kubeadm: implement mutators for "config migrate"

When upconverting from v1beta3 to v1beta4, it appears there is no
easy way to migrate some of the timeout values such as:
  ClusterConfiguration.APIServer.TimeoutForControlPlane
to a new location:
  InitConfiguration.Timeouts.<some-timeout-field>

Yes, the internal InitConfiguratio does embed a ClusterConfiguration,
but during conversion the ClusterConfiguration is converted from an
empty source.

K8s' API machinery has ways to register custom conversion functions,
such as v1beta3.ClusterConfiguration -> internal.InitConfiguration,
but these must be triggered explicitly with a decoder.
The overall migration of fields seems very awkward.

There might be hacks around that, such as storing intermediate state,
while trying to make the fuzzer rountrip happy, but instead
mutation functions can be implemented for the internal types when
calling kubeadm's migrate code. This seems much cleaner.

cmd/kubeadm/app/cmd/config.go

@@ -275,7 +275,7 @@ func newCmdConfigMigrate(out io.Writer) *cobra.Command {
 				return err
 			}
 
-			outputBytes, err := configutil.MigrateOldConfig(oldCfgBytes, allowExperimental)
+			outputBytes, err := configutil.MigrateOldConfig(oldCfgBytes, allowExperimental, nil)
 			if err != nil {
 				return err
 			}

cmd/kubeadm/app/util/config/common.go

@@ -250,9 +250,13 @@ func validateKnownGVKs(gvks []schema.GroupVersionKind) error {
 
 // MigrateOldConfig migrates an old configuration from a byte slice into a new one (returned again as a byte slice).
 // Only kubeadm kinds are migrated.
-func MigrateOldConfig(oldConfig []byte, allowExperimental bool) ([]byte, error) {
+func MigrateOldConfig(oldConfig []byte, allowExperimental bool, mutators migrateMutators) ([]byte, error) {
 	newConfig := [][]byte{}
 
+	if mutators == nil {
+		mutators = defaultMigrateMutators()
+	}
+
 	gvkmap, err := kubeadmutil.SplitYAMLDocuments(oldConfig)
 	if err != nil {
 		return []byte{}, err
@@ -277,6 +281,9 @@ func MigrateOldConfig(oldConfig []byte, allowExperimental bool) ([]byte, error)
 		if err != nil {
 			return []byte{}, err
 		}
+		if err := mutators.mutate([]any{o}); err != nil {
+			return []byte{}, err
+		}
 		b, err := MarshalKubeadmConfigObject(o, gv)
 		if err != nil {
 			return []byte{}, err
@@ -290,6 +297,9 @@ func MigrateOldConfig(oldConfig []byte, allowExperimental bool) ([]byte, error)
 		if err != nil {
 			return []byte{}, err
 		}
+		if err := mutators.mutate([]any{o}); err != nil {
+			return []byte{}, err
+		}
 		b, err := MarshalKubeadmConfigObject(o, gv)
 		if err != nil {
 			return []byte{}, err
@@ -303,6 +313,9 @@ func MigrateOldConfig(oldConfig []byte, allowExperimental bool) ([]byte, error)
 		if err != nil {
 			return []byte{}, err
 		}
+		if err := mutators.mutate([]any{o}); err != nil {
+			return []byte{}, err
+		}
 		b, err := MarshalKubeadmConfigObject(o, gv)
 		if err != nil {
 			return []byte{}, err
@@ -370,3 +383,97 @@ func isKubeadmPrereleaseVersion(versionInfo *apimachineryversion.Info, k8sVersio
 	}
 	return false
 }
+
+// migrateMutator can be used to mutate a slice of configuration objects.
+// The mutation is applied in-place and no copies are made.
+type migrateMutator struct {
+	in         []any
+	mutateFunc func(in []any) error
+}
+
+// migrateMutators holds a list of registered mutators.
+type migrateMutators []migrateMutator
+
+// mutate can be called on a list of registered mutators to find a suitable one to perform
+// a configuration object mutation.
+func (mutators migrateMutators) mutate(in []any) error {
+	var mutator *migrateMutator
+	for idx, m := range mutators {
+		if len(m.in) != len(in) {
+			continue
+		}
+		inputMatch := true
+		for idx := range m.in {
+			if reflect.TypeOf(m.in[idx]) != reflect.TypeOf(in[idx]) {
+				inputMatch = false
+				break
+			}
+		}
+		if inputMatch {
+			mutator = &mutators[idx]
+			break
+		}
+	}
+	if mutator == nil {
+		return errors.Errorf("could not find a mutator for input: %#v", in)
+	}
+	return mutator.mutateFunc(in)
+}
+
+// addEmpty adds an empty migrate mutator for a given input.
+func (mutators *migrateMutators) addEmpty(in []any) {
+	mutator := migrateMutator{
+		in:         in,
+		mutateFunc: func(in []any) error { return nil },
+	}
+	*mutators = append(*mutators, mutator)
+}
+
+// defaultMutators returns the default list of mutators for known configuration objects.
+// TODO: make this function return defaultEmptyMutators() when v1beta3 is removed.
+func defaultMigrateMutators() migrateMutators {
+	var (
+		mutators migrateMutators
+		mutator  migrateMutator
+	)
+
+	// mutator for InitConfiguration, ClusterConfiguration.
+	mutator = migrateMutator{
+		in: []any{(*kubeadmapi.InitConfiguration)(nil)},
+		mutateFunc: func(in []any) error {
+			a := in[0].(*kubeadmapi.InitConfiguration)
+			a.Timeouts.ControlPlaneComponentHealthCheck.Duration = a.APIServer.TimeoutForControlPlane.Duration
+			a.APIServer.TimeoutForControlPlane = nil
+			return nil
+		},
+	}
+	mutators = append(mutators, mutator)
+
+	// mutator for JoinConfiguration.
+	mutator = migrateMutator{
+		in: []any{(*kubeadmapi.JoinConfiguration)(nil)},
+		mutateFunc: func(in []any) error {
+			a := in[0].(*kubeadmapi.JoinConfiguration)
+			a.Timeouts.Discovery.Duration = a.Discovery.Timeout.Duration
+			a.Discovery.Timeout = nil
+			return nil
+		},
+	}
+	mutators = append(mutators, mutator)
+
+	// empty mutator for ResetConfiguration.
+	mutators.addEmpty([]any{(*kubeadmapi.ResetConfiguration)(nil)})
+
+	return mutators
+}
+
+// defaultEmptyMigrateMutators returns a list of empty mutators for known types.
+func defaultEmptyMigrateMutators() migrateMutators {
+	mutators := &migrateMutators{}
+
+	mutators.addEmpty([]any{(*kubeadmapi.InitConfiguration)(nil)})
+	mutators.addEmpty([]any{(*kubeadmapi.JoinConfiguration)(nil)})
+	mutators.addEmpty([]any{(*kubeadmapi.ResetConfiguration)(nil)})
+
+	return *mutators
+}

cmd/kubeadm/app/util/config/common_test.go

@@ -20,8 +20,12 @@ import (
 	"fmt"
 	"reflect"
 	"testing"
+	"time"
 
+	"github.com/google/go-cmp/cmp"
 	"github.com/lithammer/dedent"
+
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/apimachinery/pkg/util/version"
 	apimachineryversion "k8s.io/apimachinery/pkg/version"
@@ -454,7 +458,7 @@ func TestMigrateOldConfig(t *testing.T) {
 
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
-			b, err := MigrateOldConfig([]byte(test.oldCfg), test.allowExperimental)
+			b, err := MigrateOldConfig([]byte(test.oldCfg), test.allowExperimental, defaultEmptyMigrateMutators())
 			if test.expectErr {
 				if err == nil {
 					t.Fatalf("unexpected success:\n%s", b)
@@ -751,3 +755,106 @@ func TestNormalizeKubernetesVersion(t *testing.T) {
 		})
 	}
 }
+
+// TODO: update the test cases for this test once v1beta3 is removed.
+func TestDefaultMigrateMutators(t *testing.T) {
+	tests := []struct {
+		name          string
+		mutators      migrateMutators
+		input         []any
+		expected      []any
+		expectedDiff  bool
+		expectedError bool
+	}{
+		{
+			name:     "mutate InitConfiguration",
+			mutators: defaultMigrateMutators(),
+			input: []any{&kubeadmapi.InitConfiguration{
+				ClusterConfiguration: kubeadmapi.ClusterConfiguration{
+					APIServer: kubeadmapi.APIServer{
+						TimeoutForControlPlane: &metav1.Duration{
+							Duration: 1234 * time.Millisecond,
+						},
+					},
+				},
+				Timeouts: &kubeadmapi.Timeouts{
+					ControlPlaneComponentHealthCheck: &metav1.Duration{},
+				},
+			}},
+			expected: []any{&kubeadmapi.InitConfiguration{
+				Timeouts: &kubeadmapi.Timeouts{
+					ControlPlaneComponentHealthCheck: &metav1.Duration{
+						Duration: 1234 * time.Millisecond,
+					},
+				},
+			}},
+		},
+		{
+			name:     "mutate JoinConfiguration",
+			mutators: defaultMigrateMutators(),
+			input: []any{&kubeadmapi.JoinConfiguration{
+				Discovery: kubeadmapi.Discovery{
+					Timeout: &metav1.Duration{
+						Duration: 1234 * time.Microsecond,
+					},
+				},
+				Timeouts: &kubeadmapi.Timeouts{
+					Discovery: &metav1.Duration{},
+				},
+			}},
+			expected: []any{&kubeadmapi.JoinConfiguration{
+				Timeouts: &kubeadmapi.Timeouts{
+					Discovery: &metav1.Duration{
+						Duration: 1234 * time.Microsecond,
+					},
+				},
+			}},
+		},
+		{
+			name:     "diff when mutating InitConfiguration",
+			mutators: defaultMigrateMutators(),
+			input: []any{&kubeadmapi.InitConfiguration{
+				ClusterConfiguration: kubeadmapi.ClusterConfiguration{
+					APIServer: kubeadmapi.APIServer{
+						TimeoutForControlPlane: &metav1.Duration{
+							Duration: 1234 * time.Millisecond,
+						},
+					},
+				},
+				Timeouts: &kubeadmapi.Timeouts{
+					ControlPlaneComponentHealthCheck: &metav1.Duration{},
+				},
+			}},
+			expected: []any{&kubeadmapi.InitConfiguration{
+				Timeouts: &kubeadmapi.Timeouts{
+					ControlPlaneComponentHealthCheck: &metav1.Duration{
+						Duration: 1 * time.Millisecond, // a different value
+					},
+				},
+			}},
+			expectedDiff: true,
+		},
+		{
+			name:          "expect an error for a missing mutator",
+			mutators:      migrateMutators{}, // empty list of mutators
+			input:         []any{&kubeadmapi.ResetConfiguration{}},
+			expectedError: true,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			err := tc.mutators.mutate(tc.input)
+			if (err != nil) != tc.expectedError {
+				t.Fatalf("expected error: %v, got: %v, error: %v", tc.expectedError, (err != nil), err)
+			}
+			if err != nil {
+				return
+			}
+			diff := cmp.Diff(tc.expected, tc.input)
+			if (len(diff) > 0) != tc.expectedDiff {
+				t.Fatalf("got a diff with the expected config (-want,+got):\n%s", diff)
+			}
+		})
+	}
+}