OP#792 initial commit after fork

internal/once/singleton.go

@@ -0,0 +1,46 @@
+package once
+
+// Singleton contains a pointer to T that must be set once.
+// Until the value is set all Get() calls will block.
+type Singleton[T any] struct {
+	v   *T
+	set chan struct{}
+}
+
+// NewSingleton creates a new unset singleton.
+func NewSingleton[T any]() *Singleton[T] {
+	return &Singleton[T]{set: make(chan struct{}), v: nil}
+}
+
+// Get will return the singleton value.
+func (s *Singleton[T]) Get() *T {
+	<-s.set
+	return s.v
+}
+
+// GetNonBlocking will return the singleton value or nil if not set yet.
+func (s *Singleton[T]) GetNonBlocking() *T {
+	select {
+	case <-s.set:
+		return s.v
+	default:
+		return nil
+	}
+}
+
+// IsSet will return whether the singleton has been set.
+func (s *Singleton[T]) IsSet() bool {
+	select {
+	case <-s.set:
+		return true
+	default:
+		return false
+	}
+}
+
+// Set the value and unblock all Get requests.
+// This may only be called once, a second call will panic.
+func (s *Singleton[T]) Set(v *T) {
+	s.v = v
+	close(s.set)
+}