Cache supports conditional requests

cache.go

@@ -14,41 +14,62 @@ func CacheMiddleware(next http.Handler) http.Handler {
 		requestUrl, _ := url.ParseRequestURI(r.RequestURI)
 		path := SlashTrimmedPath(r)
 		if appConfig.cache.enable &&
-			// Check bypass query
+			// check bypass query
 			!(requestUrl != nil && requestUrl.Query().Get("cache") == "0") {
 			cacheTime, header, body := getCache(path, r.Context())
 			if cacheTime == 0 {
 				recorder := httptest.NewRecorder()
 				next.ServeHTTP(recorder, r)
-				// Copy values from recorder
+				// copy values from recorder
 				code := recorder.Code
-				// Send response
+				// send response
 				for k, v := range recorder.Header() {
 					w.Header()[k] = v
 				}
+				now := time.Now()
+				setCacheHeaders(w, now.Format(time.RFC1123), time.Unix(now.Unix()+appConfig.cache.expiration, 0).Format(time.RFC1123))
 				w.Header().Set("GoBlog-Cache", "MISS")
 				w.WriteHeader(code)
 				_, _ = w.Write(recorder.Body.Bytes())
 				// Save cache
 				if code == http.StatusOK {
-					saveCache(path, recorder.Header(), recorder.Body.Bytes())
+					saveCache(path, now, recorder.Header(), recorder.Body.Bytes())
 				}
 				return
 			} else {
-				expiresTime := time.Unix(cacheTime+appConfig.cache.expiration, 0).Format(time.RFC1123)
+				cacheTimeString := time.Unix(cacheTime, 0).Format(time.RFC1123)
+				expiresTimeString := time.Unix(cacheTime+appConfig.cache.expiration, 0).Format(time.RFC1123)
+				// check conditional request
+				ifModifiedSinceHeader := r.Header.Get("If-Modified-Since")
+				if ifModifiedSinceHeader != "" && ifModifiedSinceHeader == cacheTimeString {
+					setCacheHeaders(w, cacheTimeString, expiresTimeString)
+					// send 304
+					w.WriteHeader(http.StatusNotModified)
+					return
+				}
+				// copy cached headers
 				for k, v := range header {
 					w.Header()[k] = v
 				}
-				w.Header().Set("Expires", expiresTime)
+				setCacheHeaders(w, cacheTimeString, expiresTimeString)
 				w.Header().Set("GoBlog-Cache", "HIT")
+				// write cached body
 				_, _ = w.Write(body)
+				return
 			}
 		} else {
 			next.ServeHTTP(w, r)
+			return
 		}
 	})
 }
 
+func setCacheHeaders(w http.ResponseWriter, cacheTimeString string, expiresTimeString string) {
+	w.Header().Set("Cache-Control", "public")
+	w.Header().Set("Last-Modified", cacheTimeString)
+	w.Header().Set("Expires", expiresTimeString)
+}
+
 func getCache(path string, context context.Context) (creationTime int64, header map[string][]string, body []byte) {
 	var headerBytes []byte
 	allowedTime := time.Now().Unix() - appConfig.cache.expiration
@@ -59,16 +80,15 @@ func getCache(path string, context context.Context) (creationTime int64, header
 	return
 }
 
-func saveCache(path string, header map[string][]string, body []byte) {
-	now := time.Now().Unix()
+func saveCache(path string, now time.Time, header map[string][]string, body []byte) {
 	headerBytes, _ := json.Marshal(header)
 	startWritingToDb()
 	tx, err := appDb.Begin()
 	if err != nil {
 		return
 	}
-	_, _ = tx.Exec("delete from cache where time<?;", now-appConfig.cache.expiration)
-	_, _ = tx.Exec("insert or replace into cache (path, time, header, body) values (?, ?, ?, ?);", path, now, headerBytes, body)
+	_, _ = tx.Exec("delete from cache where time<?;", now.Unix()-appConfig.cache.expiration)
+	_, _ = tx.Exec("insert or replace into cache (path, time, header, body) values (?, ?, ?, ?);", path, now.Unix(), headerBytes, body)
 	_ = tx.Commit()
 	finishWritingToDb()
 }