add Cancel as an example of an injector

injectors.go

@@ -0,0 +1,21 @@
+package ntest
+
+import (
+	"context"
+)
+
+// This file contains example injectors that may be useful
+
+// Cancel is the injected type for the function type that will cancel
+// a Context that has been augmented with AutoCancel.
+type Cancel func()
+
+// AutoCancel adjusts context.Context so that it will be cancelled
+// when the test finishes. It can be cancelled early by calling
+// the returned Cancel function.
+func AutoCancel(ctx context.Context, t T) (context.Context, Cancel) {
+	ctx, cancel := context.WithCancel(ctx)
+	onlyOnce := onceFunc(cancel)
+	t.Cleanup(onlyOnce)
+	return ctx, onlyOnce
+}

injectors_test.go

@@ -0,0 +1,29 @@
+package ntest_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/memsql/ntest"
+)
+
+func TestCancel(t *testing.T) {
+	t.Parallel()
+	ntest.RunTest(t, context.Background, ntest.AutoCancel, func(
+		ctx context.Context,
+		cancel ntest.Cancel,
+	) {
+		select {
+		case <-ctx.Done():
+			t.Fatal("context is cancelled before cancel()")
+		default:
+		}
+		cancel()
+		select {
+		case <-ctx.Done():
+			t.Log("context is cancelled")
+		default:
+			t.Fatal("context is not cancelled after cancel()")
+		}
+	})
+}

once.go

@@ -0,0 +1,7 @@
+//go:build go1.21
+
+package ntest
+
+import "sync"
+
+var onceFunc = sync.OnceFunc

once_old.go

@@ -0,0 +1,12 @@
+//go:build !go1.21
+
+package ntest
+
+import "sync"
+
+func onceFunc(f func()) func() {
+	var once sync.Once
+	return func() {
+		once.Do(f)
+	}
+}