Watch Action instead of polling Server Status

We have recevied an issue from one of our customers that their
autoscaler pod regularly crashes due to a nil pointer dereference panic.
Analyzing the code we found out that the autoscaler polls the server
status to find out if a server is running.

The Hetzner Cloud Go client is implemented in such a way that it does
not return an error if a resource could not be found. Instead it returns
nil for the error and the resource. Ususally this is not an issue.
However, in case the server creation fails the server gets deleted from
Hetzner Cloud. This in turn leads to nil being returned and the
abovementioned panic.

The Hetzner Cloud API implements a concept called Actions. Whenever a
long running process is triggered we return an object which can
be used to get information about the progress of the task. The Action
object reliably allows to detect if a server has been created and
provides access to any error that may have occured.

This commit replaces polling the server status with using the action
object.

cluster-autoscaler/cloudprovider/hetzner/hetzner_node_group.go

@@ -18,6 +18,10 @@ package hetzner
 
 import (
 	"fmt"
+	"math/rand"
+	"sync"
+	"time"
+
 	apiv1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/api/resource"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
@@ -26,10 +30,6 @@ import (
 	"k8s.io/autoscaler/cluster-autoscaler/config"
 	"k8s.io/klog/v2"
 	schedulerframework "k8s.io/kubernetes/pkg/scheduler/framework"
-	"math/rand"
-	"strconv"
-	"sync"
-	"time"
 )
 
 // hetznerNodeGroup implements cloudprovider.NodeGroup interface. hetznerNodeGroup contains
@@ -365,14 +365,15 @@ func createServer(n *hetznerNodeGroup) error {
 	if n.manager.network != nil {
 		opts.Networks = []*hcloud.Network{n.manager.network}
 	}
-	serverCreateResult, _, err := n.manager.client.Server.Create(n.manager.apiCallContext, opts)
 
+	serverCreateResult, _, err := n.manager.client.Server.Create(n.manager.apiCallContext, opts)
 	if err != nil {
 		return fmt.Errorf("could not create server type %s in region %s: %v", n.instanceType, n.region, err)
 	}
 
+	action := serverCreateResult.Action
 	server := serverCreateResult.Server
-	err = waitForServerStatus(n.manager, server, hcloud.ServerStatusRunning)
+	err = waitForServerAction(n.manager, server.Name, action)
 	if err != nil {
 		_ = n.manager.deleteServer(server)
 		return fmt.Errorf("failed to start server %s error: %v", server.Name, err)
@@ -381,32 +382,39 @@ func createServer(n *hetznerNodeGroup) error {
 	return nil
 }
 
-func waitForServerStatus(m *hetznerManager, server *hcloud.Server, status hcloud.ServerStatus) error {
-	errorResult := make(chan error)
-
-	go func() {
-		for {
-			serverResponse, _, err := m.client.Server.Get(m.apiCallContext, strconv.Itoa(server.ID))
-			if err != nil {
-				errorResult <- fmt.Errorf("failed to get server %s status error: %v", server.Name, err)
-				return
-			}
-
-			if serverResponse.Status == status {
-				errorResult <- nil
-				return
-			}
-
-			time.Sleep(1 * time.Second)
-		}
-	}()
-
+func waitForServerAction(m *hetznerManager, serverName string, action *hcloud.Action) error {
+	// The implementation of the Hetzner Cloud action client's WatchProgress
+	// method may be a little puzzling. The following comment thus explains how
+	// waitForServerAction works.
+	//
+	// WatchProgress returns two channels. The first channel is used to send a
+	// ballpark estimate for the action progress, the second to send any error
+	// that may occur.
+	//
+	// WatchProgress is implemented in such a way, that the first channel can
+	// be ignored. It is not necessary to consume it to avoid a deadlock in
+	// WatchProgress. Any write to this channel is wrapped in a select.
+	// Progress updates are simply not sent if nothing reads from the other
+	// side.
+	//
+	// Once the action completes successfully nil is send through the second
+	// channel. Then both channels are closed.
+	//
+	// The following code therefore only watches the second channel. If it
+	// reads an error from the channel the action is failed. Otherwise the
+	// action is successful.
+	_, errChan := m.client.Action.WatchProgress(m.apiCallContext, action)
 	select {
-	case res := <-errorResult:
-		return res
+	case err := <-errChan:
+		if err != nil {
+			return fmt.Errorf("error while waiting for server action: %s: %v", serverName, err)
+		}
+		return nil
 	case <-time.After(serverCreateTimeout):
-		return fmt.Errorf("waiting for server %s status %s timeout", server.Name, status)
+		return fmt.Errorf("timeout waiting for server %s", serverName)
 	}
+
+	return nil
 }
 
 func (n *hetznerNodeGroup) resetTargetSize(expectedDelta int) {