Support dry run in admission webhooks

pkg/apis/admission/types.go

@@ -73,6 +73,11 @@ type AdmissionRequest struct {
 	// OldObject is the existing object. Only populated for UPDATE requests.
 	// +optional
 	OldObject runtime.Object
+	// DryRun indicates that modifications will definitely not be persisted for this request.
+	// Calls to webhooks must have no side effects if DryRun is true.
+	// Defaults to false.
+	// +optional
+	DryRun *bool
 }
 
 // AdmissionResponse describes an admission response.

pkg/apis/admissionregistration/fuzzer/fuzzer.go

@@ -30,6 +30,8 @@ var Funcs = func(codecs runtimeserializer.CodecFactory) []interface{} {
 			c.FuzzNoCustom(obj) // fuzz self without calling this function again
 			p := admissionregistration.FailurePolicyType("Fail")
 			obj.FailurePolicy = &p
+			s := admissionregistration.SideEffectClassUnknown
+			obj.SideEffects = &s
 		},
 	}
 }

pkg/apis/admissionregistration/types.go

@@ -112,6 +112,22 @@ const (
 	Fail FailurePolicyType = "Fail"
 )
 
+type SideEffectClass string
+
+const (
+	// SideEffectClassUnknown means that no information is known about the side effects of calling the webhook.
+	// If a request with the dry-run attribute would trigger a call to this webhook, the request will instead fail.
+	SideEffectClassUnknown SideEffectClass = "Unknown"
+	// SideEffectClassNone means that calling the webhook will have no side effects.
+	SideEffectClassNone SideEffectClass = "None"
+	// SideEffectClassSome means that calling the webhook will possibly have side effects.
+	// If a request with the dry-run attribute would trigger a call to this webhook, the request will instead fail.
+	SideEffectClassSome SideEffectClass = "Some"
+	// SideEffectClassNoneOnDryRun means that calling the webhook will possibly have side effects, but if the
+	// request being reviewed has the dry-run attribute, the side effects will be suppressed.
+	SideEffectClassNoneOnDryRun SideEffectClass = "NoneOnDryRun"
+)
+
 // +genclient
 // +genclient:nonNamespaced
 // +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object
@@ -235,6 +251,15 @@ type Webhook struct {
 	// Default to the empty LabelSelector, which matches everything.
 	// +optional
 	NamespaceSelector *metav1.LabelSelector
+
+	// SideEffects states whether this webhookk has side effects.
+	// Acceptable values are: Unknown, None, Some, NoneOnDryRun
+	// Webhooks with side effects MUST implement a reconciliation system, since a request may be
+	// rejected by a future step in the admission change and the side effects therefore need to be undone.
+	// Requests with the dryRun attribute will be auto-rejected if they match a webhook with
+	// sideEffects == Unknown or Some. Defaults to Unknown.
+	// +optional
+	SideEffects *SideEffectClass
 }
 
 // RuleWithOperations is a tuple of Operations and Resources. It is recommended to make

pkg/apis/admissionregistration/v1beta1/defaults.go

@@ -35,4 +35,9 @@ func SetDefaults_Webhook(obj *admissionregistrationv1beta1.Webhook) {
 		selector := metav1.LabelSelector{}
 		obj.NamespaceSelector = &selector
 	}
+	if obj.SideEffects == nil {
+		// TODO: revisit/remove this default and possibly make the field required when promoting to v1
+		unknown := admissionregistrationv1beta1.SideEffectClassUnknown
+		obj.SideEffects = &unknown
+	}
 }

pkg/apis/admissionregistration/validation/validation.go

@@ -192,6 +192,9 @@ func validateWebhook(hook *admissionregistration.Webhook, fldPath *field.Path) f
 	if hook.FailurePolicy != nil && !supportedFailurePolicies.Has(string(*hook.FailurePolicy)) {
 		allErrors = append(allErrors, field.NotSupported(fldPath.Child("failurePolicy"), *hook.FailurePolicy, supportedFailurePolicies.List()))
 	}
+	if hook.SideEffects != nil && !supportedSideEffectClasses.Has(string(*hook.SideEffects)) {
+		allErrors = append(allErrors, field.NotSupported(fldPath.Child("sideEffects"), *hook.SideEffects, supportedSideEffectClasses.List()))
+	}
 
 	if hook.NamespaceSelector != nil {
 		allErrors = append(allErrors, metav1validation.ValidateLabelSelector(hook.NamespaceSelector, fldPath.Child("namespaceSelector"))...)
@@ -291,6 +294,13 @@ var supportedFailurePolicies = sets.NewString(
 	string(admissionregistration.Fail),
 )
 
+var supportedSideEffectClasses = sets.NewString(
+	string(admissionregistration.SideEffectClassUnknown),
+	string(admissionregistration.SideEffectClassNone),
+	string(admissionregistration.SideEffectClassSome),
+	string(admissionregistration.SideEffectClassNoneOnDryRun),
+)
+
 var supportedOperations = sets.NewString(
 	string(admissionregistration.OperationAll),
 	string(admissionregistration.Create),

pkg/apis/admissionregistration/validation/validation_test.go

@@ -499,6 +499,21 @@ func TestValidateValidatingWebhookConfiguration(t *testing.T) {
 				}),
 			expectedError: `webhooks[0].failurePolicy: Unsupported value: "other": supported values: "Fail", "Ignore"`,
 		},
+		{
+			name: "SideEffects can only be \"Unknown\", \"None\", \"Some\", or \"NoneOnDryRun\"",
+			config: newValidatingWebhookConfiguration(
+				[]admissionregistration.Webhook{
+					{
+						Name:         "webhook.k8s.io",
+						ClientConfig: validClientConfig,
+						SideEffects: func() *admissionregistration.SideEffectClass {
+							r := admissionregistration.SideEffectClass("other")
+							return &r
+						}(),
+					},
+				}),
+			expectedError: `webhooks[0].sideEffects: Unsupported value: "other": supported values: "None", "NoneOnDryRun", "Some", "Unknown"`,
+		},
 		{
 			name: "both service and URL missing",
 			config: newValidatingWebhookConfiguration(