feat: detect the lowest Kubernetes version in upgrade-k8s CLI command

Scan all pods in `kube-system` and find `kube-proxy`, `kube-scheduler`,
`kube-controller-manager` and `kube-apiserver` ones, then check the
lowest version amongst them.

Signed-off-by: Artem Chernyshev <artem.0xD2@gmail.com>

cmd/talosctl/cmd/talos/upgrade-k8s.go

@@ -34,7 +34,6 @@ func init() {
 	upgradeK8sCmd.Flags().StringVar(&upgradeOptions.FromVersion, "from", "", "the Kubernetes control plane version to upgrade from")
 	upgradeK8sCmd.Flags().StringVar(&upgradeOptions.ToVersion, "to", constants.DefaultKubernetesVersion, "the Kubernetes control plane version to upgrade to")
 	upgradeK8sCmd.Flags().StringVar(&upgradeOptions.ControlPlaneEndpoint, "endpoint", "", "the cluster control plane endpoint")
-	cli.Should(upgradeK8sCmd.MarkFlagRequired("from"))
 	cli.Should(upgradeK8sCmd.MarkFlagRequired("to"))
 	addCommand(upgradeK8sCmd)
 }
@@ -56,6 +55,17 @@ func upgradeKubernetes(ctx context.Context, c *client.Client) error {
 		},
 	}
 
+	var err error
+
+	if upgradeOptions.FromVersion == "" {
+		upgradeOptions.FromVersion, err = k8s.DetectLowestVersion(ctx, &state, upgradeOptions)
+		if err != nil {
+			return fmt.Errorf("error detecting the lowest Kubernetes version %w", err)
+		}
+
+		upgradeOptions.Log("automatically detected the lowest Kubernetes version %s", upgradeOptions.FromVersion)
+	}
+
 	selfHosted, err := k8s.IsSelfHostedControlPlane(ctx, &state, Nodes[0])
 	if err != nil {
 		return fmt.Errorf("error checking self-hosted status: %w", err)

pkg/cluster/kubernetes/detect.go

@@ -0,0 +1,71 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package kubernetes
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"github.com/coreos/go-semver/semver"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+// DetectLowestVersion returns lowest Kubernetes components versions in the cluster.
+//nolint:gocyclo
+func DetectLowestVersion(ctx context.Context, cluster UpgradeProvider, options UpgradeOptions) (string, error) {
+	k8sClient, err := cluster.K8sHelper(ctx)
+	if err != nil {
+		return "", fmt.Errorf("error building kubernetes client: %w", err)
+	}
+
+	apps := map[string]struct{}{
+		"kube-apiserver":          {},
+		"kube-controller-manager": {},
+		"kube-proxy":              {},
+		"kube-scheduler":          {},
+	}
+
+	pods, err := k8sClient.CoreV1().Pods("kube-system").List(ctx, metav1.ListOptions{})
+	if err != nil {
+		return "", err
+	}
+
+	version, err := semver.NewVersion(options.ToVersion)
+	if err != nil {
+		return "", err
+	}
+
+	for _, pod := range pods.Items {
+		app := pod.GetObjectMeta().GetLabels()["k8s-app"]
+		if _, ok := apps[app]; !ok {
+			continue
+		}
+
+		for _, container := range pod.Spec.Containers {
+			if container.Name != app {
+				continue
+			}
+
+			parts := strings.Split(container.Image, ":")
+			if len(parts) == 1 {
+				continue
+			}
+
+			v, err := semver.NewVersion(strings.TrimLeft(parts[1], "v"))
+			if err != nil {
+				options.Log("failed to parse %s container version %s", app, err)
+
+				continue
+			}
+
+			if v.LessThan(*version) {
+				version = v
+			}
+		}
+	}
+
+	return version.String(), nil
+}