Minimize API requests by skipping to check a given image ID

Save on API requests and stop querying for the image ID from the DO API
if the numeric ID is already given: a failure to reference the image
would show up during the droplet create request and lead to the same
backoff that we run into today.

Also refactor the code and improve error messages slightly as a drive-by
change.

Author: timoreimann

cloud/services/computes/droplets.go

@@ -66,7 +66,7 @@ func (s *Service) CreateDroplet(scope *scope.MachineScope) (*godo.Droplet, error
 	clusterName := infrav1.DOSafeName(s.scope.Name())
 	instanceName := infrav1.DOSafeName(scope.Name())
 
-	image, err := s.GetImage(scope.DOMachine.Spec.Image)
+	imageID, err := s.GetImageID(scope.DOMachine.Spec.Image)
 	if err != nil {
 		return nil, errors.Wrap(err, "failed getting image")
 	}
@@ -102,7 +102,7 @@ func (s *Service) CreateDroplet(scope *scope.MachineScope) (*godo.Droplet, error
 		Size:    scope.DOMachine.Spec.Size,
 		SSHKeys: sshkeys,
 		Image: godo.DropletCreateImage{
-			ID: image.ID,
+			ID: imageID,
 		},
 		UserData:          bootstrapData,
 		PrivateNetworking: true,

cloud/services/computes/images.go

@@ -17,27 +17,30 @@ limitations under the License.
 package computes
 
 import (
+	"fmt"
+
 	"github.com/digitalocean/godo"
 	"github.com/pkg/errors"
 
 	"k8s.io/apimachinery/pkg/util/intstr"
 )
 
-func (s *Service) GetImage(imageSpec intstr.IntOrString) (*godo.Image, error) {
+func (s *Service) GetImageID(imageSpec intstr.IntOrString) (int, error) {
 	var image *godo.Image
-	var reterr error
 
 	if imageSpec.IntValue() != 0 { // nolint
-		image, _, reterr = s.scope.Images.GetByID(s.ctx, imageSpec.IntValue())
-	} else if imageSpec.String() != "" && imageSpec.String() != "0" {
-		image, _, reterr = s.scope.Images.GetBySlug(s.ctx, imageSpec.String())
-	} else {
-		reterr = errors.New("Unable to get image")
+		return imageSpec.IntValue(), nil
+	}
+
+	imageSpecStr := imageSpec.String()
+	if imageSpecStr == "" || imageSpecStr == "0" {
+		return 0, fmt.Errorf("invalid image spec string %q", imageSpecStr)
 	}
 
-	if reterr != nil {
-		return nil, errors.Wrap(reterr, "Unable to get image")
+	image, _, err := s.scope.Images.GetBySlug(s.ctx, imageSpecStr)
+	if err != nil {
+		return 0, errors.Wrap(err, "Unable to get image")
 	}
 
-	return image, nil
+	return image.ID, nil
 }