sync: Test idempotency of Gate.Close

sync/gate_test.go

@@ -1,6 +1,7 @@
 package usync
 
 import "time"
+import "errors"
 import "testing"
 
 func TestGate(test *testing.T) {
@@ -50,3 +51,13 @@ func TestGateCloseEarly(test *testing.T) {
 		test.Fatal("timed out waiting for sender goroutine to stop")
 	}
 }
+
+func TestGateCloseIdempotent(test *testing.T) {
+	gate := NewGate[int]()
+	err := gate.Close()
+	if err != nil { test.Fatal(err) }
+	err = gate.Close()
+	if !errors.Is(err, ErrAlreadyClosed) {
+		test.Fatal("wrong error: ", err)
+	}
+}