Warn/prevent if the version of etcd is unsupported with etcd-manager

Should prevent the scenario where etcd-manager can't come up because of a different version.

Can be bypassed with the SkipEtcdVersionCheck feature flag.

pkg/featureflag/featureflag.go

@@ -82,6 +82,8 @@ var (
 	VPCSkipEnableDNSSupport = New("VPCSkipEnableDNSSupport", Bool(false))
 	// VSphereCloudProvider enables the vsphere cloud provider
 	VSphereCloudProvider = New("VSphereCloudProvider", Bool(false))
+	// SkipEtcdVersionCheck will bypass the check that etcd-manager is using a supported etcd version
+	SkipEtcdVersionCheck = New("SkipEtcdVersionCheck", Bool(false))
 )
 
 // FeatureFlag defines a feature flag

pkg/model/components/etcdmanager/BUILD.bazel

@@ -12,6 +12,7 @@ go_library(
         "//pkg/apis/kops:go_default_library",
         "//pkg/assets:go_default_library",
         "//pkg/dns:go_default_library",
+        "//pkg/featureflag:go_default_library",
         "//pkg/flagbuilder:go_default_library",
         "//pkg/k8scodecs:go_default_library",
         "//pkg/kubemanifest:go_default_library",

pkg/model/components/etcdmanager/options.go

@@ -17,7 +17,12 @@ limitations under the License.
 package etcdmanager
 
 import (
+	"fmt"
+	"strings"
+
+	"k8s.io/klog"
 	"k8s.io/kops/pkg/apis/kops"
+	"k8s.io/kops/pkg/featureflag"
 	"k8s.io/kops/pkg/model/components"
 	"k8s.io/kops/pkg/urls"
 	"k8s.io/kops/upup/pkg/fi/loader"
@@ -59,7 +64,29 @@ func (b *EtcdManagerOptionsBuilder) BuildOptions(o interface{}) error {
 				etcdCluster.Version = "2.2.1"
 			}
 		}
+
+		if !etcdVersionIsSupported(etcdCluster.Version) {
+			if featureflag.SkipEtcdVersionCheck.Enabled() {
+				klog.Warningf("etcd version %q is not known to be supported, but ignoring because of SkipEtcdVersionCheck feature flag", etcdCluster.Version)
+			} else {
+				klog.Warningf("unsupported etcd version %q detected; please update etcd version.  Use export KOPS_FEATURE_FLAGS=SkipEtcdVersionCheck to override this check", etcdCluster.Version)
+				return fmt.Errorf("etcd version %q is not supported with etcd-manager, please specify a supported version or remove the value to use the default version.  Supported versions: %s", etcdCluster.Version, strings.Join(supportedEtcdVersions, ", "))
+			}
+
+		}
 	}
 
 	return nil
 }
+
+var supportedEtcdVersions = []string{"2.2.1", "3.1.12", "3.2.18", "3.2.24", "3.3.10", "3.3.13"}
+
+func etcdVersionIsSupported(version string) bool {
+	version = strings.TrimPrefix(version, "v")
+	for _, v := range supportedEtcdVersions {
+		if v == version {
+			return true
+		}
+	}
+	return false
+}