sync: Don't panic on double close of Gate, return error instead

sync/gate.go

@@ -2,6 +2,16 @@ package usync
 
 import "sync"
 
+// Error defines errors that this package can produce
+type Error string; const (
+	ErrAlreadyClosed Error = "AlreadyClosed"
+)
+
+// Error fullfills the error interface.
+func (err Error) Error () string {
+	return string(err)
+}
+
 // Gate wraps a channel and allows the receiver to abruptly stop receiving
 // messages without causing the sender to lock up.
 type Gate[T any] struct {
@@ -47,6 +57,7 @@ func (this *Gate[T]) Receive () <- chan T {
 // Close closes the gate, drains all remaining messages, and closes the channel.
 func (this *Gate[T]) Close () error {
 	this.lock.Lock()
+	if !this.open { return ErrAlreadyClosed }
 	this.open = false
 	this.lock.Unlock()
 	for len(this.channel) > 0 { <- this.channel }