// Package server implements Jarl's HTTP API and serves the web UI.
package server

import (
	"context"
	"crypto/tls"
	"errors"
	"fmt"
	"io/fs"
	"log/slog"
	"net"
	"net/http"
	"net/netip"
	"os"
	"strings"
	"sync"
	"time"

	"jarl/internal/api"
	"jarl/internal/auth"
	"jarl/internal/engine"
	"jarl/internal/nscheck"
	"jarl/internal/stats"
	"jarl/internal/store"
)

// Config configures the HTTP server.
type Config struct {
	Listen          []string
	TLSCertFile     string
	TLSKeyFile      string
	TrustedProxies  []netip.Prefix
	SessionLifetime time.Duration
	SessionIdle     time.Duration
	// CookieSecure is "auto" (secure when TLS is used or the request came
	// through a trusted HTTPS proxy), "always" or "never".
	CookieSecure string
	MaxBodyBytes int64
	Version      string
	// Stats is the query statistics collector (nil when disabled).
	Stats *stats.Collector
	// HTTPReq configures the endpoint for lego's httpreq DNS provider.
	HTTPReq HTTPReq
	// Socket is the local unix socket's path ("" for none); see socket.go.
	Socket string
	// BackupDir and BackupWarnAfter are the backup settings the dashboard
	// reports on (backup.dir and backup.warn_after).
	BackupDir       string
	BackupWarnAfter time.Duration
	// TOTPRequire is totp.require (none, admins or all) and TOTPIssuer the
	// name authenticator apps show (h_totp.go).
	TOTPRequire string
	TOTPIssuer  string
}

// Server is the HTTP server.
type Server struct {
	cfg     Config
	eng     *engine.Engine
	store   *store.Store
	log     *slog.Logger
	mux     *http.ServeMux
	limiter *rateLimiter
	web     fs.FS
	certs   *certReloader
	statsQ  *stats.Querier
	// oldSecretWarn limits warnings about rotated tokens' old secrets.
	oldSecretWarn throttle
	// expiryWarn limits warnings about expiring tokens to one a day each.
	expiryWarn throttle
	// pendingLogins are logins waiting for their TOTP code.
	pendingLogins pendingLogins
	// nsChecker, when set, replaces the default name server checker
	// (tests point it at local DNS servers).
	nsChecker *nscheck.Checker

	mu   sync.Mutex
	srvs []*http.Server
}

// New creates a server. web holds the static web UI (may be nil).
func New(cfg Config, eng *engine.Engine, log *slog.Logger, web fs.FS) *Server {
	if cfg.SessionLifetime == 0 {
		cfg.SessionLifetime = 12 * time.Hour
	}
	if cfg.SessionIdle == 0 {
		cfg.SessionIdle = 2 * time.Hour
	}
	if cfg.MaxBodyBytes == 0 {
		cfg.MaxBodyBytes = 16 << 20
	}
	if cfg.CookieSecure == "" {
		cfg.CookieSecure = "auto"
	}
	if cfg.TOTPRequire == "" {
		cfg.TOTPRequire = "none"
	}
	if cfg.TOTPIssuer == "" {
		cfg.TOTPIssuer = "Jarl"
	}
	if log == nil {
		log = slog.Default()
	}
	s := &Server{cfg: cfg, eng: eng, store: eng.Store(), log: log, mux: http.NewServeMux(), limiter: newRateLimiter(), web: web}
	s.expiryWarn.every = 24 * time.Hour
	if cfg.Stats != nil {
		s.statsQ = stats.NewQuerier(cfg.Stats)
	}
	s.routes()
	return s
}

// Handler returns the server's HTTP handler with all middleware.
func (s *Server) Handler() http.Handler {
	return s.recoverer(s.securityHeaders(s.logRequests(s.mux)))
}

// Serve listens on all configured addresses until ctx is canceled.
func (s *Server) Serve(ctx context.Context, ready func()) error {
	if len(s.cfg.Listen) == 0 {
		return errors.New("no listen addresses configured")
	}
	var tlsCfg *tls.Config
	if s.cfg.TLSCertFile != "" {
		cr, err := newCertReloader(s.cfg.TLSCertFile, s.cfg.TLSKeyFile, s.log)
		if err != nil {
			return err
		}
		s.certs = cr
		tlsCfg = &tls.Config{MinVersion: tls.VersionTLS12, GetCertificate: cr.get}
	}
	errc := make(chan error, len(s.cfg.Listen)+1)
	var listeners []net.Listener
	for _, addr := range s.cfg.Listen {
		ln, err := net.Listen("tcp", addr)
		if err != nil {
			for _, l := range listeners {
				l.Close()
			}
			return fmt.Errorf("listening on %s: %w", addr, err)
		}
		listeners = append(listeners, ln)
	}
	for i, ln := range listeners {
		srv := &http.Server{
			Handler:           s.Handler(),
			ReadHeaderTimeout: 10 * time.Second,
			ReadTimeout:       2 * time.Minute,
			WriteTimeout:      5 * time.Minute,
			IdleTimeout:       2 * time.Minute,
			TLSConfig:         tlsCfg,
			ErrorLog:          slog.NewLogLogger(s.log.Handler(), slog.LevelWarn),
		}
		s.mu.Lock()
		s.srvs = append(s.srvs, srv)
		s.mu.Unlock()
		scheme := "http"
		if tlsCfg != nil {
			scheme = "https"
		}
		s.log.Info("listening", "addr", s.cfg.Listen[i], "url", scheme+"://"+ln.Addr().String())
		go func(srv *http.Server, ln net.Listener) {
			var err error
			if tlsCfg != nil {
				err = srv.ServeTLS(ln, "", "")
			} else {
				err = srv.Serve(ln)
			}
			if !errors.Is(err, http.ErrServerClosed) {
				errc <- err
			}
		}(srv, ln)
	}
	if tlsCfg == nil {
		for _, a := range s.cfg.Listen {
			if host, _, err := net.SplitHostPort(a); err == nil {
				if ip, err := netip.ParseAddr(host); host == "" || err == nil && !ip.IsLoopback() {
					s.log.Warn("serving plain HTTP on a non-loopback address: passwords and API tokens travel unencrypted; configure tls or a TLS reverse proxy", "addr", a)
				}
			}
		}
	}
	s.serveSocket(errc)
	s.logHTTPReq(tlsCfg != nil)
	if ready != nil {
		ready()
	}
	go s.housekeeping(ctx)
	select {
	case <-ctx.Done():
	case err := <-errc:
		s.shutdown()
		return err
	}
	s.shutdown()
	return nil
}

// serveSocket serves the API on the local unix socket. A socket that
// can't be created (e.g. /run/jarl missing outside systemd) is logged, not
// fatal: the network listeners still work.
func (s *Server) serveSocket(errc chan error) {
	if s.cfg.Socket == "" {
		return
	}
	ln, err := listenSocket(s.cfg.Socket)
	if err != nil {
		s.log.Warn("not listening on the local socket", "path", s.cfg.Socket, "err", err)
		return
	}
	srv := &http.Server{
		Handler:           s.Handler(),
		ConnContext:       socketConnContext,
		ReadHeaderTimeout: 10 * time.Second,
		WriteTimeout:      5 * time.Minute,
		IdleTimeout:       2 * time.Minute,
		ErrorLog:          slog.NewLogLogger(s.log.Handler(), slog.LevelWarn),
	}
	s.mu.Lock()
	s.srvs = append(s.srvs, srv)
	s.mu.Unlock()
	s.log.Info("listening", "socket", s.cfg.Socket)
	go func() {
		if err := srv.Serve(ln); !errors.Is(err, http.ErrServerClosed) {
			errc <- err
		}
	}()
}

func (s *Server) shutdown() {
	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
	defer cancel()
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, srv := range s.srvs {
		srv.Shutdown(ctx)
	}
	if s.cfg.Socket != "" {
		os.Remove(s.cfg.Socket)
	}
}

// ReloadTLS rereads the TLS certificate (on SIGHUP).
func (s *Server) ReloadTLS() {
	if s.certs != nil {
		s.certs.reload()
	}
}

func (s *Server) housekeeping(ctx context.Context) {
	t := time.NewTicker(10 * time.Minute)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			if err := s.store.PurgeSessions(ctx, time.Now()); err != nil {
				s.log.Warn("purging sessions failed", "err", err)
			}
			s.limiter.cleanup()
			s.warnExpiringTokens(ctx, time.Now())
			if s.certs != nil {
				s.certs.reloadIfChanged()
			}
		}
	}
}

// certReloader serves a certificate that is reread when the files change,
// so renewed certificates (e.g. from Let's Encrypt) are picked up.
type certReloader struct {
	certFile, keyFile string
	log               *slog.Logger
	mu                sync.RWMutex
	cert              *tls.Certificate
	modTime           time.Time
}

func newCertReloader(certFile, keyFile string, log *slog.Logger) (*certReloader, error) {
	cr := &certReloader{certFile: certFile, keyFile: keyFile, log: log}
	if err := cr.load(); err != nil {
		return nil, err
	}
	return cr, nil
}

func (cr *certReloader) load() error {
	cert, err := tls.LoadX509KeyPair(cr.certFile, cr.keyFile)
	if err != nil {
		return fmt.Errorf("loading TLS certificate: %w", err)
	}
	fi, _ := os.Stat(cr.certFile)
	cr.mu.Lock()
	cr.cert = &cert
	if fi != nil {
		cr.modTime = fi.ModTime()
	}
	cr.mu.Unlock()
	return nil
}

func (cr *certReloader) reload() {
	if err := cr.load(); err != nil {
		cr.log.Error("reloading TLS certificate failed", "err", err)
		return
	}
	cr.log.Info("TLS certificate reloaded", "file", cr.certFile)
}

func (cr *certReloader) reloadIfChanged() {
	fi, err := os.Stat(cr.certFile)
	if err != nil {
		return
	}
	cr.mu.RLock()
	changed := fi.ModTime().After(cr.modTime)
	cr.mu.RUnlock()
	if changed {
		cr.reload()
	}
}

func (cr *certReloader) get(*tls.ClientHelloInfo) (*tls.Certificate, error) {
	cr.mu.RLock()
	defer cr.mu.RUnlock()
	return cr.cert, nil
}

// ---- middleware

type ctxKey int

const (
	keyPrincipal ctxKey = iota
	keySession
)

func (s *Server) recoverer(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		defer func() {
			if v := recover(); v != nil {
				if v == http.ErrAbortHandler {
					panic(v)
				}
				s.log.Error("panic serving request", "path", r.URL.Path, "panic", v)
				writeError(w, http.StatusInternalServerError, "internal", "internal server error")
			}
		}()
		next.ServeHTTP(w, r)
	})
}

func (s *Server) securityHeaders(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		h := w.Header()
		// The version is no secret: /healthz reports it too.
		h.Set("Server", "Jarl/"+s.cfg.Version)
		h.Set("X-Content-Type-Options", "nosniff")
		h.Set("X-Frame-Options", "DENY")
		h.Set("Referrer-Policy", "no-referrer")
		h.Set("Content-Security-Policy", "default-src 'self'; img-src 'self' data:; style-src 'self'; script-src 'self'; connect-src 'self'; frame-ancestors 'none'; base-uri 'none'; form-action 'self'")
		h.Set("Cross-Origin-Opener-Policy", "same-origin")
		if s.isHTTPS(r) {
			h.Set("Strict-Transport-Security", "max-age=31536000")
		}
		next.ServeHTTP(w, r)
	})
}

type statusWriter struct {
	http.ResponseWriter
	status int
	bytes  int
}

func (w *statusWriter) WriteHeader(code int) {
	w.status = code
	w.ResponseWriter.WriteHeader(code)
}

func (w *statusWriter) Write(b []byte) (int, error) {
	if w.status == 0 {
		w.status = http.StatusOK
	}
	n, err := w.ResponseWriter.Write(b)
	w.bytes += n
	return n, err
}

func (s *Server) logRequests(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		start := time.Now()
		sw := &statusWriter{ResponseWriter: w}
		next.ServeHTTP(sw, r)
		if r.URL.Path == "/healthz" || strings.HasPrefix(r.URL.Path, "/assets/") {
			return
		}
		user := ""
		if p, ok := r.Context().Value(keyPrincipal).(*auth.Principal); ok {
			user = p.Name()
		}
		s.log.Debug("http", "method", r.Method, "path", r.URL.Path, "status", sw.status, "bytes", sw.bytes, "duration", time.Since(start).Round(time.Millisecond), "ip", s.clientIP(r), "user", user)
	})
}

// clientIP returns the client's address, honoring X-Forwarded-For from
// trusted proxies.
func (s *Server) clientIP(r *http.Request) string {
	if peerOf(r.Context()) != nil {
		return "local"
	}
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		host = r.RemoteAddr
	}
	peer, err := netip.ParseAddr(host)
	if err != nil || !s.trusted(peer) {
		return host
	}
	xff := r.Header.Get("X-Forwarded-For")
	if xff == "" {
		return host
	}
	parts := strings.Split(xff, ",")
	// Walk from the right, skipping trusted proxies.
	for i := len(parts) - 1; i >= 0; i-- {
		a, err := netip.ParseAddr(strings.TrimSpace(parts[i]))
		if err != nil {
			return host
		}
		if !s.trusted(a) {
			return a.String()
		}
	}
	return strings.TrimSpace(parts[0])
}

func (s *Server) trusted(a netip.Addr) bool {
	a = a.Unmap()
	for _, p := range s.cfg.TrustedProxies {
		if p.Contains(a) {
			return true
		}
	}
	return false
}

// logHTTPReq says at startup whether the httpreq endpoint can work.
func (s *Server) logHTTPReq(tls bool) {
	h := s.cfg.HTTPReq
	switch {
	case !h.Enabled:
	case h.AllowPlainHTTP:
		s.log.Warn("acme.httpreq accepts plain HTTP (require_https: false): API tokens may travel unencrypted", "path", api.Prefix+"/acme/httpreq")
	case !tls && len(s.cfg.TrustedProxies) == 0 && !h.AllowLoopbackHTTP:
		s.log.Warn("acme.httpreq is enabled but jarld has no tls and no trusted_proxies, so every request will be refused as not HTTPS", "path", api.Prefix+"/acme/httpreq")
	default:
		s.log.Info("acme.httpreq endpoint enabled", "path", api.Prefix+"/acme/httpreq", "tls", tls, "trusted_proxies", len(s.cfg.TrustedProxies), "allow_loopback_http", h.AllowLoopbackHTTP)
	}
}

func (s *Server) isHTTPS(r *http.Request) bool {
	https, _ := s.transport(r)
	return https
}

// transport reports whether r reached Jarl over HTTPS, and explains how
// that was decided (for error messages and the httpreq status check).
//
// A request is HTTPS when jarld terminated TLS itself (tls in jarl.yaml),
// or when it came straight from a trusted proxy (trusted_proxies) that
// says the client used HTTPS: X-Forwarded-Proto, or, if that is absent,
// the proto parameter of Forwarded (RFC 7239). Proxies append to these
// headers, so the last value is the one the nearest proxy set; anything a
// client sent itself comes earlier and is ignored. From any other address
// the headers are ignored, since anyone could send them.
func (s *Server) transport(r *http.Request) (https bool, why string) {
	if r.TLS != nil {
		return true, "HTTPS to jarld (tls in jarl.yaml)"
	}
	if peerOf(r.Context()) != nil {
		return true, "jarld's local unix socket (never crosses a network)"
	}
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		host = r.RemoteAddr
	}
	peer, err := netip.ParseAddr(host)
	if err != nil || !s.trusted(peer) {
		if r.Header.Get("X-Forwarded-Proto") != "" || r.Header.Get("Forwarded") != "" {
			return false, fmt.Sprintf("plain HTTP from %s, which sent X-Forwarded-Proto or Forwarded but isn't in trusted_proxies, so the header was ignored", host)
		}
		return false, fmt.Sprintf("plain HTTP from %s (jarld has no tls configured, and %s isn't a trusted proxy)", host, host)
	}
	proto, header := forwardedProto(r)
	switch {
	case proto == "":
		return false, fmt.Sprintf("trusted proxy %s, which sent neither X-Forwarded-Proto nor Forwarded: proto=, so the client's protocol is unknown", host)
	case proto == "https":
		return true, fmt.Sprintf("HTTPS to trusted proxy %s (%s: https)", host, header)
	default:
		return false, fmt.Sprintf("trusted proxy %s, which says the client used %s (%s: %s)", host, proto, header, proto)
	}
}

// forwardedProto returns the client's protocol as the nearest proxy
// reported it, and the header it came from.
func forwardedProto(r *http.Request) (proto, header string) {
	if vals := r.Header.Values("X-Forwarded-Proto"); len(vals) > 0 {
		parts := strings.Split(strings.Join(vals, ","), ",")
		return strings.ToLower(strings.TrimSpace(parts[len(parts)-1])), "X-Forwarded-Proto"
	}
	if vals := r.Header.Values("Forwarded"); len(vals) > 0 {
		elems := strings.Split(strings.Join(vals, ","), ",")
		for _, pair := range strings.Split(elems[len(elems)-1], ";") {
			k, v, _ := strings.Cut(strings.TrimSpace(pair), "=")
			if strings.EqualFold(k, "proto") {
				return strings.ToLower(strings.Trim(strings.TrimSpace(v), `"`)), "Forwarded"
			}
		}
	}
	return "", ""
}