().Del("Content-Length")
 	w.Header().Del("Content-Encoding")
 	w.Header().Del("Vary")
-	w.Header().Del("ETag")
+	//w.Header().Del("ETag")
 
 	// Set new headers
 	w.Header().Set("Content-Length", strconv.Itoa(len(newImage)))