helper/resource: Allow unknown pending states (#13099)

Sometimes when waiting on a target state, the set of valid states
through which a value will transition is unknown. This commit adds
support for an empty Pending slice and will treat any states that are not
the target as valid provided the timeouts are not exceeded.

helper/resource/state.go

@@ -141,7 +141,7 @@ func (conf *StateChangeConf) WaitForState() (interface{}, error) {
 					}
 				}
 
-				if !found {
+				if !found && len(conf.Pending) > 0 {
 					result.Error = &UnexpectedStateError{
 						LastError:     err,
 						State:         result.State,

helper/resource/state_test.go

@@ -70,6 +70,23 @@ func InconsistentStateRefreshFunc() StateRefreshFunc {
 	}
 }
 
+func UnknownPendingStateRefreshFunc() StateRefreshFunc {
+	sequence := []string{
+		"unknown1", "unknown2", "done",
+	}
+
+	r := NewStateGenerator(sequence)
+
+	return func() (interface{}, string, error) {
+		idx, s, err := r.NextState()
+		if err != nil {
+			return nil, "", err
+		}
+
+		return idx, s, nil
+	}
+}
+
 func TestWaitForState_inconsistent_positive(t *testing.T) {
 	conf := &StateChangeConf{
 		Pending:                   []string{"replicating"},
@@ -154,6 +171,22 @@ func TestWaitForState_success(t *testing.T) {
 	}
 }
 
+func TestWaitForState_successUnknownPending(t *testing.T) {
+	conf := &StateChangeConf{
+		Target:  []string{"done"},
+		Refresh: UnknownPendingStateRefreshFunc(),
+		Timeout: 200 * time.Second,
+	}
+
+	obj, err := conf.WaitForState()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if obj == nil {
+		t.Fatalf("should return obj")
+	}
+}
+
 func TestWaitForState_successEmpty(t *testing.T) {
 	conf := &StateChangeConf{
 		Pending: []string{"pending", "incomplete"},