torrent/client.go

package torrent

import (
	"bufio"
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"log"
	"math/big"
	mathRand "math/rand"
	"net"
	"net/url"
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/anacrolix/missinggo"
	"github.com/anacrolix/missinggo/pproffd"
	"github.com/anacrolix/missinggo/pubsub"
	"github.com/anacrolix/sync"
	"github.com/anacrolix/utp"

	"github.com/anacrolix/torrent/bencode"
	"github.com/anacrolix/torrent/dht"
	"github.com/anacrolix/torrent/iplist"
	"github.com/anacrolix/torrent/metainfo"
	"github.com/anacrolix/torrent/mse"
	pp "github.com/anacrolix/torrent/peer_protocol"
	"github.com/anacrolix/torrent/storage"
	"github.com/anacrolix/torrent/tracker"
)

// Currently doesn't really queue, but should in the future.
func (cl *Client) queuePieceCheck(t *Torrent, pieceIndex int) {
	piece := &t.pieces[pieceIndex]
	if piece.QueuedForHash {
		return
	}
	piece.QueuedForHash = true
	t.publishPieceChange(pieceIndex)
	go cl.verifyPiece(t, pieceIndex)
}

// Queue a piece check if one isn't already queued, and the piece has never
// been checked before.
func (cl *Client) queueFirstHash(t *Torrent, piece int) {
	p := &t.pieces[piece]
	if p.EverHashed || p.Hashing || p.QueuedForHash || t.pieceComplete(piece) {
		return
	}
	cl.queuePieceCheck(t, piece)
}

// Clients contain zero or more Torrents. A Client manages a blocklist, the
// TCP/UDP protocol ports, and DHT as desired.
type Client struct {
	halfOpenLimit int
	peerID        [20]byte
	// The net.Addr.String part that should be common to all active listeners.
	listenAddr     string
	tcpListener    net.Listener
	utpSock        *utp.Socket
	dHT            *dht.Server
	ipBlockList    iplist.Ranger
	config         Config
	extensionBytes peerExtensionBytes
	// Set of addresses that have our client ID. This intentionally will
	// include ourselves if we end up trying to connect to our own address
	// through legitimate channels.
	dopplegangerAddrs map[string]struct{}

	defaultStorage storage.I

	mu     sync.RWMutex
	event  sync.Cond
	closed missinggo.Event

	torrents map[metainfo.Hash]*Torrent
}

func (cl *Client) IPBlockList() iplist.Ranger {
	cl.mu.Lock()
	defer cl.mu.Unlock()
	return cl.ipBlockList
}

func (cl *Client) SetIPBlockList(list iplist.Ranger) {
	cl.mu.Lock()
	defer cl.mu.Unlock()
	cl.ipBlockList = list
	if cl.dHT != nil {
		cl.dHT.SetIPBlockList(list)
	}
}

func (cl *Client) PeerID() string {
	return string(cl.peerID[:])
}

type torrentAddr string

func (me torrentAddr) Network() string { return "" }

func (me torrentAddr) String() string { return string(me) }

func (cl *Client) ListenAddr() net.Addr {
	if cl.listenAddr == "" {
		return nil
	}
	return torrentAddr(cl.listenAddr)
}

type hashSorter struct {
	Hashes []metainfo.Hash
}

func (hs hashSorter) Len() int {
	return len(hs.Hashes)
}

func (hs hashSorter) Less(a, b int) bool {
	return (&big.Int{}).SetBytes(hs.Hashes[a][:]).Cmp((&big.Int{}).SetBytes(hs.Hashes[b][:])) < 0
}

func (hs hashSorter) Swap(a, b int) {
	hs.Hashes[a], hs.Hashes[b] = hs.Hashes[b], hs.Hashes[a]
}

func (cl *Client) sortedTorrents() (ret []*Torrent) {
	var hs hashSorter
	for ih := range cl.torrents {
		hs.Hashes = append(hs.Hashes, ih)
	}
	sort.Sort(hs)
	for _, ih := range hs.Hashes {
		ret = append(ret, cl.torrent(ih))
	}
	return
}

// Writes out a human readable status of the client, such as for writing to a
// HTTP status page.
func (cl *Client) WriteStatus(_w io.Writer) {
	cl.mu.RLock()
	defer cl.mu.RUnlock()
	w := bufio.NewWriter(_w)
	defer w.Flush()
	if addr := cl.ListenAddr(); addr != nil {
		fmt.Fprintf(w, "Listening on %s\n", cl.ListenAddr())
	} else {
		fmt.Fprintln(w, "Not listening!")
	}
	fmt.Fprintf(w, "Peer ID: %+q\n", cl.peerID)
	if cl.dHT != nil {
		dhtStats := cl.dHT.Stats()
		fmt.Fprintf(w, "DHT nodes: %d (%d good, %d banned)\n", dhtStats.Nodes, dhtStats.GoodNodes, dhtStats.BadNodes)
		fmt.Fprintf(w, "DHT Server ID: %x\n", cl.dHT.ID())
		fmt.Fprintf(w, "DHT port: %d\n", missinggo.AddrPort(cl.dHT.Addr()))
		fmt.Fprintf(w, "DHT announces: %d\n", dhtStats.ConfirmedAnnounces)
		fmt.Fprintf(w, "Outstanding transactions: %d\n", dhtStats.OutstandingTransactions)
	}
	fmt.Fprintf(w, "# Torrents: %d\n", len(cl.torrents))
	fmt.Fprintln(w)
	for _, t := range cl.sortedTorrents() {
		if t.name() == "" {
			fmt.Fprint(w, "<unknown name>")
		} else {
			fmt.Fprint(w, t.name())
		}
		fmt.Fprint(w, "\n")
		if t.haveInfo() {
			fmt.Fprintf(w, "%f%% of %d bytes", 100*(1-float64(t.bytesLeft())/float64(t.length)), t.length)
		} else {
			w.WriteString("<missing metainfo>")
		}
		fmt.Fprint(w, "\n")
		t.writeStatus(w, cl)
		fmt.Fprintln(w)
	}
}

func listenUTP(networkSuffix, addr string) (*utp.Socket, error) {
	return utp.NewSocket("udp"+networkSuffix, addr)
}

func listenTCP(networkSuffix, addr string) (net.Listener, error) {
	return net.Listen("tcp"+networkSuffix, addr)
}

func listenBothSameDynamicPort(networkSuffix, host string) (tcpL net.Listener, utpSock *utp.Socket, listenedAddr string, err error) {
	for {
		tcpL, err = listenTCP(networkSuffix, net.JoinHostPort(host, "0"))
		if err != nil {
			return
		}
		listenedAddr = tcpL.Addr().String()
		utpSock, err = listenUTP(networkSuffix, listenedAddr)
		if err == nil {
			return
		}
		tcpL.Close()
		if !strings.Contains(err.Error(), "address already in use") {
			return
		}
	}
}

func listen(tcp, utp bool, networkSuffix, addr string) (tcpL net.Listener, utpSock *utp.Socket, listenedAddr string, err error) {
	if addr == "" {
		addr = ":50007"
	}
	host, port, err := missinggo.ParseHostPort(addr)
	if err != nil {
		return
	}
	if tcp && utp && port == 0 {
		return listenBothSameDynamicPort(networkSuffix, host)
	}
	listenedAddr = addr
	if tcp {
		tcpL, err = listenTCP(networkSuffix, addr)
		if err != nil {
			return
		}
	}
	if utp {
		utpSock, err = listenUTP(networkSuffix, addr)
		if err != nil && tcp {
			tcpL.Close()
		}
	}
	return
}

// Creates a new client.
func NewClient(cfg *Config) (cl *Client, err error) {
	if cfg == nil {
		cfg = &Config{}
	}

	defer func() {
		if err != nil {
			cl = nil
		}
	}()
	cl = &Client{
		halfOpenLimit:     socketsPerTorrent,
		config:            *cfg,
		defaultStorage:    cfg.DefaultStorage,
		dopplegangerAddrs: make(map[string]struct{}),
		torrents:          make(map[metainfo.Hash]*Torrent),
	}
	missinggo.CopyExact(&cl.extensionBytes, defaultExtensionBytes)
	cl.event.L = &cl.mu
	if cl.defaultStorage == nil {
		cl.defaultStorage = storage.NewFile(cfg.DataDir)
	}
	if cfg.IPBlocklist != nil {
		cl.ipBlockList = cfg.IPBlocklist
	}

	if cfg.PeerID != "" {
		missinggo.CopyExact(&cl.peerID, cfg.PeerID)
	} else {
		o := copy(cl.peerID[:], bep20)
		_, err = rand.Read(cl.peerID[o:])
		if err != nil {
			panic("error generating peer id")
		}
	}

	cl.tcpListener, cl.utpSock, cl.listenAddr, err = listen(
		!cl.config.DisableTCP,
		!cl.config.DisableUTP,
		func() string {
			if cl.config.DisableIPv6 {
				return "4"
			} else {
				return ""
			}
		}(),
		cl.config.ListenAddr)
	if err != nil {
		return
	}
	if cl.tcpListener != nil {
		go cl.acceptConnections(cl.tcpListener, false)
	}
	if cl.utpSock != nil {
		go cl.acceptConnections(cl.utpSock, true)
	}
	if !cfg.NoDHT {
		dhtCfg := cfg.DHTConfig
		if dhtCfg.IPBlocklist == nil {
			dhtCfg.IPBlocklist = cl.ipBlockList
		}
		if dhtCfg.Addr == "" {
			dhtCfg.Addr = cl.listenAddr
		}
		if dhtCfg.Conn == nil && cl.utpSock != nil {
			dhtCfg.Conn = cl.utpSock
		}
		cl.dHT, err = dht.NewServer(&dhtCfg)
		if err != nil {
			return
		}
	}

	return
}

// Stops the client. All connections to peers are closed and all activity will
// come to a halt.
func (cl *Client) Close() {
	cl.mu.Lock()
	defer cl.mu.Unlock()
	cl.closed.Set()
	if cl.dHT != nil {
		cl.dHT.Close()
	}
	if cl.utpSock != nil {
		cl.utpSock.Close()
	}
	if cl.tcpListener != nil {
		cl.tcpListener.Close()
	}
	for _, t := range cl.torrents {
		t.close()
	}
	cl.event.Broadcast()
}

var ipv6BlockRange = iplist.Range{Description: "non-IPv4 address"}

func (cl *Client) ipBlockRange(ip net.IP) (r iplist.Range, blocked bool) {
	if cl.ipBlockList == nil {
		return
	}
	ip4 := ip.To4()
	// If blocklists are enabled, then block non-IPv4 addresses, because
	// blocklists do not yet support IPv6.
	if ip4 == nil {
		if missinggo.CryHeard() {
			log.Printf("blocking non-IPv4 address: %s", ip)
		}
		r = ipv6BlockRange
		blocked = true
		return
	}
	return cl.ipBlockList.Lookup(ip4)
}

func (cl *Client) waitAccept() {
	cl.mu.Lock()
	defer cl.mu.Unlock()
	for {
		for _, t := range cl.torrents {
			if cl.wantConns(t) {
				return
			}
		}
		if cl.closed.IsSet() {
			return
		}
		cl.event.Wait()
	}
}

func (cl *Client) acceptConnections(l net.Listener, utp bool) {
	for {
		cl.waitAccept()
		conn, err := l.Accept()
		conn = pproffd.WrapNetConn(conn)
		if cl.closed.IsSet() {
			if conn != nil {
				conn.Close()
			}
			return
		}
		if err != nil {
			log.Print(err)
			// I think something harsher should happen here? Our accept
			// routine just fucked off.
			return
		}
		if utp {
			acceptUTP.Add(1)
		} else {
			acceptTCP.Add(1)
		}
		cl.mu.RLock()
		doppleganger := cl.dopplegangerAddr(conn.RemoteAddr().String())
		_, blocked := cl.ipBlockRange(missinggo.AddrIP(conn.RemoteAddr()))
		cl.mu.RUnlock()
		if blocked || doppleganger {
			acceptReject.Add(1)
			// log.Printf("inbound connection from %s blocked by %s", conn.RemoteAddr(), blockRange)
			conn.Close()
			continue
		}
		go cl.incomingConnection(conn, utp)
	}
}

func (cl *Client) incomingConnection(nc net.Conn, utp bool) {
	defer nc.Close()
	if tc, ok := nc.(*net.TCPConn); ok {
		tc.SetLinger(0)
	}
	c := cl.newConnection(nc)
	c.Discovery = peerSourceIncoming
	c.uTP = utp
	err := cl.runReceivedConn(c)
	if err != nil {
		// log.Print(err)
	}
}

// Returns a handle to the given torrent, if it's present in the client.
func (cl *Client) Torrent(ih metainfo.Hash) (t *Torrent, ok bool) {
	cl.mu.Lock()
	defer cl.mu.Unlock()
	t, ok = cl.torrents[ih]
	return
}

func (cl *Client) torrent(ih metainfo.Hash) *Torrent {
	return cl.torrents[ih]
}

type dialResult struct {
	Conn net.Conn
	UTP  bool
}

func doDial(dial func(addr string, t *Torrent) (net.Conn, error), ch chan dialResult, utp bool, addr string, t *Torrent) {
	conn, err := dial(addr, t)
	if err != nil {
		if conn != nil {
			conn.Close()
		}
		conn = nil // Pedantic
	}
	ch <- dialResult{conn, utp}
	if err == nil {
		successfulDials.Add(1)
		return
	}
	unsuccessfulDials.Add(1)
}

func reducedDialTimeout(max time.Duration, halfOpenLimit int, pendingPeers int) (ret time.Duration) {
	ret = max / time.Duration((pendingPeers+halfOpenLimit)/halfOpenLimit)
	if ret < minDialTimeout {
		ret = minDialTimeout
	}
	return
}

// Returns whether an address is known to connect to a client with our own ID.
func (cl *Client) dopplegangerAddr(addr string) bool {
	_, ok := cl.dopplegangerAddrs[addr]
	return ok
}

// Start the process of connecting to the given peer for the given torrent if
// appropriate.
func (cl *Client) initiateConn(peer Peer, t *Torrent) {
	if peer.Id == cl.peerID {
		return
	}
	addr := net.JoinHostPort(peer.IP.String(), fmt.Sprintf("%d", peer.Port))
	if cl.dopplegangerAddr(addr) || t.addrActive(addr) {
		duplicateConnsAvoided.Add(1)
		return
	}
	if r, ok := cl.ipBlockRange(peer.IP); ok {
		log.Printf("outbound connect to %s blocked by IP blocklist rule %s", peer.IP, r)
		return
	}
	t.halfOpen[addr] = struct{}{}
	go cl.outgoingConnection(t, addr, peer.Source)
}

func (cl *Client) dialTimeout(t *Torrent) time.Duration {
	cl.mu.Lock()
	pendingPeers := len(t.peers)
	cl.mu.Unlock()
	return reducedDialTimeout(nominalDialTimeout, cl.halfOpenLimit, pendingPeers)
}

func (cl *Client) dialTCP(addr string, t *Torrent) (c net.Conn, err error) {
	c, err = net.DialTimeout("tcp", addr, cl.dialTimeout(t))
	if err == nil {
		c.(*net.TCPConn).SetLinger(0)
	}
	return
}

func (cl *Client) dialUTP(addr string, t *Torrent) (c net.Conn, err error) {
	return cl.utpSock.DialTimeout(addr, cl.dialTimeout(t))
}

// Returns a connection over UTP or TCP, whichever is first to connect.
func (cl *Client) dialFirst(addr string, t *Torrent) (conn net.Conn, utp bool) {
	// Initiate connections via TCP and UTP simultaneously. Use the first one
	// that succeeds.
	left := 0
	if !cl.config.DisableUTP {
		left++
	}
	if !cl.config.DisableTCP {
		left++
	}
	resCh := make(chan dialResult, left)
	if !cl.config.DisableUTP {
		go doDial(cl.dialUTP, resCh, true, addr, t)
	}
	if !cl.config.DisableTCP {
		go doDial(cl.dialTCP, resCh, false, addr, t)
	}
	var res dialResult
	// Wait for a successful connection.
	for ; left > 0 && res.Conn == nil; left-- {
		res = <-resCh
	}
	if left > 0 {
		// There are still incompleted dials.
		go func() {
			for ; left > 0; left-- {
				conn := (<-resCh).Conn
				if conn != nil {
					conn.Close()
				}
			}
		}()
	}
	conn = res.Conn
	utp = res.UTP
	return
}

func (cl *Client) noLongerHalfOpen(t *Torrent, addr string) {
	if _, ok := t.halfOpen[addr]; !ok {
		panic("invariant broken")
	}
	delete(t.halfOpen, addr)
	cl.openNewConns(t)
}

// Performs initiator handshakes and returns a connection. Returns nil
// *connection if no connection for valid reasons.
func (cl *Client) handshakesConnection(nc net.Conn, t *Torrent, encrypted, utp bool) (c *connection, err error) {
	c = cl.newConnection(nc)
	c.encrypted = encrypted
	c.uTP = utp
	err = nc.SetDeadline(time.Now().Add(handshakesTimeout))
	if err != nil {
		return
	}
	ok, err := cl.initiateHandshakes(c, t)
	if !ok {
		c = nil
	}
	return
}

// Returns nil connection and nil error if no connection could be established
// for valid reasons.
func (cl *Client) establishOutgoingConn(t *Torrent, addr string) (c *connection, err error) {
	nc, utp := cl.dialFirst(addr, t)
	if nc == nil {
		return
	}
	c, err = cl.handshakesConnection(nc, t, !cl.config.DisableEncryption, utp)
	if err != nil {
		nc.Close()
		return
	} else if c != nil {
		return
	}
	nc.Close()
	if cl.config.DisableEncryption {
		// We already tried without encryption.
		return
	}
	// Try again without encryption, using whichever protocol type worked last
	// time.
	if utp {
		nc, err = cl.dialUTP(addr, t)
	} else {
		nc, err = cl.dialTCP(addr, t)
	}
	if err != nil {
		err = fmt.Errorf("error dialing for unencrypted connection: %s", err)
		return
	}
	c, err = cl.handshakesConnection(nc, t, false, utp)
	if err != nil || c == nil {
		nc.Close()
	}
	return
}

// Called to dial out and run a connection. The addr we're given is already
// considered half-open.
func (cl *Client) outgoingConnection(t *Torrent, addr string, ps peerSource) {
	c, err := cl.establishOutgoingConn(t, addr)
	cl.mu.Lock()
	defer cl.mu.Unlock()
	// Don't release lock between here and addConnection, unless it's for
	// failure.
	cl.noLongerHalfOpen(t, addr)
	if err != nil {
		if cl.config.Debug {
			log.Printf("error establishing outgoing connection: %s", err)
		}
		return
	}
	if c == nil {
		return
	}
	defer c.Close()
	c.Discovery = ps
	err = cl.runInitiatedHandshookConn(c, t)
	if err != nil {
		if cl.config.Debug {
			log.Printf("error in established outgoing connection: %s", err)
		}
	}
}

// The port number for incoming peer connections. 0 if the client isn't
// listening.
func (cl *Client) incomingPeerPort() int {
	if cl.listenAddr == "" {
		return 0
	}
	_, port, err := missinggo.ParseHostPort(cl.listenAddr)
	if err != nil {
		panic(err)
	}
	return port
}

// Convert a net.Addr to its compact IP representation. Either 4 or 16 bytes
// per "yourip" field of http://www.bittorrent.org/beps/bep_0010.html.
func addrCompactIP(addr net.Addr) (string, error) {
	host, _, err := net.SplitHostPort(addr.String())
	if err != nil {
		return "", err
	}
	ip := net.ParseIP(host)
	if v4 := ip.To4(); v4 != nil {
		if len(v4) != 4 {
			panic(v4)
		}
		return string(v4), nil
	}
	return string(ip.To16()), nil
}

func handshakeWriter(w io.Writer, bb <-chan []byte, done chan<- error) {
	var err error
	for b := range bb {
		_, err = w.Write(b)
		if err != nil {
			break
		}
	}
	done <- err
}

type (
	peerExtensionBytes [8]byte
	peerID             [20]byte
)

func (pex *peerExtensionBytes) SupportsExtended() bool {
	return pex[5]&0x10 != 0
}

func (pex *peerExtensionBytes) SupportsDHT() bool {
	return pex[7]&0x01 != 0
}

func (pex *peerExtensionBytes) SupportsFast() bool {
	return pex[7]&0x04 != 0
}

type handshakeResult struct {
	peerExtensionBytes
	peerID
	metainfo.Hash
}

// ih is nil if we expect the peer to declare the InfoHash, such as when the
// peer initiated the connection. Returns ok if the handshake was successful,
// and err if there was an unexpected condition other than the peer simply
// abandoning the handshake.
func handshake(sock io.ReadWriter, ih *metainfo.Hash, peerID [20]byte, extensions peerExtensionBytes) (res handshakeResult, ok bool, err error) {
	// Bytes to be sent to the peer. Should never block the sender.
	postCh := make(chan []byte, 4)
	// A single error value sent when the writer completes.
	writeDone := make(chan error, 1)
	// Performs writes to the socket and ensures posts don't block.
	go handshakeWriter(sock, postCh, writeDone)

	defer func() {
		close(postCh) // Done writing.
		if !ok {
			return
		}
		if err != nil {
			panic(err)
		}
		// Wait until writes complete before returning from handshake.
		err = <-writeDone
		if err != nil {
			err = fmt.Errorf("error writing: %s", err)
		}
	}()

	post := func(bb []byte) {
		select {
		case postCh <- bb:
		default:
			panic("mustn't block while posting")
		}
	}

	post([]byte(pp.Protocol))
	post(extensions[:])
	if ih != nil { // We already know what we want.
		post(ih[:])
		post(peerID[:])
	}
	var b [68]byte
	_, err = io.ReadFull(sock, b[:68])
	if err != nil {
		err = nil
		return
	}
	if string(b[:20]) != pp.Protocol {
		return
	}
	missinggo.CopyExact(&res.peerExtensionBytes, b[20:28])
	missinggo.CopyExact(&res.Hash, b[28:48])
	missinggo.CopyExact(&res.peerID, b[48:68])
	peerExtensions.Add(hex.EncodeToString(res.peerExtensionBytes[:]), 1)

	// TODO: Maybe we can just drop peers here if we're not interested. This
	// could prevent them trying to reconnect, falsely believing there was
	// just a problem.
	if ih == nil { // We were waiting for the peer to tell us what they wanted.
		post(res.Hash[:])
		post(peerID[:])
	}

	ok = true
	return
}

// Wraps a raw connection and provides the interface we want for using the
// connection in the message loop.
type deadlineReader struct {
	nc net.Conn
	r  io.Reader
}

func (r deadlineReader) Read(b []byte) (n int, err error) {
	// Keep-alives should be received every 2 mins. Give a bit of gracetime.
	err = r.nc.SetReadDeadline(time.Now().Add(150 * time.Second))
	if err != nil {
		err = fmt.Errorf("error setting read deadline: %s", err)
	}
	n, err = r.r.Read(b)
	// Convert common errors into io.EOF.
	// if err != nil {
	// 	if opError, ok := err.(*net.OpError); ok && opError.Op == "read" && opError.Err == syscall.ECONNRESET {
	// 		err = io.EOF
	// 	} else if netErr, ok := err.(net.Error); ok && netErr.Timeout() {
	// 		if n != 0 {
	// 			panic(n)
	// 		}
	// 		err = io.EOF
	// 	}
	// }
	return
}

type readWriter struct {
	io.Reader
	io.Writer
}

func maybeReceiveEncryptedHandshake(rw io.ReadWriter, skeys [][]byte) (ret io.ReadWriter, encrypted bool, err error) {
	var protocol [len(pp.Protocol)]byte
	_, err = io.ReadFull(rw, protocol[:])
	if err != nil {
		return
	}
	ret = readWriter{
		io.MultiReader(bytes.NewReader(protocol[:]), rw),
		rw,
	}
	if string(protocol[:]) == pp.Protocol {
		return
	}
	encrypted = true
	ret, err = mse.ReceiveHandshake(ret, skeys)
	return
}

func (cl *Client) receiveSkeys() (ret [][]byte) {
	for ih := range cl.torrents {
		ret = append(ret, ih[:])
	}
	return
}

func (cl *Client) initiateHandshakes(c *connection, t *Torrent) (ok bool, err error) {
	if c.encrypted {
		c.rw, err = mse.InitiateHandshake(c.rw, t.infoHash[:], nil)
		if err != nil {
			return
		}
	}
	ih, ok, err := cl.connBTHandshake(c, &t.infoHash)
	if ih != t.infoHash {
		ok = false
	}
	return
}

// Do encryption and bittorrent handshakes as receiver.
func (cl *Client) receiveHandshakes(c *connection) (t *Torrent, err error) {
	cl.mu.Lock()
	skeys := cl.receiveSkeys()
	cl.mu.Unlock()
	if !cl.config.DisableEncryption {
		c.rw, c.encrypted, err = maybeReceiveEncryptedHandshake(c.rw, skeys)
		if err != nil {
			if err == mse.ErrNoSecretKeyMatch {
				err = nil
			}
			return
		}
	}
	ih, ok, err := cl.connBTHandshake(c, nil)
	if err != nil {
		err = fmt.Errorf("error during bt handshake: %s", err)
		return
	}
	if !ok {
		return
	}
	cl.mu.Lock()
	t = cl.torrents[ih]
	cl.mu.Unlock()
	return
}

// Returns !ok if handshake failed for valid reasons.
func (cl *Client) connBTHandshake(c *connection, ih *metainfo.Hash) (ret metainfo.Hash, ok bool, err error) {
	res, ok, err := handshake(c.rw, ih, cl.peerID, cl.extensionBytes)
	if err != nil || !ok {
		return
	}
	ret = res.Hash
	c.PeerExtensionBytes = res.peerExtensionBytes
	c.PeerID = res.peerID
	c.completedHandshake = time.Now()
	return
}

func (cl *Client) runInitiatedHandshookConn(c *connection, t *Torrent) (err error) {
	if c.PeerID == cl.peerID {
		// Only if we initiated the connection is the remote address a
		// listen addr for a doppleganger.
		connsToSelf.Add(1)
		addr := c.conn.RemoteAddr().String()
		cl.dopplegangerAddrs[addr] = struct{}{}
		return
	}
	return cl.runHandshookConn(c, t)
}

func (cl *Client) runReceivedConn(c *connection) (err error) {
	err = c.conn.SetDeadline(time.Now().Add(handshakesTimeout))
	if err != nil {
		return
	}
	t, err := cl.receiveHandshakes(c)
	if err != nil {
		err = fmt.Errorf("error receiving handshakes: %s", err)
		return
	}
	if t == nil {
		return
	}
	cl.mu.Lock()
	defer cl.mu.Unlock()
	if c.PeerID == cl.peerID {
		return
	}
	return cl.runHandshookConn(c, t)
}

func (cl *Client) runHandshookConn(c *connection, t *Torrent) (err error) {
	c.conn.SetWriteDeadline(time.Time{})
	c.rw = readWriter{
		deadlineReader{c.conn, c.rw},
		c.rw,
	}
	completedHandshakeConnectionFlags.Add(c.connectionFlags(), 1)
	if !cl.addConnection(t, c) {
		return
	}
	defer cl.dropConnection(t, c)
	go c.writer(time.Minute)
	cl.sendInitialMessages(c, t)
	err = cl.connectionLoop(t, c)
	if err != nil {
		err = fmt.Errorf("error during connection loop: %s", err)
	}
	return
}

func (cl *Client) sendInitialMessages(conn *connection, torrent *Torrent) {
	if conn.PeerExtensionBytes.SupportsExtended() && cl.extensionBytes.SupportsExtended() {
		conn.Post(pp.Message{
			Type:       pp.Extended,
			ExtendedID: pp.HandshakeExtendedID,
			ExtendedPayload: func() []byte {
				d := map[string]interface{}{
					"m": func() (ret map[string]int) {
						ret = make(map[string]int, 2)
						ret["ut_metadata"] = metadataExtendedId
						if !cl.config.DisablePEX {
							ret["ut_pex"] = pexExtendedId
						}
						return
					}(),
					"v": extendedHandshakeClientVersion,
					// No upload queue is implemented yet.
					"reqq": 64,
				}
				if !cl.config.DisableEncryption {
					d["e"] = 1
				}
				if torrent.metadataSizeKnown() {
					d["metadata_size"] = torrent.metadataSize()
				}
				if p := cl.incomingPeerPort(); p != 0 {
					d["p"] = p
				}
				yourip, err := addrCompactIP(conn.remoteAddr())
				if err != nil {
					log.Printf("error calculating yourip field value in extension handshake: %s", err)
				} else {
					d["yourip"] = yourip
				}
				// log.Printf("sending %v", d)
				b, err := bencode.Marshal(d)
				if err != nil {
					panic(err)
				}
				return b
			}(),
		})
	}
	if torrent.haveAnyPieces() {
		conn.Bitfield(torrent.bitfield())
	} else if cl.extensionBytes.SupportsFast() && conn.PeerExtensionBytes.SupportsFast() {
		conn.Post(pp.Message{
			Type: pp.HaveNone,
		})
	}
	if conn.PeerExtensionBytes.SupportsDHT() && cl.extensionBytes.SupportsDHT() && cl.dHT != nil {
		conn.Post(pp.Message{
			Type: pp.Port,
			Port: uint16(missinggo.AddrPort(cl.dHT.Addr())),
		})
	}
}

func (cl *Client) peerUnchoked(torrent *Torrent, conn *connection) {
	conn.updateRequests()
}

func (cl *Client) connCancel(t *Torrent, cn *connection, r request) (ok bool) {
	ok = cn.Cancel(r)
	if ok {
		postedCancels.Add(1)
	}
	return
}

func (cl *Client) connDeleteRequest(t *Torrent, cn *connection, r request) bool {
	if !cn.RequestPending(r) {
		return false
	}
	delete(cn.Requests, r)
	return true
}

func (cl *Client) requestPendingMetadata(t *Torrent, c *connection) {
	if t.haveInfo() {
		return
	}
	if c.PeerExtensionIDs["ut_metadata"] == 0 {
		// Peer doesn't support this.
		return
	}
	// Request metadata pieces that we don't have in a random order.
	var pending []int
	for index := 0; index < t.metadataPieceCount(); index++ {
		if !t.haveMetadataPiece(index) && !c.requestedMetadataPiece(index) {
			pending = append(pending, index)
		}
	}
	for _, i := range mathRand.Perm(len(pending)) {
		c.requestMetadataPiece(pending[i])
	}
}

// Process incoming ut_metadata message.
func (cl *Client) gotMetadataExtensionMsg(payload []byte, t *Torrent, c *connection) (err error) {
	var d map[string]int
	err = bencode.Unmarshal(payload, &d)
	if err != nil {
		err = fmt.Errorf("error unmarshalling payload: %s: %q", err, payload)
		return
	}
	msgType, ok := d["msg_type"]
	if !ok {
		err = errors.New("missing msg_type field")
		return
	}
	piece := d["piece"]
	switch msgType {
	case pp.DataMetadataExtensionMsgType:
		if !c.requestedMetadataPiece(piece) {
			err = fmt.Errorf("got unexpected piece %d", piece)
			return
		}
		c.metadataRequests[piece] = false
		begin := len(payload) - metadataPieceSize(d["total_size"], piece)
		if begin < 0 || begin >= len(payload) {
			err = fmt.Errorf("data has bad offset in payload: %d", begin)
			return
		}
		t.saveMetadataPiece(piece, payload[begin:])
		c.UsefulChunksReceived++
		c.lastUsefulChunkReceived = time.Now()
		t.maybeMetadataCompleted()
	case pp.RequestMetadataExtensionMsgType:
		if !t.haveMetadataPiece(piece) {
			c.Post(t.newMetadataExtensionMessage(c, pp.RejectMetadataExtensionMsgType, d["piece"], nil))
			break
		}
		start := (1 << 14) * piece
		c.Post(t.newMetadataExtensionMessage(c, pp.DataMetadataExtensionMsgType, piece, t.metadataBytes[start:start+t.metadataPieceSize(piece)]))
	case pp.RejectMetadataExtensionMsgType:
	default:
		err = errors.New("unknown msg_type value")
	}
	return
}

func (cl *Client) upload(t *Torrent, c *connection) {
	if cl.config.NoUpload {
		return
	}
	if !c.PeerInterested {
		return
	}
	seeding := cl.seeding(t)
	if !seeding && !t.connHasWantedPieces(c) {
		return
	}
another:
	for seeding || c.chunksSent < c.UsefulChunksReceived+6 {
		c.Unchoke()
		for r := range c.PeerRequests {
			err := cl.sendChunk(t, c, r)
			if err != nil {
				if t.pieceComplete(int(r.Index)) && err == io.ErrUnexpectedEOF {
					// We had the piece, but not anymore.
				} else {
					log.Printf("error sending chunk %+v to peer: %s", r, err)
				}
				// If we failed to send a chunk, choke the peer to ensure they
				// flush all their requests. We've probably dropped a piece,
				// but there's no way to communicate this to the peer. If they
				// ask for it again, we'll kick them to allow us to send them
				// an updated bitfield.
				break another
			}
			delete(c.PeerRequests, r)
			goto another
		}
		return
	}
	c.Choke()
}

func (cl *Client) sendChunk(t *Torrent, c *connection, r request) error {
	// Count the chunk being sent, even if it isn't.
	b := make([]byte, r.Length)
	p := t.info.Piece(int(r.Index))
	n, err := t.readAt(b, p.Offset()+int64(r.Begin))
	if n != len(b) {
		if err == nil {
			panic("expected error")
		}
		return err
	}
	c.Post(pp.Message{
		Type:  pp.Piece,
		Index: r.Index,
		Begin: r.Begin,
		Piece: b,
	})
	c.chunksSent++
	uploadChunksPosted.Add(1)
	c.lastChunkSent = time.Now()
	return nil
}

// Processes incoming bittorrent messages. The client lock is held upon entry
// and exit.
func (cl *Client) connectionLoop(t *Torrent, c *connection) error {
	decoder := pp.Decoder{
		R:         bufio.NewReader(c.rw),
		MaxLength: 256 * 1024,
	}
	for {
		cl.mu.Unlock()
		var msg pp.Message
		err := decoder.Decode(&msg)
		cl.mu.Lock()
		if cl.closed.IsSet() || c.closed.IsSet() || err == io.EOF {
			return nil
		}
		if err != nil {
			return err
		}
		c.lastMessageReceived = time.Now()
		if msg.Keepalive {
			receivedKeepalives.Add(1)
			continue
		}
		receivedMessageTypes.Add(strconv.FormatInt(int64(msg.Type), 10), 1)
		switch msg.Type {
		case pp.Choke:
			c.PeerChoked = true
			c.Requests = nil
			// We can then reset our interest.
			c.updateRequests()
		case pp.Reject:
			cl.connDeleteRequest(t, c, newRequest(msg.Index, msg.Begin, msg.Length))
			c.updateRequests()
		case pp.Unchoke:
			c.PeerChoked = false
			cl.peerUnchoked(t, c)
		case pp.Interested:
			c.PeerInterested = true
			cl.upload(t, c)
		case pp.NotInterested:
			c.PeerInterested = false
			c.Choke()
		case pp.Have:
			err = c.peerSentHave(int(msg.Index))
		case pp.Request:
			if c.Choked {
				break
			}
			if !c.PeerInterested {
				err = errors.New("peer sent request but isn't interested")
				break
			}
			if !t.havePiece(msg.Index.Int()) {
				// This isn't necessarily them screwing up. We can drop pieces
				// from our storage, and can't communicate this to peers
				// except by reconnecting.
				requestsReceivedForMissingPieces.Add(1)
				err = errors.New("peer requested piece we don't have")
				break
			}
			if c.PeerRequests == nil {
				c.PeerRequests = make(map[request]struct{}, maxRequests)
			}
			c.PeerRequests[newRequest(msg.Index, msg.Begin, msg.Length)] = struct{}{}
			cl.upload(t, c)
		case pp.Cancel:
			req := newRequest(msg.Index, msg.Begin, msg.Length)
			if !c.PeerCancel(req) {
				unexpectedCancels.Add(1)
			}
		case pp.Bitfield:
			err = c.peerSentBitfield(msg.Bitfield)
		case pp.HaveAll:
			err = c.peerSentHaveAll()
		case pp.HaveNone:
			err = c.peerSentHaveNone()
		case pp.Piece:
			cl.downloadedChunk(t, c, &msg)
		case pp.Extended:
			switch msg.ExtendedID {
			case pp.HandshakeExtendedID:
				// TODO: Create a bencode struct for this.
				var d map[string]interface{}
				err = bencode.Unmarshal(msg.ExtendedPayload, &d)
				if err != nil {
					err = fmt.Errorf("error decoding extended message payload: %s", err)
					break
				}
				// log.Printf("got handshake from %q: %#v", c.Socket.RemoteAddr().String(), d)
				if reqq, ok := d["reqq"]; ok {
					if i, ok := reqq.(int64); ok {
						c.PeerMaxRequests = int(i)
					}
				}
				if v, ok := d["v"]; ok {
					c.PeerClientName = v.(string)
				}
				m, ok := d["m"]
				if !ok {
					err = errors.New("handshake missing m item")
					break
				}
				mTyped, ok := m.(map[string]interface{})
				if !ok {
					err = errors.New("handshake m value is not dict")
					break
				}
				if c.PeerExtensionIDs == nil {
					c.PeerExtensionIDs = make(map[string]byte, len(mTyped))
				}
				for name, v := range mTyped {
					id, ok := v.(int64)
					if !ok {
						log.Printf("bad handshake m item extension ID type: %T", v)
						continue
					}
					if id == 0 {
						delete(c.PeerExtensionIDs, name)
					} else {
						if c.PeerExtensionIDs[name] == 0 {
							supportedExtensionMessages.Add(name, 1)
						}
						c.PeerExtensionIDs[name] = byte(id)
					}
				}
				metadata_sizeUntyped, ok := d["metadata_size"]
				if ok {
					metadata_size, ok := metadata_sizeUntyped.(int64)
					if !ok {
						log.Printf("bad metadata_size type: %T", metadata_sizeUntyped)
					} else {
						t.setMetadataSize(metadata_size, cl)
					}
				}
				if _, ok := c.PeerExtensionIDs["ut_metadata"]; ok {
					cl.requestPendingMetadata(t, c)
				}
			case metadataExtendedId:
				err = cl.gotMetadataExtensionMsg(msg.ExtendedPayload, t, c)
				if err != nil {
					err = fmt.Errorf("error handling metadata extension message: %s", err)
				}
			case pexExtendedId:
				if cl.config.DisablePEX {
					break
				}
				var pexMsg peerExchangeMessage
				err = bencode.Unmarshal(msg.ExtendedPayload, &pexMsg)
				if err != nil {
					err = fmt.Errorf("error unmarshalling PEX message: %s", err)
					break
				}
				go func() {
					cl.mu.Lock()
					cl.addPeers(t, func() (ret []Peer) {
						for i, cp := range pexMsg.Added {
							p := Peer{
								IP:     make([]byte, 4),
								Port:   cp.Port,
								Source: peerSourcePEX,
							}
							if i < len(pexMsg.AddedFlags) && pexMsg.AddedFlags[i]&0x01 != 0 {
								p.SupportsEncryption = true
							}
							missinggo.CopyExact(p.IP, cp.IP[:])
							ret = append(ret, p)
						}
						return
					}())
					cl.mu.Unlock()
				}()
			default:
				err = fmt.Errorf("unexpected extended message ID: %v", msg.ExtendedID)
			}
			if err != nil {
				// That client uses its own extension IDs for outgoing message
				// types, which is incorrect.
				if bytes.HasPrefix(c.PeerID[:], []byte("-SD0100-")) ||
					strings.HasPrefix(string(c.PeerID[:]), "-XL0012-") {
					return nil
				}
			}
		case pp.Port:
			if cl.dHT == nil {
				break
			}
			pingAddr, err := net.ResolveUDPAddr("", c.remoteAddr().String())
			if err != nil {
				panic(err)
			}
			if msg.Port != 0 {
				pingAddr.Port = int(msg.Port)
			}
			cl.dHT.Ping(pingAddr)
		default:
			err = fmt.Errorf("received unknown message type: %#v", msg.Type)
		}
		if err != nil {
			return err
		}
	}
}

// Returns true if connection is removed from torrent.Conns.
func (cl *Client) deleteConnection(t *Torrent, c *connection) bool {
	for i0, _c := range t.conns {
		if _c != c {
			continue
		}
		i1 := len(t.conns) - 1
		if i0 != i1 {
			t.conns[i0] = t.conns[i1]
		}
		t.conns = t.conns[:i1]
		return true
	}
	return false
}

func (cl *Client) dropConnection(t *Torrent, c *connection) {
	cl.event.Broadcast()
	c.Close()
	if cl.deleteConnection(t, c) {
		cl.openNewConns(t)
	}
}

// Returns true if the connection is added.
func (cl *Client) addConnection(t *Torrent, c *connection) bool {
	if cl.closed.IsSet() {
		return false
	}
	select {
	case <-t.ceasingNetworking:
		return false
	default:
	}
	if !cl.wantConns(t) {
		return false
	}
	for _, c0 := range t.conns {
		if c.PeerID == c0.PeerID {
			// Already connected to a client with that ID.
			duplicateClientConns.Add(1)
			return false
		}
	}
	if len(t.conns) >= socketsPerTorrent {
		c := t.worstBadConn(cl)
		if c == nil {
			return false
		}
		if cl.config.Debug && missinggo.CryHeard() {
			log.Printf("%s: dropping connection to make room for new one:\n    %s", t, c)
		}
		c.Close()
		cl.deleteConnection(t, c)
	}
	if len(t.conns) >= socketsPerTorrent {
		panic(len(t.conns))
	}
	t.conns = append(t.conns, c)
	c.t = t
	return true
}

func (cl *Client) usefulConn(t *Torrent, c *connection) bool {
	if c.closed.IsSet() {
		return false
	}
	if !t.haveInfo() {
		return c.supportsExtension("ut_metadata")
	}
	if cl.seeding(t) {
		return c.PeerInterested
	}
	return t.connHasWantedPieces(c)
}

func (cl *Client) wantConns(t *Torrent) bool {
	if !cl.seeding(t) && !t.needData() {
		return false
	}
	if len(t.conns) < socketsPerTorrent {
		return true
	}
	return t.worstBadConn(cl) != nil
}

func (cl *Client) openNewConns(t *Torrent) {
	select {
	case <-t.ceasingNetworking:
		return
	default:
	}
	for len(t.peers) != 0 {
		if !cl.wantConns(t) {
			return
		}
		if len(t.halfOpen) >= cl.halfOpenLimit {
			return
		}
		var (
			k peersKey
			p Peer
		)
		for k, p = range t.peers {
			break
		}
		delete(t.peers, k)
		cl.initiateConn(p, t)
	}
	t.wantPeers.Broadcast()
}

func (cl *Client) addPeers(t *Torrent, peers []Peer) {
	for _, p := range peers {
		if cl.dopplegangerAddr(net.JoinHostPort(
			p.IP.String(),
			strconv.FormatInt(int64(p.Port), 10),
		)) {
			continue
		}
		if _, ok := cl.ipBlockRange(p.IP); ok {
			continue
		}
		if p.Port == 0 {
			// The spec says to scrub these yourselves. Fine.
			continue
		}
		t.addPeer(p, cl)
	}
}

// Prepare a Torrent without any attachment to a Client. That means we can
// initialize fields all fields that don't require the Client without locking
// it.
func (cl *Client) newTorrent(ih metainfo.Hash) (t *Torrent) {
	t = &Torrent{
		cl:        cl,
		infoHash:  ih,
		chunkSize: defaultChunkSize,
		peers:     make(map[peersKey]Peer),

		closing:           make(chan struct{}),
		ceasingNetworking: make(chan struct{}),

		halfOpen:          make(map[string]struct{}),
		pieceStateChanges: pubsub.NewPubSub(),

		storageOpener: cl.defaultStorage,
	}
	t.wantPeers.L = &cl.mu
	return
}

func init() {
	// For shuffling the tracker tiers.
	mathRand.Seed(time.Now().Unix())
}

type trackerTier []string

// The trackers within each tier must be shuffled before use.
// http://stackoverflow.com/a/12267471/149482
// http://www.bittorrent.org/beps/bep_0012.html#order-of-processing
func shuffleTier(tier trackerTier) {
	for i := range tier {
		j := mathRand.Intn(i + 1)
		tier[i], tier[j] = tier[j], tier[i]
	}
}

func copyTrackers(base []trackerTier) (copy []trackerTier) {
	for _, tier := range base {
		copy = append(copy, append(trackerTier(nil), tier...))
	}
	return
}

func mergeTier(tier trackerTier, newURLs []string) trackerTier {
nextURL:
	for _, url := range newURLs {
		for _, trURL := range tier {
			if trURL == url {
				continue nextURL
			}
		}
		tier = append(tier, url)
	}
	return tier
}

// A file-like handle to some torrent data resource.
type Handle interface {
	io.Reader
	io.Seeker
	io.Closer
	io.ReaderAt
}

// Specifies a new torrent for adding to a client. There are helpers for
// magnet URIs and torrent metainfo files.
type TorrentSpec struct {
	// The tiered tracker URIs.
	Trackers [][]string
	InfoHash metainfo.Hash
	Info     *metainfo.InfoEx
	// The name to use if the Name field from the Info isn't available.
	DisplayName string
	// The chunk size to use for outbound requests. Defaults to 16KiB if not
	// set.
	ChunkSize int
	Storage   storage.I
}

func TorrentSpecFromMagnetURI(uri string) (spec *TorrentSpec, err error) {
	m, err := metainfo.ParseMagnetURI(uri)
	if err != nil {
		return
	}
	spec = &TorrentSpec{
		Trackers:    [][]string{m.Trackers},
		DisplayName: m.DisplayName,
		InfoHash:    m.InfoHash,
	}
	return
}

func TorrentSpecFromMetaInfo(mi *metainfo.MetaInfo) (spec *TorrentSpec) {
	spec = &TorrentSpec{
		Trackers:    mi.AnnounceList,
		Info:        &mi.Info,
		DisplayName: mi.Info.Name,
		InfoHash:    mi.Info.Hash(),
	}
	if len(spec.Trackers) == 0 {
		spec.Trackers = [][]string{[]string{mi.Announce}}
	} else {
		spec.Trackers[0] = append(spec.Trackers[0], mi.Announce)
	}
	return
}

func (cl *Client) AddTorrentInfoHash(infoHash metainfo.Hash) (t *Torrent, new bool) {
	cl.mu.Lock()
	defer cl.mu.Unlock()
	t, ok := cl.torrents[infoHash]
	if ok {
		return
	}
	new = true
	t = cl.newTorrent(infoHash)
	if !cl.config.DisableTrackers {
		go cl.announceTorrentTrackers(t)
	}
	if cl.dHT != nil {
		go cl.announceTorrentDHT(t, true)
	}
	cl.torrents[infoHash] = t
	return
}

// Add or merge a torrent spec. If the torrent is already present, the
// trackers will be merged with the existing ones. If the Info isn't yet
// known, it will be set. The display name is replaced if the new spec
// provides one. Returns new if the torrent wasn't already in the client.
func (cl *Client) AddTorrentSpec(spec *TorrentSpec) (t *Torrent, new bool, err error) {
	t, new = cl.AddTorrentInfoHash(spec.InfoHash)
	if spec.DisplayName != "" {
		t.SetDisplayName(spec.DisplayName)
	}
	if spec.Info != nil {
		err = t.SetInfoBytes(spec.Info.Bytes)
		if err != nil {
			return
		}
	}
	cl.mu.Lock()
	defer cl.mu.Unlock()
	if spec.ChunkSize != 0 {
		t.chunkSize = pp.Integer(spec.ChunkSize)
	}
	t.addTrackers(spec.Trackers)
	t.maybeNewConns()
	return
}

func (cl *Client) dropTorrent(infoHash metainfo.Hash) (err error) {
	t, ok := cl.torrents[infoHash]
	if !ok {
		err = fmt.Errorf("no such torrent")
		return
	}
	err = t.close()
	if err != nil {
		panic(err)
	}
	delete(cl.torrents, infoHash)
	return
}

// Returns true when peers are required, or false if the torrent is closing.
func (cl *Client) waitWantPeers(t *Torrent) bool {
	cl.mu.Lock()
	defer cl.mu.Unlock()
	for {
		select {
		case <-t.ceasingNetworking:
			return false
		default:
		}
		if len(t.peers) > torrentPeersLowWater {
			goto wait
		}
		if t.needData() || cl.seeding(t) {
			return true
		}
	wait:
		t.wantPeers.Wait()
	}
}

// Returns whether the client should make effort to seed the torrent.
func (cl *Client) seeding(t *Torrent) bool {
	if cl.config.NoUpload {
		return false
	}
	if !cl.config.Seed {
		return false
	}
	if t.needData() {
		return false
	}
	return true
}

func (cl *Client) announceTorrentDHT(t *Torrent, impliedPort bool) {
	for cl.waitWantPeers(t) {
		// log.Printf("getting peers for %q from DHT", t)
		ps, err := cl.dHT.Announce(string(t.infoHash[:]), cl.incomingPeerPort(), impliedPort)
		if err != nil {
			log.Printf("error getting peers from dht: %s", err)
			return
		}
		// Count all the unique addresses we got during this announce.
		allAddrs := make(map[string]struct{})
	getPeers:
		for {
			select {
			case v, ok := <-ps.Peers:
				if !ok {
					break getPeers
				}
				addPeers := make([]Peer, 0, len(v.Peers))
				for _, cp := range v.Peers {
					if cp.Port == 0 {
						// Can't do anything with this.
						continue
					}
					addPeers = append(addPeers, Peer{
						IP:     cp.IP[:],
						Port:   cp.Port,
						Source: peerSourceDHT,
					})
					key := (&net.UDPAddr{
						IP:   cp.IP[:],
						Port: cp.Port,
					}).String()
					allAddrs[key] = struct{}{}
				}
				cl.mu.Lock()
				cl.addPeers(t, addPeers)
				numPeers := len(t.peers)
				cl.mu.Unlock()
				if numPeers >= torrentPeersHighWater {
					break getPeers
				}
			case <-t.ceasingNetworking:
				ps.Close()
				return
			}
		}
		ps.Close()
		// log.Printf("finished DHT peer scrape for %s: %d peers", t, len(allAddrs))
	}
}

func (cl *Client) trackerBlockedUnlocked(trRawURL string) (blocked bool, err error) {
	url_, err := url.Parse(trRawURL)
	if err != nil {
		return
	}
	host, _, err := net.SplitHostPort(url_.Host)
	if err != nil {
		host = url_.Host
	}
	addr, err := net.ResolveIPAddr("ip", host)
	if err != nil {
		return
	}
	cl.mu.RLock()
	_, blocked = cl.ipBlockRange(addr.IP)
	cl.mu.RUnlock()
	return
}

func (cl *Client) announceTorrentSingleTracker(tr string, req *tracker.AnnounceRequest, t *Torrent) error {
	blocked, err := cl.trackerBlockedUnlocked(tr)
	if err != nil {
		return fmt.Errorf("error determining if tracker blocked: %s", err)
	}
	if blocked {
		return fmt.Errorf("tracker blocked: %s", tr)
	}
	resp, err := tracker.Announce(tr, req)
	if err != nil {
		return fmt.Errorf("error announcing: %s", err)
	}
	var peers []Peer
	for _, peer := range resp.Peers {
		peers = append(peers, Peer{
			IP:   peer.IP,
			Port: peer.Port,
		})
	}
	cl.mu.Lock()
	cl.addPeers(t, peers)
	cl.mu.Unlock()

	// log.Printf("%s: %d new peers from %s", t, len(peers), tr)

	time.Sleep(time.Second * time.Duration(resp.Interval))
	return nil
}

func (cl *Client) announceTorrentTrackersFastStart(req *tracker.AnnounceRequest, trackers []trackerTier, t *Torrent) (atLeastOne bool) {
	oks := make(chan bool)
	outstanding := 0
	for _, tier := range trackers {
		for _, tr := range tier {
			outstanding++
			go func(tr string) {
				err := cl.announceTorrentSingleTracker(tr, req, t)
				oks <- err == nil
			}(tr)
		}
	}
	for outstanding > 0 {
		ok := <-oks
		outstanding--
		if ok {
			atLeastOne = true
		}
	}
	return
}

// Announce torrent to its trackers.
func (cl *Client) announceTorrentTrackers(t *Torrent) {
	req := tracker.AnnounceRequest{
		Event:    tracker.Started,
		NumWant:  -1,
		Port:     uint16(cl.incomingPeerPort()),
		PeerId:   cl.peerID,
		InfoHash: t.infoHash,
	}
	if !cl.waitWantPeers(t) {
		return
	}
	cl.mu.RLock()
	req.Left = t.bytesLeftAnnounce()
	trackers := t.trackers
	cl.mu.RUnlock()
	if cl.announceTorrentTrackersFastStart(&req, trackers, t) {
		req.Event = tracker.None
	}
newAnnounce:
	for cl.waitWantPeers(t) {
		cl.mu.RLock()
		req.Left = t.bytesLeftAnnounce()
		trackers = t.trackers
		cl.mu.RUnlock()
		numTrackersTried := 0
		for _, tier := range trackers {
			for trIndex, tr := range tier {
				numTrackersTried++
				err := cl.announceTorrentSingleTracker(tr, &req, t)
				if err != nil {
					continue
				}
				// Float the successful announce to the top of the tier. If
				// the trackers list has been changed, we'll be modifying an
				// old copy so it won't matter.
				cl.mu.Lock()
				tier[0], tier[trIndex] = tier[trIndex], tier[0]
				cl.mu.Unlock()

				req.Event = tracker.None
				continue newAnnounce
			}
		}
		if numTrackersTried != 0 {
			log.Printf("%s: all trackers failed", t)
		}
		// TODO: Wait until trackers are added if there are none.
		time.Sleep(10 * time.Second)
	}
}

func (cl *Client) allTorrentsCompleted() bool {
	for _, t := range cl.torrents {
		if !t.haveInfo() {
			return false
		}
		if t.numPiecesCompleted() != t.numPieces() {
			return false
		}
	}
	return true
}

// Returns true when all torrents are completely downloaded and false if the
// client is stopped before that.
func (cl *Client) WaitAll() bool {
	cl.mu.Lock()
	defer cl.mu.Unlock()
	for !cl.allTorrentsCompleted() {
		if cl.closed.IsSet() {
			return false
		}
		cl.event.Wait()
	}
	return true
}

// Handle a received chunk from a peer.
func (cl *Client) downloadedChunk(t *Torrent, c *connection, msg *pp.Message) {
	chunksReceived.Add(1)

	req := newRequest(msg.Index, msg.Begin, pp.Integer(len(msg.Piece)))

	// Request has been satisfied.
	if cl.connDeleteRequest(t, c, req) {
		defer c.updateRequests()
	} else {
		unexpectedChunksReceived.Add(1)
	}

	index := int(req.Index)
	piece := &t.pieces[index]

	// Do we actually want this chunk?
	if !t.wantPiece(req) {
		unwantedChunksReceived.Add(1)
		c.UnwantedChunksReceived++
		return
	}

	c.UsefulChunksReceived++
	c.lastUsefulChunkReceived = time.Now()

	cl.upload(t, c)

	// Need to record that it hasn't been written yet, before we attempt to do
	// anything with it.
	piece.incrementPendingWrites()
	// Record that we have the chunk.
	piece.unpendChunkIndex(chunkIndex(req.chunkSpec, t.chunkSize))

	// Cancel pending requests for this chunk.
	for _, c := range t.conns {
		if cl.connCancel(t, c, req) {
			c.updateRequests()
		}
	}

	cl.mu.Unlock()
	// Write the chunk out.
	err := t.writeChunk(int(msg.Index), int64(msg.Begin), msg.Piece)
	cl.mu.Lock()

	piece.decrementPendingWrites()

	if err != nil {
		log.Printf("%s: error writing chunk %v: %s", t, req, err)
		t.pendRequest(req)
		t.updatePieceCompletion(int(msg.Index))
		return
	}

	// It's important that the piece is potentially queued before we check if
	// the piece is still wanted, because if it is queued, it won't be wanted.
	if t.pieceAllDirty(index) {
		cl.queuePieceCheck(t, int(req.Index))
	}

	if c.peerTouchedPieces == nil {
		c.peerTouchedPieces = make(map[int]struct{})
	}
	c.peerTouchedPieces[index] = struct{}{}

	cl.event.Broadcast()
	t.publishPieceChange(int(req.Index))
	return
}

// Return the connections that touched a piece, and clear the entry while
// doing it.
func (cl *Client) reapPieceTouches(t *Torrent, piece int) (ret []*connection) {
	for _, c := range t.conns {
		if _, ok := c.peerTouchedPieces[piece]; ok {
			ret = append(ret, c)
			delete(c.peerTouchedPieces, piece)
		}
	}
	return
}

func (cl *Client) pieceHashed(t *Torrent, piece int, correct bool) {
	p := &t.pieces[piece]
	if p.EverHashed {
		// Don't score the first time a piece is hashed, it could be an
		// initial check.
		if correct {
			pieceHashedCorrect.Add(1)
		} else {
			log.Printf("%s: piece %d (%x) failed hash", t, piece, p.Hash)
			pieceHashedNotCorrect.Add(1)
		}
	}
	p.EverHashed = true
	touchers := cl.reapPieceTouches(t, piece)
	if correct {
		err := p.Storage().MarkComplete()
		if err != nil {
			log.Printf("%T: error completing piece %d: %s", t.storage, piece, err)
		}
		t.updatePieceCompletion(piece)
	} else if len(touchers) != 0 {
		log.Printf("dropping %d conns that touched piece", len(touchers))
		for _, c := range touchers {
			cl.dropConnection(t, c)
		}
	}
	cl.pieceChanged(t, piece)
}

func (cl *Client) onCompletedPiece(t *Torrent, piece int) {
	t.pendingPieces.Remove(piece)
	t.pendAllChunkSpecs(piece)
	for _, conn := range t.conns {
		conn.Have(piece)
		for r := range conn.Requests {
			if int(r.Index) == piece {
				conn.Cancel(r)
			}
		}
		// Could check here if peer doesn't have piece, but due to caching
		// some peers may have said they have a piece but they don't.
		cl.upload(t, conn)
	}
}

func (cl *Client) onFailedPiece(t *Torrent, piece int) {
	if t.pieceAllDirty(piece) {
		t.pendAllChunkSpecs(piece)
	}
	if !t.wantPieceIndex(piece) {
		return
	}
	cl.openNewConns(t)
	for _, conn := range t.conns {
		if conn.PeerHasPiece(piece) {
			conn.updateRequests()
		}
	}
}

func (cl *Client) pieceChanged(t *Torrent, piece int) {
	correct := t.pieceComplete(piece)
	defer cl.event.Broadcast()
	if correct {
		cl.onCompletedPiece(t, piece)
	} else {
		cl.onFailedPiece(t, piece)
	}
	if t.updatePiecePriority(piece) {
		t.piecePriorityChanged(piece)
	}
	t.publishPieceChange(piece)
}

func (cl *Client) verifyPiece(t *Torrent, piece int) {
	cl.mu.Lock()
	defer cl.mu.Unlock()
	p := &t.pieces[piece]
	for p.Hashing || t.storage == nil {
		cl.event.Wait()
	}
	p.QueuedForHash = false
	if t.isClosed() || t.pieceComplete(piece) {
		t.updatePiecePriority(piece)
		t.publishPieceChange(piece)
		return
	}
	p.Hashing = true
	t.publishPieceChange(piece)
	cl.mu.Unlock()
	sum := t.hashPiece(piece)
	cl.mu.Lock()
	select {
	case <-t.closing:
		return
	default:
	}
	p.Hashing = false
	cl.pieceHashed(t, piece, sum == p.Hash)
}

// Returns handles to all the torrents loaded in the Client.
func (cl *Client) Torrents() (ret []*Torrent) {
	cl.mu.Lock()
	for _, t := range cl.torrents {
		ret = append(ret, t)
	}
	cl.mu.Unlock()
	return
}

func (cl *Client) AddMagnet(uri string) (T *Torrent, err error) {
	spec, err := TorrentSpecFromMagnetURI(uri)
	if err != nil {
		return
	}
	T, _, err = cl.AddTorrentSpec(spec)
	return
}

func (cl *Client) AddTorrent(mi *metainfo.MetaInfo) (T *Torrent, err error) {
	T, _, err = cl.AddTorrentSpec(TorrentSpecFromMetaInfo(mi))
	var ss []string
	missinggo.CastSlice(&ss, mi.Nodes)
	cl.AddDHTNodes(ss)
	return
}

func (cl *Client) AddTorrentFromFile(filename string) (T *Torrent, err error) {
	mi, err := metainfo.LoadFromFile(filename)
	if err != nil {
		return
	}
	return cl.AddTorrent(mi)
}

func (cl *Client) DHT() *dht.Server {
	return cl.dHT
}

func (cl *Client) AddDHTNodes(nodes []string) {
	for _, n := range nodes {
		hmp := missinggo.SplitHostMaybePort(n)
		ip := net.ParseIP(hmp.Host)
		if ip == nil {
			log.Printf("won't add DHT node with bad IP: %q", hmp.Host)
			continue
		}
		ni := dht.NodeInfo{
			Addr: dht.NewAddr(&net.UDPAddr{
				IP:   ip,
				Port: hmp.Port,
			}),
		}
		cl.DHT().AddNode(ni)
	}
}