NextMyDay/internal/core/server/server.go

// Package server is MyDay's ONE door (SPEC-003 §1/§4): it turns an inbound
// request into a dispatched module function, applying identity · session · role
// and every cross-cutting concern. The door RESOLVES and GATES; a module never
// sees a request it should not (D-002, D-039).
//
// The pipeline (SPEC-003 §4):
//
//  1. correlation id      -> context + response header
//  2. client IP           -> from server.trusted_proxies (SET XFF, A6)
//  3. security headers
//  4. rate limit          -> /{GUID}, /access, /setup (A3)
//  5. routing.Dispatch    -> the pure model (SPEC-002)
//  6. the Kind switch     -> Redirect · NotFound · Setup · Enroll · Route/Action
//     Route/Action under a GUID -> the identity/session/role gate:
//     session valid? (non-empty, not expired/revoked — A2)
//     session.user_id == the GUID's user?
//     role permits the function?
//     attach audit.Meta -> dispatch
//
// Spec: docs/specs/spec-003-core-door.md.
package server

import (
	"errors"
	"log/slog"
	"net/http"
	"strconv"
	"strings"
	"time"

	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/audit"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/auth"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/config"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/httpx"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/middleware"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/routing"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/users"
)

// Deps assembles the door's dependencies.
type Deps struct {
	// Config is the loaded configuration.
	Config *config.Config
	// Resolver answers the pure model's GUID question (SPEC-002 §3.1).
	Resolver routing.Resolver
	// Registry is the module + core-handler dispatch surface.
	Registry Registry
	// Sessions validates the session cookie.
	Sessions auth.Sessions
	// Identities resolves a GUID to a user (the door's own lookup, distinct
	// from the pure Resolver surface).
	Identities auth.Identities
	// Users loads the authenticated user's role for the role gate.
	Users users.Users
	// Audit receives the door's and the services' audit rows.
	Audit audit.Audit
	// CSRF verifies action requests (A8).
	CSRF *httpx.CSRF
	// PreSession verifies the anonymous enroll/login double-submit token
	// (A15). When nil the `auth` action module is unavailable (404).
	PreSession *httpx.PreSessionCSRF
	// TrustedProxies overrides the set parsed from Config.Server.TrustedProxies.
	TrustedProxies httpx.TrustedProxies
	// SetupMode reports whether the WIZARD is enabled — the config toggle
	// `setup` (D-045). When nil it defaults to Config.Setup; the entrypoint
	// also clears it in-process once the wizard completes.
	SetupMode func() bool
	// LockedMode reports whether the instance is LOCKED: `setup: false` and no
	// admin exists yet (D-045). In that state `GET /` and `GET /setup` render
	// the "enable setup" boilerplate and every other route is 404 — the state
	// is deliberately not publicly claimable. When nil it is never locked.
	LockedMode func() bool
	// SetupGate is the first-run gate: the OPTIONAL setup code + the signed
	// unlock cookie, and the first-run pages (D-044/D-045). When nil, setup
	// mode keeps the strict behaviour (only `/setup` served, everything else
	// 404) — the entrypoint always wires it.
	SetupGate SetupGate
	// RoleGate enforces per-function authorisation. Defaults to
	// AllowAuthenticated.
	RoleGate RoleGate
	// Logger receives structured logs. Defaults to slog.Default().
	Logger *slog.Logger
	// Now is the clock (tests). Defaults to time.Now.
	Now func() time.Time
	// RateLimit / RateWindow tune the auth-sensitive limiter (A3).
	RateLimit  int
	RateWindow time.Duration
}

// Server is the door.
type Server struct {
	cfg      *config.Config
	disp     *routing.Dispatcher
	reg      Registry
	sessions auth.Sessions
	ids      auth.Identities
	users    users.Users
	audit    audit.Audit
	csrf     *httpx.CSRF
	precsrf  *httpx.PreSessionCSRF
	trusted  httpx.TrustedProxies
	setup    func() bool
	locked   func() bool
	gate     SetupGate
	roles    RoleGate
	log      *slog.Logger
	now      func() time.Time
	limiter  *middleware.RateLimiter
}

// SetupGate is the first-run gate the door consults. It reports whether a
// required setup code has been unlocked and renders the first-run pages that
// replace a bare 404 (D-044/D-045). It is implemented by *setup.Gate;
// declaring the seam here keeps the door free of the setup package's internals.
type SetupGate interface {
	// Required reports whether a setup code is required to reach the wizard
	// (D-045: `setup_code` is set). When false the `setup` toggle alone admits.
	Required() bool
	// Unlocked reports whether the request carries a valid setup-code unlock
	// (D-044).
	Unlocked(r *http.Request) bool
	// CodeForm renders the setup-code entry page (setup_code set, not yet
	// unlocked).
	CodeForm(w http.ResponseWriter, r *http.Request) error
	// Landing renders the page `GET /` shows while setup is enabled.
	Landing(w http.ResponseWriter, r *http.Request) error
	// Locked renders the "enable setup and restart" page (setup: false, no
	// admin).
	Locked(w http.ResponseWriter, r *http.Request) error
}

// New validates the dependencies and builds the door.
func New(d Deps) (*Server, error) {
	if d.Config == nil {
		return nil, errors.New("server: Config is required")
	}
	if d.Resolver == nil {
		return nil, errors.New("server: Resolver is required")
	}
	if d.Registry == nil {
		return nil, errors.New("server: Registry is required")
	}
	if d.Sessions == nil {
		return nil, errors.New("server: Sessions is required")
	}
	if d.Identities == nil {
		return nil, errors.New("server: Identities is required")
	}
	if d.Users == nil {
		return nil, errors.New("server: Users is required")
	}
	if d.CSRF == nil {
		return nil, errors.New("server: CSRF is required")
	}

	trusted := d.TrustedProxies
	if trusted == nil {
		var err error
		trusted, err = httpx.ParseTrustedProxies(d.Config.Server.TrustedProxies)
		if err != nil {
			return nil, err
		}
	}

	setup := d.SetupMode
	if setup == nil {
		cfg := d.Config
		setup = func() bool { return cfg.Setup }
	}
	locked := d.LockedMode
	if locked == nil {
		locked = func() bool { return false }
	}
	logger := d.Logger
	if logger == nil {
		logger = slog.Default()
	}
	clock := d.Now
	if clock == nil {
		clock = time.Now
	}
	roles := d.RoleGate
	if roles == nil {
		roles = AllowAuthenticated{}
	}

	return &Server{
		cfg:      d.Config,
		disp:     routing.New(d.Resolver, d.Registry),
		reg:      d.Registry,
		sessions: d.Sessions,
		ids:      d.Identities,
		users:    d.Users,
		audit:    d.Audit,
		csrf:     d.CSRF,
		precsrf:  d.PreSession,
		trusted:  trusted,
		setup:    setup,
		locked:   locked,
		gate:     d.SetupGate,
		roles:    roles,
		log:      logger,
		now:      clock,
		limiter:  middleware.NewRateLimiter(d.RateLimit, d.RateWindow),
	}, nil
}

// Handler wraps the door with the correlation-id middleware (SPEC-003 §4).
func (s *Server) Handler() http.Handler { return middleware.CorrelationID(s) }

// Serve binds listen and serves the door until it fails. Any address may be
// bound — including `0.0.0.0:8080` behind a reverse proxy (D-043); the
// bootstrap is gated per request by the `setup` toggle (D-045), not by a bind
// check.
func (s *Server) Serve(listen string) error {
	s.log.Info("myday: listening", "addr", listen, "setup_mode", s.setup())
	srv := &http.Server{
		Addr:              listen,
		Handler:           s.Handler(),
		ReadHeaderTimeout: 10 * time.Second,
	}
	return srv.ListenAndServe()
}

// ServeHTTP is the door pipeline (SPEC-003 §4).
func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	start := s.now()

	ctx := r.Context()
	cid := middleware.CorrelationFrom(ctx)
	if cid == "" {
		cid = middleware.NewCorrelationID()
		ctx = middleware.WithCorrelation(ctx, cid)
	}
	w.Header().Set(middleware.CorrelationHeader, cid)

	// 2. client IP (SET XFF, never append — A6).
	ip := httpx.ClientIP(r, s.trusted)

	// The scheme is resolved per request (D-043): TLS, or a trusted proxy's
	// X-Forwarded-Proto. Every cookie writer reads it back off the context.
	secure := httpx.RequestIsHTTPS(r, s.trusted)
	ctx = httpx.WithSecure(ctx, secure)

	// 3. security headers.
	httpx.SetSecurityHeaders(w, secure)

	// The door attaches audit.Meta before any service call (R-1). The actor is
	// filled in once the identity gate resolves it.
	ctx = middleware.AttachAudit(ctx, 0, ip, cid)
	r = r.WithContext(ctx)

	sw := &statusWriter{ResponseWriter: w, status: http.StatusOK}
	var res routing.Resolution
	defer func() {
		s.log.Info("request",
			"correlation_id", cid,
			"method", r.Method,
			"path", httpx.RedactPath(r.URL.Path),
			"query", httpx.RedactQuery(r.URL.RawQuery),
			"client_ip", ip,
			"status", sw.status,
			"workflow", res.Workflow,
			"module", res.Module,
			"function", res.Function,
			"duration_ms", s.now().Sub(start).Milliseconds(),
		)
	}()

	// 4. rate limit the auth-sensitive routes (A3, AC-9).
	if class := middleware.Class(r.URL.Path); class != "" {
		key := ip + "|" + class
		if !s.limiter.Allow(key) {
			if secs := s.limiter.RetryAfter(key); secs > 0 {
				sw.Header().Set("Retry-After", strconv.Itoa(secs))
			}
			s.writeError(sw, http.StatusTooManyRequests, "too many requests")
			return
		}
	}

	// 5. the pure model.
	res = s.disp.Dispatch(r.URL.Path, r.FormValue("do"), r.FormValue("action"))

	// 6. the Kind switch.
	s.dispatch(sw, r, res, cid, ip)
}

// dispatch is the Kind switch (SPEC-003 §4 step 5).
func (s *Server) dispatch(w http.ResponseWriter, r *http.Request, res routing.Resolution, cid, ip string) {
	// SPEC-005 §3 / D-045: the setup gate is the config toggle `setup`,
	// evaluated per request.
	//
	//   * wizard mode (`setup: true`) — `GET /setup` serves the wizard. If a
	//     `setup_code` is set it must be unlocked first (D-044); `GET /` points
	//     at the wizard; every other route is 404.
	//   * locked mode (`setup: false`, no admin) — `GET /` and `GET /setup`
	//     render the "enable setup" boilerplate; every other route is 404.
	//     Deliberately not claimable: the operator must flip the toggle.
	if s.setup() {
		admitted := s.setupAdmitted(r)
		// Static assets are the first-run pages' stylesheet — served in every
		// first-run state (SPEC-005 §3).
		static := res.Kind == routing.KindRoute && res.Workflow == "static"

		switch {
		case res.Kind == routing.KindRedirect && res.Redirect == "/home":
			// `GET /` — point at the wizard (never the app, D-045).
			s.serveLanding(w, r, cid, ip)
			return
		case res.Kind == routing.KindSetup && !admitted:
			// `GET /setup` with a required code not yet entered.
			s.serveCodeForm(w, r, cid, ip)
			return
		case static:
			// fall through — the wizard's stylesheet.
		case !s.isSetupUnlock(res) && (!admitted || !setupAllowed(res)):
			// SPEC-005 §3: once admitted, the app serves ONLY /setup; every
			// other route is 404 (delete, don't redirect). The unlock action is
			// the one exception — it is what grants the unlock (D-044).
			s.log.Warn("setup request refused: the wizard is not admitted",
				"correlation_id", cid, "client_ip", ip,
				"path", httpx.RedactPath(r.URL.Path))
			s.writeError(w, http.StatusNotFound, "not found")
			return
		}
	} else if s.locked() {
		// D-045: `setup: false` and no admin — a friendly, non-claimable
		// boilerplate at `/` and `/setup`; 404 elsewhere (static assets are the
		// one exception, so the boilerplate renders inside the shell).
		switch {
		case res.Kind == routing.KindRedirect && res.Redirect == "/home":
			s.serveLocked(w, r, cid, ip)
			return
		case res.Kind == routing.KindSetup:
			s.serveLocked(w, r, cid, ip)
			return
		case res.Kind == routing.KindRoute && res.Workflow == "static":
			// fall through — the boilerplate's stylesheet (SPEC-005 §3).
		default:
			s.writeError(w, http.StatusNotFound, "not found")
			return
		}
	}

	switch res.Kind {
	case routing.KindRedirect:
		// The ONE canonical redirect ("/" -> "/home").
		http.Redirect(w, r, res.Redirect, http.StatusFound)

	case routing.KindNotFound:
		s.writeError(w, http.StatusNotFound, "not found")

	case routing.KindSetup:
		// /setup is served ONLY in setup mode; otherwise it 404s (delete,
		// don't redirect — SPEC-005 §5).
		if !s.setup() {
			s.writeError(w, http.StatusNotFound, "not found")
			return
		}
		s.serveCore(w, r, "setup", res, cid, ip)

	case routing.KindEnroll:
		// GUID found but not enrolled -> the enrollment screen (SPEC-006).
		s.serveCore(w, r, "enroll", res, cid, ip)

	case routing.KindLogin:
		s.serveCore(w, r, "login", res, cid, ip)

	case routing.KindRoute, routing.KindAction:
		// /setup is setup-mode-only through BOTH doors — a path request
		// classifies as KindSetup, but `do`/`action` under /setup classifies as
		// KindAction with Workflow "setup". Productive mode 404s either way
		// (delete, don't redirect — SPEC-005 §5).
		if res.Workflow == "setup" && !s.setup() {
			s.writeError(w, http.StatusNotFound, "not found")
			return
		}
		// SPEC-005: the wizard's steps are actions under the reserved `setup`
		// workflow. They run anonymously (pre-session CSRF, A15).
		if res.Kind == routing.KindAction && res.Workflow == "setup" {
			s.serveSetupAction(w, r, res, cid, ip)
			return
		}
		// M1c/L3s — pin the setup MODULE at the door: the wizard's functions are
		// reachable ONLY through the reserved `setup` workflow. Any other
		// resolution naming `do=setup` (a GUID path, a reserved workflow) is
		// invalid in every mode and 404s rather than falling through to the
		// module registry or the GUID door.
		if res.Module == setupModule {
			s.writeError(w, http.StatusNotFound, "not found")
			return
		}
		// SPEC-006: the `auth` module's actions are the enroll/login flows.
		// They run BEFORE the session gate (the whole point is to obtain a
		// session); logout is the exception and stays session-gated.
		if res.Kind == routing.KindAction && res.Module == authModule &&
			!routing.IsReservedWorkflow(res.Workflow) {
			s.serveAuthAction(w, r, res, cid, ip)
			return
		}
		// SPEC-007: the anonymous /access recovery action — the recovery
		// request form (`do=auth`). It runs before any session (pre-session
		// CSRF, A15) and is always a generic response (A4). Only the `auth`
		// module is served here: the session-gated approve/deny actions must
		// ride a GUID path and are deliberately NOT reachable at /access.
		if res.Kind == routing.KindAction && res.Workflow == "access" && res.Module == authModule {
			s.serveAccessAction(w, r, res, cid, ip)
			return
		}
		// A reserved level-1 workflow (home · access · api · static · …) is
		// core-owned and needs no identity gate.
		if routing.IsReservedWorkflow(res.Workflow) {
			s.serveCore(w, r, res.Workflow, res, cid, ip)
			return
		}
		// Otherwise it is a GUID branch: the identity/session/role gate.
		s.serveGUID(w, r, res, cid, ip)

	default:
		s.writeError(w, http.StatusNotFound, "not found")
	}
}

// setupAllowed reports whether a Resolution is the wizard itself — the
// reserved `setup` workflow, path or action (SPEC-005 §3). Used while the
// wizard is enabled: everything else is 404. Static assets are handled
// separately (they are served in every first-run state).
func setupAllowed(res routing.Resolution) bool {
	switch res.Kind {
	case routing.KindSetup:
		return true
	case routing.KindAction:
		return res.Workflow == "setup"
	}
	return false
}

// setupAdmitted reports whether the request may reach the wizard while
// `setup: true` (D-045): always when no `setup_code` is required, otherwise
// only with a valid code unlock (D-044).
func (s *Server) setupAdmitted(r *http.Request) bool {
	if s.gate == nil || !s.gate.Required() {
		return true
	}
	return s.gate.Unlocked(r)
}

// isSetupUnlock reports whether res is the D-044 unlock action — the ONE request
// served while the wizard is enabled without an unlock, because it is what
// grants the unlock. The door still pins it to the `setup` module
// (serveSetupAction), so a `do=core` under /setup cannot ride this carve-out.
func (s *Server) isSetupUnlock(res routing.Resolution) bool {
	return res.Kind == routing.KindAction &&
		res.Workflow == "setup" &&
		res.Module == setupModule &&
		res.Function == "unlock"
}

// serveCodeForm renders the setup-code entry page — `GET /setup` with a required
// code not yet entered. A nil gate (not wired) keeps the strict 404; the
// entrypoint always wires it.
func (s *Server) serveCodeForm(w http.ResponseWriter, r *http.Request, cid, ip string) {
	s.serveGatePage(w, r, cid, ip, "code form", func(g SetupGate) error { return g.CodeForm(w, r) })
}

// serveLanding renders the page `GET /` shows while `setup: true` (D-045).
func (s *Server) serveLanding(w http.ResponseWriter, r *http.Request, cid, ip string) {
	s.serveGatePage(w, r, cid, ip, "setup landing", func(g SetupGate) error { return g.Landing(w, r) })
}

// serveLocked renders the "enable setup" boilerplate (D-045).
func (s *Server) serveLocked(w http.ResponseWriter, r *http.Request, cid, ip string) {
	s.serveGatePage(w, r, cid, ip, "locked boilerplate", func(g SetupGate) error { return g.Locked(w, r) })
}

// serveGatePage hands off to a first-run page rendered by the gate, mapping a
// render fault to a logged 500. A nil gate keeps the strict 404 — the
// entrypoint always wires it.
func (s *Server) serveGatePage(w http.ResponseWriter, r *http.Request, cid, ip, label string, render func(SetupGate) error) {
	if s.gate == nil {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	if err := render(s.gate); err != nil {
		s.log.Error("setup: render "+label, "correlation_id", cid, "client_ip", ip, "error", err)
		s.writeError(w, http.StatusInternalServerError, "internal error")
	}
}

// serveCore hands off to a core-owned workflow handler.
func (s *Server) serveCore(w http.ResponseWriter, r *http.Request, workflow string, res routing.Resolution, cid, ip string) {
	h, ok := s.reg.CoreHandler(workflow)
	if !ok {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	s.invoke(w, r, h, newRequestContext(res, 0, cid, ip))
}

// serveAuthAction runs the anonymous enroll/login actions (SPEC-006 §4/§5).
// It runs BEFORE the identity/session gate — the whole point is to obtain a
// session — so it verifies the signed pre-session double-submit token (A15)
// instead of the session-bound one. `logout` is session-gated (serveGUID).
func (s *Server) serveAuthAction(w http.ResponseWriter, r *http.Request, res routing.Resolution, cid, ip string) {
	if s.precsrf == nil {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	// `logout` revokes an existing session, and the SPEC-006 §8 passkey-nudge
	// actions (passkey.add / passkey.later / passkey.never) require one — they
	// are session-gated (SPEC-003 §6 CSRF), NOT anonymous like the enroll/login
	// actions above.
	if res.Function == "logout" || strings.HasPrefix(res.Function, "passkey.") {
		s.serveGUID(w, r, res, cid, ip)
		return
	}
	if !s.precsrf.Valid(r, r.FormValue("csrf")) {
		s.writeError(w, http.StatusForbidden, "forbidden")
		return
	}
	h, ok := s.reg.Handler(res.Module, res.Function)
	if !ok {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	s.invoke(w, r, h, newRequestContext(res, 0, cid, ip))
}

// serveSetupAction runs a SPEC-005 wizard step. The wizard is anonymous — no
// session exists before the admin is created — so it verifies the signed
// pre-session double-submit token (A15) rather than the session-bound CSRF. The
// door has already gated this to setup mode.
func (s *Server) serveSetupAction(w http.ResponseWriter, r *http.Request, res routing.Resolution, cid, ip string) {
	if !s.setup() {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	// M1c: the door pins the action door to the `setup` module. Without this a
	// `do=core&action=modules.toggle` under /setup would reach a core action
	// that the session-gated role gate (admin-only) would otherwise protect.
	if res.Module != setupModule {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	if s.precsrf == nil {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	if !s.precsrf.Valid(r, r.FormValue("csrf")) {
		s.writeError(w, http.StatusForbidden, "forbidden")
		return
	}
	h, ok := s.reg.Handler(res.Module, res.Function)
	if !ok {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	s.invoke(w, r, h, newRequestContext(res, 0, cid, ip))
}

// serveAccessAction runs a SPEC-007 /access action. The recovery entry is
// anonymous — no session exists yet — so it verifies the signed pre-session
// double-submit token (A15) rather than the session-bound CSRF (§6). The
// response is generic whatever the email matches (AC-4).
func (s *Server) serveAccessAction(w http.ResponseWriter, r *http.Request, res routing.Resolution, cid, ip string) {
	if s.precsrf == nil {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	if !s.precsrf.Valid(r, r.FormValue("csrf")) {
		s.writeError(w, http.StatusForbidden, "forbidden")
		return
	}
	h, ok := s.reg.Handler(res.Module, res.Function)
	if !ok {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	s.invoke(w, r, h, newRequestContext(res, 0, cid, ip))
}

// serveGUID runs the identity/session/role gate, then dispatches to the module
// function (SPEC-003 §4 step 5).
func (s *Server) serveGUID(w http.ResponseWriter, r *http.Request, res routing.Resolution, cid, ip string) {
	ctx := r.Context()

	// (a) load the session cookie; an absent or empty id is rejected (A2).
	// Over https the cookie name carries the `__Host-` prefix (L1s/D-043).
	secure := httpx.SecureFromRequest(r)
	cookieName := httpx.SessionCookieNameFor(secure)
	cookie, err := r.Cookie(cookieName)
	if err != nil || cookie.Value == "" {
		s.loginRequired(w, r, res, cid, ip)
		return
	}

	// Validate: exists · not expired · not revoked · non-empty (A2/AC-5).
	sess, err := s.sessions.Validate(ctx, cookie.Value)
	if err != nil {
		httpx.ClearSessionCookie(w, cookieName, secure)
		s.loginRequired(w, r, res, cid, ip)
		return
	}

	// Resolve the GUID's user and require an ACTIVE identity.
	idn, err := s.ids.ByGUID(ctx, res.Workflow)
	if err != nil || idn.Status != auth.StatusActive {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}

	// (b) the identity match: the session must belong to the GUID's user
	// (AC-4). The rejection is generic — no oracle about the session's owner.
	if sess.UserID != idn.UserID {
		s.loginRequired(w, r, res, cid, ip)
		return
	}

	// (d) the role gate.
	u, err := s.users.Get(ctx, sess.UserID)
	if err != nil {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	if !s.roles.Allowed(u.Role, res.Module, res.Function) {
		s.writeError(w, http.StatusForbidden, "forbidden")
		return
	}

	// Attach the actor now that the identity is known (R-1).
	ctx = middleware.AttachAudit(ctx, sess.UserID, ip, cid)

	// CSRF on every action (A8/AC-6). The routing fields are not a control.
	if res.Kind == routing.KindAction {
		if !s.csrf.Valid(sess.ID, r.FormValue("csrf")) {
			s.writeError(w, http.StatusForbidden, "invalid csrf token")
			return
		}
	}

	// (e) build the RequestContext and dispatch.
	h, ok := s.reg.Handler(res.Module, res.Function)
	if !ok {
		s.writeError(w, http.StatusNotFound, "not found")
		return
	}
	ctx = httpx.WithToken(ctx, s.csrf.Token(sess.ID))
	r = r.WithContext(ctx)

	s.invoke(w, r, h, newRequestContext(res, sess.UserID, cid, ip))
}

// loginRequired hands off to the login screen without revealing why the session
// was rejected (missing, empty, invalid, or belonging to another user). The
// screen is a RENDERED PAGE and answers 200; only a failed credential attempt is
// 401/400 (F-1). The 401 below is a degenerate fallback for a registry that
// carries no login screen at all.
func (s *Server) loginRequired(w http.ResponseWriter, r *http.Request, res routing.Resolution, cid, ip string) {
	h, ok := s.reg.CoreHandler("login")
	if !ok {
		s.writeError(w, http.StatusUnauthorized, "login required")
		return
	}
	s.invoke(w, r, h, newRequestContext(res, 0, cid, ip))
}

// invoke calls a handler and maps a returned error to a 500 (logged).
func (s *Server) invoke(w http.ResponseWriter, r *http.Request, h Handler, rc *RequestContext) {
	if err := h(w, r, rc); err != nil {
		s.log.Error("handler error",
			"correlation_id", rc.Correlation,
			"workflow", rc.Workflow,
			"function", rc.Function,
			"error", err,
		)
		s.writeError(w, http.StatusInternalServerError, "internal error")
	}
}

// writeError writes a terse plain-text error response.
func (s *Server) writeError(w http.ResponseWriter, status int, msg string) {
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	w.Header().Set("X-Content-Type-Options", "nosniff")
	w.WriteHeader(status)
	_, _ = w.Write([]byte(msg + "\n"))
}

// statusWriter captures the response status for the access log.
type statusWriter struct {
	http.ResponseWriter
	status int
	wrote  bool
}

func (w *statusWriter) WriteHeader(code int) {
	if w.wrote {
		return
	}
	w.status = code
	w.wrote = true
	w.ResponseWriter.WriteHeader(code)
}

func (w *statusWriter) Write(b []byte) (int, error) {
	if !w.wrote {
		w.WriteHeader(http.StatusOK)
	}
	return w.ResponseWriter.Write(b)
}

var _ http.Handler = (*Server)(nil)