libct/cg/sd: fix dbus error handling

This fixes isDbusError function, introduced by commit bacfc2c. Due to a
type error it was not working at all.

This also fixes the whole "retry on dbus disconnect" logic.

This also fixes a regression in startUnit (and cgroupManager.Apply()),
which should never return "unit already exists" error but it did.

Fixes: bacfc2c
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/common.go

@@ -313,7 +313,7 @@ func getUnitName(c *configs.Cgroup) string {
 // isDbusError returns true if the error is a specific dbus error.
 func isDbusError(err error, name string) bool {
 	if err != nil {
-		var derr *dbus.Error
+		var derr dbus.Error
 		if errors.As(err, &derr) {
 			return strings.Contains(derr.Name, name)
 		}

libcontainer/cgroups/systemd/systemd_test.go

@@ -168,3 +168,28 @@ func TestSkipDevicesFalse(t *testing.T) {
 		"cat: /dev/null: Operation not permitted",
 	})
 }
+
+func TestUnitExistsIgnored(t *testing.T) {
+	if !IsRunningSystemd() {
+		t.Skip("Test requires systemd.")
+	}
+	if os.Geteuid() != 0 {
+		t.Skip("Test requires root.")
+	}
+
+	podConfig := &configs.Cgroup{
+		Parent:    "system.slice",
+		Name:      "system-runc_test_exists.slice",
+		Resources: &configs.Resources{},
+	}
+	// Create "pods" cgroup (a systemd slice to hold containers).
+	pm := newManager(podConfig)
+	defer pm.Destroy() //nolint:errcheck
+
+	// create twice to make sure "UnitExists" error is ignored.
+	for i := 0; i < 2; i++ {
+		if err := pm.Apply(-1); err != nil {
+			t.Fatal(err)
+		}
+	}
+}