Auto refresh todo editor.

todo.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -12,11 +14,23 @@ import (
 // widget treats as "render nothing"); POST replaces the whole file atomically.
 // Unlike the index.md save path, empty content does NOT delete the file: the
 // widget has no in-UI way to recreate it, so an empty save keeps an empty file.
+//
+// Both GET and POST carry an ETag (a content hash) so the widget can poll for
+// external edits: it revalidates GET with If-None-Match and reloads only when
+// another device actually changed the bytes. The POST response echoes the new
+// ETag so a device's own save comes back 304 rather than reloading itself.
 
 func (h *handler) todoPath() string {
 	return filepath.Join(h.root, "todo.txt")
 }
 
+// todoETag is a strong validator derived from the file content — not mtime, so
+// a save of identical bytes (or a touch) does not look like a change.
+func todoETag(data []byte) string {
+	sum := sha256.Sum256(data)
+	return `"` + hex.EncodeToString(sum[:])[:16] + `"`
+}
+
 func (h *handler) handleTodo(w http.ResponseWriter, r *http.Request) {
 	if !h.checkAuth(w, r) {
 		return
@@ -41,9 +55,16 @@ func (h *handler) handleTodoGet(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "read failed: "+err.Error(), http.StatusInternalServerError)
 		return
 	}
-	// The rail re-fetches on every page load; a heuristically cached copy
-	// would show stale todos after an edit.
+	// The rail re-fetches on every page load and polls for external edits; a
+	// heuristically cached copy would show stale todos. Revalidation is driven
+	// by the ETag (If-None-Match) rather than the browser's own HTTP cache.
+	etag := todoETag(data)
+	w.Header().Set("ETag", etag)
 	w.Header().Set("Cache-Control", "no-store")
+	if r.Header.Get("If-None-Match") == etag {
+		w.WriteHeader(http.StatusNotModified)
+		return
+	}
 	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
 	w.Write(data)
 }
@@ -58,5 +79,8 @@ func (h *handler) handleTodoPost(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "write failed: "+err.Error(), http.StatusInternalServerError)
 		return
 	}
+	// Echo the new ETag so the saving client can adopt it and skip reloading
+	// its own write on the next poll.
+	w.Header().Set("ETag", todoETag([]byte(content)))
 	w.WriteHeader(http.StatusNoContent)
 }