streamdata: edit metadata

This edits metadata in a robust way.

Signed-off-by: Yonah <contrib@gensokyo.uk>

streamdata.go

@@ -224,6 +224,46 @@ func (c *Channel) Add(ident *Ident, f func(v *VOD, w io.Writer) error) error {
 	return nil
 }
 
+// Edit loads a [VOD] by its [Ident], and writes the modified [VOD] back to the
+// on-disk representation.
+func (c *Channel) Edit(ident *Ident, f func(v *VOD) error) error {
+	v, err := c.Load(ident)
+	if err != nil {
+		return err
+	}
+
+	err = f(v)
+	if err != nil {
+		return err
+	}
+
+	pathname := path.Join(
+		channelPathVOD,
+		"."+ident.String(),
+	)
+
+	var w *os.File
+	if w, err = c.root.OpenFile(
+		pathname,
+		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
+		0444,
+	); err != nil {
+		return err
+	} else if err = json.NewEncoder(w).Encode(v); err != nil {
+		_ = w.Close()
+		_ = c.root.Remove(pathname)
+		return err
+	} else if err = w.Close(); err != nil {
+		_ = c.root.Remove(pathname)
+		return err
+	} else {
+		return c.root.Rename(pathname, path.Join(
+			channelPathVOD,
+			ident.String(),
+		))
+	}
+}
+
 // Path returns a pathname by [Ident].
 func (c *Channel) Path(ident *Ident) string {
 	return path.Join(