Add the ability to spin up Hetzner servers from custom snapshots

This comes in handy when using tools such as Packer to generate customized
images.

cluster-autoscaler/cloudprovider/hetzner/README.md

@@ -5,11 +5,17 @@ The cluster autoscaler for Hetzner Cloud scales worker nodes.
 # Configuration
 
 `HCLOUD_TOKEN` Required Hetzner Cloud token.
+
 `HCLOUD_CLOUD_INIT` Base64 encoded Cloud Init yaml with commands to join the cluster, Sample [examples/cloud-init.txt for (Kubernetes 1.20.1)](examples/cloud-init.txt)
-`HCLOUD_IMAGE` Defaults to `ubuntu-20.04`, @see https://docs.hetzner.cloud/#images
+
+`HCLOUD_IMAGE` Defaults to `ubuntu-20.04`, @see https://docs.hetzner.cloud/#images. You can also use an image ID here (e.g. `15512617`), or a label selector associated with a custom snapshot (e.g. `customized_ubuntu=true`).
+
 `HCLOUD_NETWORK` Default empty , The name of the network that is used in the cluster , @see https://docs.hetzner.cloud/#networks
+
 `HCLOUD_SSH_KEY` Default empty , This SSH Key will have access to the fresh created server, @see https://docs.hetzner.cloud/#ssh-keys
+
 Node groups must be defined with the `--nodes=<min-servers>:<max-servers>:<instance-type>:<region>:<name>` flag.
+
 Multiple flags will create multiple node pools. For example:
 ```
 --nodes=1:10:CPX51:FSN1:pool1

cluster-autoscaler/cloudprovider/hetzner/hetzner_manager.go

@@ -39,7 +39,7 @@ type hetznerManager struct {
 	nodeGroups     map[string]*hetznerNodeGroup
 	apiCallContext context.Context
 	cloudInit      string
-	image          string
+	image          *hcloud.Image
 	sshKey         *hcloud.SSHKey
 	network        *hcloud.Network
 }
@@ -55,18 +55,47 @@ func newManager() (*hetznerManager, error) {
 		return nil, errors.New("`HCLOUD_CLOUD_INIT` is not specified")
 	}
 
-	image := os.Getenv("HCLOUD_IMAGE")
-	if image == "" {
-		image = "ubuntu-20.04"
-	}
-
 	client := hcloud.NewClient(hcloud.WithToken(token))
 	ctx := context.Background()
 	cloudInit, err := base64.StdEncoding.DecodeString(cloudInitBase64)
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse cloud init error: %s", err)
 	}
 
+	imageName := os.Getenv("HCLOUD_IMAGE")
+	if imageName == "" {
+		imageName = "ubuntu-20.04"
+	}
+
+	// Search for an image ID corresponding to the supplied HCLOUD_IMAGE env
+	// variable. This value can either be an image ID itself (an int), a name
+	// (e.g. "ubuntu-20.04"), or a label selector associated with an image
+	// snapshot. In the latter case it will use the most recent snapshot.
+	image, _, err := client.Image.Get(ctx, imageName)
+	if err != nil || image == nil {
+		images, err := client.Image.AllWithOpts(ctx, hcloud.ImageListOpts{
+			Type:   []hcloud.ImageType{hcloud.ImageTypeSnapshot},
+			Status: []hcloud.ImageStatus{hcloud.ImageStatusAvailable},
+			Sort:   []string{"created:desc"},
+		})
+
+		labelSelector := strings.Split(imageName, "=")
+		if len(labelSelector) != 2 {
+			return nil, fmt.Errorf("unable to find image %s: invalid label selector", imageName)
+		}
+
+		for _, i := range images {
+			if i.Labels[labelSelector[0]] == labelSelector[1] {
+				image = i
+				break
+			}
+		}
+
+		if err != nil || image == nil {
+			return nil, fmt.Errorf("unable to find image %s: %v", imageName, err)
+		}
+	}
+
 	var network *hcloud.Network
 	networkName := os.Getenv("HCLOUD_NETWORK")
 

cluster-autoscaler/cloudprovider/hetzner/hetzner_node_group.go

@@ -353,7 +353,7 @@ func createServer(n *hetznerNodeGroup) error {
 		UserData:         n.manager.cloudInit,
 		Location:         &hcloud.Location{Name: n.region},
 		ServerType:       &hcloud.ServerType{Name: n.instanceType},
-		Image:            &hcloud.Image{Name: n.manager.image},
+		Image:            n.manager.image,
 		StartAfterCreate: &StartAfterCreate,
 		Labels: map[string]string{
 			nodeGroupLabel: n.id,