Add identity transformation packages idtransform and celformer

Implements Supervisor identity transformations helpers using CEL.

go.mod

@@ -14,6 +14,7 @@ require (
 	github.com/go-logr/zapr v1.2.3
 	github.com/gofrs/flock v0.8.1
 	github.com/golang/mock v1.6.0
+	github.com/google/cel-go v0.13.0
 	github.com/google/go-cmp v0.5.9
 	github.com/google/gofuzz v1.2.0
 	github.com/google/uuid v1.3.0
@@ -85,7 +86,6 @@ require (
 	github.com/golang/glog v1.0.0 // indirect
 	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
 	github.com/golang/protobuf v1.5.2 // indirect
-	github.com/google/cel-go v0.12.6 // indirect
 	github.com/google/gnostic v0.6.9 // indirect
 	github.com/grpc-ecosystem/go-grpc-prometheus v1.2.0 // indirect
 	github.com/grpc-ecosystem/grpc-gateway/v2 v2.7.0 // indirect
@@ -146,8 +146,8 @@ require (
 	golang.org/x/time v0.0.0-20220411224347-583f2d630306 // indirect
 	golang.org/x/tools v0.4.0 // indirect
 	google.golang.org/appengine v1.6.7 // indirect
-	google.golang.org/genproto v0.0.0-20220616135557-88e70c0c3a90 // indirect
+	google.golang.org/genproto v0.0.0-20221027153422-115e99e71e1c // indirect
-	google.golang.org/grpc v1.49.0 // indirect
+	google.golang.org/grpc v1.50.1 // indirect
 	google.golang.org/protobuf v1.28.1 // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect
 	gopkg.in/ini.v1 v1.66.6 // indirect

go.sum

@@ -260,8 +260,8 @@ github.com/golang/snappy v0.0.3/go.mod h1:/XxbfmMg8lxefKM7IXC3fBNl/7bRcc72aCRzEW
 github.com/google/btree v0.0.0-20180813153112-4030bb1f1f0c/go.mod h1:lNA+9X1NB3Zf8V7Ke586lFgjr2dZNuvo3lPJSGZ5JPQ=
 github.com/google/btree v1.0.0/go.mod h1:lNA+9X1NB3Zf8V7Ke586lFgjr2dZNuvo3lPJSGZ5JPQ=
 github.com/google/btree v1.0.1 h1:gK4Kx5IaGY9CD5sPJ36FHiBJ6ZXl0kilRiiCj+jdYp4=
-github.com/google/cel-go v0.12.6 h1:kjeKudqV0OygrAqA9fX6J55S8gj+Jre2tckIm5RoG4M=
+github.com/google/cel-go v0.13.0 h1:z+8OBOcmh7IeKyqwT/6IlnMvy621fYUqnTVPEdegGlU=
-github.com/google/cel-go v0.12.6/go.mod h1:Jk7ljRzLBhkmiAwBoUxB1sZSCVBAzkqPF25olK/iRDw=
+github.com/google/cel-go v0.13.0/go.mod h1:K2hpQgEjDp18J76a2DKFRlPBPpgRZgi6EbnpDgIhJ8s=
 github.com/google/gnostic v0.6.9 h1:ZK/5VhkoX835RikCHpSUJV9a+S3e1zLh59YnyWeBW+0=
 github.com/google/gnostic v0.6.9/go.mod h1:Nm8234We1lq6iB9OmlgNv3nH91XLLVZHCDayfA3xq+E=
 github.com/google/go-cmp v0.2.0/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=
@@ -1079,8 +1079,9 @@ google.golang.org/genproto v0.0.0-20220505152158-f39f71e6c8f3/go.mod h1:RAyBrSAP
 google.golang.org/genproto v0.0.0-20220518221133-4f43b3371335/go.mod h1:RAyBrSAP7Fh3Nc84ghnVLDPuV51xc9agzmm4Ph6i0Q4=
 google.golang.org/genproto v0.0.0-20220523171625-347a074981d8/go.mod h1:RAyBrSAP7Fh3Nc84ghnVLDPuV51xc9agzmm4Ph6i0Q4=
 google.golang.org/genproto v0.0.0-20220608133413-ed9918b62aac/go.mod h1:KEWEmljWE5zPzLBa/oHl6DaEt9LmfH6WtH1OHIvleBA=
-google.golang.org/genproto v0.0.0-20220616135557-88e70c0c3a90 h1:4SPz2GL2CXJt28MTF8V6Ap/9ZiVbQlJeGSd9qtA7DLs=
 google.golang.org/genproto v0.0.0-20220616135557-88e70c0c3a90/go.mod h1:KEWEmljWE5zPzLBa/oHl6DaEt9LmfH6WtH1OHIvleBA=
+google.golang.org/genproto v0.0.0-20221027153422-115e99e71e1c h1:QgY/XxIAIeccR+Ca/rDdKubLIU9rcJ3xfy1DC/Wd2Oo=
+google.golang.org/genproto v0.0.0-20221027153422-115e99e71e1c/go.mod h1:CGI5F/G+E5bKwmfYo09AXuVN4dD894kIKUFmVbP2/Fo=
 google.golang.org/grpc v1.19.0/go.mod h1:mqu4LbDTu4XGKhr4mRzUsmM4RtVoemTSY81AxZiDr8c=
 google.golang.org/grpc v1.20.1/go.mod h1:10oTOabMzJvdu6/UiuZezV6QK5dSlG84ov/aaiqXj38=
 google.golang.org/grpc v1.21.0/go.mod h1:oYelfM1adQP15Ek0mdvEgi9Df8B9CZIaU1084ijfRaM=
@@ -1115,8 +1116,8 @@ google.golang.org/grpc v1.45.0/go.mod h1:lN7owxKUQEqMfSyQikvvk5tf/6zMPsrK+ONuO11
 google.golang.org/grpc v1.46.0/go.mod h1:vN9eftEi1UMyUsIF80+uQXhHjbXYbm0uXoFCACuMGWk=
 google.golang.org/grpc v1.46.2/go.mod h1:vN9eftEi1UMyUsIF80+uQXhHjbXYbm0uXoFCACuMGWk=
 google.golang.org/grpc v1.47.0/go.mod h1:vN9eftEi1UMyUsIF80+uQXhHjbXYbm0uXoFCACuMGWk=
-google.golang.org/grpc v1.49.0 h1:WTLtQzmQori5FUH25Pq4WT22oCsv8USpQ+F6rqtsmxw=
+google.golang.org/grpc v1.50.1 h1:DS/BukOZWp8s6p4Dt/tOaJaTQyPyOoCcrjroHuCeLzY=
-google.golang.org/grpc v1.49.0/go.mod h1:ZgQEeidpAuNRZ8iRrlBKXZQP1ghovWIVhdJRyCDK+GI=
+google.golang.org/grpc v1.50.1/go.mod h1:ZgQEeidpAuNRZ8iRrlBKXZQP1ghovWIVhdJRyCDK+GI=
 google.golang.org/grpc/cmd/protoc-gen-go-grpc v1.1.0/go.mod h1:6Kw0yEErY5E/yWrBtf03jp27GLLJujG4z/JK95pnjjw=
 google.golang.org/protobuf v0.0.0-20200109180630-ec00e32a8dfd/go.mod h1:DFci5gLYBciE7Vtevhsrf46CRTquxDuWsQurQQe4oz8=
 google.golang.org/protobuf v0.0.0-20200221191635-4d8936d0db64/go.mod h1:kwYJMbMJ01Woi6D6+Kah6886xMZcty6N08ah7+eCXa0=

internal/celtransformer/celformer.go

@@ -0,0 +1,278 @@
+// Copyright 2023 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package celtransformer is an implementation of upstream-to-downstream identity transformations
+// and policies using CEL scripts.
+//
+// The CEL language is documented in https://github.com/google/cel-spec/blob/master/doc/langdef.md
+// with optional extensions documented in https://github.com/google/cel-go/tree/master/ext.
+package celtransformer
+
+import (
+	"context"
+	"fmt"
+	"reflect"
+	"strings"
+	"time"
+
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/ext"
+
+	"go.pinniped.dev/internal/idtransform"
+)
+
+const (
+	usernameVariableName = "username"
+	groupsVariableName   = "groups"
+
+	defaultPolicyRejectedAuthMessage = "Authentication was rejected by a configured policy"
+)
+
+// CELTransformer can compile any number of transformation expression pipelines.
+// Each compiled pipeline can be cached in memory for later thread-safe evaluation.
+type CELTransformer struct {
+	compiler             *cel.Env
+	maxExpressionRuntime time.Duration
+}
+
+// NewCELTransformer returns a CELTransformer.
+// A running process should only need one instance of a CELTransformer.
+func NewCELTransformer(maxExpressionRuntime time.Duration) (*CELTransformer, error) {
+	env, err := newEnv()
+	if err != nil {
+		return nil, err
+	}
+	return &CELTransformer{compiler: env, maxExpressionRuntime: maxExpressionRuntime}, nil
+}
+
+// CompileTransformation compiles a CEL-based identity transformation expression.
+// The compiled transform can be cached in memory and executed repeatedly and in a thread-safe way.
+func (c *CELTransformer) CompileTransformation(t CELTransformation) (idtransform.IdentityTransformation, error) {
+	return t.compile(c)
+}
+
+// CELTransformation can be compiled into an IdentityTransformation.
+type CELTransformation interface {
+	compile(transformer *CELTransformer) (idtransform.IdentityTransformation, error)
+}
+
+// UsernameTransformation is a CEL expression that can transform a username (or leave it unchanged).
+// It implements CELTransformation.
+type UsernameTransformation struct {
+	Expression string
+}
+
+// GroupsTransformation is a CEL expression that can transform a list of group names (or leave it unchanged).
+// It implements CELTransformation.
+type GroupsTransformation struct {
+	Expression string
+}
+
+// AllowAuthenticationPolicy is a CEL expression that can allow the authentication to proceed by returning true.
+// It implements CELTransformation. When the CEL expression returns false, the authentication is rejected and the
+// RejectedAuthenticationMessage is used. When RejectedAuthenticationMessage is empty, a default message will be
+// used for rejected authentications.
+type AllowAuthenticationPolicy struct {
+	Expression                    string
+	RejectedAuthenticationMessage string
+}
+
+func compileProgram(transformer *CELTransformer, expectedExpressionType *cel.Type, expr string) (cel.Program, error) {
+	if strings.TrimSpace(expr) == "" {
+		return nil, fmt.Errorf("cannot compile empty CEL expression")
+	}
+
+	// compile does both parsing and type checking. The parsing phase indicates whether the expression is
+	// syntactically valid and expands any macros present within the environment. Parsing and checking are
+	// more computationally expensive than evaluation, so parsing and checking are done in advance.
+	ast, issues := transformer.compiler.Compile(expr)
+	if issues != nil {
+		return nil, fmt.Errorf("CEL expression compile error: %s", issues.String())
+	}
+
+	// The compiler's type checker has determined the type of the expression's result.
+	// Check that it matches the type that we expect.
+	if ast.OutputType().String() != expectedExpressionType.String() {
+		return nil, fmt.Errorf("CEL expression should return type %q but returns type %q", expectedExpressionType, ast.OutputType())
+	}
+
+	// The cel.Program is stateless, thread-safe, and cachable.
+	program, err := transformer.compiler.Program(ast,
+		cel.InterruptCheckFrequency(100), // Kubernetes uses 100 here, so we'll copy that setting.
+		cel.EvalOptions(cel.OptOptimize), // Optimize certain things now rather than at evaluation time.
+	)
+	if err != nil {
+		return nil, fmt.Errorf("CEL expression program construction error: %w", err)
+	}
+	return program, nil
+}
+
+func (t *UsernameTransformation) compile(transformer *CELTransformer) (idtransform.IdentityTransformation, error) {
+	program, err := compileProgram(transformer, cel.StringType, t.Expression)
+	if err != nil {
+		return nil, err
+	}
+	return &compiledUsernameTransformation{
+		program:              program,
+		maxExpressionRuntime: transformer.maxExpressionRuntime,
+	}, nil
+}
+
+func (t *GroupsTransformation) compile(transformer *CELTransformer) (idtransform.IdentityTransformation, error) {
+	program, err := compileProgram(transformer, cel.ListType(cel.StringType), t.Expression)
+	if err != nil {
+		return nil, err
+	}
+	return &compiledGroupsTransformation{
+		program:              program,
+		maxExpressionRuntime: transformer.maxExpressionRuntime,
+	}, nil
+}
+
+func (t *AllowAuthenticationPolicy) compile(transformer *CELTransformer) (idtransform.IdentityTransformation, error) {
+	program, err := compileProgram(transformer, cel.BoolType, t.Expression)
+	if err != nil {
+		return nil, err
+	}
+	return &compiledAllowAuthenticationPolicy{
+		program:                       program,
+		maxExpressionRuntime:          transformer.maxExpressionRuntime,
+		rejectedAuthenticationMessage: t.RejectedAuthenticationMessage,
+	}, nil
+}
+
+// Implements idtransform.IdentityTransformation.
+type compiledUsernameTransformation struct {
+	program              cel.Program
+	maxExpressionRuntime time.Duration
+}
+
+// Implements idtransform.IdentityTransformation.
+type compiledGroupsTransformation struct {
+	program              cel.Program
+	maxExpressionRuntime time.Duration
+}
+
+// Implements idtransform.IdentityTransformation.
+type compiledAllowAuthenticationPolicy struct {
+	program                       cel.Program
+	maxExpressionRuntime          time.Duration
+	rejectedAuthenticationMessage string
+}
+
+func evalProgram(ctx context.Context, program cel.Program, maxExpressionRuntime time.Duration, username string, groups []string) (ref.Val, error) {
+	// Limit the runtime of a CEL expression to avoid accidental very expensive expressions.
+	timeoutCtx, cancel := context.WithTimeout(ctx, maxExpressionRuntime)
+	defer cancel()
+
+	// Evaluation is thread-safe and side effect free. Many inputs can be sent to the same cel.Program
+	// and if fields are present in the input, but not referenced in the expression, they are ignored.
+	// The argument to Eval may either be an `interpreter.Activation` or a `map[string]interface{}`.
+	val, _, err := program.ContextEval(timeoutCtx, map[string]interface{}{
+		usernameVariableName: username,
+		groupsVariableName:   groups,
+	})
+	return val, err
+}
+
+func (c *compiledUsernameTransformation) Evaluate(ctx context.Context, username string, groups []string) (*idtransform.TransformationResult, error) {
+	val, err := evalProgram(ctx, c.program, c.maxExpressionRuntime, username, groups)
+	if err != nil {
+		return nil, err
+	}
+	nativeValue, err := val.ConvertToNative(reflect.TypeOf(""))
+	if err != nil {
+		return nil, fmt.Errorf("could not convert expression result to string: %w", err)
+	}
+	stringValue, ok := nativeValue.(string)
+	if !ok {
+		return nil, fmt.Errorf("could not convert expression result to string")
+	}
+	return &idtransform.TransformationResult{
+		Username:              stringValue,
+		Groups:                groups, // groups are not modified by username transformations
+		AuthenticationAllowed: true,
+	}, nil
+}
+
+func (c *compiledGroupsTransformation) Evaluate(ctx context.Context, username string, groups []string) (*idtransform.TransformationResult, error) {
+	val, err := evalProgram(ctx, c.program, c.maxExpressionRuntime, username, groups)
+	if err != nil {
+		return nil, err
+	}
+	nativeValue, err := val.ConvertToNative(reflect.TypeOf([]string{}))
+	if err != nil {
+		return nil, fmt.Errorf("could not convert expression result to []string: %w", err)
+	}
+	stringSliceValue, ok := nativeValue.([]string)
+	if !ok {
+		return nil, fmt.Errorf("could not convert expression result to []string")
+	}
+	return &idtransform.TransformationResult{
+		Username:              username, // username is not modified by groups transformations
+		Groups:                stringSliceValue,
+		AuthenticationAllowed: true,
+	}, nil
+}
+
+func (c *compiledAllowAuthenticationPolicy) Evaluate(ctx context.Context, username string, groups []string) (*idtransform.TransformationResult, error) {
+	val, err := evalProgram(ctx, c.program, c.maxExpressionRuntime, username, groups)
+	if err != nil {
+		return nil, err
+	}
+	nativeValue, err := val.ConvertToNative(reflect.TypeOf(true))
+	if err != nil {
+		return nil, fmt.Errorf("could not convert expression result to bool: %w", err)
+	}
+	boolValue, ok := nativeValue.(bool)
+	if !ok {
+		return nil, fmt.Errorf("could not convert expression result to bool")
+	}
+	result := &idtransform.TransformationResult{
+		Username:              username, // username is not modified by policies
+		Groups:                groups,   // groups are not modified by policies
+		AuthenticationAllowed: boolValue,
+	}
+	if !boolValue {
+		if len(c.rejectedAuthenticationMessage) == 0 {
+			result.RejectedAuthenticationMessage = defaultPolicyRejectedAuthMessage
+		} else {
+			result.RejectedAuthenticationMessage = c.rejectedAuthenticationMessage
+		}
+	}
+	return result, nil
+}
+
+func newEnv() (*cel.Env, error) {
+	// Note that Kubernetes uses CEL in several places, which are helpful to see as an example of
+	// how to configure the CEL compiler for production usage. Examples:
+	// https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/apiserver/pkg/admission/plugin/validatingadmissionpolicy/compiler.go
+	// https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/apiextensions-apiserver/pkg/apiserver/schema/cel/compilation.go
+	return cel.NewEnv(
+		// Declare our variable without giving them values yet. By declaring them here, the type is known during
+		// the parsing/checking phase.
+		cel.Variable(usernameVariableName, cel.StringType),
+		cel.Variable(groupsVariableName, cel.ListType(cel.StringType)),
+
+		// Enable the strings extensions.
+		// See https://github.com/google/cel-go/tree/master/ext#strings
+		// CEL also has other extensions for bas64 encoding/decoding and for math that we could choose to enable.
+		// See https://github.com/google/cel-go/tree/master/ext
+		// Kubernetes adds more extensions for extra regexp helpers, URLs, and extra list helpers that we could also
+		// consider enabling. Note that if we added their regexp extension, then we would also need to add
+		// cel.OptimizeRegex(library.ExtensionLibRegexOptimizations...) as an option when we call cel.Program.
+		// See https://github.com/kubernetes/kubernetes/tree/master/staging/src/k8s.io/apiserver/pkg/cel/library
+		ext.Strings(),
+
+		// Just in case someone converts a string to a timestamp, make any time operations which do not include
+		// an explicit timezone argument default to UTC.
+		cel.DefaultUTCTimeZone(true),
+
+		// Check list and map literal entry types during type-checking.
+		cel.HomogeneousAggregateLiterals(),
+
+		// Check for collisions in declarations now instead of later.
+		cel.EagerlyValidateDeclarations(true),
+	)
+}

internal/celtransformer/celformer_test.go

@@ -0,0 +1,734 @@
+// Copyright 2023 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package celtransformer
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"go.pinniped.dev/internal/here"
+	"go.pinniped.dev/internal/idtransform"
+)
+
+func TestTransformer(t *testing.T) {
+	var veryLargeGroupList []string
+	for i := 0; i < 10000; i++ {
+		veryLargeGroupList = append(veryLargeGroupList, fmt.Sprintf("g%d", i))
+	}
+
+	alreadyCancelledContext, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	tests := []struct {
+		name       string
+		username   string
+		groups     []string
+		transforms []CELTransformation
+		ctx        context.Context
+
+		wantUsername            string
+		wantGroups              []string
+		wantAuthRejected        bool
+		wantAuthRejectedMessage string
+		wantCompileErr          string
+		wantEvaluationErr       string
+	}{
+		{
+			name:         "empty transforms list does not change the identity and allows auth",
+			username:     "ryan",
+			groups:       []string{"admins", "developers", "other"},
+			transforms:   []CELTransformation{},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "other"},
+		},
+		{
+			name:     "simple transforms which do not change the identity and allows auth",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `username`},
+				&GroupsTransformation{Expression: `groups`},
+				&AllowAuthenticationPolicy{Expression: `true`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "other"},
+		},
+		{
+			name:     "transformations run in the order that they are given and accumulate results",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `"a:" + username`},
+				&UsernameTransformation{Expression: `"b:" + username`},
+				&GroupsTransformation{Expression: `groups.map(g, "a:" + g)`},
+				&GroupsTransformation{Expression: `groups.map(g, "b:" + g)`},
+			},
+			wantUsername: "b:a:ryan",
+			wantGroups:   []string{"b:a:admins", "b:a:developers", "b:a:other"},
+		},
+		{
+			name:     "policies which return false cause the pipeline to stop running and return a rejected auth result",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `"a:" + username`},
+				&AllowAuthenticationPolicy{Expression: `true`, RejectedAuthenticationMessage: `Everyone is allowed`},
+				&GroupsTransformation{Expression: `groups.map(g, "a:" + g)`},
+				&AllowAuthenticationPolicy{Expression: `username == "admin"`, RejectedAuthenticationMessage: `Only the username "admin" is allowed`},
+				&GroupsTransformation{Expression: `groups.map(g, "b:" + g)`}, // does not get evaluated
+			},
+			wantUsername:            "a:ryan",
+			wantGroups:              []string{"a:admins", "a:developers", "a:other"},
+			wantAuthRejected:        true,
+			wantAuthRejectedMessage: `Only the username "admin" is allowed`,
+		},
+		{
+			name:     "policies without a RejectedAuthenticationMessage get a default message",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `username == "admin"`, RejectedAuthenticationMessage: ""},
+			},
+			wantUsername:            "ryan",
+			wantGroups:              []string{"admins", "developers", "other"},
+			wantAuthRejected:        true,
+			wantAuthRejectedMessage: `Authentication was rejected by a configured policy`,
+		},
+		{
+			name:     "any transformations can use the username and group variables",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `groups[0] == "admins" && username == "ryan"`},
+				&GroupsTransformation{Expression: `groups + [username]`},
+				&UsernameTransformation{Expression: `groups[2]`},               // changes the username to "other"
+				&GroupsTransformation{Expression: `groups + [username + "2"]`}, // by the time this expression runs, the username was already changed to "other"
+			},
+			wantUsername: "other",
+			wantGroups:   []string{"admins", "developers", "other", "ryan", "other2"},
+		},
+		{
+			name:     "the CEL string extensions are enabled for use in the expressions",
+			username: " ryan ",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `groups.map(g, g.replace("mins", "ministrators"))`},
+				&UsernameTransformation{Expression: `username.upperAscii()`},
+				&AllowAuthenticationPolicy{Expression: `(username.lowerAscii()).trim() == "ryan"`, RejectedAuthenticationMessage: `Silly example`},
+				&UsernameTransformation{Expression: `username.trim()`},
+			},
+			wantUsername: "RYAN",
+			wantGroups:   []string{"administrators", "developers", "other"},
+		},
+		{
+			name:     "UTC is the default time zone for time operations",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `string(timestamp("2023-01-16T10:00:20.021-08:00").getHours())`},
+			},
+			// Without the compiler option cel.DefaultUTCTimeZone, this result would be 10.
+			// With the option, this result is the original hour from the timestamp string (10), plus the effect
+			// of the timezone (8), to move the hour into the UTC time zone.
+			wantUsername: "18",
+			wantGroups:   []string{"admins", "developers", "other"},
+		},
+		{
+			name:     "the default UTC time zone for time operations can be overridden by passing the time zone as an argument to the operation",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `string(timestamp("2023-01-16T10:00:20.021-08:00").getHours("US/Mountain"))`},
+			},
+			// This is the hour of the timestamp in Mountain time, which is one time zone over from Pacific (-08:00),
+			// hence it is one larger than the original "10" from the timestamp string.
+			wantUsername: "11",
+			wantGroups:   []string{"admins", "developers", "other"},
+		},
+		{
+			name:     "quick expressions are finished by CEL before CEL even looks at the cancel context",
+			username: "ryan",
+			groups:   veryLargeGroupList,
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `["one group"]`},
+			},
+			ctx:          alreadyCancelledContext,
+			wantUsername: "ryan",
+			wantGroups:   []string{"one group"},
+		},
+
+		//
+		// Unit tests to demonstrate practical examples of useful CEL expressions.
+		//
+		{
+			name:     "can prefix username and all groups",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `"username_prefix:" + username`},
+				&GroupsTransformation{Expression: `groups.map(g, "group_prefix:" + g)`},
+			},
+			wantUsername: "username_prefix:ryan",
+			wantGroups:   []string{"group_prefix:admins", "group_prefix:developers", "group_prefix:other"},
+		},
+		{
+			name:     "can suffix username and all groups",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `username + ":username_suffix"`},
+				&GroupsTransformation{Expression: `groups.map(g, g + ":group_suffix")`},
+			},
+			wantUsername: "ryan:username_suffix",
+			wantGroups:   []string{"admins:group_suffix", "developers:group_suffix", "other:group_suffix"},
+		},
+		{
+			name:     "can change case of username and all groups",
+			username: "rYan 🚀",
+			groups:   []string{"aDmins", "dEvelopers", "oTher"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `username.lowerAscii()`},
+				&GroupsTransformation{Expression: `groups.map(g, g.upperAscii())`},
+			},
+			wantUsername: "ryan 🚀",
+			wantGroups:   []string{"ADMINS", "DEVELOPERS", "OTHER"},
+		},
+		{
+			name:     "can replace whitespace",
+			username: " r\ty a n \n",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `username.replace(" ", "").replace("\n", "").replace("\t", "")`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "other"},
+		},
+		{
+			name:     "can filter groups based on an allow list",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `groups.filter(g, g in ["admins", "developers"])`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers"},
+		},
+		{
+			name:     "can filter groups based on a disallow list",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `groups.filter(g, !(g in ["admins", "developers"]))`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"other"},
+		},
+		{
+			name:     "can filter groups based on a disallowed prefixes",
+			username: "ryan",
+			groups:   []string{"disallowed1:admins", "disallowed2:developers", "other"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `groups.filter(group, !(["disallowed1:", "disallowed2:"].exists(prefix, group.startsWith(prefix))))`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"other"},
+		},
+		{
+			name:     "can add a group",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `groups + ["new-group"]`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "other", "new-group"},
+		},
+		{
+			name:     "can add a group but only if they already belong to another group - when the user does belong to that other group",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `"other" in groups ? groups + ["new-group"] : groups`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "other", "new-group"},
+		},
+		{
+			name:     "can add a group but only if they already belong to another group - when the user does NOT belong to that other group",
+			username: "ryan",
+			groups:   []string{"admins", "developers"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `"other" in groups ? groups + ["new-group"] : groups`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers"},
+		},
+		{
+			name:     "can rename a group",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `groups.map(g, g == "other" ? "other-renamed" : g)`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "other-renamed"},
+		},
+		{
+			name:     "can reject auth based on belonging to one group - when the user meets the criteria",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other", "super-admins"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `"super-admins" in groups`, RejectedAuthenticationMessage: `Only users who belong to the "super-admins" group are allowed`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "other", "super-admins"},
+		},
+		{
+			name:     "can reject auth based on belonging to one group - when the user does NOT meet the criteria",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `"super-admins" in groups`, RejectedAuthenticationMessage: `Only users who belong to the "super-admins" group are allowed`},
+			},
+			wantUsername:            "ryan",
+			wantGroups:              []string{"admins", "developers", "other"},
+			wantAuthRejected:        true,
+			wantAuthRejectedMessage: `Only users who belong to the "super-admins" group are allowed`,
+		},
+		{
+			name:     "can reject auth unless the user belongs to any one of the groups in a list - when the user meets the criteria",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "foobar", "other"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `groups.exists(g, g in ["foobar", "foobaz", "foobat"])`, RejectedAuthenticationMessage: `Only users who belong to any of the groups in a list are allowed`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "foobar", "other"},
+		},
+		{
+			name:     "can reject auth unless the user belongs to any one of the groups in a list - when the user does NOT meet the criteria",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `groups.exists(g, g in ["foobar", "foobaz", "foobat"])`, RejectedAuthenticationMessage: `Only users who belong to any of the groups in a list are allowed`},
+			},
+			wantUsername:            "ryan",
+			wantGroups:              []string{"admins", "developers", "other"},
+			wantAuthRejected:        true,
+			wantAuthRejectedMessage: `Only users who belong to any of the groups in a list are allowed`,
+		},
+		{
+			name:     "can reject auth unless the user belongs to all of the groups in a list - when the user meets the criteria",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other", "foobar", "foobaz", "foobat"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `["foobar", "foobaz", "foobat"].all(g, g in groups)`, RejectedAuthenticationMessage: `Only users who belong to all groups in a list are allowed`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "other", "foobar", "foobaz", "foobat"},
+		},
+		{
+			name:     "can reject auth unless the user belongs to all of the groups in a list - when the user does NOT meet the criteria",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other", "foobaz", "foobat"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `["foobar", "foobaz", "foobat"].all(g, g in groups)`, RejectedAuthenticationMessage: `Only users who belong to all groups in a list are allowed`},
+			},
+			wantUsername:            "ryan",
+			wantGroups:              []string{"admins", "developers", "other", "foobaz", "foobat"},
+			wantAuthRejected:        true,
+			wantAuthRejectedMessage: `Only users who belong to all groups in a list are allowed`,
+		},
+		{
+			name:     "can reject auth if the user belongs to any groups in a disallowed groups list - when the user meets the criteria",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `!groups.exists(g, g in ["foobar", "foobaz"])`, RejectedAuthenticationMessage: `Only users who do not belong to any of the groups in a list are allowed`},
+			},
+			wantUsername: "ryan",
+			wantGroups:   []string{"admins", "developers", "other"},
+		},
+		{
+			name:     "can reject auth if the user belongs to any groups in a disallowed groups list - when the user does NOT meet the criteria",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other", "foobaz"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `!groups.exists(g, g in ["foobar", "foobaz"])`, RejectedAuthenticationMessage: `Only users who do not belong to any of the groups in a list are allowed`},
+			},
+			wantUsername:            "ryan",
+			wantGroups:              []string{"admins", "developers", "other", "foobaz"},
+			wantAuthRejected:        true,
+			wantAuthRejectedMessage: `Only users who do not belong to any of the groups in a list are allowed`,
+		},
+		{
+			name:     "can reject auth unless the username is in an allowed users list - when the user meets the criteria",
+			username: "foobaz",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `username in ["foobar", "foobaz"]`, RejectedAuthenticationMessage: `Only certain usernames allowed`},
+			},
+			wantUsername: "foobaz",
+			wantGroups:   []string{"admins", "developers", "other"},
+		},
+		{
+			name:     "can reject auth unless the username is in an allowed users list - when the user does NOT meet the criteria",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `username in ["foobar", "foobaz"]`, RejectedAuthenticationMessage: `Only certain usernames allowed`},
+			},
+			wantUsername:            "ryan",
+			wantGroups:              []string{"admins", "developers", "other"},
+			wantAuthRejected:        true,
+			wantAuthRejectedMessage: `Only certain usernames allowed`,
+		},
+
+		//
+		// Error cases
+		//
+		{
+			name:     "username transformation returns an empty string as the new username",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `""`},
+			},
+			wantEvaluationErr: "identity transformation returned an empty username, which is not allowed",
+		},
+		{
+			name:     "username transformation returns a string containing only whitespace as the new username",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `" \n \t "`},
+			},
+			wantEvaluationErr: "identity transformation returned an empty username, which is not allowed",
+		},
+		{
+			name: "username transformation compiles to return null, which is not a string so it has the wrong type",
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `null`},
+			},
+			wantCompileErr: `CEL expression should return type "string" but returns type "null_type"`,
+		},
+		{
+			name: "groups transformation compiles to return null, which is not a string so it has the wrong type",
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `null`},
+			},
+			wantCompileErr: `CEL expression should return type "list(string)" but returns type "null_type"`,
+		},
+		{
+			name: "policy transformation compiles to return null, which is not a string so it has the wrong type",
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `null`},
+			},
+			wantCompileErr: `CEL expression should return type "bool" but returns type "null_type"`,
+		},
+		{
+			name: "username transformation has empty expression",
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: ``},
+			},
+			wantCompileErr: `cannot compile empty CEL expression`,
+		},
+		{
+			name: "groups transformation has empty expression",
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: ``},
+			},
+			wantCompileErr: `cannot compile empty CEL expression`,
+		},
+		{
+			name: "policy transformation has empty expression",
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: ``},
+			},
+			wantCompileErr: `cannot compile empty CEL expression`,
+		},
+		{
+			name: "username transformation has expression which contains only whitespace",
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: " \n\t "},
+			},
+			wantCompileErr: `cannot compile empty CEL expression`,
+		},
+		{
+			name: "groups transformation has expression which contains only whitespace",
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: " \n\t "},
+			},
+			wantCompileErr: `cannot compile empty CEL expression`,
+		},
+		{
+			name: "policy transformation has expression which contains only whitespace",
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: " \n\t "},
+			},
+			wantCompileErr: `cannot compile empty CEL expression`,
+		},
+		{
+			name:     "slow username transformation expressions are canceled by the cancel context after partial evaluation",
+			username: "ryan",
+			groups:   veryLargeGroupList,
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `groups.filter(x, groups.all(x, true))[0]`},
+			},
+			ctx:               alreadyCancelledContext,
+			wantEvaluationErr: `identity transformation at index 0: operation interrupted`,
+		},
+		{
+			name:     "slow groups transformation expressions are canceled by the cancel context after partial evaluation",
+			username: "ryan",
+			groups:   veryLargeGroupList,
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `groups.filter(x, groups.all(x, true))`},
+			},
+			ctx:               alreadyCancelledContext,
+			wantEvaluationErr: `identity transformation at index 0: operation interrupted`,
+		},
+		{
+			name:     "slow policy expressions are canceled by the cancel context after partial evaluation",
+			username: "ryan",
+			groups:   veryLargeGroupList,
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: "username"},
+				&AllowAuthenticationPolicy{Expression: `groups.all(x, groups.all(x, true))`}, // this is the slow one
+			},
+			ctx:               alreadyCancelledContext,
+			wantEvaluationErr: `identity transformation at index 1: operation interrupted`,
+		},
+		{
+			name:     "slow transformation expressions are canceled and the rest of the expressions do not run",
+			username: "ryan",
+			groups:   veryLargeGroupList,
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `username`}, // quick expressions are allowed to run even though the context is cancelled
+				&UsernameTransformation{Expression: `groups.filter(x, groups.all(x, true))[0]`},
+				&UsernameTransformation{Expression: `groups.filter(x, groups.all(x, true))[0]`},
+				&UsernameTransformation{Expression: `groups.filter(x, groups.all(x, true))[0]`},
+			},
+			ctx:               alreadyCancelledContext,
+			wantEvaluationErr: `identity transformation at index 1: operation interrupted`,
+		},
+		{
+			name:     "slow username transformation expressions are canceled after a maximum allowed duration",
+			username: "ryan",
+			groups:   veryLargeGroupList,
+			transforms: []CELTransformation{
+				// On my laptop, evaluating this expression would take ~20 seconds if we allowed it to evaluate to completion.
+				&UsernameTransformation{Expression: `groups.filter(x, groups.all(x, true))[0]`},
+			},
+			wantEvaluationErr: `identity transformation at index 0: operation interrupted`,
+		},
+		{
+			name:     "slow groups transformation expressions are canceled after a maximum allowed duration",
+			username: "ryan",
+			groups:   veryLargeGroupList,
+			transforms: []CELTransformation{
+				// On my laptop, evaluating this expression would take ~20 seconds if we allowed it to evaluate to completion.
+				&GroupsTransformation{Expression: `groups.filter(x, groups.all(x, true))`},
+			},
+			wantEvaluationErr: `identity transformation at index 0: operation interrupted`,
+		},
+		{
+			name:     "slow policy transformation expressions are canceled after a maximum allowed duration",
+			username: "ryan",
+			groups:   veryLargeGroupList,
+			transforms: []CELTransformation{
+				// On my laptop, evaluating this expression would take ~20 seconds if we allowed it to evaluate to completion.
+				&AllowAuthenticationPolicy{Expression: `groups.all(x, groups.all(x, true))`},
+			},
+			wantEvaluationErr: `identity transformation at index 0: operation interrupted`,
+		},
+		{
+			name: "compile errors are returned by the compile step for a username transform",
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: `foobar.junk()`},
+			},
+			wantCompileErr: here.Doc(`
+				CEL expression compile error: ERROR: <input>:1:1: undeclared reference to 'foobar' (in container '')
+				 | foobar.junk()
+				 | ^
+				ERROR: <input>:1:12: undeclared reference to 'junk' (in container '')
+				 | foobar.junk()
+				 | ...........^`,
+			),
+		},
+		{
+			name: "compile errors are returned by the compile step for a groups transform",
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `foobar.junk()`},
+			},
+			wantCompileErr: here.Doc(`
+				CEL expression compile error: ERROR: <input>:1:1: undeclared reference to 'foobar' (in container '')
+				 | foobar.junk()
+				 | ^
+				ERROR: <input>:1:12: undeclared reference to 'junk' (in container '')
+				 | foobar.junk()
+				 | ...........^`,
+			),
+		},
+		{
+			name: "compile errors are returned by the compile step for a policy",
+			transforms: []CELTransformation{
+				&AllowAuthenticationPolicy{Expression: `foobar.junk()`},
+			},
+			wantCompileErr: here.Doc(`
+				CEL expression compile error: ERROR: <input>:1:1: undeclared reference to 'foobar' (in container '')
+				 | foobar.junk()
+				 | ^
+				ERROR: <input>:1:12: undeclared reference to 'junk' (in container '')
+				 | foobar.junk()
+				 | ...........^`,
+			),
+		},
+		{
+			name:     "evaluation errors stop the pipeline and return an error",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&UsernameTransformation{Expression: "username"},
+				&AllowAuthenticationPolicy{Expression: `1 / 0 == 7`},
+			},
+			wantEvaluationErr: `identity transformation at index 1: division by zero`,
+		},
+		{
+			name:     "HomogeneousAggregateLiterals compiler setting is enabled to help the user avoid type mistakes in expressions",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `groups.all(g, g in ["admins", 1])`},
+			},
+			wantCompileErr: here.Doc(`
+				CEL expression compile error: ERROR: <input>:1:31: expected type 'string' but found 'int'
+				 | groups.all(g, g in ["admins", 1])
+				 | ..............................^`,
+			),
+		},
+		{
+			name:     "when an expression's type cannot be determined at compile time, e.g. due to the use of dynamic types",
+			username: "ryan",
+			groups:   []string{"admins", "developers", "other"},
+			transforms: []CELTransformation{
+				&GroupsTransformation{Expression: `groups.map(g, {"admins": dyn(1), "developers":"a"}[g])`},
+			},
+			wantCompileErr: `CEL expression should return type "list(string)" but returns type "list(dyn)"`,
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			transformer, err := NewCELTransformer(100 * time.Millisecond)
+			require.NoError(t, err)
+
+			pipeline := idtransform.NewTransformationPipeline()
+
+			for _, transform := range tt.transforms {
+				compiledTransform, err := transformer.CompileTransformation(transform)
+				if tt.wantCompileErr != "" {
+					require.EqualError(t, err, tt.wantCompileErr)
+					return // the rest of the test doesn't make sense when there was a compile error
+				}
+				require.NoError(t, err, "got an unexpected compile error")
+				pipeline.AppendTransformation(compiledTransform)
+			}
+
+			ctx := context.Background()
+			if tt.ctx != nil {
+				ctx = tt.ctx
+			}
+
+			result, err := pipeline.Evaluate(ctx, tt.username, tt.groups)
+			if tt.wantEvaluationErr != "" {
+				require.EqualError(t, err, tt.wantEvaluationErr)
+				return // the rest of the test doesn't make sense when there was an evaluation error
+			}
+			require.NoError(t, err, "got an unexpected evaluation error")
+
+			require.Equal(t, tt.wantUsername, result.Username)
+			require.Equal(t, tt.wantGroups, result.Groups)
+			require.Equal(t, !tt.wantAuthRejected, result.AuthenticationAllowed, "AuthenticationAllowed had unexpected value")
+			require.Equal(t, tt.wantAuthRejectedMessage, result.RejectedAuthenticationMessage)
+		})
+	}
+}
+
+func TestTypicalPerformanceAndThreadSafety(t *testing.T) {
+	t.Parallel()
+
+	transformer, err := NewCELTransformer(100 * time.Millisecond)
+	require.NoError(t, err)
+
+	pipeline := idtransform.NewTransformationPipeline()
+
+	var compiledTransform idtransform.IdentityTransformation
+	compiledTransform, err = transformer.CompileTransformation(&UsernameTransformation{Expression: `"username_prefix:" + username`})
+	require.NoError(t, err)
+	pipeline.AppendTransformation(compiledTransform)
+	compiledTransform, err = transformer.CompileTransformation(&GroupsTransformation{Expression: `groups.map(g, "group_prefix:" + g)`})
+	require.NoError(t, err)
+	pipeline.AppendTransformation(compiledTransform)
+	compiledTransform, err = transformer.CompileTransformation(&AllowAuthenticationPolicy{Expression: `username == "username_prefix:ryan"`})
+	require.NoError(t, err)
+	pipeline.AppendTransformation(compiledTransform)
+
+	var groups []string
+	var wantGroups []string
+	for i := 0; i < 100; i++ {
+		groups = append(groups, fmt.Sprintf("g%d", i))
+		wantGroups = append(wantGroups, fmt.Sprintf("group_prefix:g%d", i))
+	}
+
+	// Before looking at performance, check that the behavior of the function is correct.
+	result, err := pipeline.Evaluate(context.Background(), "ryan", groups)
+	require.NoError(t, err)
+	require.Equal(t, "username_prefix:ryan", result.Username)
+	require.Equal(t, wantGroups, result.Groups)
+	require.True(t, result.AuthenticationAllowed)
+	require.Empty(t, result.RejectedAuthenticationMessage)
+
+	// This loop is meant to give a sense of typical runtime of CEL expressions which transforms a username
+	// and 100 group names. It is not meant to be a pass/fail test or scientific benchmark test.
+	iterations := 1000
+	start := time.Now()
+	for i := 0; i < iterations; i++ {
+		_, _ = pipeline.Evaluate(context.Background(), "ryan", groups)
+	}
+	elapsed := time.Since(start)
+	t.Logf("TestTypicalPerformanceAndThreadSafety %d iterations of Evaluate took %s; average runtime %s", iterations, elapsed, elapsed/time.Duration(iterations))
+	// On my laptop this prints: TestTypicalPerformanceAndThreadSafety 1000 iterations of Evaluate took 257.981421ms; average runtime 257.981µs
+
+	// Now use the transformations pipeline from different goroutines at the same time. Hopefully the race detector
+	// will complain if this is not thread safe in some way. Use the pipeline enough that it will be very likely that
+	// there will be several parallel invocations of the Evaluate function. Every invocation should also yield the
+	// exact same result, since they are all using the same inputs. This assumes that the unit tests are run using
+	// the race detector.
+	var wg sync.WaitGroup
+	numGoroutines := 10
+	for i := 0; i < numGoroutines; i++ {
+		wg.Add(1) // increment WaitGroup counter for each goroutine
+		go func() {
+			defer wg.Done() // decrement WaitGroup counter when this goroutine finishes
+			for j := 0; j < iterations*2; j++ {
+				localResult, localErr := pipeline.Evaluate(context.Background(), "ryan", groups)
+				require.NoError(t, localErr)
+				require.Equal(t, "username_prefix:ryan", localResult.Username)
+				require.Equal(t, wantGroups, localResult.Groups)
+				require.True(t, localResult.AuthenticationAllowed)
+				require.Empty(t, localResult.RejectedAuthenticationMessage)
+			}
+		}()
+	}
+	wg.Wait() // wait for the counter to reach zero, indicating the all goroutines are finished
+}

internal/idtransform/identity_transformations.go

@@ -0,0 +1,72 @@
+// Copyright 2023 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package idtransform defines upstream-to-downstream identity transformations which could be
+// implemented using various approaches or languages.
+package idtransform
+
+import (
+	"context"
+	"fmt"
+	"strings"
+)
+
+// TransformationResult is the result of evaluating a transformation against some inputs.
+type TransformationResult struct {
+	Username                      string   // the new username for an allowed auth
+	Groups                        []string // the new group names for an allowed auth
+	AuthenticationAllowed         bool     // when false, disallow this authentication attempt
+	RejectedAuthenticationMessage string   // should be set when AuthenticationAllowed is false
+}
+
+// IdentityTransformation is an individual identity transformation which can be evaluated.
+type IdentityTransformation interface {
+	Evaluate(ctx context.Context, username string, groups []string) (*TransformationResult, error)
+}
+
+// TransformationPipeline is a list of identity transforms, which can be evaluated in order against some given input
+// values.
+type TransformationPipeline struct {
+	transforms []IdentityTransformation
+}
+
+// NewTransformationPipeline creates an empty TransformationPipeline.
+func NewTransformationPipeline() *TransformationPipeline {
+	return &TransformationPipeline{transforms: []IdentityTransformation{}}
+}
+
+// AppendTransformation adds a transformation to the end of the list of transformations for this pipeline.
+// This is not thread-safe, so be sure to add all transformations from a single goroutine before using Evaluate
+// from multiple goroutines.
+func (p *TransformationPipeline) AppendTransformation(t IdentityTransformation) {
+	p.transforms = append(p.transforms, t)
+}
+
+// Evaluate runs the transformation pipeline for a given input identity. It returns a potentially transformed or
+// rejected identity, or an error. If any transformation in the list rejects the authentication, then the list is
+// short-circuited but no error is returned. Only unexpected errors are returned as errors. This is safe to call
+// from multiple goroutines.
+func (p *TransformationPipeline) Evaluate(ctx context.Context, username string, groups []string) (*TransformationResult, error) {
+	accumulatedResult := &TransformationResult{
+		Username:              username,
+		Groups:                groups,
+		AuthenticationAllowed: true,
+	}
+	var err error
+	for i, transform := range p.transforms {
+		accumulatedResult, err = transform.Evaluate(ctx, accumulatedResult.Username, accumulatedResult.Groups)
+		if err != nil {
+			// There was an unexpected error evaluating a transformation.
+			return nil, fmt.Errorf("identity transformation at index %d: %w", i, err)
+		}
+		if !accumulatedResult.AuthenticationAllowed {
+			// Auth has been rejected by a policy. Stop evaluating the rest of the transformations.
+			return accumulatedResult, nil
+		}
+		if strings.TrimSpace(accumulatedResult.Username) == "" {
+			return nil, fmt.Errorf("identity transformation returned an empty username, which is not allowed")
+		}
+	}
+	// There were no unexpected errors and no policy which rejected auth.
+	return accumulatedResult, nil
+}