Uninstall() should not fail if a cgroup directory doesn't exist

It can be occurred if two controllers are mounted together or if Uninstall() is called on a error path.

PiperOrigin-RevId: 218723886
Change-Id: I69d7a3c0685a7da38527ea8b7b301dbe96268285

runsc/cgroup/cgroup.go

@@ -229,7 +229,11 @@ func (c *Cgroup) Uninstall() error {
 		defer cancel()
 		b := backoff.WithContext(backoff.NewConstantBackOff(100*time.Millisecond), ctx)
 		if err := backoff.Retry(func() error {
-			return syscall.Rmdir(path)
+			err := syscall.Rmdir(path)
+			if os.IsNotExist(err) {
+				return nil
+			}
+			return err
 		}, b); err != nil {
 			return fmt.Errorf("error removing cgroup path %q: %v", path, err)
 		}

runsc/cgroup/cgroup_test.go

@@ -18,6 +18,17 @@ import (
 	"testing"
 )
 
+func TestUninstallEnoent(t *testing.T) {
+	c := Cgroup{
+		// set a non-existent name
+		Name: "runsc-test-uninstall-656e6f656e740a",
+		Own:  true,
+	}
+	if err := c.Uninstall(); err != nil {
+		t.Errorf("Uninstall() failed: %v", err)
+	}
+}
+
 func TestCountCpuset(t *testing.T) {
 	for _, tc := range []struct {
 		str   string