func testCallbackGC(t *testing.T) {
	nestedCall(runtime.GC)
}

func testCallbackPanic(t *testing.T) {
	// Make sure panic during callback unwinds properly.
	if lockedOSThread() {
		t.Fatal("locked OS thread on entry to TestCallbackPanic")
	}
	defer func() {
		s := recover()
		if s == nil {
			t.Fatal("did not panic")
		}
		if s.(string) != "callback panic" {
			t.Fatal("wrong panic:", s)
		}
		if lockedOSThread() {