ValidatingAdmissionPolicy: Variable Composition (#118642)

* [API REVIEW] Variable Composition

* lazy map.

* variable composition implementation.

* check variables during VAP validation.

* generated: ./hack/update-vendor.sh

* generated: UPDATE_COMPATIBILITY_FIXTURE_DATA

(cd staging/src/k8s.io/api/ && env UPDATE_COMPATIBILITY_FIXTURE_DATA=true go test)

* cost calucation.

* tests for cost calculations.

* e2e test for variables.

* fix doc for Validation.Expression.

* generated: ./hack/update-codegen.sh

* fix missing utilruntime import.

* generated: ./hack/update-openapi-spec.sh

api/openapi-spec/v3/apis__admissionregistration.k8s.io__v1alpha1_openapi.json

@@ -490,6 +490,24 @@
             },
             "type": "array",
             "x-kubernetes-list-type": "atomic"
+          },
+          "variables": {
+            "description": "Variables contain definitions of variables that can be used in composition of other expressions. Each variable is defined as a named CEL expression. The variables defined here will be available under `variables` in other expressions of the policy except MatchConditions because MatchConditions are evaluated before the rest of the policy.\n\nThe expression of a variable can refer to other variables defined earlier in the list but not those after. Thus, Variables must be sorted by the order of first appearance and acyclic.",
+            "items": {
+              "allOf": [
+                {
+                  "$ref": "#/components/schemas/io.k8s.api.admissionregistration.v1alpha1.Variable"
+                }
+              ],
+              "default": {}
+            },
+            "type": "array",
+            "x-kubernetes-list-map-keys": [
+              "name"
+            ],
+            "x-kubernetes-list-type": "map",
+            "x-kubernetes-patch-merge-key": "name",
+            "x-kubernetes-patch-strategy": "merge"
           }
         },
         "type": "object"
@@ -534,7 +552,7 @@
         "properties": {
           "expression": {
             "default": "",
-            "description": "Expression represents the expression which will be evaluated by CEL. ref: https://github.com/google/cel-spec CEL expressions have access to the contents of the API request/response, organized into CEL variables as well as some other useful variables:\n\n- 'object' - The object from the incoming request. The value is null for DELETE requests. - 'oldObject' - The existing object. The value is null for CREATE requests. - 'request' - Attributes of the API request([ref](/pkg/apis/admission/types.go#AdmissionRequest)). - 'params' - Parameter resource referred to by the policy binding being evaluated. Only populated if the policy has a ParamKind. - 'authorizer' - A CEL Authorizer. May be used to perform authorization checks for the principal (user or service account) of the request.\n  See https://pkg.go.dev/k8s.io/apiserver/pkg/cel/library#Authz\n- 'authorizer.requestResource' - A CEL ResourceCheck constructed from the 'authorizer' and configured with the\n  request resource.\n\nThe `apiVersion`, `kind`, `metadata.name` and `metadata.generateName` are always accessible from the root of the object. No other metadata properties are accessible.\n\nOnly property names of the form `[a-zA-Z_.-/][a-zA-Z0-9_.-/]*` are accessible. Accessible property names are escaped according to the following rules when accessed in the expression: - '__' escapes to '__underscores__' - '.' escapes to '__dot__' - '-' escapes to '__dash__' - '/' escapes to '__slash__' - Property names that exactly match a CEL RESERVED keyword escape to '__{keyword}__'. The keywords are:\n\t  \"true\", \"false\", \"null\", \"in\", \"as\", \"break\", \"const\", \"continue\", \"else\", \"for\", \"function\", \"if\",\n\t  \"import\", \"let\", \"loop\", \"package\", \"namespace\", \"return\".\nExamples:\n  - Expression accessing a property named \"namespace\": {\"Expression\": \"object.__namespace__ > 0\"}\n  - Expression accessing a property named \"x-prop\": {\"Expression\": \"object.x__dash__prop > 0\"}\n  - Expression accessing a property named \"redact__d\": {\"Expression\": \"object.redact__underscores__d > 0\"}\n\nEquality on arrays with list type of 'set' or 'map' ignores element order, i.e. [1, 2] == [2, 1]. Concatenation on arrays with x-kubernetes-list-type use the semantics of the list type:\n  - 'set': `X + Y` performs a union where the array positions of all elements in `X` are preserved and\n    non-intersecting elements in `Y` are appended, retaining their partial order.\n  - 'map': `X + Y` performs a merge where the array positions of all keys in `X` are preserved but the values\n    are overwritten by values in `Y` when the key sets of `X` and `Y` intersect. Elements in `Y` with\n    non-intersecting keys are appended, retaining their partial order.\nRequired.",
+            "description": "Expression represents the expression which will be evaluated by CEL. ref: https://github.com/google/cel-spec CEL expressions have access to the contents of the API request/response, organized into CEL variables as well as some other useful variables:\n\n- 'object' - The object from the incoming request. The value is null for DELETE requests. - 'oldObject' - The existing object. The value is null for CREATE requests. - 'request' - Attributes of the API request([ref](/pkg/apis/admission/types.go#AdmissionRequest)). - 'params' - Parameter resource referred to by the policy binding being evaluated. Only populated if the policy has a ParamKind. - 'variables' - Map of composited variables, from its name to its lazily evaluated value.\n  For example, a variable named 'foo' can be accessed as 'variables.foo'.\n- 'authorizer' - A CEL Authorizer. May be used to perform authorization checks for the principal (user or service account) of the request.\n  See https://pkg.go.dev/k8s.io/apiserver/pkg/cel/library#Authz\n- 'authorizer.requestResource' - A CEL ResourceCheck constructed from the 'authorizer' and configured with the\n  request resource.\n\nThe `apiVersion`, `kind`, `metadata.name` and `metadata.generateName` are always accessible from the root of the object. No other metadata properties are accessible.\n\nOnly property names of the form `[a-zA-Z_.-/][a-zA-Z0-9_.-/]*` are accessible. Accessible property names are escaped according to the following rules when accessed in the expression: - '__' escapes to '__underscores__' - '.' escapes to '__dot__' - '-' escapes to '__dash__' - '/' escapes to '__slash__' - Property names that exactly match a CEL RESERVED keyword escape to '__{keyword}__'. The keywords are:\n\t  \"true\", \"false\", \"null\", \"in\", \"as\", \"break\", \"const\", \"continue\", \"else\", \"for\", \"function\", \"if\",\n\t  \"import\", \"let\", \"loop\", \"package\", \"namespace\", \"return\".\nExamples:\n  - Expression accessing a property named \"namespace\": {\"Expression\": \"object.__namespace__ > 0\"}\n  - Expression accessing a property named \"x-prop\": {\"Expression\": \"object.x__dash__prop > 0\"}\n  - Expression accessing a property named \"redact__d\": {\"Expression\": \"object.redact__underscores__d > 0\"}\n\nEquality on arrays with list type of 'set' or 'map' ignores element order, i.e. [1, 2] == [2, 1]. Concatenation on arrays with x-kubernetes-list-type use the semantics of the list type:\n  - 'set': `X + Y` performs a union where the array positions of all elements in `X` are preserved and\n    non-intersecting elements in `Y` are appended, retaining their partial order.\n  - 'map': `X + Y` performs a merge where the array positions of all keys in `X` are preserved but the values\n    are overwritten by values in `Y` when the key sets of `X` and `Y` intersect. Elements in `Y` with\n    non-intersecting keys are appended, retaining their partial order.\nRequired.",
             "type": "string"
           },
           "message": {
@@ -555,6 +573,26 @@
         ],
         "type": "object"
       },
+      "io.k8s.api.admissionregistration.v1alpha1.Variable": {
+        "description": "Variable is the definition of a variable that is used for composition.",
+        "properties": {
+          "expression": {
+            "default": "",
+            "description": "Expression is the expression that will be evaluated as the value of the variable. The CEL expression has access to the same identifiers as the CEL expressions in Validation.",
+            "type": "string"
+          },
+          "name": {
+            "default": "",
+            "description": "Name is the name of the variable. The name must be a valid CEL identifier and unique among all variables. The variable can be accessed in other expressions through `variables` For example, if name is \"foo\", the variable will be available as `variables.foo`",
+            "type": "string"
+          }
+        },
+        "required": [
+          "name",
+          "expression"
+        ],
+        "type": "object"
+      },
       "io.k8s.apimachinery.pkg.apis.meta.v1.APIResource": {
         "description": "APIResource specifies the name of a resource and whether it is namespaced.",
         "properties": {