Provide a truly lazy restmapper

This commit adds a rest mapper that will lazily query the provided
client for discovery information to do REST mappings.

pkg/client/apiutil/lazyrestmapper/lazyrestmapper.go

@@ -0,0 +1,252 @@
+/*
+Copyright 2023 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package lazyrestmapper
+
+import (
+	"fmt"
+	"sync"
+
+	"k8s.io/apimachinery/pkg/api/meta"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/client-go/discovery"
+	"k8s.io/client-go/rest"
+	"k8s.io/client-go/restmapper"
+)
+
+// LazyRESTMapper is a RESTMapper that will lazily query the provided
+// client for discovery information to do REST mappings.
+type LazyRESTMapper struct {
+	mapper      meta.RESTMapper
+	client      *discovery.DiscoveryClient
+	knownGroups map[string]*restmapper.APIGroupResources
+	apiGroups   *metav1.APIGroupList
+
+	// mutex to provide thread-safe mapper reloading
+	mu sync.Mutex
+}
+
+// NewLazyRESTMapper initializes a LazyRESTMapper.
+func NewLazyRESTMapper(c *rest.Config) (meta.RESTMapper, error) {
+	discoveryClient, err := discovery.NewDiscoveryClientForConfig(c)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create discovery client: %w", err)
+	}
+
+	return NewLazyRESTMapperWithClient(discoveryClient)
+}
+
+// NewLazyRESTMapperWithClient initializes a LazyRESTMapper with a custom discovery client.
+func NewLazyRESTMapperWithClient(discoveryClient *discovery.DiscoveryClient) (meta.RESTMapper, error) {
+	return &LazyRESTMapper{
+		mapper:      restmapper.NewDiscoveryRESTMapper([]*restmapper.APIGroupResources{}),
+		client:      discoveryClient,
+		knownGroups: map[string]*restmapper.APIGroupResources{},
+	}, nil
+}
+
+// KindFor implements Mapper.KindFor.
+func (m *LazyRESTMapper) KindFor(resource schema.GroupVersionResource) (schema.GroupVersionKind, error) {
+	res, err := m.mapper.KindFor(resource)
+	if meta.IsNoMatchError(err) {
+		if err = m.addKnownGroupAndReload(resource.Group, resource.Version); err != nil {
+			return res, err
+		}
+
+		res, err = m.mapper.KindFor(resource)
+	}
+
+	return res, err
+}
+
+// KindsFor implements Mapper.KindsFor.
+func (m *LazyRESTMapper) KindsFor(resource schema.GroupVersionResource) ([]schema.GroupVersionKind, error) {
+	res, err := m.mapper.KindsFor(resource)
+	if meta.IsNoMatchError(err) {
+		if err = m.addKnownGroupAndReload(resource.Group, resource.Version); err != nil {
+			return res, err
+		}
+
+		res, err = m.mapper.KindsFor(resource)
+	}
+
+	return res, err
+}
+
+// ResourceFor implements Mapper.ResourceFor.
+func (m *LazyRESTMapper) ResourceFor(input schema.GroupVersionResource) (schema.GroupVersionResource, error) {
+	res, err := m.mapper.ResourceFor(input)
+	if meta.IsNoMatchError(err) {
+		if err = m.addKnownGroupAndReload(input.Group, input.Version); err != nil {
+			return res, err
+		}
+
+		res, err = m.mapper.ResourceFor(input)
+	}
+
+	return res, err
+}
+
+// ResourcesFor implements Mapper.ResourcesFor.
+func (m *LazyRESTMapper) ResourcesFor(input schema.GroupVersionResource) ([]schema.GroupVersionResource, error) {
+	res, err := m.mapper.ResourcesFor(input)
+	if meta.IsNoMatchError(err) {
+		if err = m.addKnownGroupAndReload(input.Group, input.Version); err != nil {
+			return res, err
+		}
+
+		res, err = m.mapper.ResourcesFor(input)
+	}
+
+	return res, err
+}
+
+// RESTMapping implements Mapper.RESTMapping.
+func (m *LazyRESTMapper) RESTMapping(gk schema.GroupKind, versions ...string) (*meta.RESTMapping, error) {
+	res, err := m.mapper.RESTMapping(gk, versions...)
+	if meta.IsNoMatchError(err) {
+		if err = m.addKnownGroupAndReload(gk.Group, versions...); err != nil {
+			return res, err
+		}
+
+		res, err = m.mapper.RESTMapping(gk, versions...)
+	}
+
+	return res, err
+}
+
+// RESTMappings implements Mapper.RESTMappings.
+func (m *LazyRESTMapper) RESTMappings(gk schema.GroupKind, versions ...string) ([]*meta.RESTMapping, error) {
+	res, err := m.mapper.RESTMappings(gk, versions...)
+	if meta.IsNoMatchError(err) {
+		if err = m.addKnownGroupAndReload(gk.Group, versions...); err != nil {
+			return res, err
+		}
+
+		res, err = m.mapper.RESTMappings(gk, versions...)
+	}
+
+	return res, err
+}
+
+// ResourceSingularizer implements Mapper.ResourceSingularizer.
+func (m *LazyRESTMapper) ResourceSingularizer(resource string) (string, error) {
+	return m.mapper.ResourceSingularizer(resource)
+}
+
+// addKnownGroupAndReload reloads the mapper with updated information about missing API group.
+// versions can be specified for partial updates, for instance for v1beta1 version only.
+func (m *LazyRESTMapper) addKnownGroupAndReload(groupName string, versions ...string) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// First, find information about requested group and its versions. Fail immediately if there is no such group.
+	apiGroup, err := m.findAPIGroupByName(groupName)
+	if err != nil {
+		return err
+	}
+
+	// If no specific versions are set by user, we will scan all available ones for the API group.
+	if len(versions) == 0 {
+		for _, version := range apiGroup.Versions {
+			versions = append(versions, version.Version)
+		}
+	}
+
+	// Second, get resources. The number of API calls is equal to the number of versions: /apis/<group>/<version>.
+	groupVersionResources, err := m.fetchGroupVersionResources(apiGroup.Name, versions...)
+	if err != nil {
+		return fmt.Errorf("failed to get API group resources: %w", err)
+	}
+
+	groupResources := &restmapper.APIGroupResources{
+		Group:              apiGroup,
+		VersionedResources: make(map[string][]metav1.APIResource),
+	}
+	for version, resources := range groupVersionResources {
+		groupResources.VersionedResources[version.Version] = resources.APIResources
+	}
+
+	// Add new known API group or just append the resources to the existing group.
+	if _, ok := m.knownGroups[groupName]; !ok {
+		m.knownGroups[groupName] = groupResources
+	} else {
+		for version, resources := range groupResources.VersionedResources {
+			m.knownGroups[groupName].VersionedResources[version] = resources
+		}
+	}
+
+	// Finally, update the group with received information and regenerate the mapper.
+	updatedGroupResources := make([]*restmapper.APIGroupResources, 0, len(m.knownGroups))
+	for _, v := range m.knownGroups {
+		updatedGroupResources = append(updatedGroupResources, v)
+	}
+
+	m.mapper = restmapper.NewDiscoveryRESTMapper(updatedGroupResources)
+
+	return nil
+}
+
+// findAPIGroupByName returns API group by its name.
+func (m *LazyRESTMapper) findAPIGroupByName(groupName string) (metav1.APIGroup, error) {
+	// Ensure that required info about existing API groups is received and stored in the mapper.
+	// It will make 2 API calls to /api and /apis, but only once.
+	if m.apiGroups == nil {
+		apiGroups, err := m.client.ServerGroups()
+		if err != nil {
+			return metav1.APIGroup{}, fmt.Errorf("failed to get server groups: %w", err)
+		}
+		if len(apiGroups.Groups) == 0 {
+			return metav1.APIGroup{}, fmt.Errorf("received an empty API groups list")
+		}
+
+		m.apiGroups = apiGroups
+	}
+
+	for i := range m.apiGroups.Groups {
+		if groupName == (&m.apiGroups.Groups[i]).Name {
+			return m.apiGroups.Groups[i], nil
+		}
+	}
+
+	return metav1.APIGroup{}, fmt.Errorf("failed to find API group %s", groupName)
+}
+
+// fetchGroupVersionResources fetchs the resources for the specified group and its versions.
+func (m *LazyRESTMapper) fetchGroupVersionResources(groupName string, versions ...string) (map[schema.GroupVersion]*metav1.APIResourceList, error) {
+	groupVersionResources := make(map[schema.GroupVersion]*metav1.APIResourceList)
+	failedGroups := make(map[schema.GroupVersion]error)
+
+	for _, version := range versions {
+		groupVersion := schema.GroupVersion{Group: groupName, Version: version}
+
+		apiResourceList, err := m.client.ServerResourcesForGroupVersion(groupVersion.String())
+		if err != nil {
+			failedGroups[groupVersion] = err
+		}
+		if apiResourceList != nil {
+			// even in case of error, some fallback might have been returned
+			groupVersionResources[groupVersion] = apiResourceList
+		}
+	}
+
+	if len(failedGroups) > 0 {
+		return nil, &discovery.ErrGroupDiscoveryFailed{Groups: failedGroups}
+	}
+
+	return groupVersionResources, nil
+}