/*
Copyright 2016 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 apiserver

import (
	"context"
	"fmt"
	"net/http"
	"sync"
	"time"

	apierrors "k8s.io/apimachinery/pkg/api/errors"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/runtime/schema"
	"k8s.io/apimachinery/pkg/util/sets"
	"k8s.io/apimachinery/pkg/util/wait"
	"k8s.io/apiserver/pkg/endpoints/discovery/aggregated"
	genericfeatures "k8s.io/apiserver/pkg/features"
	peerreconcilers "k8s.io/apiserver/pkg/reconcilers"
	genericapiserver "k8s.io/apiserver/pkg/server"
	"k8s.io/apiserver/pkg/server/dynamiccertificates"
	"k8s.io/apiserver/pkg/server/egressselector"
	serverstorage "k8s.io/apiserver/pkg/server/storage"
	utilfeature "k8s.io/apiserver/pkg/util/feature"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/transport"
	"k8s.io/component-base/metrics/legacyregistry"
	"k8s.io/component-base/tracing"
	v1 "k8s.io/kube-aggregator/pkg/apis/apiregistration/v1"
	v1helper "k8s.io/kube-aggregator/pkg/apis/apiregistration/v1/helper"
	"k8s.io/kube-aggregator/pkg/apis/apiregistration/v1beta1"
	aggregatorscheme "k8s.io/kube-aggregator/pkg/apiserver/scheme"
	"k8s.io/kube-aggregator/pkg/client/clientset_generated/clientset"
	informers "k8s.io/kube-aggregator/pkg/client/informers/externalversions"
	listers "k8s.io/kube-aggregator/pkg/client/listers/apiregistration/v1"
	openapicontroller "k8s.io/kube-aggregator/pkg/controllers/openapi"
	openapiaggregator "k8s.io/kube-aggregator/pkg/controllers/openapi/aggregator"
	openapiv3controller "k8s.io/kube-aggregator/pkg/controllers/openapiv3"
	openapiv3aggregator "k8s.io/kube-aggregator/pkg/controllers/openapiv3/aggregator"
	localavailability "k8s.io/kube-aggregator/pkg/controllers/status/local"
	availabilitymetrics "k8s.io/kube-aggregator/pkg/controllers/status/metrics"
	remoteavailability "k8s.io/kube-aggregator/pkg/controllers/status/remote"
	apiservicerest "k8s.io/kube-aggregator/pkg/registry/apiservice/rest"
	openapicommon "k8s.io/kube-openapi/pkg/common"
)

// making sure we only register metrics once into legacy registry
var registerIntoLegacyRegistryOnce sync.Once

func init() {
	// we need to add the options (like ListOptions) to empty v1
	metav1.AddToGroupVersion(aggregatorscheme.Scheme, schema.GroupVersion{Group: "", Version: "v1"})

	unversioned := schema.GroupVersion{Group: "", Version: "v1"}
	aggregatorscheme.Scheme.AddUnversionedTypes(unversioned,
		&metav1.Status{},
		&metav1.APIVersions{},
		&metav1.APIGroupList{},
		&metav1.APIGroup{},
		&metav1.APIResourceList{},
	)
}

const (
	// legacyAPIServiceName is the fixed name of the only non-groupified API version
	legacyAPIServiceName = "v1."
	// StorageVersionPostStartHookName is the name of the storage version updater post start hook.
	StorageVersionPostStartHookName = "built-in-resources-storage-version-updater"
)

// ExtraConfig represents APIServices-specific configuration
type ExtraConfig struct {
	// PeerAdvertiseAddress is the IP for this kube-apiserver which is used by peer apiservers to route a request
	// to this apiserver. This happens in cases where the peer is not able to serve the request due to
	// version skew. If unset, AdvertiseAddress/BindAddress will be used.
	PeerAdvertiseAddress peerreconcilers.PeerAdvertiseAddress

	// ProxyClientCert/Key are the client cert used to identify this proxy. Backing APIServices use
	// this to confirm the proxy's identity
	ProxyClientCertFile string
	ProxyClientKeyFile  string

	// If present, the Dial method will be used for dialing out to delegate
	// apiservers.
	ProxyTransport *http.Transport

	// Mechanism by which the Aggregator will resolve services. Required.
	ServiceResolver ServiceResolver

	RejectForwardingRedirects bool

	// DisableRemoteAvailableConditionController disables the controller that updates the Available conditions for
	// remote APIServices via querying endpoints of the referenced services. In generic controlplane use-cases,
	// the concept of services and endpoints might differ, and might require another implementation of this
	// controller. Local APIService are reconciled nevertheless.
	DisableRemoteAvailableConditionController bool
}

// Config represents the configuration needed to create an APIAggregator.
type Config struct {
	GenericConfig *genericapiserver.RecommendedConfig
	ExtraConfig   ExtraConfig
}

type completedConfig struct {
	GenericConfig genericapiserver.CompletedConfig
	ExtraConfig   *ExtraConfig
}

// CompletedConfig same as Config, just to swap private object.
type CompletedConfig struct {
	// Embed a private pointer that cannot be instantiated outside of this package.
	*completedConfig
}

type runnable interface {
	RunWithContext(ctx context.Context) error
}

// preparedGenericAPIServer is a private wrapper that enforces a call of PrepareRun() before Run can be invoked.
type preparedAPIAggregator struct {
	*APIAggregator
	runnable runnable
}

// APIAggregator contains state for a Kubernetes cluster master/api server.
type APIAggregator struct {
	GenericAPIServer *genericapiserver.GenericAPIServer

	// provided for easier embedding
	APIRegistrationInformers informers.SharedInformerFactory

	delegateHandler http.Handler

	// proxyCurrentCertKeyContent holds he client cert used to identify this proxy. Backing APIServices use this to confirm the proxy's identity
	proxyCurrentCertKeyContent certKeyFunc
	proxyTransportDial         *transport.DialHolder

	// proxyHandlers are the proxy handlers that are currently registered, keyed by apiservice.name
	proxyHandlers map[string]*proxyHandler
	// handledGroupVersions contain the groups that already have routes. The key is the name of the group and the value
	// is the versions for the group.
	handledGroupVersions map[string]sets.Set[string]

	// lister is used to add group handling for /apis/<group> aggregator lookups based on
	// controller state
	lister listers.APIServiceLister

	// Information needed to determine routing for the aggregator
	serviceResolver ServiceResolver

	// Enable swagger and/or OpenAPI if these configs are non-nil.
	openAPIConfig *openapicommon.Config

	// Enable OpenAPI V3 if these configs are non-nil
	openAPIV3Config *openapicommon.OpenAPIV3Config

	// openAPIAggregationController downloads and merges OpenAPI v2 specs.
	openAPIAggregationController *openapicontroller.AggregationController

	// openAPIV3AggregationController downloads and caches OpenAPI v3 specs.
	openAPIV3AggregationController *openapiv3controller.AggregationController

	// discoveryAggregationController downloads and caches discovery documents
	// from all aggregated apiservices so they are available from /apis endpoint
	// when discovery with resources are requested
	discoveryAggregationController DiscoveryAggregationController

	// rejectForwardingRedirects is whether to allow to forward redirect response
	rejectForwardingRedirects bool

	// tracerProvider is used to wrap the proxy transport and handler with tracing
	tracerProvider tracing.TracerProvider
}

// Complete fills in any fields not set that are required to have valid data. It's mutating the receiver.
func (cfg *Config) Complete() CompletedConfig {
	c := completedConfig{
		cfg.GenericConfig.Complete(),
		&cfg.ExtraConfig,
	}

	// the kube aggregator wires its own discovery mechanism
	// TODO eventually collapse this by extracting all of the discovery out
	c.GenericConfig.EnableDiscovery = false

	return CompletedConfig{&c}
}

// NewWithDelegate returns a new instance of APIAggregator from the given config.
func (c completedConfig) NewWithDelegate(delegationTarget genericapiserver.DelegationTarget) (*APIAggregator, error) {
	genericServer, err := c.GenericConfig.New("kube-aggregator", delegationTarget)
	if err != nil {
		return nil, err
	}

	apiregistrationClient, err := clientset.NewForConfig(c.GenericConfig.LoopbackClientConfig)
	if err != nil {
		return nil, err
	}
	informerFactory := informers.NewSharedInformerFactory(
		apiregistrationClient,
		5*time.Minute, // this is effectively used as a refresh interval right now.  Might want to do something nicer later on.
	)

	// apiServiceRegistrationControllerInitiated is closed when APIServiceRegistrationController has finished "installing" all known APIServices.
	// At this point we know that the proxy handler knows about APIServices and can handle client requests.
	// Before it might have resulted in a 404 response which could have serious consequences for some controllers like  GC and NS
	//
	// Note that the APIServiceRegistrationController waits for APIServiceInformer to synced before doing its work.
	apiServiceRegistrationControllerInitiated := make(chan struct{})
	if err := genericServer.RegisterMuxAndDiscoveryCompleteSignal("APIServiceRegistrationControllerInitiated", apiServiceRegistrationControllerInitiated); err != nil {
		return nil, err
	}

	var proxyTransportDial *transport.DialHolder
	if c.GenericConfig.EgressSelector != nil {
		egressDialer, err := c.GenericConfig.EgressSelector.Lookup(egressselector.Cluster.AsNetworkContext())
		if err != nil {
			return nil, err
		}
		if egressDialer != nil {
			proxyTransportDial = &transport.DialHolder{Dial: egressDialer}
		}
	} else if c.ExtraConfig.ProxyTransport != nil && c.ExtraConfig.ProxyTransport.DialContext != nil {
		proxyTransportDial = &transport.DialHolder{Dial: c.ExtraConfig.ProxyTransport.DialContext}
	}

	s := &APIAggregator{
		GenericAPIServer:           genericServer,
		delegateHandler:            delegationTarget.UnprotectedHandler(),
		proxyTransportDial:         proxyTransportDial,
		proxyHandlers:              map[string]*proxyHandler{},
		handledGroupVersions:       map[string]sets.Set[string]{},
		lister:                     informerFactory.Apiregistration().V1().APIServices().Lister(),
		APIRegistrationInformers:   informerFactory,
		serviceResolver:            c.ExtraConfig.ServiceResolver,
		openAPIConfig:              c.GenericConfig.OpenAPIConfig,
		openAPIV3Config:            c.GenericConfig.OpenAPIV3Config,
		proxyCurrentCertKeyContent: func() (bytes []byte, bytes2 []byte) { return nil, nil },
		rejectForwardingRedirects:  c.ExtraConfig.RejectForwardingRedirects,
		tracerProvider:             c.GenericConfig.TracerProvider,
	}

	apiGroupInfo := apiservicerest.NewRESTStorage(c.GenericConfig.MergedResourceConfig, c.GenericConfig.RESTOptionsGetter, false)
	if err := s.GenericAPIServer.InstallAPIGroup(&apiGroupInfo); err != nil {
		return nil, err
	}

	enabledVersions := sets.NewString()
	for v := range apiGroupInfo.VersionedResourcesStorageMap {
		enabledVersions.Insert(v)
	}
	if !enabledVersions.Has(v1.SchemeGroupVersion.Version) {
		return nil, fmt.Errorf("API group/version %s must be enabled", v1.SchemeGroupVersion.String())
	}

	apisHandler := &apisHandler{
		codecs:         aggregatorscheme.Codecs,
		lister:         s.lister,
		discoveryGroup: discoveryGroup(enabledVersions),
	}

	apisHandlerWithAggregationSupport := aggregated.WrapAggregatedDiscoveryToHandler(apisHandler, s.GenericAPIServer.AggregatedDiscoveryGroupManager)
	s.GenericAPIServer.Handler.NonGoRestfulMux.Handle("/apis", apisHandlerWithAggregationSupport)
	s.GenericAPIServer.Handler.NonGoRestfulMux.UnlistedHandle("/apis/", apisHandler)

	apiserviceRegistrationController := NewAPIServiceRegistrationController(informerFactory.Apiregistration().V1().APIServices(), s)
	if len(c.ExtraConfig.ProxyClientCertFile) > 0 && len(c.ExtraConfig.ProxyClientKeyFile) > 0 {
		aggregatorProxyCerts, err := dynamiccertificates.NewDynamicServingContentFromFiles("aggregator-proxy-cert", c.ExtraConfig.ProxyClientCertFile, c.ExtraConfig.ProxyClientKeyFile)
		if err != nil {
			return nil, err
		}
		// We are passing the context to ProxyCerts.RunOnce as it needs to implement RunOnce(ctx) however the
		// context is not used at all. So passing a empty context shouldn't be a problem
		if err := aggregatorProxyCerts.RunOnce(context.Background()); err != nil {
			return nil, err
		}
		aggregatorProxyCerts.AddListener(apiserviceRegistrationController)
		s.proxyCurrentCertKeyContent = aggregatorProxyCerts.CurrentCertKeyContent

		s.GenericAPIServer.AddPostStartHookOrDie("aggregator-reload-proxy-client-cert", func(postStartHookContext genericapiserver.PostStartHookContext) error {
			go aggregatorProxyCerts.Run(postStartHookContext, 1)
			return nil
		})
	}

	s.GenericAPIServer.AddPostStartHookOrDie("start-kube-aggregator-informers", func(context genericapiserver.PostStartHookContext) error {
		informerFactory.Start(context.Done())
		c.GenericConfig.SharedInformerFactory.Start(context.Done())
		return nil
	})

	// create shared (remote and local) availability metrics
	// TODO: decouple from legacyregistry
	metrics := availabilitymetrics.New()
	registerIntoLegacyRegistryOnce.Do(func() { err = metrics.Register(legacyregistry.Register, legacyregistry.CustomRegister) })
	if err != nil {
		return nil, err
	}

	// always run local availability controller
	local, err := localavailability.New(
		informerFactory.Apiregistration().V1().APIServices(),
		apiregistrationClient.ApiregistrationV1(),
		metrics,
	)
	if err != nil {
		return nil, err
	}
	s.GenericAPIServer.AddPostStartHookOrDie("apiservice-status-local-available-controller", func(context genericapiserver.PostStartHookContext) error {
		// if we end up blocking for long periods of time, we may need to increase workers.
		go local.Run(5, context.Done())
		return nil
	})

	// conditionally run remote availability controller. This could be replaced in certain
	// generic controlplane use-cases where there is another concept of services and/or endpoints.
	if !c.ExtraConfig.DisableRemoteAvailableConditionController {
		remote, err := remoteavailability.New(
			informerFactory.Apiregistration().V1().APIServices(),
			c.GenericConfig.SharedInformerFactory.Core().V1().Services(),
			c.GenericConfig.SharedInformerFactory.Discovery().V1().EndpointSlices(),
			apiregistrationClient.ApiregistrationV1(),
			proxyTransportDial,
			(func() ([]byte, []byte))(s.proxyCurrentCertKeyContent),
			s.serviceResolver,
			metrics,
		)
		if err != nil {
			return nil, err
		}
		s.GenericAPIServer.AddPostStartHookOrDie("apiservice-status-remote-available-controller", func(context genericapiserver.PostStartHookContext) error {
			// if we end up blocking for long periods of time, we may need to increase workers.
			go remote.Run(5, context.Done())
			return nil
		})
	}

	s.GenericAPIServer.AddPostStartHookOrDie("apiservice-registration-controller", func(context genericapiserver.PostStartHookContext) error {
		go apiserviceRegistrationController.Run(context.Done(), apiServiceRegistrationControllerInitiated)
		select {
		case <-context.Done():
		case <-apiServiceRegistrationControllerInitiated:
		}

		return nil
	})

	s.discoveryAggregationController = NewDiscoveryManager(
		// Use aggregator as the source name to avoid overwriting native/CRD
		// groups
		s.GenericAPIServer.AggregatedDiscoveryGroupManager.WithSource(aggregated.AggregatorSource),
	)

	// Setup discovery endpoint
	s.GenericAPIServer.AddPostStartHookOrDie("apiservice-discovery-controller", func(context genericapiserver.PostStartHookContext) error {
		// Discovery aggregation depends on the apiservice registration controller
		// having the full list of APIServices already synced
		select {
		case <-context.Done():
			return nil
		// Context cancelled, should abort/clean goroutines
		case <-apiServiceRegistrationControllerInitiated:
		}

		// Run discovery manager's worker to watch for new/removed/updated
		// APIServices to the discovery document can be updated at runtime
		// When discovery is ready, all APIServices will be present, with APIServices
		// that have not successfully synced discovery to be present but marked as Stale.
		discoverySyncedCh := make(chan struct{})
		go s.discoveryAggregationController.Run(context.Done(), discoverySyncedCh)

		select {
		case <-context.Done():
			return nil
		// Context cancelled, should abort/clean goroutines
		case <-discoverySyncedCh:
			// API services successfully sync
		}
		return nil
	})

	if utilfeature.DefaultFeatureGate.Enabled(genericfeatures.StorageVersionAPI) &&
		utilfeature.DefaultFeatureGate.Enabled(genericfeatures.APIServerIdentity) {
		// Spawn a goroutine in aggregator apiserver to update storage version for
		// all built-in resources
		s.GenericAPIServer.AddPostStartHookOrDie(StorageVersionPostStartHookName, func(hookContext genericapiserver.PostStartHookContext) error {
			// Wait for apiserver-identity to exist first before updating storage
			// versions, to avoid storage version GC accidentally garbage-collecting
			// storage versions.
			kubeClient, err := kubernetes.NewForConfig(hookContext.LoopbackClientConfig)
			if err != nil {
				return err
			}
			if err := wait.PollImmediateUntil(100*time.Millisecond, func() (bool, error) {
				_, err := kubeClient.CoordinationV1().Leases(metav1.NamespaceSystem).Get(
					context.TODO(), s.GenericAPIServer.APIServerID, metav1.GetOptions{})
				if apierrors.IsNotFound(err) {
					return false, nil
				}
				if err != nil {
					return false, err
				}
				return true, nil
			}, hookContext.Done()); err != nil {
				return fmt.Errorf("failed to wait for apiserver-identity lease %s to be created: %v",
					s.GenericAPIServer.APIServerID, err)
			}
			// Technically an apiserver only needs to update storage version once during bootstrap.
			// Reconcile StorageVersion objects every 10 minutes will help in the case that the
			// StorageVersion objects get accidentally modified/deleted by a different agent. In that
			// case, the reconciliation ensures future storage migration still works. If nothing gets
			// changed, the reconciliation update is a noop and gets short-circuited by the apiserver,
			// therefore won't change the resource version and trigger storage migration.
			go wait.PollImmediateUntil(10*time.Minute, func() (bool, error) {
				// All apiservers (aggregator-apiserver, kube-apiserver, apiextensions-apiserver)
				// share the same generic apiserver config. The same StorageVersion manager is used
				// to register all built-in resources when the generic apiservers install APIs.
				s.GenericAPIServer.StorageVersionManager.UpdateStorageVersions(hookContext.LoopbackClientConfig, s.GenericAPIServer.APIServerID)
				return false, nil
			}, hookContext.Done())
			// Once the storage version updater finishes the first round of update,
			// the PostStartHook will return to unblock /healthz. The handler chain
			// won't block write requests anymore. Check every second since it's not
			// expensive.
			wait.PollImmediateUntil(1*time.Second, func() (bool, error) {
				return s.GenericAPIServer.StorageVersionManager.Completed(), nil
			}, hookContext.Done())
			return nil
		})
	}

	return s, nil
}

// PrepareRun prepares the aggregator to run, by setting up the OpenAPI spec &
// aggregated discovery document and calling the generic PrepareRun.
func (s *APIAggregator) PrepareRun() (preparedAPIAggregator, error) {
	// add post start hook before generic PrepareRun in order to be before /healthz installation
	if s.openAPIConfig != nil {
		s.GenericAPIServer.AddPostStartHookOrDie("apiservice-openapi-controller", func(context genericapiserver.PostStartHookContext) error {
			go s.openAPIAggregationController.Run(context.Done())
			return nil
		})
	}

	if s.openAPIV3Config != nil {
		s.GenericAPIServer.AddPostStartHookOrDie("apiservice-openapiv3-controller", func(context genericapiserver.PostStartHookContext) error {
			go s.openAPIV3AggregationController.Run(context.Done())
			return nil
		})
	}

	prepared := s.GenericAPIServer.PrepareRun()

	// delay OpenAPI setup until the delegate had a chance to setup their OpenAPI handlers
	if s.openAPIConfig != nil {
		specDownloader := openapiaggregator.NewDownloader()
		openAPIAggregator, err := openapiaggregator.BuildAndRegisterAggregator(
			&specDownloader,
			s.GenericAPIServer.NextDelegate(),
			s.GenericAPIServer.Handler.GoRestfulContainer.RegisteredWebServices(),
			s.openAPIConfig,
			s.GenericAPIServer.Handler.NonGoRestfulMux)
		if err != nil {
			return preparedAPIAggregator{}, err
		}
		s.openAPIAggregationController = openapicontroller.NewAggregationController(&specDownloader, openAPIAggregator)
	}

	if s.openAPIV3Config != nil {
		specDownloaderV3 := openapiv3aggregator.NewDownloader()
		openAPIV3Aggregator, err := openapiv3aggregator.BuildAndRegisterAggregator(
			specDownloaderV3,
			s.GenericAPIServer.NextDelegate(),
			s.GenericAPIServer.Handler.GoRestfulContainer,
			s.openAPIV3Config,
			s.GenericAPIServer.Handler.NonGoRestfulMux)
		if err != nil {
			return preparedAPIAggregator{}, err
		}
		s.openAPIV3AggregationController = openapiv3controller.NewAggregationController(openAPIV3Aggregator)
	}

	return preparedAPIAggregator{APIAggregator: s, runnable: prepared}, nil
}

func (s preparedAPIAggregator) Run(ctx context.Context) error {
	return s.runnable.RunWithContext(ctx)
}

// AddAPIService adds an API service.  It is not thread-safe, so only call it on one thread at a time please.
// It's a slow moving API, so its ok to run the controller on a single thread
func (s *APIAggregator) AddAPIService(apiService *v1.APIService) error {
	// if the proxyHandler already exists, it needs to be updated. The aggregation bits do not
	// since they are wired against listers because they require multiple resources to respond
	if proxyHandler, exists := s.proxyHandlers[apiService.Name]; exists {
		proxyHandler.updateAPIService(apiService)
		if s.openAPIAggregationController != nil {
			s.openAPIAggregationController.UpdateAPIService(proxyHandler, apiService)
		}
		if s.openAPIV3AggregationController != nil {
			s.openAPIV3AggregationController.UpdateAPIService(proxyHandler, apiService)
		}
		// Forward calls to discovery manager to update discovery document
		if s.discoveryAggregationController != nil {
			handlerCopy := *proxyHandler
			handlerCopy.setServiceAvailable()
			s.discoveryAggregationController.AddAPIService(apiService, &handlerCopy)
		}
		return nil
	}

	proxyPath := "/apis/" + apiService.Spec.Group + "/" + apiService.Spec.Version
	// v1. is a special case for the legacy API.  It proxies to a wider set of endpoints.
	if apiService.Name == legacyAPIServiceName {
		proxyPath = "/api"
	}

	// register the proxy handler
	proxyHandler := &proxyHandler{
		localDelegate:              s.delegateHandler,
		proxyCurrentCertKeyContent: s.proxyCurrentCertKeyContent,
		proxyTransportDial:         s.proxyTransportDial,
		serviceResolver:            s.serviceResolver,
		rejectForwardingRedirects:  s.rejectForwardingRedirects,
		tracerProvider:             s.tracerProvider,
	}
	proxyHandler.updateAPIService(apiService)
	if s.openAPIAggregationController != nil {
		s.openAPIAggregationController.AddAPIService(proxyHandler, apiService)
	}
	if s.openAPIV3AggregationController != nil {
		s.openAPIV3AggregationController.AddAPIService(proxyHandler, apiService)
	}
	if s.discoveryAggregationController != nil {
		s.discoveryAggregationController.AddAPIService(apiService, proxyHandler)
	}

	s.proxyHandlers[apiService.Name] = proxyHandler
	s.GenericAPIServer.Handler.NonGoRestfulMux.Handle(proxyPath, proxyHandler)
	s.GenericAPIServer.Handler.NonGoRestfulMux.UnlistedHandlePrefix(proxyPath+"/", proxyHandler)

	// if we're dealing with the legacy group, we're done here
	if apiService.Name == legacyAPIServiceName {
		return nil
	}

	// if we've already registered the path with the handler, we don't want to do it again.
	versions, exist := s.handledGroupVersions[apiService.Spec.Group]
	if exist {
		versions.Insert(apiService.Spec.Version)
		return nil
	}

	// it's time to register the group aggregation endpoint
	groupPath := "/apis/" + apiService.Spec.Group
	groupDiscoveryHandler := &apiGroupHandler{
		codecs:    aggregatorscheme.Codecs,
		groupName: apiService.Spec.Group,
		lister:    s.lister,
		delegate:  s.delegateHandler,
	}
	// aggregation is protected
	s.GenericAPIServer.Handler.NonGoRestfulMux.Handle(groupPath, groupDiscoveryHandler)
	s.GenericAPIServer.Handler.NonGoRestfulMux.UnlistedHandle(groupPath+"/", groupDiscoveryHandler)
	s.handledGroupVersions[apiService.Spec.Group] = sets.New[string](apiService.Spec.Version)
	return nil
}

// RemoveAPIService removes the APIService from being handled.  It is not thread-safe, so only call it on one thread at a time please.
// It's a slow moving API, so it's ok to run the controller on a single thread.
func (s *APIAggregator) RemoveAPIService(apiServiceName string) {
	// Forward calls to discovery manager to update discovery document
	if s.discoveryAggregationController != nil {
		s.discoveryAggregationController.RemoveAPIService(apiServiceName)
	}

	version := v1helper.APIServiceNameToGroupVersion(apiServiceName)

	proxyPath := "/apis/" + version.Group + "/" + version.Version
	// v1. is a special case for the legacy API.  It proxies to a wider set of endpoints.
	if apiServiceName == legacyAPIServiceName {
		proxyPath = "/api"
	}
	s.GenericAPIServer.Handler.NonGoRestfulMux.Unregister(proxyPath)
	s.GenericAPIServer.Handler.NonGoRestfulMux.Unregister(proxyPath + "/")
	if s.openAPIAggregationController != nil {
		s.openAPIAggregationController.RemoveAPIService(apiServiceName)
	}
	if s.openAPIV3AggregationController != nil {
		s.openAPIV3AggregationController.RemoveAPIService(apiServiceName)
	}
	delete(s.proxyHandlers, apiServiceName)

	versions, exist := s.handledGroupVersions[version.Group]
	if !exist {
		return
	}
	versions.Delete(version.Version)
	if versions.Len() > 0 {
		return
	}
	delete(s.handledGroupVersions, version.Group)
	groupPath := "/apis/" + version.Group
	s.GenericAPIServer.Handler.NonGoRestfulMux.Unregister(groupPath)
	s.GenericAPIServer.Handler.NonGoRestfulMux.Unregister(groupPath + "/")
}

// DefaultAPIResourceConfigSource returns default configuration for an APIResource.
func DefaultAPIResourceConfigSource() *serverstorage.ResourceConfig {
	ret := serverstorage.NewResourceConfig()
	// NOTE: GroupVersions listed here will be enabled by default. Don't put alpha versions in the list.
	ret.EnableVersions(
		v1.SchemeGroupVersion,
		v1beta1.SchemeGroupVersion,
	)

	return ret
}
