package relay

import (
	"encoding/binary"
	"errors"
	"fmt"
	"math/rand/v2"
	"net"
	"sort"
	"strconv"
	"sync"
	"time"

	"github.com/pion/logging"
	"github.com/pion/stun/v3"
	"github.com/pion/turn/v4"
)

// Realm is the TURN realm every credential is minted and verified under.
const Realm = "carlos"

// pendingLife is how long a socket or an admission may sit unclaimed
// before tick reclaims it. Both are consumed within one request in the
// normal path; this only catches an allocation that failed after pion had
// already asked us for a socket.
const pendingLife = 30 * time.Second

// authMemory is how long the last parsed username for a typed source is
// kept once that source has no allocation, so a client that authenticates
// and never allocates cannot grow the map without bound.
const authMemory = 10 * time.Minute

const (
	// maxRefusalIdentities and maxLoggedRefusals bound the two maps an
	// unauthenticated client can otherwise grow: pion calls AuthHandler
	// with the raw username before it checks MESSAGE-INTEGRITY, so every
	// counter keyed on something the caller chose is remote memory. Only
	// identities in the authority record get their own row; everything
	// else shares the zero identity, and these caps are the backstop.
	maxRefusalIdentities = 4096
	maxLoggedRefusals    = 4096

	// refusalLogEvery is how often one identity's one refusal reason may
	// put a line in the log.
	refusalLogEvery = time.Minute

	// minLastAuthEntries is the floor under Limits.BoxLive*2 for how many
	// typed sources may hold a remembered username at once.
	minLastAuthEntries = 256
)

var errTCPAllocUnsupported = errors.New("relay: TCP relay allocations are not supported")

// ServerConfig is everything the relay needs from its process: where to
// listen, what to advertise, and the four live views (masters, authority,
// lease, limits) it reads on every request.
type ServerConfig struct {
	Region           string
	RelayIP          net.IP       // the EIP advertised to clients
	UDP, TCP         string       // listen addresses, e.g. ":3478"; empty disables
	MinPort, MaxPort uint16       // 49152, 65535
	DenyCIDRs        []*net.IPNet // from CARLOS_RELAY_DENY_CIDRS, on top of the built-in private ranges
	ExtraAllow       []*net.IPNet // peer ranges allowed despite the built-in list; empty in production
	Masters          func() *Masters
	Authority        func() *Authority
	Lease            func(now time.Time) bool
	Limits           Limits
	Now              func() time.Time
	Log              func(string, ...any)

	// Meter is the byte meter (Task 7); nil disables metering. It is an
	// interface so this file does not depend on the meter's shape.
	Meter interface {
		Add(id Identity, in, out, rejected int64)
	}
}

// Server is the relay: a pion TURN server whose every socket we own, so
// that an allocation can be ended, attributed and bounded from outside
// pion, which exposes no way to do any of it.
type Server struct {
	cfg ServerConfig
	now func() time.Time
	log func(string, ...any)

	adm         *Admission
	box         *Bucket
	ports       *portSet
	deny        []*net.IPNet
	allow       []*net.IPNet
	local       []net.IP // our listeners' own addresses, when not wildcard
	maxLastAuth int

	turn      *turn.Server
	udp       net.PacketConn
	tcp       net.Listener
	listeners []string

	mu       sync.Mutex
	seq      int64
	pending  map[int]pendingConn        // relay port -> socket awaiting OnAllocationCreated
	lastAuth map[string]authRec         // typed source -> the username it last authenticated with
	admits   map[string]admitRec        // typed source -> an admission not yet owned by an allocation
	bound    map[string]*Binding        // typed source -> the identity it is bound to
	allocs   map[string]*allocEntry     // typed source -> its live allocation
	owners   map[*relayConn]*allocEntry // socket -> what closing it must undo
	byPort   map[int]*relayConn         // relay port -> its socket, however far the allocation got
	refusals map[Identity]map[string]int
	logged   map[logKey]time.Time
	shedding map[Identity]bool
}

type logKey struct {
	id     Identity
	reason string
}

type allocEntry struct {
	conn  *relayConn
	id    Identity
	key   string // the typed source this allocation was created for
	seq   int64
	timer *time.Timer
	flow  *flow
}

type pendingConn struct {
	conn *relayConn
	flow *flow
	at   time.Time
}

// flow is one allocation's peer-to-client accounting. pion's allocation
// reads a peer packet and then either forwards it to the client or drops
// it for want of a permission, both on the allocation's own goroutine, so
// a packet that is still unforwarded when the next one arrives (or when
// the socket closes) is a packet that was dropped. That is what spec §3
// counts as `rejected`, and it is never charged to a budget.
type flow struct {
	mu       sync.Mutex
	awaiting int64 // size of the peer packet read and not yet forwarded
}

// read records a peer packet and returns the bytes of the previous one if
// it was never forwarded.
func (f *flow) read(n int) int64 {
	f.mu.Lock()
	defer f.mu.Unlock()
	dropped := f.awaiting
	f.awaiting = int64(n)
	return dropped
}

// forwarded clears the packet the listener has just written to the client.
func (f *flow) forwarded() {
	f.mu.Lock()
	f.awaiting = 0
	f.mu.Unlock()
}

// flush is the last packet's verdict when the socket closes.
func (f *flow) flush() int64 {
	f.mu.Lock()
	defer f.mu.Unlock()
	dropped := f.awaiting
	f.awaiting = 0
	return dropped
}

// readCounter is the relay socket's peer-ingress leg. It sits under
// relayConn so that relayConn keeps counting only what it forwards.
type readCounter struct {
	net.PacketConn
	f      *flow
	onDrop func(int64)
}

func (r *readCounter) ReadFrom(p []byte) (int, net.Addr, error) {
	n, addr, err := r.PacketConn.ReadFrom(p)
	if n > 0 {
		if dropped := r.f.read(n); dropped > 0 && r.onDrop != nil {
			r.onDrop(dropped)
		}
	}
	return n, addr, err
}

type authRec struct {
	u  Username
	at time.Time
}

type admitRec struct {
	id Identity
	at time.Time
}

// builtinDeny is the peer policy every relay carries whatever the operator
// configures: loopback, link-local (the IMDS address included), RFC 1918,
// unique-local and carrier-grade NAT space; "this network" (0.0.0.0/8, which
// Linux delivers to the local host, not only the unspecified address), the
// IETF protocol-assignment block (192.0.0.0/24) and the benchmarking range
// (198.18.0.0/15), none of which is a public peer (discussion #220).
//
// IPv6 forms that carry an IPv4 address — NAT64 (64:ff9b::/96) and 6to4
// (2002::/16) — are not listed: they are judged by the address they embed
// (see embeddedV4), so a NAT64 gateway cannot be used to reach 10.0.0.1 and
// a public address behind either one still works.
var builtinDeny = mustCIDRs(
	"127.0.0.0/8", "::1/128",
	"169.254.0.0/16", "fe80::/10",
	"10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16",
	"fc00::/7", "100.64.0.0/10",
	"0.0.0.0/8", "192.0.0.0/24", "198.18.0.0/15",
)

// Prefixes whose addresses embed an IPv4 address: the NAT64 well-known
// prefix carries it in the last 32 bits (RFC 6052), 6to4 in bits 16-47
// (RFC 3056).
var (
	nat64Prefix = mustCIDRs("64:ff9b::/96")[0]
	sixToFour   = mustCIDRs("2002::/16")[0]
)

// embeddedV4 returns the IPv4 address a NAT64 or 6to4 peer stands for, or
// nil for any other address. An IPv4-mapped address (::ffff:a.b.c.d) needs
// no help: net.IPNet.Contains already matches it against the IPv4 ranges.
func embeddedV4(ip net.IP) net.IP {
	ip16 := ip.To16()
	if ip16 == nil || ip.To4() != nil {
		return nil
	}
	switch {
	case nat64Prefix.Contains(ip16):
		return net.IPv4(ip16[12], ip16[13], ip16[14], ip16[15])
	case sixToFour.Contains(ip16):
		return net.IPv4(ip16[2], ip16[3], ip16[4], ip16[5])
	}
	return nil
}

func mustCIDRs(ss ...string) []*net.IPNet {
	out := make([]*net.IPNet, 0, len(ss))
	for _, s := range ss {
		_, n, err := net.ParseCIDR(s)
		if err != nil {
			panic("relay: bad built-in CIDR " + s + ": " + err.Error())
		}
		out = append(out, n)
	}
	return out
}

func NewServer(cfg ServerConfig) (*Server, error) {
	if cfg.Now == nil {
		cfg.Now = time.Now
	}
	if cfg.Log == nil {
		cfg.Log = func(string, ...any) {}
	}
	if cfg.MinPort == 0 {
		cfg.MinPort = 49152
	}
	if cfg.MaxPort == 0 {
		cfg.MaxPort = 65535
	}
	switch {
	case cfg.RelayIP == nil:
		return nil, errors.New("relay: RelayIP is required")
	case cfg.MinPort > cfg.MaxPort:
		return nil, fmt.Errorf("relay: port range %d-%d is empty", cfg.MinPort, cfg.MaxPort)
	case cfg.Masters == nil || cfg.Authority == nil || cfg.Lease == nil:
		return nil, errors.New("relay: Masters, Authority and Lease are required")
	case cfg.UDP == "" && cfg.TCP == "":
		return nil, errors.New("relay: no listener configured")
	}

	maxLastAuth := cfg.Limits.BoxLive * 2
	if maxLastAuth < minLastAuthEntries {
		maxLastAuth = minLastAuthEntries
	}
	s := &Server{
		cfg:         cfg,
		now:         cfg.Now,
		log:         cfg.Log,
		adm:         NewAdmission(cfg.Limits, cfg.Now),
		box:         NewBucket(cfg.Limits.BoxBitsPerSec, cfg.Now),
		ports:       &portSet{m: map[int]bool{}},
		deny:        append(append([]*net.IPNet{}, builtinDeny...), cfg.DenyCIDRs...),
		allow:       append([]*net.IPNet{}, cfg.ExtraAllow...),
		maxLastAuth: maxLastAuth,
		pending:     map[int]pendingConn{},
		lastAuth:    map[string]authRec{},
		admits:      map[string]admitRec{},
		bound:       map[string]*Binding{},
		allocs:      map[string]*allocEntry{},
		owners:      map[*relayConn]*allocEntry{},
		byPort:      map[int]*relayConn{},
		refusals:    map[Identity]map[string]int{},
		logged:      map[logKey]time.Time{},
		shedding:    map[Identity]bool{},
	}

	var packets []turn.PacketConnConfig
	var listeners []turn.ListenerConfig
	if cfg.UDP != "" {
		pc, err := net.ListenPacket("udp", cfg.UDP)
		if err != nil {
			return nil, fmt.Errorf("relay: listen udp %s: %w", cfg.UDP, err)
		}
		s.udp = pc
		s.noteLocal(pc.LocalAddr())
		s.listeners = append(s.listeners, "udp:"+portOf(pc.LocalAddr()))
		packets = append(packets, turn.PacketConnConfig{
			PacketConn:            &listenerConn{PacketConn: pc, s: s},
			RelayAddressGenerator: s,
			PermissionHandler:     s.permit,
		})
	}
	if cfg.TCP != "" {
		l, err := net.Listen("tcp", cfg.TCP)
		if err != nil {
			if s.udp != nil {
				s.udp.Close()
			}
			return nil, fmt.Errorf("relay: listen tcp %s: %w", cfg.TCP, err)
		}
		s.tcp = l
		s.noteLocal(l.Addr())
		s.listeners = append(s.listeners, "tcp:"+portOf(l.Addr()))
		listeners = append(listeners, turn.ListenerConfig{
			Listener:              &countingListener{Listener: l, s: s},
			RelayAddressGenerator: s,
			PermissionHandler:     s.permit,
		})
	}

	ts, err := turn.NewServer(turn.ServerConfig{
		Realm:             Realm,
		AuthHandler:       s.auth,
		QuotaHandler:      s.quota,
		EventHandler:      turn.EventHandler{OnAllocationCreated: s.created},
		PacketConnConfigs: packets,
		ListenerConfigs:   listeners,
		LoggerFactory:     logging.NewDefaultLoggerFactory(),
	})
	if err != nil {
		if s.udp != nil {
			s.udp.Close()
		}
		if s.tcp != nil {
			s.tcp.Close()
		}
		return nil, fmt.Errorf("relay: pion server: %w", err)
	}
	s.turn = ts
	return s, nil
}

func (s *Server) noteLocal(a net.Addr) {
	var ip net.IP
	switch v := a.(type) {
	case *net.UDPAddr:
		ip = v.IP
	case *net.TCPAddr:
		ip = v.IP
	}
	if ip != nil && !ip.IsUnspecified() {
		s.local = append(s.local, ip)
	}
}

func portOf(a net.Addr) string {
	switch v := a.(type) {
	case *net.UDPAddr:
		return strconv.Itoa(v.Port)
	case *net.TCPAddr:
		return strconv.Itoa(v.Port)
	}
	_, p, err := net.SplitHostPort(a.String())
	if err != nil {
		return a.String()
	}
	return p
}

// Listeners names the transports that are up, e.g. ["udp:3478","tcp:3478"],
// for the heartbeat.
func (s *Server) Listeners() []string { return append([]string{}, s.listeners...) }

// UDPAddr is the UDP listener's bound address, or nil if UDP is disabled.
func (s *Server) UDPAddr() net.Addr {
	if s.udp == nil {
		return nil
	}
	return s.udp.LocalAddr()
}

// Live is the number of allocations the relay is carrying.
func (s *Server) Live() int {
	s.mu.Lock()
	defer s.mu.Unlock()
	return len(s.allocs)
}

// LiveByIdentity is Live broken down per app, for the meter's `allocs`
// gauge (spec §3, "Metering"). The Meter the server holds is narrowed to
// Add, deliberately — the server counts bytes as it forwards them and knows
// nothing about sample boundaries — so the gauge is the serve loop's job to
// set, and this is where it reads it from.
func (s *Server) LiveByIdentity() map[Identity]int {
	s.mu.Lock()
	defer s.mu.Unlock()
	out := make(map[Identity]int, len(s.allocs))
	for _, e := range s.allocs {
		out[e.id]++
	}
	return out
}

// Seed replays metered bytes into the rolling-hour windows, so a restart
// resumes its budgets from the metrics objects it wrote.
func (s *Server) Seed(id Identity, at time.Time, n int64) { s.adm.Seed(id, at, n) }

// Close ends every allocation and stops listening.
func (s *Server) Close() error {
	s.EndAll("server closing")
	s.mu.Lock()
	pending := make([]*relayConn, 0, len(s.pending))
	for _, p := range s.pending {
		pending = append(pending, p.conn)
	}
	s.mu.Unlock()
	for _, c := range pending {
		c.Close()
	}
	if s.turn == nil {
		return nil
	}
	return s.turn.Close()
}

// EndAll ends every allocation: a lease lapse, or shutdown.
func (s *Server) EndAll(reason string) {
	s.EndWhere(func(*Binding) bool { return true }, reason)
}

// EndWhere ends every allocation whose binding matches: a disable, an
// incarnation change, this region leaving an app's list, a byte budget, a
// retired master. The predicate is called on a copy of the binding, off
// the server's lock.
func (s *Server) EndWhere(match func(*Binding) bool, reason string) {
	type candidate struct {
		conn *relayConn
		b    Binding
	}
	s.mu.Lock()
	cands := make([]candidate, 0, len(s.allocs))
	for _, e := range s.allocs {
		cands = append(cands, candidate{conn: e.conn, b: *e.conn.b})
	}
	s.mu.Unlock()

	n := 0
	for _, c := range cands {
		b := c.b
		if match(&b) {
			c.conn.Close()
			n++
		}
	}
	if n > 0 {
		s.log("relay: ended %d allocation(s): %s", n, reason)
	}
}

// ShedOldest ends the n oldest allocations, for memory pressure.
func (s *Server) ShedOldest(n int) {
	if n <= 0 {
		return
	}
	s.mu.Lock()
	entries := make([]*allocEntry, 0, len(s.allocs))
	for _, e := range s.allocs {
		entries = append(entries, e)
	}
	s.mu.Unlock()
	sort.Slice(entries, func(i, j int) bool { return entries[i].seq < entries[j].seq })
	if n > len(entries) {
		n = len(entries)
	}
	for _, e := range entries[:n] {
		e.conn.Close()
	}
	if n > 0 {
		s.log("relay: shed %d oldest allocation(s)", n)
	}
}

// tick is the sweep: it ends allocations whose credential has expired
// (belt for the per-allocation timers), whose master has retired, whose app
// or account is over budget, and every allocation at all if the serving
// lease has lapsed. It also reclaims sockets and admissions that a failed
// allocation left behind.
func (s *Server) tick() {
	now := s.now()
	masters := s.cfg.Masters()
	leaseDead := !s.cfg.Lease(now)

	type victim struct {
		conn   *relayConn
		reason string
	}
	var victims []victim
	var stale []*relayConn

	// A masters file that failed to reload leaves masters nil; that is a
	// reason to keep serving on the keys we have, not to end anything.
	retired := func(kid string) bool {
		if masters == nil || kid == "" {
			return false
		}
		_, ok := masters.Lookup(kid, now)
		return !ok
	}

	s.mu.Lock()
	for _, e := range s.allocs {
		b := e.conn.b
		switch {
		case leaseDead:
			victims = append(victims, victim{e.conn, "serving lease lapsed"})
		case !b.Expiry.IsZero() && !b.Expiry.After(now):
			victims = append(victims, victim{e.conn, "credential expired"})
		case retired(b.Kid):
			victims = append(victims, victim{e.conn, "master " + b.Kid + " retired"})
		case s.adm.OverBudget(e.id, now):
			victims = append(victims, victim{e.conn, "byte budget"})
		}
	}
	for port, p := range s.pending {
		if now.Sub(p.at) > pendingLife {
			delete(s.pending, port)
			stale = append(stale, p.conn)
		}
	}
	var lapsed []Identity
	for key, a := range s.admits {
		if now.Sub(a.at) > pendingLife {
			delete(s.admits, key)
			lapsed = append(lapsed, a.id)
		}
	}
	for key, a := range s.lastAuth {
		if _, live := s.allocs[key]; !live && now.Sub(a.at) > authMemory {
			delete(s.lastAuth, key)
		}
	}
	// Refusal counters live only for apps the authority still names; the
	// zero identity is the shared bucket for everything else and stays.
	for id := range s.refusals {
		if id == (Identity{}) {
			continue
		}
		if _, ok := s.cfg.Authority().Lookup(id.Account, id.App); !ok {
			delete(s.refusals, id)
		}
	}
	for k, at := range s.logged {
		if now.Sub(at) >= refusalLogEvery {
			delete(s.logged, k)
		}
	}
	s.mu.Unlock()

	for _, id := range lapsed {
		s.adm.Release(id)
	}
	for _, c := range stale {
		c.Close()
	}
	for _, v := range victims {
		s.log("relay: ending allocation for %s/%s: %s", v.conn.b.ID.Account, v.conn.b.ID.App, v.reason)
		v.conn.Close()
	}
}

// --- turn.RelayAddressGenerator ------------------------------------------

func (s *Server) Validate() error {
	if s.cfg.RelayIP == nil {
		return errors.New("relay: RelayIP is required")
	}
	if s.cfg.MinPort > s.cfg.MaxPort {
		return fmt.Errorf("relay: port range %d-%d is empty", s.cfg.MinPort, s.cfg.MaxPort)
	}
	return nil
}

// AllocatePacketConn binds one allocation's relay socket in the configured
// port range and hands pion our wrapper for it: the wrapper is how the
// allocation is ended, metered and bounded. The advertised address is the
// EIP, which is not an address on the box, so the socket itself binds the
// wildcard.
func (s *Server) AllocatePacketConn(network string, requestedPort int) (net.PacketConn, net.Addr, error) {
	if network == "" {
		network = "udp4"
	}
	lo, hi := int(s.cfg.MinPort), int(s.cfg.MaxPort)
	span := hi - lo + 1
	tries := span
	if tries > 64 {
		tries = 64
	}
	ports := make([]int, 0, tries+1)
	if requestedPort >= lo && requestedPort <= hi {
		ports = append(ports, requestedPort)
	}
	start := rand.IntN(span)
	for i := 0; i < tries; i++ {
		ports = append(ports, lo+(start+i)%span)
	}

	for _, port := range ports {
		if s.ports.has(port) {
			continue
		}
		pc, err := net.ListenPacket(network, net.JoinHostPort("", strconv.Itoa(port)))
		if err != nil {
			continue
		}
		bucket := NewBucket(s.cfg.Limits.AllocBitsPerSec, s.now)
		f := &flow{}
		var c *relayConn
		// The socket counts peer ingress under the wrapper, so a peer
		// packet pion drops for want of a permission is still seen.
		counted := &readCounter{PacketConn: pc, f: f, onDrop: func(n int64) { s.countRejected(c.b.ID, n) }}
		c = newRelayConn(counted, &Binding{}, s.cfg.RelayIP, s.ports, bucket, s.box,
			s.countIn, s.countGated, func(rc *relayConn) { s.release(port, rc) })
		s.ports.add(port)
		s.mu.Lock()
		s.pending[port] = pendingConn{conn: c, flow: f, at: s.now()}
		s.byPort[port] = c
		s.mu.Unlock()
		return c, &net.UDPAddr{IP: s.cfg.RelayIP, Port: port}, nil
	}
	return nil, nil, fmt.Errorf("relay: no free port in %d-%d", lo, hi)
}

// AllocateConn would be a TCP relay allocation (RFC 6062), which we do not
// offer: a TURN client may reach us over TCP, but every relayed leg is UDP.
func (s *Server) AllocateConn(string, int) (net.Conn, net.Addr, error) {
	return nil, nil, errTCPAllocUnsupported
}

// --- pion handlers -------------------------------------------------------

// auth is pion's AuthHandler: called on every authenticated request, and
// the whole of spec §4's verification. It returns the key pion checks
// MESSAGE-INTEGRITY against, so a wrong credential fails inside pion and
// never reaches here.
func (s *Server) auth(username, realm string, src net.Addr) ([]byte, bool) {
	now := s.now()
	u, err := ParseUsername(username)
	if err != nil {
		return s.refused(Identity{}, "bad shape", "relay: auth refused from %s: %v", src, err)
	}
	id := Identity{Account: u.Account, App: u.App}

	if !u.Expiry.After(now) || u.Expiry.After(now.Add(MaxCredentialLife)) {
		return s.refused(id, "expired", "relay: auth refused for %s/%s: expiry %s outside (now, now+%s]",
			id.Account, id.App, u.Expiry.UTC().Format(time.RFC3339), MaxCredentialLife)
	}

	s.mu.Lock()
	bound, isBound := s.bound[typedSrc(src)]
	s.mu.Unlock()
	if isBound && (bound.ID != id || bound.Incarnation != u.Incarnation) {
		return s.refused(id, "bound elsewhere", "relay: auth refused for %s/%s: source %s is bound to %s/%s",
			id.Account, id.App, typedSrc(src), bound.ID.Account, bound.ID.App)
	}

	app, ok := s.cfg.Authority().Lookup(u.Account, u.App)
	switch {
	case !ok:
		return s.refused(id, "unknown app", "relay: auth refused for %s/%s: not in the authority record", id.Account, id.App)
	case app.Incarnation != u.Incarnation:
		return s.refused(id, "wrong incarnation", "relay: auth refused for %s/%s: incarnation %s, record has %s",
			id.Account, id.App, u.Incarnation, app.Incarnation)
	case !contains(app.Regions, s.cfg.Region):
		return s.refused(id, "wrong region", "relay: auth refused for %s/%s: region %s not in %v",
			id.Account, id.App, s.cfg.Region, app.Regions)
	}

	masters := s.cfg.Masters()
	if masters == nil {
		return s.refused(id, "unknown kid", "relay: auth refused for %s/%s: no masters loaded", id.Account, id.App)
	}
	master, ok := masters.Lookup(u.Kid, now)
	if !ok {
		return s.refused(id, "unknown kid", "relay: auth refused for %s/%s: kid %s is unknown or retired",
			id.Account, id.App, u.Kid)
	}

	key := DeriveKey(master.Key, u.Account, u.App, u.Incarnation)
	s.rememberAuth(typedSrc(src), u, now)
	return turn.GenerateAuthKey(username, realm, Credential(key, username)), true
}

// rememberAuth stores the username a source last authenticated with, for
// the allocation event that follows. pion verifies MESSAGE-INTEGRITY only
// after the handler returns, so this map is written for sources that have
// not proved anything yet: tick ages entries out, and this caps them at
// twice the box's live-allocation limit, dropping the oldest quarter when
// it fills.
func (s *Server) rememberAuth(key string, u Username, now time.Time) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if _, known := s.lastAuth[key]; !known && len(s.lastAuth) >= s.maxLastAuth {
		type aged struct {
			key string
			at  time.Time
		}
		all := make([]aged, 0, len(s.lastAuth))
		for k, r := range s.lastAuth {
			all = append(all, aged{k, r.at})
		}
		sort.Slice(all, func(i, j int) bool { return all[i].at.Before(all[j].at) })
		drop := len(all) / 4
		if drop == 0 {
			drop = 1
		}
		for _, a := range all[:drop] {
			delete(s.lastAuth, a.key)
		}
	}
	s.lastAuth[key] = authRec{u: u, at: now}
}

// quota is pion's QuotaHandler, called on Allocate only: a refusal answers
// 486. The lease comes first, because a relay whose authority read is
// stale must take nothing new on.
func (s *Server) quota(username, _ string, src net.Addr) bool {
	now := s.now()
	if !s.cfg.Lease(now) {
		s.log("relay: allocation refused: serving lease has lapsed")
		return false
	}
	u, err := ParseUsername(username)
	if err != nil {
		return false // auth would already have refused this
	}
	id := Identity{Account: u.Account, App: u.App}
	ok, reason := s.adm.Admit(id)
	if !ok {
		s.countRefusal(id, reason)
		s.log("relay: allocation refused for %s/%s: %s", id.Account, id.App, reason)
		return false
	}
	key := typedSrc(src)
	s.mu.Lock()
	old, had := s.admits[key]
	s.admits[key] = admitRec{id: id, at: now}
	s.mu.Unlock()
	if had {
		// A previous attempt from this source was admitted and never
		// became an allocation; it owns no socket, so release it here.
		s.adm.Release(old.id)
	}
	return true
}

// created is pion's OnAllocationCreated, called with the relay address
// before pion starts reading from the socket. It is where the wrapper we
// handed out earlier learns whose allocation it is.
func (s *Server) created(src, _ net.Addr, _, _, _ string, relayAddr net.Addr, _ int) {
	ua, ok := relayAddr.(*net.UDPAddr)
	if !ok {
		s.log("relay: allocation created with a non-UDP relay address %v", relayAddr)
		return
	}
	key := typedSrc(src)
	now := s.now()

	s.mu.Lock()
	p, havePending := s.pending[ua.Port]
	if !havePending {
		// The sweep already reclaimed this socket, or it was never ours.
		// Either way pion now owns an allocation no deadline of ours can
		// end, so end it here by closing the socket it reads from.
		orphan := s.byPort[ua.Port]
		s.mu.Unlock()
		if orphan != nil {
			s.log("relay: allocation created on port %d after its socket was reclaimed: ending it", ua.Port)
			orphan.Close()
		} else {
			s.log("relay: allocation created on port %d with no socket of ours", ua.Port)
		}
		return
	}
	delete(s.pending, ua.Port)
	rec, haveAuth := s.lastAuth[key]
	if !haveAuth {
		s.mu.Unlock()
		s.log("relay: allocation created for %s with no authenticated username", key)
		p.conn.Close()
		return
	}
	c := p.conn
	c.b.ID = Identity{Account: rec.u.Account, App: rec.u.App}
	c.b.Expiry = rec.u.Expiry
	c.b.Kid = rec.u.Kid
	c.b.Incarnation = rec.u.Incarnation
	c.b.Src = key
	s.seq++
	e := &allocEntry{conn: c, id: c.b.ID, key: key, seq: s.seq, flow: p.flow}
	s.allocs[key] = e
	s.owners[c] = e
	s.bound[key] = c.b
	delete(s.admits, key)
	expiry := rec.u.Expiry
	s.mu.Unlock()

	// The credential's own deadline. Set outside the lock: a timer whose
	// deadline has already passed fires on the spot, and closing takes the
	// lock we would still be holding.
	t := time.AfterFunc(expiry.Sub(now), func() { c.Close() })
	s.mu.Lock()
	if cur, ok := s.allocs[key]; ok && cur == e {
		e.timer = t
	} else {
		t.Stop()
	}
	s.mu.Unlock()
	s.log("relay: allocation for %s/%s on %s from %s until %s",
		c.b.ID.Account, c.b.ID.App, relayAddr, key, expiry.UTC().Format(time.RFC3339))
}

// release is the wrapped socket's onClose, whoever closed it: pion on a
// read error, a timer, tick, EndWhere. Its state is keyed on this socket,
// not on the typed source, so a replacement allocation bound for the same
// source while this one was closing (pion removes an allocation from its
// manager before closing it) keeps its binding and this one still gives
// its admission back.
func (s *Server) release(port int, c *relayConn) {
	s.ports.remove(port)
	var id Identity
	var release bool
	var last int64
	s.mu.Lock()
	if p, ok := s.pending[port]; ok && p.conn == c {
		delete(s.pending, port)
		last = p.flow.flush()
	}
	if cur, ok := s.byPort[port]; ok && cur == c {
		delete(s.byPort, port)
	}
	if e, ok := s.owners[c]; ok {
		last = e.flow.flush()
		delete(s.owners, c)
		if e.timer != nil {
			e.timer.Stop()
		}
		if cur, ok := s.allocs[e.key]; ok && cur == e {
			delete(s.allocs, e.key)
		}
		if b, ok := s.bound[e.key]; ok && b == c.b {
			delete(s.bound, e.key)
		}
		id, release = e.id, true
	}
	s.mu.Unlock()
	if release {
		s.adm.Release(id)
	}
	if last > 0 {
		// The last peer packet was read and never forwarded.
		s.countRejected(c.b.ID, last)
	}
}

// permit is pion's PermissionHandler: it sees only a peer IP, so the port
// half of the EIP rule lives in relayConn.WriteTo.
func (s *Server) permit(client net.Addr, peer net.IP) bool {
	deny := func(why string) bool {
		s.mu.Lock()
		b, ok := s.bound[typedSrc(client)]
		var id Identity
		if ok {
			id = b.ID
		}
		s.mu.Unlock()
		s.countRefusal(id, "peer denied")
		s.log("relay: permission refused for %s to %s: %s", client, peer, why)
		return false
	}
	if peer == nil {
		return deny("unspecified address")
	}
	// Our own EIP is allowed: two clients that both relay here hold relay
	// candidates on it and must reach each other.
	if s.cfg.RelayIP != nil && peer.Equal(s.cfg.RelayIP) {
		return true
	}
	if why := s.peerRefusal(peer); why != "" {
		return deny(why)
	}
	// A NAT64 or 6to4 address is a route to the IPv4 address inside it, so
	// that address has to pass too.
	if v4 := embeddedV4(peer); v4 != nil {
		if why := s.peerRefusal(v4); why != "" {
			return deny(why + " (embedded in " + peer.String() + ")")
		}
	}
	return true
}

// peerRefusal is permit's verdict on one address, "" when it may be reached.
func (s *Server) peerRefusal(peer net.IP) string {
	switch {
	case peer.IsUnspecified():
		return "unspecified address"
	case peer.IsMulticast():
		return "multicast address"
	}
	// ExtraAllow is the operator's (and the test fixture's) way to permit
	// a range the built-in list denies. Empty on a real box.
	for _, n := range s.allow {
		if n.Contains(peer) {
			return ""
		}
	}
	for _, n := range s.deny {
		if n.Contains(peer) {
			return "denied range " + n.String()
		}
	}
	for _, ip := range s.local {
		if peer.Equal(ip) {
			return "the relay's own address"
		}
	}
	return ""
}

// --- metering ------------------------------------------------------------

// countIn is the client-to-peer leg, counted at the relay socket after
// pion forwarded it.
func (s *Server) countIn(id Identity, n int) {
	if s.cfg.Meter != nil {
		s.cfg.Meter.Add(id, int64(n), 0, 0)
	}
	if s.adm.AddBytes(id, int64(n)) {
		s.overBudget(id)
	}
}

// countOut is the peer-to-client leg, counted at the listener socket and
// attributed through the owner binding.
func (s *Server) countOut(id Identity, n int) {
	if s.cfg.Meter != nil {
		s.cfg.Meter.Add(id, 0, int64(n), 0)
	}
	if s.adm.AddBytes(id, int64(n)) {
		s.overBudget(id)
	}
}

// countRejected is spec §3's `rejected`: peer bytes that reached a relay
// socket and were dropped for want of a permission. Metered, never charged
// to a budget.
func (s *Server) countRejected(id Identity, n int64) {
	if s.cfg.Meter != nil {
		s.cfg.Meter.Add(id, 0, 0, n)
	}
}

// countGated is the other kind of drop: a client packet aimed at our own
// EIP on a port that is not a live allocation. It is a policy refusal, not
// a meter line, so it is counted per app and goes no further.
func (s *Server) countGated(id Identity, _ int) {
	s.countRefusal(id, "gated")
}

// overBudget ends an app's allocations once its rolling-hour budget is
// crossed, because a budget that only stops new allocations bounds nothing.
// It runs off the forwarding path, once per identity at a time.
func (s *Server) overBudget(id Identity) {
	s.mu.Lock()
	if s.shedding[id] {
		s.mu.Unlock()
		return
	}
	s.shedding[id] = true
	s.mu.Unlock()
	go func() {
		s.EndWhere(func(b *Binding) bool { return b.ID == id }, "byte budget")
		s.mu.Lock()
		delete(s.shedding, id)
		s.mu.Unlock()
	}()
}

// attrib is which identity a refusal may be counted against. pion hands
// the AuthHandler whatever username arrived, before any integrity check,
// so an unauthenticated sender picks the account and app strings. Only an
// app the authority record actually names gets a row of its own; every
// other refusal shares the zero identity.
func (s *Server) attrib(id Identity) Identity {
	if id.Account == "" || id.App == "" {
		return Identity{}
	}
	if _, ok := s.cfg.Authority().Lookup(id.Account, id.App); !ok {
		return Identity{}
	}
	return id
}

func (s *Server) countRefusal(id Identity, reason string) {
	id = s.attrib(id)
	s.mu.Lock()
	m := s.refusals[id]
	if m == nil && len(s.refusals) >= maxRefusalIdentities {
		id = Identity{}
		m = s.refusals[id]
	}
	if m == nil {
		m = map[string]int{}
		s.refusals[id] = m
	}
	m[reason]++
	s.mu.Unlock()
}

// mayLog rate-limits one identity's one refusal reason to a line a minute,
// so a flood of refusals cannot be turned into a flood of log.
func (s *Server) mayLog(id Identity, reason string) bool {
	k := logKey{id: s.attrib(id), reason: reason}
	now := s.now()
	s.mu.Lock()
	defer s.mu.Unlock()
	if at, ok := s.logged[k]; ok {
		if now.Sub(at) < refusalLogEvery {
			return false
		}
	} else if len(s.logged) >= maxLoggedRefusals {
		return false
	}
	s.logged[k] = now
	return true
}

// refused counts a refusal against the app and logs why. Its return values
// are the AuthHandler's, so a refusal is one line at the call site.
func (s *Server) refused(id Identity, reason, format string, args ...any) ([]byte, bool) {
	s.countRefusal(id, reason)
	if s.mayLog(id, reason) {
		s.log(format, args...)
	}
	return nil, false
}

func (s *Server) refusalCount(id Identity, reason string) int {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.refusals[id][reason]
}

func (s *Server) refusalIdentities() int {
	s.mu.Lock()
	defer s.mu.Unlock()
	return len(s.refusals)
}

// entryFor is the owner binding lookup the out leg is attributed through.
func (s *Server) entryFor(key string) *allocEntry {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.allocs[key]
}

// --- listener wrappers ---------------------------------------------------

// listenerConn wraps the UDP listener pion answers clients on. It mirrors
// relayConn without the EIP gate: everything written here goes to a client
// that asked us for it, so the only questions are whose bytes they are and
// whether the buckets still have room.
type listenerConn struct {
	net.PacketConn
	s *Server
}

func (c *listenerConn) WriteTo(p []byte, addr net.Addr) (int, error) {
	// A TURN response is the client asking us something, not relayed
	// media: bucketing it would drop Refresh responses at the app's rate
	// cap and end calls for a reason spec §3 does not list, and metering
	// it would bill the client for its own signalling.
	if !relayedPayload(p) {
		return c.PacketConn.WriteTo(p, addr)
	}
	e := c.s.entryFor(typedSrc(addr))
	if e != nil {
		// Peek both, then spend both: a box refusal must not leave the
		// app's bucket debited (see relayConn.WriteTo).
		if e.conn.appBucket != nil && !e.conn.appBucket.Peek(len(p)) {
			return len(p), nil
		}
		if c.s.box != nil && !c.s.box.Peek(len(p)) {
			return len(p), nil
		}
		if e.conn.appBucket != nil {
			e.conn.appBucket.Spend(len(p))
		}
		if c.s.box != nil {
			c.s.box.Spend(len(p))
		}
	}
	n, err := c.PacketConn.WriteTo(p, addr)
	if n > 0 && e != nil {
		e.flow.forwarded()
		c.s.countOut(e.id, n)
	}
	return n, err
}

// relayedPayload reports whether a packet bound for a client carries peer
// media rather than TURN signalling: a ChannelData message (first two bits
// 0b01, so 0x40 to 0x7f) or a Data indication.
func relayedPayload(p []byte) bool {
	if len(p) == 0 {
		return false
	}
	if p[0]&0xc0 == 0x40 {
		return true
	}
	if !stun.IsMessage(p) {
		return false
	}
	var t stun.MessageType
	t.ReadValue(binary.BigEndian.Uint16(p[0:2]))
	return t.Method == stun.MethodData && t.Class == stun.ClassIndication
}

// countingListener wraps the TCP listener so the out leg is metered for
// TCP clients too. A TCP stream cannot have bytes dropped out of it the
// way a datagram can, so the buckets are spent but never enforced here.
type countingListener struct {
	net.Listener
	s *Server
}

func (l *countingListener) Accept() (net.Conn, error) {
	c, err := l.Listener.Accept()
	if err != nil {
		return nil, err
	}
	return &countingConn{Conn: c, s: l.s, key: typedSrc(c.RemoteAddr())}, nil
}

type countingConn struct {
	net.Conn
	s   *Server
	key string
}

func (c *countingConn) Write(p []byte) (int, error) {
	payload := relayedPayload(p)
	n, err := c.Conn.Write(p)
	if n > 0 && payload {
		if e := c.s.entryFor(c.key); e != nil {
			if e.conn.appBucket != nil {
				e.conn.appBucket.Spend(n)
			}
			if c.s.box != nil {
				c.s.box.Spend(n)
			}
			e.flow.forwarded()
			c.s.countOut(e.id, n)
		}
	}
	return n, err
}

func contains(ss []string, s string) bool {
	for _, x := range ss {
		if x == s {
			return true
		}
	}
	return false
}