access-engine/cmd/flex-auth/readback.go

package main

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"strings"

	"github.com/netkingdom/flex-auth/internal/audit"
	"github.com/netkingdom/flex-auth/internal/callerauth"
	"github.com/netkingdom/flex-auth/internal/policy"
	"github.com/netkingdom/flex-auth/internal/readback"
	"github.com/netkingdom/flex-auth/internal/registry"
)

// readbackBinding is the caller-binding key the readback routes authorize
// against. It is not a protected system, so a principal bound to it can never
// represent a system on any pin's /v1/check (FLEX-WP-0035-T02).
const readbackBinding = "application-readback"

type repeatedFlag []string

func (f *repeatedFlag) String() string { return strings.Join(*f, ",") }

func (f *repeatedFlag) Set(value string) error {
	*f = append(*f, value)
	return nil
}

// runServeReadback serves the advisory application-access readback. It mounts
// no /v1/check or /v1/batch_check: this pin cannot produce an enforcement
// decision (FLEX-DEC-2026-019).
func runServeReadback(args []string, _, stderr io.Writer) int {
	fs := newFlagSet("serve-readback", stderr)
	addr := fs.String("addr", "127.0.0.1:8080", "HTTP listen address")
	logPath := fs.String("log", "", "optional JSONL decision log path")
	callerAuthMode := fs.String("caller-auth-mode", "disabled", "disabled, warn, or enforce")
	callerAudience := fs.String("caller-audience", "flex-auth", "required caller token audience")
	callerKubernetesURL := fs.String("caller-kubernetes-url", "https://kubernetes.default.svc", "Kubernetes API base URL for TokenReview")
	callerReviewerTokenFile := fs.String("caller-reviewer-token-file", "/var/run/secrets/flex-auth-reviewer/token", "projected Kubernetes API credential")
	callerCAFile := fs.String("caller-ca-file", "/var/run/secrets/flex-auth-reviewer/ca.crt", "Kubernetes API CA bundle")
	readbackCaller := fs.String("readback-caller", "", "authenticated Kubernetes principal admitted to the readback routes")
	var registries repeatedFlag
	fs.Var(&registries, "registry", "registry snapshot JSON file (repeatable; system ids must not repeat)")
	var packages keyValueFlags
	fs.Var(&packages, "package", "system=policy package Markdown file (repeatable)")
	if err := fs.Parse(args); err != nil {
		return 64
	}
	if len(registries) == 0 || len(packages) == 0 {
		fmt.Fprintln(stderr, "serve-readback requires --registry and --package")
		return 64
	}
	bindings := map[string]string{}
	if *readbackCaller != "" {
		bindings[readbackBinding] = *readbackCaller
	}

	service, err := buildReadback(context.Background(), registries, packages.StringMap())
	if err != nil {
		return fail(stderr, err)
	}
	if *logPath != "" {
		service.SetDecisionLog(audit.NewJSONLDecisionLog(*logPath))
	}
	authenticator, err := buildCallerAuthenticator(
		callerauth.Mode(*callerAuthMode),
		*callerAudience,
		*callerKubernetesURL,
		*callerReviewerTokenFile,
		*callerCAFile,
		bindings,
		stderr,
	)
	if err != nil {
		return fail(stderr, err)
	}

	fmt.Fprintf(stderr, "flex-auth readback serving on http://%s\n", *addr)
	if err := http.ListenAndServe(*addr, newReadbackMux(service, authenticator)); err != nil {
		return fail(stderr, err)
	}
	return 0
}

func buildReadback(ctx context.Context, registryPaths []string, packagePaths map[string]string) (*readback.Service, error) {
	merged := registry.NewStore()
	seen := map[string]string{}
	for _, path := range registryPaths {
		store, err := registry.LoadFile(path)
		if err != nil {
			return nil, err
		}
		snapshot := store.Snapshot()
		for _, system := range snapshot.Systems {
			if first, dup := seen[system.ID]; dup {
				return nil, fmt.Errorf("system %q appears in both %s and %s", system.ID, first, path)
			}
			seen[system.ID] = path
		}
		if err := merged.ApplySnapshot(snapshot); err != nil {
			return nil, fmt.Errorf("merge %s: %w", path, err)
		}
	}
	loaded := make(map[string]*policy.Package, len(packagePaths))
	for system, path := range packagePaths {
		pkg, err := policy.LoadAndValidateFile(ctx, path)
		if err != nil {
			return nil, err
		}
		loaded[system] = pkg
	}
	return readback.New(merged, loaded)
}

func newReadbackMux(service *readback.Service, authenticator *callerauth.Authenticator) *http.ServeMux {
	mux := http.NewServeMux()
	mux.HandleFunc("/healthz", func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("content-type", "application/json")
		_ = json.NewEncoder(w).Encode(map[string]string{"status": "ok"})
	})
	authorize := func(w http.ResponseWriter, r *http.Request) (context.Context, bool) {
		identity, err := authenticator.Authorize(r.Context(), r.Header.Get("Authorization"), []string{readbackBinding})
		if err != nil {
			writeCallerAuthError(w, err)
			return nil, false
		}
		return callerauth.WithRecord(r.Context(), authenticator.Record(identity)), true
	}
	mux.HandleFunc("/v1/readback/application_access", func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodPost {
			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
			return
		}
		var request readback.Request
		decoder := json.NewDecoder(r.Body)
		decoder.DisallowUnknownFields() // no action, resource or purpose can be smuggled in
		if err := decoder.Decode(&request); err != nil {
			http.Error(w, err.Error(), http.StatusBadRequest)
			return
		}
		ctx, ok := authorize(w, r)
		if !ok {
			return
		}
		response, err := service.Evaluate(ctx, request)
		if errors.Is(err, readback.ErrInvalidRequest) {
			http.Error(w, err.Error(), http.StatusBadRequest)
			return
		}
		writeHTTP(w, response, err)
	})
	mux.HandleFunc("/v1/readback/systems", func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodGet {
			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
			return
		}
		if _, ok := authorize(w, r); !ok {
			return
		}
		writeHTTP(w, service.Systems(), nil)
	})
	return mux
}