peerconn.go

package torrent

import (
	"bufio"
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"math/rand"
	"net"
	"net/netip"
	"strconv"
	"strings"
	"sync/atomic"
	"time"

	"github.com/RoaringBitmap/roaring"
	"github.com/anacrolix/generics"
	. "github.com/anacrolix/generics"
	"github.com/anacrolix/log"
	"github.com/anacrolix/missinggo/v2/bitmap"
	"github.com/anacrolix/multiless"
	"golang.org/x/exp/maps"
	"golang.org/x/time/rate"

	"github.com/anacrolix/torrent/bencode"
	"github.com/anacrolix/torrent/internal/alloclim"
	"github.com/anacrolix/torrent/metainfo"
	"github.com/anacrolix/torrent/mse"
	pp "github.com/anacrolix/torrent/peer_protocol"
	utHolepunch "github.com/anacrolix/torrent/peer_protocol/ut-holepunch"
)

// Maintains the state of a BitTorrent-protocol based connection with a peer.
type PeerConn struct {
	Peer

	// A string that should identify the PeerConn's net.Conn endpoints. The net.Conn could
	// be wrapping WebRTC, uTP, or TCP etc. Used in writing the conn status for peers.
	connString string

	// See BEP 3 etc.
	PeerID             PeerID
	PeerExtensionBytes pp.PeerExtensionBits
	PeerListenPort     int

	// The actual Conn, used for closing, and setting socket options. Do not use methods on this
	// while holding any mutexes.
	conn net.Conn
	// The Reader and Writer for this Conn, with hooks installed for stats,
	// limiting, deadlines etc.
	w io.Writer
	r io.Reader

	messageWriter peerConnMsgWriter

	PeerExtensionIDs map[pp.ExtensionName]pp.ExtensionNumber
	PeerClientName   atomic.Value
	uploadTimer      *time.Timer
	pex              pexConnState

	// The pieces the peer has claimed to have.
	_peerPieces roaring.Bitmap
	// The peer has everything. This can occur due to a special message, when
	// we may not even know the number of pieces in the torrent yet.
	peerSentHaveAll bool

	peerRequestDataAllocLimiter alloclim.Limiter

	outstandingHolepunchingRendezvous map[netip.AddrPort]struct{}
}

func (cn *PeerConn) pexStatus(lock bool) string {
	if !cn.bitExtensionEnabled(pp.ExtensionBitLtep) {
		return "extended protocol disabled"
	}
	if cn.PeerExtensionIDs == nil {
		return "pending extended handshake"
	}
	if !cn.supportsExtension(pp.ExtensionNamePex, lock) {
		return "unsupported"
	}
	if true {
		return fmt.Sprintf(
			"%v conns, %v unsent events",
			len(cn.pex.remoteLiveConns),
			cn.pex.numPending(),
		)
	} else {
		// This alternative branch prints out the remote live conn addresses.
		return fmt.Sprintf(
			"%v conns, %v unsent events",
			strings.Join(generics.SliceMap(
				maps.Keys(cn.pex.remoteLiveConns),
				func(from netip.AddrPort) string {
					return from.String()
				}), ","),
			cn.pex.numPending(),
		)
	}
}

func (cn *PeerConn) peerImplStatusLines(lock bool) []string {
	return []string{
		cn.connString,
		fmt.Sprintf("peer id: %+q", cn.PeerID),
		fmt.Sprintf("extensions: %v", cn.PeerExtensionBytes),
		fmt.Sprintf("ltep extensions: %v", cn.PeerExtensionIDs),
		fmt.Sprintf("pex: %s", cn.pexStatus(lock)),
	}
}

func (p *PeerConn) isLowOnRequests(lock bool, lockTorrent bool) bool {
	if lock {
		p.mu.RLock()
		defer p.mu.RUnlock()
	}
	return p.requestState.Requests.IsEmpty() && p.requestState.Cancelled.IsEmpty()
}

// Returns true if the connection is over IPv6.
func (cn *PeerConn) ipv6() bool {
	ip := cn.remoteIp()
	if ip.To4() != nil {
		return false
	}
	return len(ip) == net.IPv6len
}

// Returns true the if the dialer/initiator has the higher client peer ID. See
// https://github.com/arvidn/libtorrent/blame/272828e1cc37b042dfbbafa539222d8533e99755/src/bt_peer_connection.cpp#L3536-L3557.
// As far as I can tell, Transmission just keeps the oldest connection.
func (cn *PeerConn) isPreferredDirection() bool {
	// True if our client peer ID is higher than the remote's peer ID.
	return bytes.Compare(cn.PeerID[:], cn.t.cl.peerID[:]) < 0 == cn.outgoing
}

// Returns whether the left connection should be preferred over the right one,
// considering only their networking properties. If ok is false, we can't
// decide.
func (l *PeerConn) hasPreferredNetworkOver(r *PeerConn) bool {
	var ml multiless.Computation
	ml = ml.Bool(r.isPreferredDirection(), l.isPreferredDirection())
	ml = ml.Bool(l.utp(), r.utp())
	ml = ml.Bool(r.ipv6(), l.ipv6())
	return ml.Less()
}

func (cn *PeerConn) peerHasAllPieces(lock bool, lockTorrentInfo bool) (all, known bool) {
	if lock {
		cn.mu.Lock()
		defer cn.mu.Unlock()
	}

	if cn.peerSentHaveAll {
		return true, true
	}

	if !cn.t.haveInfo(lockTorrentInfo) {
		return false, false
	}

	return cn._peerPieces.GetCardinality() == uint64(cn.t.numPieces(lockTorrentInfo)), true
}

func (cn *PeerConn) onGotInfo(info *metainfo.Info, lockTorrent bool) {
	cn.setNumPieces(info.NumPieces(), lockTorrent)
}

// Correct the PeerPieces slice length. Return false if the existing slice is invalid, such as by
// receiving badly sized BITFIELD, or invalid HAVE messages.
func (cn *PeerConn) setNumPieces(num pieceIndex, lockTorrent bool) {
	if lockTorrent {
		cn.t.mu.Lock()
		defer cn.t.mu.Unlock()
	}

	cn.mu.Lock()
	cn._peerPieces.RemoveRange(bitmap.BitRange(num), bitmap.ToEnd)
	cn.mu.Unlock()

	cn.peerPiecesChanged(false)
}

func (cn *PeerConn) peerPieces(lock bool) *roaring.Bitmap {
	if lock {
		cn.mu.RLock()
		defer cn.mu.RUnlock()
	}

	return &cn._peerPieces
}

func (cn *PeerConn) connectionFlags() (ret string) {
	c := func(b byte) {
		ret += string([]byte{b})
	}
	if cn.cryptoMethod == mse.CryptoMethodRC4 {
		c('E')
	} else if cn.headerEncrypted {
		c('e')
	}
	ret += string(cn.Discovery)
	if cn.utp() {
		c('U')
	}
	return
}

func (cn *PeerConn) utp() bool {
	return parseNetworkString(cn.Network).Udp
}

func (cn *PeerConn) onClose(lockTorrent bool) {
	if cn.pex.IsEnabled() {
		cn.pex.Close()
	}
	cn.tickleWriter()
	if cn.conn != nil {
		go cn.conn.Close()
	}
	if cb := cn.callbacks.PeerConnClosed; cb != nil {
		cb(cn)
	}
}

// Writes a message into the write buffer. Returns whether it's okay to keep writing. Writing is
// done asynchronously, so it may be that we're not able to honour backpressure from this method.
func (cn *PeerConn) write(msg pp.Message, lock bool) bool {
	torrent.Add(fmt.Sprintf("messages written of type %s", msg.Type.String()), 1)
	// We don't need to track bytes here because the connection's Writer has that behaviour injected
	// (although there's some delay between us buffering the message, and the connection writer
	// flushing it out.).
	if lock {
		cn.mu.Lock()
		defer cn.mu.Unlock()
	}
	notFull := cn.messageWriter.write(msg)
	// Last I checked only Piece messages affect stats, and we don't write those.
	cn.wroteMsg(&msg)
	cn.tickleWriter()
	return notFull
}

func (cn *PeerConn) requestMetadataPiece(index int) {
	eID := cn.PeerExtensionIDs[pp.ExtensionNameMetadata]
	if eID == pp.ExtensionDeleteNumber {
		return
	}
	if index < len(cn.metadataRequests) && cn.metadataRequests[index] {
		return
	}
	cn.logger.WithDefaultLevel(log.Debug).Printf("requesting metadata piece %d", index)
	cn.write(pp.MetadataExtensionRequestMsg(eID, index), true)
	for index >= len(cn.metadataRequests) {
		cn.metadataRequests = append(cn.metadataRequests, false)
	}
	cn.metadataRequests[index] = true
}

func (cn *PeerConn) requestedMetadataPiece(index int) bool {
	return index < len(cn.metadataRequests) && cn.metadataRequests[index]
}

func (cn *PeerConn) onPeerSentCancel(r Request, lock bool) {
	if lock {
		cn.mu.Lock()
		defer cn.mu.Unlock()
	}

	if _, ok := cn.peerRequests[r]; !ok {
		torrent.Add("unexpected cancels received", 1)
		return
	}
	if cn.fastEnabled(false) {
		cn.reject(r, false)
	} else {
		delete(cn.peerRequests, r)
	}
}

func (cn *PeerConn) choke(msg messageWriter, lock bool) (more bool) {
	if lock {
		cn.mu.Lock()
		defer cn.mu.Unlock()
	}

	if cn.choking {
		return true
	}
	cn.choking = true
	more = msg(pp.Message{
		Type: pp.Choke,
	}, false)
	if !cn.fastEnabled(false) {
		cn.deleteAllPeerRequests(false)
	}
	return
}

func (cn *PeerConn) deleteAllPeerRequests(lock bool) {
	if lock {
		cn.mu.Lock()
		defer cn.mu.Unlock()
	}

	for _, state := range cn.peerRequests {
		state.allocReservation.Drop()
	}
	cn.peerRequests = nil
}

func (cn *PeerConn) unchoke(msg func(pp.Message, bool) bool, lock bool) bool {
	if lock {
		cn.mu.Lock()
		defer cn.mu.Unlock()
	}

	if !cn.choking {
		return true
	}
	cn.choking = false
	return msg(pp.Message{
		Type: pp.Unchoke,
	}, false)
}

func (pc *PeerConn) writeInterested(interested bool, lock bool) bool {
	return pc.write(pp.Message{
		Type: func() pp.MessageType {
			if interested {
				return pp.Interested
			} else {
				return pp.NotInterested
			}
		}(),
	}, lock)
}

func (me *PeerConn) _request(r Request, lock bool) bool {
	return me.write(pp.Message{
		Type:   pp.Request,
		Index:  r.Index,
		Begin:  r.Begin,
		Length: r.Length,
	}, lock)
}

func (me *PeerConn) _cancel(r RequestIndex, lock bool, lockTorrent bool) bool {
	me.write(makeCancelMessage(me.t.requestIndexToRequest(r, lockTorrent)), lock)
	return me.remoteRejectsCancels(false)
}

// Whether we should expect a reject message after sending a cancel.
func (me *PeerConn) remoteRejectsCancels(lock bool) bool {
	if !me.fastEnabled(lock) {
		return false
	}
	if me.remoteIsTransmission() {
		// Transmission did not send rejects for received cancels. See
		// https://github.com/transmission/transmission/pull/2275. Fixed in 4.0.0-beta.1 onward in
		// https://github.com/transmission/transmission/commit/76719bf34c255da4fca991c2ad3fa4b65d2154b1.
		// Peer ID prefix scheme described
		// https://github.com/transmission/transmission/blob/7ec7607bbcf0fa99bd4b157b9b0f0c411d59f45d/CMakeLists.txt#L128-L149.
		return me.PeerID[3] >= '4'
	}
	return true
}

func (cn *PeerConn) fillWriteBuffer() {
	cn.mu.RLock()
	bufferAboveLowWater := cn.messageWriter.writeBuffer.Len() > writeBufferLowWaterLen
	cn.mu.RUnlock()

	if bufferAboveLowWater {
		// Fully committing to our max requests requires sufficient space (see
		// maxLocalToRemoteRequests). Flush what we have instead. We also prefer always to make
		// requests than to do PEX or upload, so we short-circuit before handling those. Any update
		// request reason will not be cleared, so we'll come right back here when there's space. We
		// can't do this in maybeUpdateActualRequestState because it's a method on Peer and has no
		// knowledge of write buffers.
		return
	}
	cn.maybeUpdateActualRequestState(true)

	cn.mu.RLock()
	pex := cn.pex
	write := cn.write
	cn.mu.RUnlock()

	if pex.IsEnabled() {
		if flow := pex.Share(write, &cn.mu); !flow {
			return
		}
	}
	cn.upload(write, true)
}

func (cn *PeerConn) have(piece pieceIndex) {
	if cn.sentHaves.Get(bitmap.BitIndex(piece)) {
		return
	}
	cn.write(pp.Message{
		Type:  pp.Have,
		Index: pp.Integer(piece),
	}, true)
	cn.sentHaves.Add(bitmap.BitIndex(piece))
}

func (cn *PeerConn) postBitfield(lockTorrent bool) {
	if cn.sentHaves.Len() != 0 {
		panic("bitfield must be first have-related message sent")
	}
	if !cn.t.haveAnyPieces(lockTorrent) {
		return
	}
	cn.write(pp.Message{
		Type:     pp.Bitfield,
		Bitfield: cn.t.bitfield(lockTorrent),
	}, true)
	if lockTorrent {
		cn.t.mu.RLock()
		defer cn.t.mu.RUnlock()
	}
	cn.sentHaves = bitmap.Bitmap{RB: cn.t._completedPieces.Clone()}
}

func (cn *PeerConn) handleUpdateRequests(lockTorrent bool) {
	// The writer determines the request state as needed when it can write.
	cn.tickleWriter()
}

func (cn *PeerConn) raisePeerMinPieces(newMin pieceIndex) {
	if newMin > cn.peerMinPieces {
		cn.peerMinPieces = newMin
	}
}

func (cn *PeerConn) peerSentHave(piece pieceIndex, lockTorrent bool) error {
	if cn.t.haveInfo(true) && piece >= cn.t.numPieces(true) || piece < 0 {
		return errors.New("invalid piece")
	}
	if cn.peerHasPiece(piece, true, lockTorrent) {
		return nil
	}
	cn.raisePeerMinPieces(piece + 1)
	if !cn.peerHasPiece(piece, true, lockTorrent) {
		cn.t.incPieceAvailability(piece, true)
	}
	cn.mu.Lock()
	cn._peerPieces.Add(uint32(piece))
	cn.mu.Unlock()
	if cn.t.wantPieceIndex(piece, lockTorrent) {
		cn.updateRequests("have", lockTorrent)
	}
	cn.peerPiecesChanged(lockTorrent)
	return nil
}

func (cn *PeerConn) peerSentBitfield(bf []bool, lockTorrent bool) error {
	if lockTorrent {
		cn.t.mu.Lock()
		defer cn.t.mu.Unlock()
	}

	shouldUpdateRequests := false

	err := func() error {
		cn.mu.Lock()
		defer cn.mu.Unlock()

		if len(bf)%8 != 0 {
			panic("expected bitfield length divisible by 8")
		}
		// We know that the last byte means that at most the last 7 bits are wasted.
		cn.raisePeerMinPieces(pieceIndex(len(bf) - 7))
		if cn.t.haveInfo(false) && len(bf) > int(cn.t.numPieces(true)) {
			// Ignore known excess pieces.
			bf = bf[:cn.t.numPieces(true)]
		}
		bm := boolSliceToBitmap(bf)
		if cn.t.haveInfo(true) && pieceIndex(bm.GetCardinality()) == cn.t.numPieces(true) {
			cn.onPeerHasAllPiecesNoTriggers(false, false)
			return nil
		}
		if !bm.IsEmpty() {
			cn.raisePeerMinPieces(pieceIndex(bm.Maximum()) + 1)
		}

		if cn.peerSentHaveAll {
			if !cn.t.deleteConnWithAllPieces(&cn.Peer, false) {
				panic(cn)
			}
			cn.peerSentHaveAll = false
			if !cn._peerPieces.IsEmpty() {
				panic("if peer has all, we expect no individual peer pieces to be set")
			}
		} else {
			bm.Xor(&cn._peerPieces)
		}
		cn.peerSentHaveAll = false
		// bm is now 'on' for pieces that are changing
		bm.Iterate(func(x uint32) bool {
			pi := pieceIndex(x)
			if cn._peerPieces.Contains(x) {
				// Then we must be losing this piece
				cn.t.decPieceAvailability(pi, false)
			} else {
				if !shouldUpdateRequests && cn.t.wantPieceIndex(pieceIndex(x), false) {
					shouldUpdateRequests = true
				}
				// We must be gaining this piece
				cn.t.incPieceAvailability(pieceIndex(x), false)
			}
			return true
		})
		// Apply the changes. If we had everything previously, this should be empty, so xor is the same
		// as or.
		cn._peerPieces.Xor(&bm)

		return nil
	}()

	if err != nil {
		return err
	}

	if shouldUpdateRequests {
		cn.updateRequests("bitfield", false)
	}
	// We didn't guard this before, I see no reason to do it now.
	cn.peerPiecesChanged(false)
	return nil
}

func (cn *PeerConn) onPeerHasAllPiecesNoTriggers(lock bool, lockTorrent bool) {
	if lockTorrent {
		cn.t.mu.Lock()
		defer cn.t.mu.Unlock()
	}

	if lock {
		cn.mu.Lock()
		defer cn.mu.Unlock()
	}

	t := cn.t

	if t.haveInfo(true) {
		cn._peerPieces.Iterate(func(x uint32) bool {
			t.decPieceAvailability(pieceIndex(x), false)
			return true
		})
	}
	t.addConnWithAllPieces(&cn.Peer, false)
	cn.peerSentHaveAll = true
	cn._peerPieces.Clear()
}

func (cn *PeerConn) onPeerHasAllPieces(lockTorrent bool) {
	if lockTorrent {
		cn.t.mu.Lock()
		defer cn.t.mu.Unlock()
	}

	cn.onPeerHasAllPiecesNoTriggers(true, false)

	cn.mu.Lock()
	isEmpty := cn.t._pendingPieces.IsEmpty()
	cn.mu.Unlock()

	if !isEmpty {
		cn.updateRequests("Peer.onPeerHasAllPieces", false)
	}
	cn.peerPiecesChanged(false)
}

func (cn *PeerConn) onPeerSentHaveAll() error {
	cn.onPeerHasAllPieces(true)
	return nil
}

func (cn *PeerConn) peerSentHaveNone(lockTorrent bool) error {
	if lockTorrent {
		cn.t.mu.Lock()
		defer cn.t.mu.Unlock()
	}

	func() {
		cn.mu.Lock()
		defer cn.mu.Unlock()

		if !cn.peerSentHaveAll {
			cn.t.decPeerPieceAvailability(&cn.Peer, false, false)
		}
		cn._peerPieces.Clear()
		cn.peerSentHaveAll = false
	}()

	cn.peerPiecesChanged(false)
	return nil
}

func (c *PeerConn) requestPendingMetadata(lockTorrent bool) {
	if c.t.haveInfo(true) {
		return
	}
	if c.PeerExtensionIDs[pp.ExtensionNameMetadata] == 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 < c.t.metadataPieceCount(lockTorrent); index++ {
		if !c.t.haveMetadataPiece(index, lockTorrent) && !c.requestedMetadataPiece(index) {
			pending = append(pending, index)
		}
	}
	rand.Shuffle(len(pending), func(i, j int) { pending[i], pending[j] = pending[j], pending[i] })
	for _, i := range pending {
		c.requestMetadataPiece(i)
	}
}

func (cn *PeerConn) wroteMsg(msg *pp.Message) {
	torrent.Add(fmt.Sprintf("messages written of type %s", msg.Type.String()), 1)
	if msg.Type == pp.Extended {
		for name, id := range cn.PeerExtensionIDs {
			if id != msg.ExtendedID {
				continue
			}
			torrent.Add(fmt.Sprintf("Extended messages written for protocol %q", name), 1)
		}
	}
	cn.allStats(func(cs *ConnStats) { cs.wroteMsg(msg) })
}

func (cn *PeerConn) wroteBytes(n int64) {
	cn.allStats(add(n, func(cs *ConnStats) *Count { return &cs.BytesWritten }))
}

func (c *PeerConn) fastEnabled(lock bool) bool {
	if lock {
		c.mu.RLock()
		defer c.mu.RUnlock()
	}
	return c.PeerExtensionBytes.SupportsFast() && c.t.cl.config.Extensions.SupportsFast()
}

func (c *PeerConn) reject(r Request, lock bool) {
	if !c.fastEnabled(lock) {
		panic("fast not enabled")
	}
	c.write(r.ToMsg(pp.Reject), lock)
	// It is possible to reject a request before it is added to peer requests due to being invalid.

	if lock {
		c.mu.Lock()
		defer c.mu.Unlock()
	}

	if state, ok := c.peerRequests[r]; ok {
		state.allocReservation.Drop()
		delete(c.peerRequests, r)
	}
}

func (c *PeerConn) maximumPeerRequestChunkLength() (_ Option[int]) {
	uploadRateLimiter := c.t.cl.config.UploadRateLimiter
	if uploadRateLimiter.Limit() == rate.Inf {
		return
	}
	return Some(uploadRateLimiter.Burst())
}

// startFetch is for testing purposes currently.
func (c *PeerConn) onReadRequest(r Request, startFetch bool, lock bool) error {
	if !c.t.havePiece(pieceIndex(r.Index), true) {
		// TODO: Tell the peer we don't have the piece, and reject this request.
		requestsReceivedForMissingPieces.Add(1)
		return fmt.Errorf("peer requested piece we don't have: %v", r.Index.Int())
	}

	pieceLength := c.t.pieceLength(pieceIndex(r.Index), true)

	if lock {
		c.mu.Lock()
		defer c.mu.Unlock()
	}

	requestedChunkLengths.Add(strconv.FormatUint(r.Length.Uint64(), 10), 1)
	if _, ok := c.peerRequests[r]; ok {
		torrent.Add("duplicate requests received", 1)
		if c.fastEnabled(false) {
			return errors.New("received duplicate request with fast enabled")
		}
		return nil
	}
	if c.choking {
		torrent.Add("requests received while choking", 1)
		if c.fastEnabled(false) {
			torrent.Add("requests rejected while choking", 1)
			c.reject(r, false)
		}
		return nil
	}
	// TODO: What if they've already requested this?
	if len(c.peerRequests) >= localClientReqq {
		torrent.Add("requests received while queue full", 1)
		if c.fastEnabled(false) {
			c.reject(r, false)
		}
		// BEP 6 says we may close here if we choose.
		return nil
	}
	if opt := c.maximumPeerRequestChunkLength(); opt.Ok && int(r.Length) > opt.Value {
		err := fmt.Errorf("peer requested chunk too long (%v)", r.Length)
		c.logger.Levelf(log.Warning, err.Error())
		if c.fastEnabled(false) {
			c.reject(r, false)
			return nil
		} else {
			return err
		}
	}
	// Check this after we know we have the piece, so that the piece length will be known.
	if chunkOverflowsPiece(r.ChunkSpec, pieceLength) {
		torrent.Add("bad requests received", 1)
		return errors.New("chunk overflows piece")
	}
	if c.peerRequests == nil {
		c.peerRequests = make(map[Request]*peerRequestState, localClientReqq)
	}
	value := &peerRequestState{
		allocReservation: c.peerRequestDataAllocLimiter.Reserve(int64(r.Length)),
	}
	c.peerRequests[r] = value
	if startFetch {
		// TODO: Limit peer request data read concurrency.
		go c.peerRequestDataReader(r, value)
	}
	return nil
}

func (c *PeerConn) peerRequestDataReader(r Request, prs *peerRequestState) {
	// Should we depend on Torrent closure here? I think it's okay to get cancelled from elsewhere,
	// or fail to read and then cleanup. Also, we used to hang here if the reservation was never
	// dropped, that was fixed.
	ctx := context.Background()
	err := prs.allocReservation.Wait(ctx)
	if err != nil {
		c.logger.WithDefaultLevel(log.Debug).Levelf(log.ErrorLevel(err), "waiting for alloc limit reservation: %v", err)
		return
	}
	b, err := c.readPeerRequestData(r)
	if err != nil {
		c.peerRequestDataReadFailed(err, r, true, true)
	} else {
		if b == nil {
			panic("data must be non-nil to trigger send")
		}
		torrent.Add("peer request data read successes", 1)
		c.mu.Lock()
		prs.data = b
		c.mu.Unlock()
		// This might be required for the error case too (#752 and #753).
		c.tickleWriter()
	}
}

// If this is maintained correctly, we might be able to support optional synchronous reading for
// chunk sending, the way it used to work.
func (c *PeerConn) peerRequestDataReadFailed(err error, r Request, lock bool, lockTorrent bool) {
	if lockTorrent {
		c.t.mu.Lock()
		defer c.t.mu.Unlock()
	}

	torrent.Add("peer request data read failures", 1)
	logLevel := log.Warning
	if c.t.hasStorageCap() {
		// It's expected that pieces might drop. See
		// https://github.com/anacrolix/torrent/issues/702#issuecomment-1000953313.
		logLevel = log.Debug
	}
	c.logger.Levelf(logLevel, "error reading chunk for peer Request %v: %v", r, err)
	if c.t.closed.IsSet() {
		return
	}
	i := pieceIndex(r.Index)
	if c.t.pieceComplete(i, false) {
		// There used to be more code here that just duplicated the following break. Piece
		// completions are currently cached, so I'm not sure how helpful this update is, except to
		// pull any completion changes pushed to the storage backend in failed reads that got us
		// here.
		c.t.updatePieceCompletion(i, false)
	}
	// We've probably dropped a piece from storage, but there's no way to communicate this to the
	// peer. If they ask for it again, we kick them allowing us to send them updated piece states if
	// we reconnect. TODO: Instead, we could just try to update them with Bitfield or HaveNone and
	// if they kick us for breaking protocol, on reconnect we will be compliant again (at least
	// initially).
	if c.fastEnabled(lock) {
		c.reject(r, lock)
	} else {
		if c.choking {
			// If fast isn't enabled, I think we would have wiped all peer requests when we last
			// choked, and requests while we're choking would be ignored. It could be possible that
			// a peer request data read completed concurrently to it being deleted elsewhere.
			c.logger.WithDefaultLevel(log.Warning).Printf("already choking peer, requests might not be rejected correctly")
		}
		// Choking a non-fast peer should cause them to flush all their requests.
		c.choke(c.write, lock)
	}
}

func (c *PeerConn) readPeerRequestData(r Request) ([]byte, error) {
	b := make([]byte, r.Length)
	p := c.t.info.Piece(int(r.Index))
	n, err := c.t.readAt(b, p.Offset()+int64(r.Begin), true)
	if n == len(b) {
		if err == io.EOF {
			err = nil
		}
	} else {
		if err == nil {
			panic("expected error")
		}
	}
	return b, err
}

func (c *PeerConn) logProtocolBehaviour(level log.Level, format string, arg ...interface{}) {
	c.logger.WithContextText(fmt.Sprintf(
		"peer id %q, ext v %q", c.PeerID, c.PeerClientName.Load(),
	)).SkipCallers(1).Levelf(level, format, arg...)
}

// Processes incoming BitTorrent wire-protocol messages. The client lock is held upon entry and
// exit. Returning will end the connection.
func (c *PeerConn) mainReadLoop() (err error) {
	defer func() {
		if err != nil {
			torrent.Add("connection.mainReadLoop returned with error", 1)
		} else {
			torrent.Add("connection.mainReadLoop returned with no error", 1)
		}
	}()
	t := c.t
	cl := t.cl

	decoder := pp.Decoder{
		R:         bufio.NewReaderSize(c.r, 1<<17),
		MaxLength: 4 * pp.Integer(max(int64(t.chunkSize), defaultChunkSize)),
		Pool:      &t.chunkPool,
	}
	for {
		var msg pp.Message
		err = decoder.Decode(&msg)

		if cb := c.callbacks.ReadMessage; cb != nil && err == nil {
			cb(c, &msg)
		}

		if t.closed.IsSet() || c.closed.IsSet() {
			return nil
		}
		if err != nil {
			return err
		}

		c.mu.Lock()
		peerChoking := c.peerChoking
		c.lastMessageReceived = time.Now()
		c.mu.Unlock()

		if msg.Keepalive {
			receivedKeepalives.Add(1)
			continue
		}
		messageTypesReceived.Add(msg.Type.String(), 1)
		if msg.Type.FastExtension() && !c.fastEnabled(true) {
			runSafeExtraneous(func() { torrent.Add("fast messages received when extension is disabled", 1) })
			return fmt.Errorf("received fast extension message (type=%v) but extension is disabled", msg.Type)
		}

		switch msg.Type {
		case pp.Choke:
			if peerChoking {
				break
			}

			func() {
				c.t.mu.RLock()
				defer c.t.mu.RUnlock()

				if !c.fastEnabled(true) {
					c.deleteAllRequests("choked by non-fast PeerConn", true)
				} else {
					// We don't decrement pending requests here, let's wait for the peer to either
					// reject or satisfy the outstanding requests. Additionally, some peers may unchoke
					// us and resume where they left off, we don't want to have piled on to those chunks
					// in the meanwhile. I think a peer's ability to abuse this should be limited: they
					// could let us request a lot of stuff, then choke us and never reject, but they're
					// only a single peer, our chunk balancing should smooth over this abuse.
				}
				c.mu.Lock()
				defer c.mu.Unlock()
				c.peerChoking = true
				c.updateExpectingChunks(false)
			}()

		case pp.Unchoke:
			if !peerChoking {
				// Some clients do this for some reason. Transmission doesn't error on this, so we
				// won't for consistency.
				c.logProtocolBehaviour(log.Debug, "received unchoke when already unchoked")
				break
			}

			func() {
				c.t.mu.RLock()
				defer c.t.mu.RUnlock()

				isEmpty := func() bool {
					c.mu.Lock()
					defer c.mu.Unlock()

					preservedCount := 0
					c.requestState.Requests.Iterate(func(x RequestIndex) bool {
						if !c.peerAllowedFast.Contains(c.t.pieceIndexOfRequestIndex(x, false)) {
							preservedCount++
						}
						return true
					})
					if preservedCount != 0 {
						// TODO: Yes this is a debug log but I'm not happy with the state of the logging lib
						// right now.
						c.logger.Levelf(log.Debug,
							"%v requests were preserved while being choked (fast=%v)",
							preservedCount,
							c.fastEnabled(false))

						torrent.Add("requestsPreservedThroughChoking", int64(preservedCount))
					}
					c.peerChoking = false

					return c.t._pendingPieces.IsEmpty()
				}()

				if !isEmpty {
					c.updateRequests("unchoked", false)
				}

				c.updateExpectingChunks(false)
			}()

		case pp.Interested:
			func() {
				c.mu.Lock()
				defer c.mu.Unlock()
				c.peerInterested = true
				c.tickleWriter()
			}()
		case pp.NotInterested:
			func() {
				c.mu.Lock()
				defer c.mu.Unlock()
				c.peerInterested = false
				// We don't clear their requests since it isn't clear in the spec.
				// We'll probably choke them for this, which will clear them if
				// appropriate, and is clearly specified.
			}()
		case pp.Have:
			err = c.peerSentHave(pieceIndex(msg.Index), true)
		case pp.Bitfield:
			err = c.peerSentBitfield(msg.Bitfield, true)
		case pp.Request:
			r := newRequestFromMessage(&msg)
			err = c.onReadRequest(r, true, true)
			if err != nil {
				err = fmt.Errorf("on reading request %v: %w", r, err)
			}
		case pp.Piece:
			c.doChunkReadStats(int64(len(msg.Piece)))
			c.receiveChunk(&msg)
			if len(msg.Piece) == int(t.chunkSize) {
				t.chunkPool.Put(&msg.Piece)
			}
			if err != nil {
				err = fmt.Errorf("receiving chunk: %w", err)
			}
		case pp.Cancel:
			req := newRequestFromMessage(&msg)
			c.onPeerSentCancel(req, true)
		case pp.Port:
			ipa, ok := tryIpPortFromNetAddr(c.RemoteAddr)
			if !ok {
				break
			}
			pingAddr := net.UDPAddr{
				IP:   ipa.IP,
				Port: ipa.Port,
			}
			if msg.Port != 0 {
				pingAddr.Port = int(msg.Port)
			}
			func() {
				cl.lock()
				defer cl.unlock()
				cl.eachDhtServer(func(s DhtServer) {
					go s.Ping(&pingAddr)
				})
			}()
		case pp.Suggest:
			torrent.Add("suggests received", 1)
			log.Fmsg("peer suggested piece %d", msg.Index).AddValues(c, msg.Index).LogLevel(log.Debug, c.t.logger)
			c.updateRequests("suggested", true)
		case pp.HaveAll:
			err = c.onPeerSentHaveAll()
		case pp.HaveNone:
			err = c.peerSentHaveNone(true)
		case pp.Reject:
			req := newRequestFromMessage(&msg)
			if !c.remoteRejectedRequest(c.t.requestIndexFromRequest(req, false)) {
				err = fmt.Errorf("received invalid reject for request %v", req)
				c.logger.Levelf(log.Debug, "%v", err)
			}
		case pp.AllowedFast:
			torrent.Add("allowed fasts received", 1)
			log.Fmsg("peer allowed fast: %d", msg.Index).AddValues(c).LogLevel(log.Debug, c.t.logger)
			c.updateRequests("PeerConn.mainReadLoop allowed fast", true)
		case pp.Extended:
			err = c.onReadExtendedMsg(msg.ExtendedID, msg.ExtendedPayload)
		default:
			err = fmt.Errorf("received unknown message type: %#v", msg.Type)
		}
		if err != nil {
			return err
		}
	}
}

func (c *PeerConn) onReadExtendedMsg(id pp.ExtensionNumber, payload []byte) (err error) {
	defer func() {
		// TODO: Should we still do this?
		if err != nil {
			// These clients use their own extension IDs for outgoing message
			// types, which is incorrect.
			if bytes.HasPrefix(c.PeerID[:], []byte("-SD0100-")) || strings.HasPrefix(string(c.PeerID[:]), "-XL0012-") {
				err = nil
			}
		}
	}()
	t := c.t
	cl := t.cl
	switch id {
	case pp.HandshakeExtendedID:
		return func() error {
			var d pp.ExtendedHandshakeMessage
			if err := bencode.Unmarshal(payload, &d); err != nil {
				c.logger.Printf("error parsing extended handshake message %q: %s", payload, err)
				return fmt.Errorf("unmarshalling extended handshake payload: %w", err)
			}
			if cb := c.callbacks.ReadExtendedHandshake; cb != nil {
				cb(c, &d)
			}

			c.t.mu.RLock()
			defer c.t.mu.RUnlock()

			c.mu.Lock()
			// c.logger.WithDefaultLevel(log.Debug).Printf("received extended handshake message:\n%s", spew.Sdump(d))
			if d.Reqq != 0 {
				c.PeerMaxRequests = d.Reqq
			}
			c.PeerClientName.Store(d.V)
			if c.PeerExtensionIDs == nil {
				c.PeerExtensionIDs = make(map[pp.ExtensionName]pp.ExtensionNumber, len(d.M))
			}
			c.PeerListenPort = d.Port
			c.PeerPrefersEncryption = d.Encryption
			for name, id := range d.M {
				if _, ok := c.PeerExtensionIDs[name]; !ok {
					peersSupportingExtension.Add(
						// expvar.Var.String must produce valid JSON. "ut_payme\xeet_address" was being
						// entered here which caused problems later when unmarshalling.
						strconv.Quote(string(name)),
						1)
				}
				c.PeerExtensionIDs[name] = id
			}
			c.mu.Unlock()

			if d.MetadataSize != 0 {
				if err = t.setMetadataSize(d.MetadataSize, false); err != nil {
					return fmt.Errorf("setting metadata size to %d: %w", d.MetadataSize, err)
				}
			}
			c.requestPendingMetadata(false)
			if !t.cl.config.DisablePEX {
				c.mu.Lock()
				t.pex.Add(c, false) // we learnt enough now
				// This checks the extension is supported internally.
				c.pex.Init(c)
				c.mu.Unlock()
			}
			return nil
		}()
	case metadataExtendedId:
		err := cl.gotMetadataExtensionMsg(payload, t, c)
		if err != nil {
			return fmt.Errorf("handling metadata extension message: %w", err)
		}
		return nil
	case pexExtendedId:
		return func() error {
			c.t.mu.Lock()
			defer c.t.mu.Unlock()
			c.mu.Lock()
			enabled := c.pex.IsEnabled()
			c.mu.Unlock()

			if !enabled {
				return nil // or hang-up maybe?
			}

			// peer is passed to recv - it needs to lock it to
			// update time but unlock for torrent.addpeers
			// may lock it while deciding which peers to drop in
			// worst peers
			err = c.pex.recv(payload, &c.mu, false)
			if err != nil {
				return fmt.Errorf("receiving pex message: %w", err)
			}
			return nil
		}()
	case utHolepunchExtendedId:
		var msg utHolepunch.Msg
		err = msg.UnmarshalBinary(payload)
		if err != nil {
			err = fmt.Errorf("unmarshalling ut_holepunch message: %w", err)
			return
		}
		err = c.t.handleReceivedUtHolepunchMsg(msg, c)
		return
	default:
		return fmt.Errorf("unexpected extended message ID: %v", id)
	}
}

// Set both the Reader and Writer for the connection from a single ReadWriter.
func (cn *PeerConn) setRW(rw io.ReadWriter) {
	cn.r = rw
	cn.w = rw
}

// Returns the Reader and Writer as a combined ReadWriter.
func (cn *PeerConn) rw() io.ReadWriter {
	return struct {
		io.Reader
		io.Writer
	}{cn.r, cn.w}
}

func (c *PeerConn) uploadAllowed() bool {
	if c.t.cl.config.NoUpload {
		return false
	}
	if c.t.dataUploadDisallowed {
		return false
	}
	if c.t.seeding(true) {
		return true
	}
	if !c.peerHasWantedPieces(true, true) {
		return false
	}
	// Don't upload more than 100 KiB more than we download.
	if c._stats.BytesWrittenData.Int64() >= c._stats.BytesReadData.Int64()+100<<10 {
		return false
	}
	return true
}

func (c *PeerConn) setRetryUploadTimer(delay time.Duration) {
	if c.uploadTimer == nil {
		c.uploadTimer = time.AfterFunc(delay, c.tickleWriter)
	} else {
		c.uploadTimer.Reset(delay)
	}
}

// Also handles choking and unchoking of the remote peer.
func (c *PeerConn) upload(msg func(pp.Message, bool) bool, lock bool) bool {
	// Breaking or completing this loop means we don't want to upload to the peer anymore, and we
	// choke them.
another:
	for c.uploadAllowed() {
		// We want to upload to the peer.
		if !c.unchoke(msg, lock) {
			return false
		}

		type peerRequest struct {
			Request
			state *peerRequestState
		}

		var peerRequests []peerRequest

		func() {
			if lock {
				c.mu.RLock()
				defer c.mu.RUnlock()
			}
			peerRequests = make([]peerRequest, 0, len(c.peerRequests))
			for r, state := range c.peerRequests {
				s := *state
				peerRequests = append(peerRequests, peerRequest{r, &s})
			}
		}()

		for _, peerRequest := range peerRequests {
			if peerRequest.state.data == nil {
				continue
			}
			res := c.t.cl.config.UploadRateLimiter.ReserveN(time.Now(), int(peerRequest.Length))
			if !res.OK() {
				panic(fmt.Sprintf("upload rate limiter burst size < %d", peerRequest.Length))
			}
			delay := res.Delay()
			if delay > 0 {
				res.Cancel()
				c.setRetryUploadTimer(delay)
				// Hard to say what to return here.
				return true
			}
			more := c.sendChunk(peerRequest.Request, msg, peerRequest.state, lock)

			func() {
				if lock {
					c.mu.Lock()
					defer c.mu.Unlock()
				}
				delete(c.peerRequests, peerRequest.Request)
			}()

			if !more {
				return false
			}
			goto another
		}
		return true
	}
	return c.choke(msg, lock)
}

func (cn *PeerConn) drop(lockTorrent bool) {
	cn.t.dropConnection(cn, lockTorrent)
}

func (cn *PeerConn) ban() {
	cn.t.cl.banPeerIP(cn.remoteIp())
}

// This is called when something has changed that should wake the writer, such as putting stuff into
// the writeBuffer, or changing some state that the writer can act on.
func (c *PeerConn) tickleWriter() {
	c.messageWriter.writeCond.Broadcast()
}

func (c *PeerConn) sendChunk(r Request, msg func(pp.Message, bool) bool, state *peerRequestState, lock bool) (more bool) {
	func() {
		if lock {
			c.mu.Lock()
			defer c.mu.Unlock()
		}
		c.lastChunkSent = time.Now()
	}()
	state.allocReservation.Release()
	return msg(pp.Message{
		Type:  pp.Piece,
		Index: r.Index,
		Begin: r.Begin,
		Piece: state.data,
	}, true)
}

func (c *Peer) setTorrent(t *Torrent, lockTorrent bool) {
	if c.t != nil {
		panic("connection already associated with a torrent")
	}

	if lockTorrent {
		t.mu.RLock()
		defer t.mu.RUnlock()
	}

	c.t = t
	c.logger.WithDefaultLevel(log.Debug).Printf("set torrent=%v", t.name(false))
	t.reconcileHandshakeStats(c)
}

func (c *PeerConn) pexPeerFlags(lock bool) pp.PexPeerFlags {
	if lock {
		c.mu.RLock()
		defer c.mu.RUnlock()
	}

	f := pp.PexPeerFlags(0)
	if c.PeerPrefersEncryption {
		f |= pp.PexPrefersEncryption
	}
	if c.outgoing {
		f |= pp.PexOutgoingConn
	}
	if c.utp() {
		f |= pp.PexSupportsUtp
	}
	return f
}

// This returns the address to use if we want to dial the peer again. It incorporates the peer's
// advertised listen port.
func (c *PeerConn) dialAddr(lock bool) PeerRemoteAddr {
	if lock {
		c.mu.RLock()
		defer c.mu.RUnlock()
	}

	if c.outgoing || c.PeerListenPort == 0 {
		return c.RemoteAddr
	}
	addrPort, err := addrPortFromPeerRemoteAddr(c.RemoteAddr)
	if err != nil {
		c.logger.Levelf(
			log.Warning,
			"error parsing %q for alternate dial port: %v",
			c.RemoteAddr,
			err,
		)
		return c.RemoteAddr
	}
	return netip.AddrPortFrom(addrPort.Addr(), uint16(c.PeerListenPort))
}

func (c *PeerConn) pexEvent(t pexEventType, lock bool) (_ pexEvent, err error) {
	f := c.pexPeerFlags(lock)
	dialAddr := c.dialAddr(lock)
	addr, err := addrPortFromPeerRemoteAddr(dialAddr)
	if err != nil || !addr.IsValid() {
		err = fmt.Errorf("parsing dial addr %q: %w", dialAddr, err)
		return
	}
	return pexEvent{t, addr, f, nil}, nil
}

func (pc *PeerConn) String() string {
	return fmt.Sprintf("%T %p [id=%+q, exts=%v, v=%q]", pc, pc, pc.PeerID, pc.PeerExtensionBytes, pc.PeerClientName.Load())
}

// Returns the pieces the peer could have based on their claims. If we don't know how many pieces
// are in the torrent, it could be a very large range if the peer has sent HaveAll.
func (pc *PeerConn) PeerPieces() *roaring.Bitmap {
	return pc.newPeerPieces(true, true)
}

func (pc *PeerConn) remoteIsTransmission() bool {
	return bytes.HasPrefix(pc.PeerID[:], []byte("-TR")) && pc.PeerID[7] == '-'
}

func (pc *PeerConn) remoteDialAddrPort(lock bool) (netip.AddrPort, error) {
	dialAddr := pc.dialAddr(lock)
	return addrPortFromPeerRemoteAddr(dialAddr)
}

func (pc *PeerConn) bitExtensionEnabled(bit pp.ExtensionBit) bool {
	return pc.t.cl.config.Extensions.GetBit(bit) && pc.PeerExtensionBytes.GetBit(bit)
}

func (cn *PeerConn) peerPiecesChanged(lockTorrent bool) {
	cn.t.maybeDropMutuallyCompletePeer(cn, lockTorrent)
}

// Returns whether the connection could be useful to us. We're seeding and
// they want data, we don't have metainfo and they can provide it, etc.
func (c *PeerConn) useful(lock bool, lockTorrent bool) bool {
	t := c.t
	if c.closed.IsSet() {
		return false
	}
	if !t.haveInfo(true) {
		return c.supportsExtension("ut_metadata", lock)
	}

	if lockTorrent {
		c.t.mu.RLock()
		defer c.t.mu.RUnlock()
	}

	if lock {
		c.mu.RLock()
		defer c.mu.RUnlock()
	}

	if t.seeding(false) && c.peerInterested {
		return true
	}
	if c.peerHasWantedPieces(false, false) {
		return true
	}
	return false
}