【kubernetes/k8s源码分析】 kube-apiserver admission controller 源码分析

kubernetes git version: v1.14

初始化阶段 admission 代码流程

main

      -->  NewAPIServerCommand

                  -->  NewServerRunOptions

                             -->  kubeoptions.NewAdmissionOptions

结构体 AdmissionOptions 

    路径 k8s.io/apiserver/pkg/server/options/admission.go

• RecommendedPluginOrder：代表了有序的推荐插件列表集合
• DefaultOffPlugins：代表了默认禁止的插件
• EnablePlugins:：开启的插件列表，通过kube-apiserver 启动参数设置 --enable-admission-plugins 选项
• DisablePlugins:：禁止的插件列表，通过kube-apiserver 启动参数设置 --disable-admission-plugins 选项
• Plugins：代表了所有已经注册的插件

// AdmissionOptions holds the admission options
type AdmissionOptions struct {
	// RecommendedPluginOrder holds an ordered list of plugin names we recommend to use by default
	RecommendedPluginOrder []string
	// DefaultOffPlugins is a set of plugin names that is disabled by default
	DefaultOffPlugins sets.String

	// EnablePlugins indicates plugins to be enabled passed through `--enable-admission-plugins`.
	EnablePlugins []string
	// DisablePlugins indicates plugins to be disabled passed through `--disable-admission-plugins`.
	DisablePlugins []string
	// ConfigFile is the file path with admission control configuration.
	ConfigFile string
	// Plugins contains all registered plugins.
	Plugins *admission.Plugins
	// Decorators is a list of admission decorator to wrap around the admission plugins
	Decorators admission.Decorators
}

 

1. NewAdmissionOptions

      实例化 AdmissionOptions，包括基本通用的 admission，注册这些admission 插件，AllOrderedPlugins 定义了插件的推荐顺序集合，DefaultOffAdmissionPlugins 定义了默认关闭的 admission 插件

// NewAdmissionOptions creates a new instance of AdmissionOptions
// Note:
//  In addition it calls RegisterAllAdmissionPlugins to register
//  all kube-apiserver admission plugins.
//
//  Provides the list of RecommendedPluginOrder that holds sane values
//  that can be used by servers that don't care about admission chain.
//  Servers that do care can overwrite/append that field after creation.
func NewAdmissionOptions() *AdmissionOptions {
	options := genericoptions.NewAdmissionOptions()
	// register all admission plugins
	RegisterAllAdmissionPlugins(options.Plugins)
	// set RecommendedPluginOrder
	options.RecommendedPluginOrder = AllOrderedPlugins
	// set DefaultOffPlugins
	options.DefaultOffPlugins = DefaultOffAdmissionPlugins()

	return &AdmissionOptions{
		GenericAdmission: options,
	}
}

    1.1 NewAdmissionOptions

      路径 vendor/k8s.io/apiserver/pkg/server/options/admission.go

      实例化 AdmissionOptions，具体参数详看上述 '结构体 AdmissionOptions'

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      RecommendPluginOrder 推荐有序插件包括NamespaceLifecycle，MutatingAdmissionWebhook，ValidatingAdmissionWebhook

       分别调用 Register 函数注册这三个推荐的有序插件

// NewAdmissionOptions creates a new instance of AdmissionOptions
// Note:
//  In addition it calls RegisterAllAdmissionPlugins to register
//  all generic admission plugins.
//
//  Provides the list of RecommendedPluginOrder that holds sane values
//  that can be used by servers that don't care about admission chain.
//  Servers that do care can overwrite/append that field after creation.
func NewAdmissionOptions() *AdmissionOptions {
	options := &AdmissionOptions{
		Plugins: admission.NewPlugins(),
		// This list is mix of mutating admission plugins and validating
		// admission plugins. The apiserver always runs the validating ones
		// after all the mutating ones, so their relative order in this list
		// doesn't matter.
		RecommendedPluginOrder: []string{lifecycle.PluginName, initialization.PluginName, mutatingwebhook.PluginName, validatingwebhook.PluginName},
		DefaultOffPlugins:      sets.NewString(initialization.PluginName),
	}
	server.RegisterAllAdmissionPlugins(options.Plugins)
	return options
}

    1.1.1 NewPlugins 实例化 Plugins

     registry 记录了插件以及方法

// Factory is a function that returns an Interface for admission decisions.
// The config parameter provides an io.Reader handler to the factory in
// order to load specific configurations. If no configuration is provided
// the parameter is nil.
type Factory func(config io.Reader) (Interface, error)

type Plugins struct {
	lock     sync.Mutex
	registry map[string]Factory
}

func NewPlugins() *Plugins {
	return &Plugins{}
}

    1.1.2 RegisterAllAdmissionPlugins

       分别调用 Register 函数注册这三个推荐的有序插件，包括NamespaceLifecycle，MutatingAdmissionWebhook，ValidatingAdmissionWebhook

// RegisterAllAdmissionPlugins registers all admission plugins
func RegisterAllAdmissionPlugins(plugins *admission.Plugins) {
	lifecycle.Register(plugins)
	validatingwebhook.Register(plugins)
	mutatingwebhook.Register(plugins)
}

   1.1.3 Register 函数

     比如 lifecycle 这个插件注册NamespaceLifecycle 以及方法

// Register registers a plugin
func Register(plugins *admission.Plugins) {
	plugins.Register(PluginName, func(config io.Reader) (admission.Interface, error) {
		return NewLifecycle(sets.NewString(metav1.NamespaceDefault, metav1.NamespaceSystem, metav1.NamespacePublic))
	})
}

    1.2 RegisterAllAdmissionPlugins

      注册了一大堆 admission 插件，与 1.1.3 类同

// RegisterAllAdmissionPlugins registers all admission plugins and
// sets the recommended plugins order.
func RegisterAllAdmissionPlugins(plugins *admission.Plugins) {
	admit.Register(plugins) // DEPRECATED as no real meaning
	alwayspullimages.Register(plugins)
	antiaffinity.Register(plugins)
	defaulttolerationseconds.Register(plugins)
	deny.Register(plugins) // DEPRECATED as no real meaning
	eventratelimit.Register(plugins)
	exec.Register(plugins)
	extendedresourcetoleration.Register(plugins)
	gc.Register(plugins)
	imagepolicy.Register(plugins)
	limitranger.Register(plugins)
	autoprovision.Register(plugins)
	exists.Register(plugins)
	noderestriction.Register(plugins)
	nodetaint.Register(plugins)
	label.Register(plugins) // DEPRECATED, future PVs should not rely on labels for zone topology
	podnodeselector.Register(plugins)
	podpreset.Register(plugins)
	podtolerationrestriction.Register(plugins)
	resourcequota.Register(plugins)
	podsecuritypolicy.Register(plugins)
	podpriority.Register(plugins)
	scdeny.Register(plugins)
	serviceaccount.Register(plugins)
	setdefault.Register(plugins)
	resize.Register(plugins)
	storageobjectinuseprotection.Register(plugins)
}

   1.3 AllOrderedPlugins 定义了有序的 admission 插件列表

// AllOrderedPlugins is the list of all the plugins in order.
var AllOrderedPlugins = []string{
	admit.PluginName,                        // AlwaysAdmit
	autoprovision.PluginName,                // NamespaceAutoProvision
	lifecycle.PluginName,                    // NamespaceLifecycle
	exists.PluginName,                       // NamespaceExists
	scdeny.PluginName,                       // SecurityContextDeny
	antiaffinity.PluginName,                 // LimitPodHardAntiAffinityTopology
	podpreset.PluginName,                    // PodPreset
	limitranger.PluginName,                  // LimitRanger
	serviceaccount.PluginName,               // ServiceAccount
	noderestriction.PluginName,              // NodeRestriction
	nodetaint.PluginName,                    // TaintNodesByCondition
	alwayspullimages.PluginName,             // AlwaysPullImages
	imagepolicy.PluginName,                  // ImagePolicyWebhook
	podsecuritypolicy.PluginName,            // PodSecurityPolicy
	podnodeselector.PluginName,              // PodNodeSelector
	podpriority.PluginName,                  // Priority
	defaulttolerationseconds.PluginName,     // DefaultTolerationSeconds
	podtolerationrestriction.PluginName,     // PodTolerationRestriction
	exec.DenyEscalatingExec,                 // DenyEscalatingExec
	exec.DenyExecOnPrivileged,               // DenyExecOnPrivileged
	eventratelimit.PluginName,               // EventRateLimit
	extendedresourcetoleration.PluginName,   // ExtendedResourceToleration
	label.PluginName,                        // PersistentVolumeLabel
	setdefault.PluginName,                   // DefaultStorageClass
	storageobjectinuseprotection.PluginName, // StorageObjectInUseProtection
	gc.PluginName,                           // OwnerReferencesPermissionEnforcement
	resize.PluginName,                       // PersistentVolumeClaimResize
	mutatingwebhook.PluginName,              // MutatingAdmissionWebhook
	validatingwebhook.PluginName,            // ValidatingAdmissionWebhook
	resourcequota.PluginName,                // ResourceQuota
	deny.PluginName,                         // AlwaysDeny
}

    1.4 DefaultOffAdmissionPlugins 定义了禁止的 admission plugin 列表

     默认开启的 admission plugin 有 

• NamespaceLifecycle
• LimitRanger
• ServiceAccount
• DefaultStorageClass
• PersistentVolumeClaimResize
• DefaultTolerationSeconds
• MutatingAdmissionWebhook
• ValidatingAdmissionWebhook
• ResourceQuota

// DefaultOffAdmissionPlugins get admission plugins off by default for kube-apiserver.
func DefaultOffAdmissionPlugins() sets.String {
	defaultOnPlugins := sets.NewString(
		lifecycle.PluginName,                //NamespaceLifecycle
		limitranger.PluginName,              //LimitRanger
		serviceaccount.PluginName,           //ServiceAccount
		setdefault.PluginName,               //DefaultStorageClass
		resize.PluginName,                   //PersistentVolumeClaimResize
		defaulttolerationseconds.PluginName, //DefaultTolerationSeconds
		mutatingwebhook.PluginName,          //MutatingAdmissionWebhook
		validatingwebhook.PluginName,        //ValidatingAdmissionWebhook
		resourcequota.PluginName,            //ResourceQuota
	)

	if utilfeature.DefaultFeatureGate.Enabled(features.PodPriority) {
		defaultOnPlugins.Insert(podpriority.PluginName) //PodPriority
	}

	if utilfeature.DefaultFeatureGate.Enabled(features.TaintNodesByCondition) {
		defaultOnPlugins.Insert(nodetaint.PluginName) //TaintNodesByCondition
	}

	return sets.NewString(AllOrderedPlugins...).Difference(defaultOnPlugins)
}

buildGenericConfig

       -->  s.Admission.ApplyTo

              -->  a.GenericAdmission.ApplyTo

                     -->  a.Plugins.NewFromPlugins

                             -->  InitPlugin

2. buildGenericConfig

     Config 结构主要用来初始化 admission 插件，路径实现 pkg/kubeapiserver/admission/config.go

admissionConfig := &kubeapiserveradmission.Config{
	ExternalInformers:    versionedInformers,
	LoopbackClientConfig: genericConfig.LoopbackClientConfig,
	CloudConfigFile:      s.CloudProvider.CloudConfigFile,
}

     admissionConfig.New 函数主要用来为 admission 建立插件以及 start hook，具体如下 2.1 讲解

pluginInitializers, admissionPostStartHook, err = admissionConfig.New(proxyTransport, serviceResolver)
if err != nil {
	lastErr = fmt.Errorf("failed to create admission plugin initializer: %v", err)
	return
}

    2.1 New 函数

      NewPluginInitializer 实例化 PluginInitializer，这个结构用来初始化 admission plugin

// New sets up the plugins and admission start hooks needed for admission
func (c *Config) New(proxyTransport *http.Transport, serviceResolver webhook.ServiceResolver) ([]admission.PluginInitializer, server.PostStartHookFunc, error) {
	webhookAuthResolverWrapper := webhook.NewDefaultAuthenticationInfoResolverWrapper(proxyTransport, c.LoopbackClientConfig)
	webhookPluginInitializer := webhookinit.NewPluginInitializer(webhookAuthResolverWrapper, serviceResolver)

	var cloudConfig []byte
	if c.CloudConfigFile != "" {
		var err error
		cloudConfig, err = ioutil.ReadFile(c.CloudConfigFile)
		if err != nil {
			klog.Fatalf("Error reading from cloud configuration file %s: %#v", c.CloudConfigFile, err)
		}
	}
	internalClient, err := internalclientset.NewForConfig(c.LoopbackClientConfig)
	if err != nil {
		return nil, nil, err
	}

    2.2 NewPluginInitializer 实例化 PluginInitializer

    具体实现路径为 pkg/kubeapiserver/admission/initializer.go

discoveryClient := cacheddiscovery.NewMemCacheClient(internalClient.Discovery())
discoveryRESTMapper := restmapper.NewDeferredDiscoveryRESTMapper(discoveryClient)
kubePluginInitializer := NewPluginInitializer(
	cloudConfig,
	discoveryRESTMapper,
	quotainstall.NewQuotaConfigurationForAdmission(),
)

    2.3 admissionPostStartHook 函数是重置 cache 操作

admissionPostStartHook := func(context genericapiserver.PostStartHookContext) error {
	discoveryRESTMapper.Reset()
	go utilwait.Until(discoveryRESTMapper.Reset, 30*time.Second, context.StopCh)
	return nil
}

return []admission.PluginInitializer{webhookPluginInitializer, kubePluginInitializer}, admissionPostStartHook, nil

3. ApplyTo

    路径 k8s.io/apiserver/pkg/server/options/admission.go

// ApplyTo adds the admission chain to the server configuration.
// In case admission plugin names were not provided by a custer-admin they will be prepared from the recommended/default values.
// In addition the method lazily initializes a generic plugin that is appended to the list of pluginInitializers
// note this method uses:
//  genericconfig.Authorizer
func (a *AdmissionOptions) ApplyTo(
	c *server.Config,
	informers informers.SharedInformerFactory,
	kubeAPIServerClientConfig *rest.Config,
	pluginInitializers ...admission.PluginInitializer,
) error {
	if a == nil {
		return nil
	}

    3.1 NewFromPlugins

    InitPlugin 主要对插件进行初始化工作，分别调用 Initialize 初始化，在调用 ValidateInitialization 验证是否实现了接口方法 ValidateInitialization

    最后包裹 handlers，chainAdmissionHandler 包含了插件的 handler，实现了 Admit 和 Validate 方法，就是对所有插件 handler 调用 Admit 和 Validate 方法

• func (admissionHandler chainAdmissionHandler) Admit(a Attributes, o ObjectInterfaces) error
• func (admissionHandler chainAdmissionHandler) Validate(a Attributes, o ObjectInterfaces) error

// NewFromPlugins returns an admission.Interface that will enforce admission control decisions of all
// the given plugins.
func (ps *Plugins) NewFromPlugins(pluginNames []string, configProvider ConfigProvider, pluginInitializer PluginInitializer, decorator Decorator) (Interface, error) {
	handlers := []Interface{}
	mutationPlugins := []string{}
	validationPlugins := []string{}
	for _, pluginName := range pluginNames {
		pluginConfig, err := configProvider.ConfigFor(pluginName)
		if err != nil {
			return nil, err
		}

		plugin, err := ps.InitPlugin(pluginName, pluginConfig, pluginInitializer)
		if err != nil {
			return nil, err
		}

	return chainAdmissionHandler(handlers), nil
}

    3.2 赋值 AdmissionControl

      具体操作就是又包裹了一下 pluginHandlerWithMetrics

• func (p pluginHandlerWithMetrics) Admit(a admission.Attributes, o admission.ObjectInterfaces) error
• func (p pluginHandlerWithMetrics) Validate(a admission.Attributes, o admission.ObjectInterfaces) error

// WithStepMetrics is a decorator for a whole admission phase, i.e. admit or validation.admission step.
func WithStepMetrics(i admission.Interface) admission.Interface {
	return WithMetrics(i, Metrics.ObserveAdmissionStep)
}

// WithMetrics is a decorator for admission handlers with a generic observer func.
func WithMetrics(i admission.Interface, observer ObserverFunc, extraLabels ...string) admission.Interface {
	return &pluginHandlerWithMetrics{
		Interface:   i,
		observer:    observer,
		extraLabels: extraLabels,
	}
}

    初始化工作完成了，那究竟哪里会使用 admission 呢？ 请看下文，具体 kube-apiserver 启动源码分析不再赘述，直接关键函数开始

registerResourceHandlers

      -->  metrics.InstrumentRouteFunc

                     -->  restfulUpdateResource              PUT

                     -->  restfulPatchResource                 PATCH

                     -->  restfulCreateNamedResource    POST

                     -->  restfulCreateResource               POST

                     -->  restfulDeleteResource               DELETE

                     -->  restfulDeleteCollection               DELETECOLLECTION

                     -->  restfulConnectResource            CONNECT 

4. registerResourceHandlers

    可以得到 PUT PATCH POST DELETE DELETECOLLECTION CONNECT 这些 method 用到了 admission 

switch action.Verb {
case "PUT": // Update a resource.
	doc := "replace the specified " + kind
	if isSubresource {
		doc = "replace " + subresource + " of the specified " + kind
	}
	handler := metrics.InstrumentRouteFunc(action.Verb, group, version, resource, subresource, requestScope, metrics.APIServerComponent, restfulUpdateResource(updater, reqScope, admit))
	
case "PATCH": // Partially update a resource
	handler := metrics.InstrumentRouteFunc(action.Verb, group, version, resource, subresource, requestScope, metrics.APIServerComponent, restfulPatchResource(patcher, reqScope, admit, supportedTypes))
	
case "POST": // Create a resource.
	var handler restful.RouteFunction
	if isNamedCreater {
		handler = restfulCreateNamedResource(namedCreater, reqScope, admit)
	} else {
		handler = restfulCreateResource(creater, reqScope, admit)
	}
	handler = metrics.InstrumentRouteFunc(action.Verb, group, version, resource, subresource, requestScope, metrics.APIServerComponent, handler)
	
case "DELETE": // Delete a resource.
	handler := metrics.InstrumentRouteFunc(action.Verb, group, version, resource, subresource, requestScope, metrics.APIServerComponent, restfulDeleteResource(gracefulDeleter, isGracefulDeleter, reqScope, admit))
	
case "DELETECOLLECTION":
	handler := metrics.InstrumentRouteFunc(action.Verb, group, version, resource, subresource, requestScope, metrics.APIServerComponent, restfulDeleteCollection(collectionDeleter, isCollectionDeleter, reqScope, admit))
	
case "CONNECT":
	for _, method := range connecter.ConnectMethods() {
		handler := metrics.InstrumentRouteFunc(action.Verb, group, version, resource, subresource, requestScope, metrics.APIServerComponent, restfulConnectResource(connecter, reqScope, admit, path, isSubresource))

}

    4.1 InstrumentRouteFunc 函数

     包裹了 type RouteFunction func(*Request, *Response) ,主要关心 routeFunc(request,response),也就是传入的 restfulXXXXXResource ,如上列表所示

// InstrumentRouteFunc works like Prometheus' InstrumentHandlerFunc but wraps
// the go-restful RouteFunction instead of a HandlerFunc plus some Kubernetes endpoint specific information.
func InstrumentRouteFunc(verb, group, version, resource, subresource, scope, component string, routeFunc restful.RouteFunction) restful.RouteFunction {
	return restful.RouteFunction(func(request *restful.Request, response *restful.Response) {
		now := time.Now()

		delegate := &ResponseWriterDelegator{ResponseWriter: response.ResponseWriter}

		_, cn := response.ResponseWriter.(http.CloseNotifier)
		_, fl := response.ResponseWriter.(http.Flusher)
		_, hj := response.ResponseWriter.(http.Hijacker)
		var rw http.ResponseWriter
		if cn && fl && hj {
			rw = &fancyResponseWriterDelegator{delegate}
		} else {
			rw = delegate
		}
		response.ResponseWriter = rw

		routeFunc(request, response)

		MonitorRequest(request.Request, verb, group, version, resource, subresource, scope, component, delegate.Header().Get("Content-Type"), delegate.Status(), delegate.ContentLength(), time.Since(now))
	})
}

    4.2 restfulCreateResource 函数

      简单,假设不知道干麽的,继续看代码

func restfulCreateResource(r rest.Creater, scope handlers.RequestScope, admit admission.Interface) restful.RouteFunction {
	return func(req *restful.Request, res *restful.Response) {
		handlers.CreateResource(r, scope, admit)(res.ResponseWriter, req.Request)
	}
}

// CreateResource returns a function that will handle a resource creation.
func CreateResource(r rest.Creater, scope RequestScope, admission admission.Interface) http.HandlerFunc {
	return createHandler(&namedCreaterAdapter{r}, scope, admission, false)
}

    4.3 createHandler 函数

     路径 k8s.io/apiserver/pkg/endpoints/handlers/create.go,终于找到主要的执行函数了,这个对于 CREATE 的action

func createHandler(r rest.NamedCreater, scope RequestScope, admit admission.Interface, includeName bool) http.HandlerFunc {
	return func(w http.ResponseWriter, req *http.Request) {
		// For performance tracking purposes.
		trace := utiltrace.New("Create " + req.URL.Path)
		defer trace.LogIfLong(500 * time.Millisecond)

		if isDryRun(req.URL) && !utilfeature.DefaultFeatureGate.Enabled(features.DryRun) {
			scope.err(errors.NewBadRequest("the dryRun alpha feature is disabled"), w, req)
			return
		}

      此去省略几千字,略过一堆解析 body 等操作,只关心 admission 的处理

admissionAttributes := admission.NewAttributesRecord(obj, nil, scope.Kind, namespace, name, scope.Resource, scope.Subresource, admission.Create, dryrun.IsDryRun(options.DryRun), userInfo)

    4.4 结构体 attributeRecord 

       实现了接口 Attributes

type attributesRecord struct {
	kind        schema.GroupVersionKind
	namespace   string
	name        string
	resource    schema.GroupVersionResource
	subresource string
	operation   Operation
	dryRun      bool
	object      runtime.Object
	oldObject   runtime.Object
	userInfo    user.Info

	// other elements are always accessed in single goroutine.
	// But ValidatingAdmissionWebhook add annotations concurrently.
	annotations     map[string]string
	annotationsLock sync.RWMutex
}

    4.5 这个已经是包裹的 pluginHandlerWithMetrics 在层层拨开,具体到每个 admissin 插件的 Admit 方法

       本文只讲解具体的几个 admission plugin,其他雷同

if mutatingAdmission, ok := admit.(admission.MutationInterface); ok && mutatingAdmission.Handles(admission.Create) {
	err = mutatingAdmission.Admit(admissionAttributes, &scope)
	if err != nil {
		scope.err(err, w, req)
		return
	}
}

6. MutatingAdmissionWebhook

    group: admissionregistration.k8s.io kind: MutatingWebhookConfiguration

    6.1  NewMutatingWebhook 函数

    实例化 Plugin, 允许的包括 CONNECT CREATE DELETE UPDATE

webhooks:
  - name: kube-webhook.mutating.me
    clientConfig:
      service:
        name: kube-webhook-svc
        namespace: default
        path: "/webhook"
      caBundle: ${CA_BUNDLE}
    rules:
      - operations: [ "CREATE" ]
        apiGroups: ["apps"]
        apiVersions: ["v1"]
        resources: ["deployments"]

// NewMutatingWebhook returns a generic admission webhook plugin.
func NewMutatingWebhook(configFile io.Reader) (*Plugin, error) {
	handler := admission.NewHandler(admission.Connect, admission.Create, admission.Delete, admission.Update)
	p := &Plugin{}
	var err error
	p.Webhook, err = generic.NewWebhook(handler, configFile, configuration.NewMutatingWebhookConfigurationManager, newMutatingDispatcher(p))
	if err != nil {
		return nil, err
	}

	return p, nil
}

    6.2 Admit 函数

// Admit makes an admission decision based on the request attributes.
func (a *Plugin) Admit(attr admission.Attributes, o admission.ObjectInterfaces) error {
	return a.Webhook.Dispatch(attr, o)
}

    6.3 Dispatch 函数

        中间略过,直奔 Dispatch 这个函数,有原来实例化时 newMutatingDispatcher 就是实现了 Dispatcher 接口

// Dispatch is called by the downstream Validate or Admit methods.
func (a *Webhook) Dispatch(attr admission.Attributes, o admission.ObjectInterfaces) error {
	if rules.IsWebhookConfigurationResource(attr) {
		return nil
	}
	if !a.WaitForReady() {
		return admission.NewForbidden(attr, fmt.Errorf("not yet ready to handle request"))
	}
	hooks := a.hookSource.Webhooks()
	// TODO: Figure out if adding one second timeout make sense here.
	ctx := context.TODO()

    return a.dispatcher.Dispatch(ctx, &versionedAttr, o, relevantHooks)

    6.4 Dispatch 函数

     路径 k8s.io/apiserver/pkg/admission/plugin/webhook/mutating/dispatcher.go

     对每个 hook 调用 callAttrMutatingHook 进行处理

func (a *mutatingDispatcher) Dispatch(ctx context.Context, attr *generic.VersionedAttributes, o admission.ObjectInterfaces, relevantHooks []*v1beta1.Webhook) error {
	for _, hook := range relevantHooks {
		t := time.Now()
		err := a.callAttrMutatingHook(ctx, hook, attr, o)
		admissionmetrics.Metrics.ObserveWebhook(time.Since(t), err != nil, attr.Attributes, "admit", hook.Name)
		if err == nil {
			continue
		}

    6.5 callAttrMutatingHook 函数

// note that callAttrMutatingHook updates attr
func (a *mutatingDispatcher) callAttrMutatingHook(ctx context.Context, h *v1beta1.Webhook, attr *generic.VersionedAttributes, o admission.ObjectInterfaces) error {
	if attr.IsDryRun() {
		if h.SideEffects == nil {
			return &webhook.ErrCallingWebhook{WebhookName: h.Name, Reason: fmt.Errorf("Webhook SideEffects is nil")}
		}
		if !(*h.SideEffects == v1beta1.SideEffectClassNone || *h.SideEffects == v1beta1.SideEffectClassNoneOnDryRun) {
			return webhookerrors.NewDryRunUnsupportedErr(h.Name)
		}
	}

	// Currently dispatcher only supports `v1beta1` AdmissionReview
	// TODO: Make the dispatcher capable of sending multiple AdmissionReview versions
	if !util.HasAdmissionReviewVersion(v1beta1.SchemeGroupVersion.Version, h) {
		return &webhook.ErrCallingWebhook{WebhookName: h.Name, Reason: fmt.Errorf("webhook does not accept v1beta1 AdmissionReview")}
	}

    6.5.1 其他略过,这里是不是很熟悉

    创建 AdmissionReview 请求,发送 POST 请求,所以自己定义的 admission server 就可以接受到请求了

// AdmissionReview describes an admission review request/response.
type AdmissionReview struct {
   metav1.TypeMeta `json:",inline"`
   // Request describes the attributes for the admission request.
   // +optional
   Request *AdmissionRequest `json:"request,omitempty" protobuf:"bytes,1,opt,name=request"`
   // Response describes the attributes for the admission response.
   // +optional
   Response *AdmissionResponse `json:"response,omitempty" protobuf:"bytes,2,opt,name=response"`
}

// Make the webhook request
request := request.CreateAdmissionReview(attr)
client, err := a.cm.HookClient(util.HookClientConfigForWebhook(h))
if err != nil {
	return &webhook.ErrCallingWebhook{WebhookName: h.Name, Reason: err}
}
response := &admissionv1beta1.AdmissionReview{}
r := client.Post().Context(ctx).Body(&request)
if h.TimeoutSeconds != nil {
	r = r.Timeout(time.Duration(*h.TimeoutSeconds) * time.Second)
}
if err := r.Do().Into(response); err != nil {
	return &webhook.ErrCallingWebhook{WebhookName: h.Name, Reason: err}
}

if response.Response == nil {
	return &webhook.ErrCallingWebhook{WebhookName: h.Name, Reason: fmt.Errorf("Webhook response was absent")}
}

7. AlwaysPullImages admission 插件

    7.1 实例化 AlwaysPullImages

// AlwaysPullImages is an implementation of admission.Interface.
// It looks at all new pods and overrides each container's image pull policy to Always.
type AlwaysPullImages struct {
	*admission.Handler
}

// NewAlwaysPullImages creates a new always pull images admission control handler
func NewAlwaysPullImages() *AlwaysPullImages {
	return &AlwaysPullImages{
		Handler: admission.NewHandler(admission.Create, admission.Update),
	}
}

    7.2 Admit 函数

     比较简单就是修改了 pod 属性, 将 imagePullPolicy 修改为 Always

// Admit makes an admission decision based on the request attributes
func (a *AlwaysPullImages) Admit(attributes admission.Attributes, o admission.ObjectInterfaces) (err error) {
	// Ignore all calls to subresources or resources other than pods.
	if shouldIgnore(attributes) {
		return nil
	}
	pod, ok := attributes.GetObject().(*api.Pod)
	if !ok {
		return apierrors.NewBadRequest("Resource was marked with kind Pod but was unable to be converted")
	}

	for i := range pod.Spec.InitContainers {
		pod.Spec.InitContainers[i].ImagePullPolicy = api.PullAlways
	}

	for i := range pod.Spec.Containers {
		pod.Spec.Containers[i].ImagePullPolicy = api.PullAlways
	}

	return nil
}

    7.3 Validate 函数

      就是拿到 pod 信息, 验证是否  imagePullPolicy 为 Always

// Validate makes sure that all containers are set to always pull images
func (*AlwaysPullImages) Validate(attributes admission.Attributes, o admission.ObjectInterfaces) (err error) {
	if shouldIgnore(attributes) {
		return nil
	}

	pod, ok := attributes.GetObject().(*api.Pod)
	if !ok {
		return apierrors.NewBadRequest("Resource was marked with kind Pod but was unable to be converted")
	}

	for i := range pod.Spec.InitContainers {
		if pod.Spec.InitContainers[i].ImagePullPolicy != api.PullAlways {
			return admission.NewForbidden(attributes,
				field.NotSupported(field.NewPath("spec", "initContainers").Index(i).Child("imagePullPolicy"),
					pod.Spec.InitContainers[i].ImagePullPolicy, []string{string(api.PullAlways)},
				),
			)
		}
	}
	for i := range pod.Spec.Containers {
		if pod.Spec.Containers[i].ImagePullPolicy != api.PullAlways {
			return admission.NewForbidden(attributes,
				field.NotSupported(field.NewPath("spec", "containers").Index(i).Child("imagePullPolicy"),
					pod.Spec.Containers[i].ImagePullPolicy, []string{string(api.PullAlways)},
				),
			)
		}
	}

	return nil
}

总结:

     分析了 admission 初始化以及插件注册流程

     注册 API 时,PUT PATCH POST DELETE DELETECOLLECTION CONNECT 会加入 admission plugin 处理

     分析了 MutatingAdmissionWebhook 以及 AlwaysPullImages 插件实现