// session.go — post-passkey sessions + the middleware that gates the admin API.
//
// WebAuthn authenticates a login CEREMONY; from there we carry an opaque,
// server-side session (revocable, no secrets in the cookie). Cookie is
// Secure + HttpOnly + SameSite=Strict; state-changing requests additionally must
// echo the session's CSRF token in a header (belt-and-braces beyond same-origin).
package main

import (
	"context"
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"net/http"
	"sync"
	"time"
)

const (
	sessionCookie = "fleet_session"
	csrfHeader    = "X-Fleet-CSRF"
)

type ctxKey int

const adminCtxKey ctxKey = 0

type session struct {
	admin   string
	csrf    string
	expires time.Time
}

type sessionStore struct {
	mu   sync.Mutex
	ttl  time.Duration
	byID map[string]session
}

func newSessionStore(ttl time.Duration) *sessionStore {
	return &sessionStore{ttl: ttl, byID: map[string]session{}}
}

func randToken() string {
	b := make([]byte, 32)
	_, _ = rand.Read(b)
	return base64.RawURLEncoding.EncodeToString(b)
}

// create mints a session for a just-authenticated admin, returning the session id
// (cookie value) and the CSRF token (returned to the SPA in the login response).
func (s *sessionStore) create(admin string) (id, csrf string) {
	id, csrf = randToken(), randToken()
	s.mu.Lock()
	s.byID[id] = session{admin: admin, csrf: csrf, expires: time.Now().Add(s.ttl)}
	s.mu.Unlock()
	return id, csrf
}

func (s *sessionStore) get(id string) (session, bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	sess, ok := s.byID[id]
	if !ok {
		return session{}, false
	}
	if time.Now().After(sess.expires) {
		delete(s.byID, id)
		return session{}, false
	}
	return sess, true
}

func (s *sessionStore) delete(id string) {
	s.mu.Lock()
	delete(s.byID, id)
	s.mu.Unlock()
}

// requireSession wraps the admin API. It rejects requests without a valid session
// cookie, and (for mutating methods) without the matching CSRF header. The
// authenticated admin name is injected into the request context for audit.
func (s *server) requireSession(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Insecure dev mode has no passkey auth — open, and adminName reports the
		// dev sentinel. (main refuses insecure unless explicitly enabled.)
		if s.cfg.adminInsecure {
			next.ServeHTTP(w, r)
			return
		}
		c, err := r.Cookie(sessionCookie)
		if err != nil {
			writeErr(w, http.StatusUnauthorized, "not signed in")
			return
		}
		sess, ok := s.sessions.get(c.Value)
		if !ok {
			writeErr(w, http.StatusUnauthorized, "session expired")
			return
		}
		if isMutating(r.Method) {
			if subtle.ConstantTimeCompare([]byte(r.Header.Get(csrfHeader)), []byte(sess.csrf)) != 1 {
				writeErr(w, http.StatusForbidden, "missing or bad CSRF token")
				return
			}
		}
		next.ServeHTTP(w, r.WithContext(context.WithValue(r.Context(), adminCtxKey, sess)))
	})
}

// sessionFrom returns the authenticated session injected by requireSession.
func sessionFrom(r *http.Request) (session, bool) {
	sess, ok := r.Context().Value(adminCtxKey).(session)
	return sess, ok
}

func isMutating(method string) bool {
	switch method {
	case http.MethodPost, http.MethodPut, http.MethodPatch, http.MethodDelete:
		return true
	}
	return false
}

// setSessionCookie writes the session cookie. Secure+HttpOnly+SameSite=Strict.
func setSessionCookie(w http.ResponseWriter, id string, ttl time.Duration) {
	http.SetCookie(w, &http.Cookie{
		Name:     sessionCookie,
		Value:    id,
		Path:     "/",
		MaxAge:   int(ttl.Seconds()),
		Secure:   true,
		HttpOnly: true,
		SameSite: http.SameSiteStrictMode,
	})
}

func clearSessionCookie(w http.ResponseWriter) {
	http.SetCookie(w, &http.Cookie{Name: sessionCookie, Value: "", Path: "/", MaxAge: -1, Secure: true, HttpOnly: true, SameSite: http.SameSiteStrictMode})
}

// adminName returns the authenticated admin for audit attribution — the session
// identity in secure mode, or the dev sentinel in insecure mode.
func adminName(r *http.Request) string {
	if sess, ok := sessionFrom(r); ok && sess.admin != "" {
		return sess.admin
	}
	return "dev-insecure"
}