Add package starformer, uses Starlark for user-defined authn filters

go.mod

@@ -30,6 +30,7 @@ require (
 	github.com/spf13/pflag v1.0.5
 	github.com/stretchr/testify v1.8.1
 	github.com/tdewolff/minify/v2 v2.12.4
+	go.starlark.net v0.0.0-20210602144842-1cdb82c9e17a
 	go.uber.org/zap v1.24.0
 	golang.org/x/crypto v0.5.0
 	golang.org/x/net v0.5.0

go.sum

@@ -613,6 +613,8 @@ go.opentelemetry.io/otel/trace v1.10.0/go.mod h1:Sij3YYczqAdz+EhmGhE6TpTxUO5/F/A
 go.opentelemetry.io/proto/otlp v0.7.0/go.mod h1:PqfVotwruBrMGOCsRd/89rSnXhoiJIqeYNgFYFoEGnI=
 go.opentelemetry.io/proto/otlp v0.19.0 h1:IVN6GR+mhC4s5yfcTbmzHYODqvWAp3ZedA2SJPI1Nnw=
 go.opentelemetry.io/proto/otlp v0.19.0/go.mod h1:H7XAot3MsfNsj7EXtrA2q5xSNQ10UqI405h3+duxN4U=
+go.starlark.net v0.0.0-20210602144842-1cdb82c9e17a h1:wDtSCWGrX9tusypq2Qq9xzaA3Tf/+4D2KaWO+HQvGZE=
+go.starlark.net v0.0.0-20210602144842-1cdb82c9e17a/go.mod h1:t3mmBBPzAVvK0L0n1drDmrQsJ8FoIx4INCqVMTr/Zo0=
 go.uber.org/atomic v1.4.0/go.mod h1:gD2HeocX3+yG+ygLZcrzQJaqmWj9AIm7n08wl/qW/PE=
 go.uber.org/atomic v1.7.0/go.mod h1:fEN4uk6kAWBTFdckzkM89CLk9XfWZrxpCo0nPH17wJc=
 go.uber.org/atomic v1.10.0 h1:9qC72Qh0+3MqyJbAn8YU5xVq1frD8bn3JtD2oXtafVQ=

internal/starformer/starformer.go

@@ -0,0 +1,183 @@
+// Copyright 2021 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package starformer is an implementation of UpstreamToDownstreamTransformer using Starlark scripts.
+// See Starlark dialect language documentation here: https://github.com/google/starlark-go/blob/master/doc/spec.md
+// A video introduction to Starlark and how to integrate it into projects is here: https://www.youtube.com/watch?v=9P_YKVhncWI
+package starformer
+
+import (
+	"fmt"
+
+	"go.starlark.net/lib/json"
+	starlarkmath "go.starlark.net/lib/math"
+	"go.starlark.net/lib/time"
+	"go.starlark.net/resolve"
+	"go.starlark.net/starlark"
+
+	"go.pinniped.dev/internal/plog"
+)
+
+const (
+	maxExecutionSteps     = 10000000
+	transformFunctionName = "transform"
+)
+
+// Configure some global variables in starlark-go.
+// nolint:gochecknoinits // wish these weren't globals but oh well
+func init() {
+	// Allow the non-standard "set" data structure to be used.
+	resolve.AllowSet = true
+
+	// Note that we could allow "while" statements and recursive functions, but the language already
+	// has "for" loops so it seems unnecessary for our use case. This is currently the default
+	// value in starlark-go but repeating it here as documentation.
+	resolve.AllowRecursion = false
+}
+
+type Transformer struct {
+	hook *starlark.Function
+}
+
+// New creates an instance of Transformer. Given some Starlark source code as a string, it loads the code.
+// If there is any error during loading, it will return the error. It expects the loaded code to define
+// a Starlark function called "transform" which should take two positional arguments. The returned
+// Transformer can be safely called from multiple threads simultaneously, no matter how the Starlark
+// source code was written, because the Starlark module has been frozen (made immutable).
+func New(starlarkSourceCode string) (*Transformer, error) {
+	// Create a Starlark thread in which the source will be loaded.
+	thread := &starlark.Thread{
+		Name: "starlark script loader",
+		Print: func(thread *starlark.Thread, msg string) {
+			// When the script has a top-level print(), send it to the server log.
+			plog.Debug("debug message while loading starlark transform script", "msg", msg)
+		},
+		Load: func(thread *starlark.Thread, module string) (starlark.StringDict, error) {
+			// Allow starlark-go's custom built-in modules to be loaded by scripts if they desire.
+			switch module {
+			case "json.star":
+				return starlark.StringDict{"json": json.Module}, nil
+			case "time.star":
+				return starlark.StringDict{"time": time.Module}, nil
+			case "math.star":
+				return starlark.StringDict{"math": starlarkmath.Module}, nil
+			default:
+				// Don't allow any other file to be loaded.
+				return nil, fmt.Errorf("only the following modules may be loaded: json.star, time.star, math.star")
+			}
+		},
+	}
+
+	// Prevent the top-level statements of the Starlark script from accidentally running forever.
+	thread.SetMaxExecutionSteps(maxExecutionSteps)
+
+	// Start with empty predeclared names, aside from the built-ins.
+	predeclared := starlark.StringDict{}
+
+	// Load a Starlark script. Initialization of a script runs its top-level statements from top to bottom,
+	// and then "freezes" all of the values making them immutable. The result can be used in multiple threads
+	// simultaneously without interfering, communicating, or racing with each other. The filename given here
+	// will appear in some Starlark error messages.
+	globals, err := starlark.ExecFile(thread, "transform.star", starlarkSourceCode, predeclared)
+	if err != nil {
+		return nil, fmt.Errorf("error while loading starlark transform script: %w", err)
+	}
+
+	// Get the function called "transform" from the global state of the module that was just loaded.
+	hook, _ := globals[transformFunctionName].(*starlark.Function)
+	if hook == nil {
+		return nil, fmt.Errorf("starlark script does not define %q function", transformFunctionName)
+	}
+
+	// Check that the "transform" function takes the expected number of arguments so we can call it later.
+	if hook.NumParams() != 2 {
+		return nil, fmt.Errorf("starlark script's global %q function has %d parameters but should have 2", transformFunctionName, hook.NumParams())
+	}
+
+	return &Transformer{hook: hook}, nil
+}
+
+// Transform calls the Starlark "transform" function that was loaded by New. The username and groups params are
+// passed into the Starlark function, and the return values of the Starlark function are returned. If there is an error
+// during the call to the Starlark function (either a programming error, a runtime error, or an intentional call to
+// Starlark's `fail` built-in function) then Transform will return the error. This function is thread-safe.
+// The runtime of this function depends on the complexity of the Starlark source code, but for a typical Starlark
+// function will be something on the order of 50µs on a modern laptop.
+func (t *Transformer) Transform(username string, groups []string) (string, []string, error) {
+	// TODO: maybe add a context param for cancellation, which is supported in starlark-go by
+	//  calling thread.Cancel() from any goroutine, or maybe this doesn't matter because there is
+	//  already a maxExecutionSteps so scripts are guaranteed to finish within a reasonable time.
+
+	// Create a Starlark thread in which the function will be called.
+	thread := &starlark.Thread{
+		Name: "starlark script executor",
+		Print: func(thread *starlark.Thread, msg string) {
+			// When the script's 'transform' function has a print(), send it to the server log.
+			plog.Debug("debug message while running starlark transform script", "msg", msg)
+		},
+	}
+
+	// Prevent the Starlark function from accidentally running forever.
+	thread.SetMaxExecutionSteps(maxExecutionSteps)
+
+	// Prepare the function arguments as Starlark values.
+	groupsTuple := starlark.Tuple{}
+	for _, group := range groups {
+		groupsTuple = append(groupsTuple, starlark.String(group))
+	}
+	args := starlark.Tuple{starlark.String(username), groupsTuple}
+
+	// Call the Starlark hook function in the new thread and pass the arguments.
+	// Get back the function's return value or an error.
+	hookReturnValue, err := starlark.Call(thread, t.hook, args, nil)
+
+	// Errors could be programming mistakes in the script, or could be an intentional usage of the `fail` built-in.
+	// Either way, return an error to reject the login.
+	if err != nil {
+		return "", nil, fmt.Errorf("error while running starlark %q function: %w", transformFunctionName, err)
+	}
+
+	// The special Starlark value 'None' is interpreted here as a shortcut to mean make no edits.
+	if hookReturnValue == starlark.None {
+		return username, groups, nil
+	}
+
+	// TODO: maybe offer a way for the user to reject a login with a nice error message which we can distinguish from
+	//  an accidental coding error, for example by returning a single string from their 'transform' function instead
+	//  of a tuple, or by returning a special value that we set up in the module's state in advance like
+	//  `return rejectAuthentication(message)`
+
+	// Otherwise the function should have returned a tuple with two values.
+	returnedTuple, ok := hookReturnValue.(starlark.Tuple)
+	if !ok || returnedTuple.Len() != 2 {
+		return "", nil, fmt.Errorf("expected starlark %q function to return None or a Tuple of length 2", transformFunctionName)
+	}
+
+	// The first value in the returned tuple is the username. Turn it back into a golang string.
+	transformedUsername, ok := starlark.AsString(returnedTuple.Index(0))
+	if !ok || len(transformedUsername) == 0 {
+		return "", nil, fmt.Errorf("expected starlark %q function's return tuple to have a non-empty string as the first value", transformFunctionName)
+	}
+
+	// The second value in the returned tuple is an iterable of group names.
+	returnedGroups, ok := returnedTuple.Index(1).(starlark.Iterable)
+	if !ok {
+		return "", nil, fmt.Errorf("expected starlark %q function's return tuple to have an iterable value as the second value", transformFunctionName)
+	}
+
+	// Turn the returned iterable of group names back into a golang []string, including turning an empty iterable into an empty slice.
+	transformedGroupNames := []string{}
+	groupsIterator := returnedGroups.Iterate()
+	defer groupsIterator.Done()
+	var transformedGroup starlark.Value
+	for groupsIterator.Next(&transformedGroup) {
+		transformedGroupName, ok := starlark.AsString(transformedGroup)
+		if !ok || len(transformedGroupName) == 0 {
+			return "", nil, fmt.Errorf("expected starlark %q function's return tuple's second value to contain only non-empty strings", transformFunctionName)
+		}
+		transformedGroupNames = append(transformedGroupNames, transformedGroupName)
+	}
+
+	// Got username and group names, so return them as the transformed values.
+	return transformedUsername, transformedGroupNames, nil
+}