Set Last-Modified header for several HTML pages

database/message.go

@@ -23,6 +23,10 @@ func (msg *Message) CreatedAtRFC3339() string {
 	return sqlTimeToGoTime(msg.CreatedAt).Format(time.RFC3339)
 }
 
+func (msg *Message) GetUpdatedAt() time.Time {
+	return sqlTimeToGoTime(msg.UpdatedAt)
+}
+
 func sqlTimeToGoTime(sqlTime string) time.Time {
 	t, err := time.Parse("2006-01-02T15:04:05Z", sqlTime)
 	if err != nil {

frontend.go

@@ -103,6 +103,7 @@ func (h *Handler) LatestMessagesByRoom(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	setLastModifiedAt(w, messages)
 	data := &template.ListTemplateData{
 		Messages:    messages,
 		Rooms:       h.AllRooms(),
@@ -132,6 +133,7 @@ func (h *Handler) LatestMessagesByAuthor(w http.ResponseWriter, r *http.Request)
 		return
 	}
 
+	setLastModifiedAt(w, messages)
 	data := &template.ListTemplateData{
 		Messages:      messages,
 		Rooms:         h.AllRooms(),
@@ -189,6 +191,7 @@ func (h *Handler) MessageContext(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	setLastModifiedAt(w, subsequentMessages)
 	data := &template.ShowTemplateData{
 		PriorMessages:      priorMessages,
 		Message:            *message,

handler.go

@@ -145,3 +145,15 @@ func (h *Handler) FetchAndCacheGet(r *http.Request, key string, f messagesGetFun
 
 	return message, err
 }
+
+// Set Last-Modified header
+// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified
+func setLastModifiedAt(w http.ResponseWriter, messages []database.Message) {
+	latestTimestamp := time.Unix(0, 0)
+	for _, message := range messages {
+		if updatedAt := message.GetUpdatedAt(); updatedAt.After(latestTimestamp) {
+			latestTimestamp = updatedAt
+		}
+	}
+	w.Header().Set("Last-Modified", latestTimestamp.Format(time.RFC1123))
+}