fix(action): unexpected behaviour when watching non-existing Actions

When passed an action with an unknown ID, the
`ActionClient.WatchProgress()` and `ActionClient.WatchOverallProgress()`
methods experience unexpected behaviour:

- `WatchProgress()` would panic with a nil pointer dereference when
  accessing `a.Status`
- `WatchOverallProgress()` would go into an infinite loop, never
  finishing or closing the channels.

hcloud/action.go

@@ -216,6 +216,13 @@ func (c *ActionClient) WatchOverallProgress(ctx context.Context, actions []*Acti
 				errCh <- err
 				return
 			}
+			if len(as) == 0 {
+				// No actions returned for the provided IDs, they do not exist in the API.
+				// We need to catch and fail early for this, otherwise the loop will continue
+				// indefinitely.
+				errCh <- fmt.Errorf("failed to wait for actions: remaining actions (%v) are not returned from API", opts.ID)
+				return
+			}
 
 			for _, a := range as {
 				switch a.Status {
@@ -282,6 +289,10 @@ func (c *ActionClient) WatchProgress(ctx context.Context, action *Action) (<-cha
 				errCh <- err
 				return
 			}
+			if a == nil {
+				errCh <- fmt.Errorf("failed to wait for action %d: action not returned from API", action.ID)
+				return
+			}
 
 			switch a.Status {
 			case ActionStatusRunning:

hcloud/action_test.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"net/http"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -286,6 +287,85 @@ func TestActionClientWatchOverallProgress(t *testing.T) {
 	}
 }
 
+func TestActionClientWatchOverallProgressInvalidID(t *testing.T) {
+	env := newTestEnv()
+	defer env.Teardown()
+
+	callCount := 0
+
+	env.Mux.HandleFunc("/actions", func(w http.ResponseWriter, r *http.Request) {
+		callCount++
+		var actions []schema.Action
+
+		switch callCount {
+		case 1:
+		default:
+			t.Errorf("unexpected number of calls to the test server: %v", callCount)
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		_ = json.NewEncoder(w).Encode(struct {
+			Actions []schema.Action `json:"actions"`
+			Meta    schema.Meta     `json:"meta"`
+		}{
+			Actions: actions,
+			Meta: schema.Meta{
+				Pagination: &schema.MetaPagination{
+					Page:         1,
+					LastPage:     1,
+					PerPage:      len(actions),
+					TotalEntries: len(actions),
+				},
+			},
+		})
+	})
+
+	actions := []*Action{
+		{
+			ID:     1,
+			Status: ActionStatusRunning,
+		},
+	}
+
+	ctx := context.Background()
+	progressCh, errCh := env.Client.Action.WatchOverallProgress(ctx, actions)
+	progressUpdates := []int{}
+	errs := []error{}
+
+	moreProgress, moreErrors := true, true
+
+	for moreProgress || moreErrors {
+		var progress int
+		var err error
+
+		select {
+		case progress, moreProgress = <-progressCh:
+			if moreProgress {
+				progressUpdates = append(progressUpdates, progress)
+			}
+		case err, moreErrors = <-errCh:
+			if moreErrors {
+				errs = append(errs, err)
+			}
+		}
+	}
+
+	if len(errs) != 1 {
+		t.Fatalf("expected to receive one error: %v", errs)
+	}
+
+	err := errs[0]
+
+	if !strings.HasPrefix(err.Error(), "failed to wait for actions") {
+		t.Fatalf("expected failed to wait for actions error, but got: %#v", err)
+	}
+
+	expectedProgressUpdates := []int{}
+	if !reflect.DeepEqual(progressUpdates, expectedProgressUpdates) {
+		t.Fatalf("expected progresses %v but received %v", expectedProgressUpdates, progressUpdates)
+	}
+}
+
 func TestActionClientWatchProgress(t *testing.T) {
 	env := newTestEnv()
 	defer env.Teardown()
@@ -385,3 +465,55 @@ func TestActionClientWatchProgressError(t *testing.T) {
 		t.Fatal("expected an error")
 	}
 }
+
+func TestActionClientWatchProgressInvalidID(t *testing.T) {
+	env := newTestEnv()
+	defer env.Teardown()
+
+	callCount := 0
+
+	env.Mux.HandleFunc("/actions/1", func(w http.ResponseWriter, r *http.Request) {
+		callCount++
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusNotFound)
+		switch callCount {
+		case 1:
+			_ = json.NewEncoder(w).Encode(schema.ErrorResponse{
+				Error: schema.Error{
+					Code:    string(ErrorCodeNotFound),
+					Message: "action with ID '1' not found",
+					Details: nil,
+				},
+			})
+		default:
+			t.Errorf("unexpected number of calls to the test server: %v", callCount)
+		}
+	})
+	action := &Action{
+		ID: 1,
+	}
+
+	ctx := context.Background()
+	progressCh, errCh := env.Client.Action.WatchProgress(ctx, action)
+	var (
+		progressUpdates []int
+		err             error
+	)
+
+loop:
+	for {
+		select {
+		case progress := <-progressCh:
+			progressUpdates = append(progressUpdates, progress)
+		case err = <-errCh:
+			break loop
+		}
+	}
+
+	if !strings.HasPrefix(err.Error(), "failed to wait for action") {
+		t.Fatalf("expected failed to wait for action error, but got: %#v", err)
+	}
+	if len(progressUpdates) != 0 {
+		t.Fatalf("unexpected progress updates: %v", progressUpdates)
+	}
+}