Update code for internal system

internal/util/util.go

@@ -1,5 +1,6 @@
 package util
 
+import "io"
 import "image/color"
 
 // IndexOf returns the index of needle within haystack. If needle does not exist
@@ -99,3 +100,32 @@ func (this *Memo[T]) InvalidateTo (value T) {
 	this.Invalidate()
 	this.cache = value
 }
+
+// Cycler stores a value and an accompanying io.Closer. When the value is set,
+// the closer associated with the previous value is closed.
+type Cycler[T any] struct {
+	value  T
+	closer io.Closer
+}
+
+// Value returns the cycler's value.
+func (this *Cycler[T]) Value () T {
+	return this.value
+}
+
+// Set sets the value and associated closer, closing the previous one.
+func (this *Cycler[T]) Set (value T, closer io.Closer) (err error) {
+	if this.closer != nil {
+		err = this.closer.Close()
+	}
+	this.value  = value
+	this.closer = closer
+	return err
+}
+
+// Close closes the associated closer early.
+func (this *Cycler[T]) Close () error {
+	err := this.closer.Close()
+	this.closer = nil
+	return err
+}