[btrtrc.git] / 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() string {
        if !cn.bitExtensionEnabled(pp.ExtensionBitLtep) {
                return "extended protocol disabled"
        }
        if cn.PeerExtensionIDs == nil {
                return "pending extended handshake"
        }
        if !cn.supportsExtension(pp.ExtensionNamePex) {
                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() []string {
        return []string{
                cn.connString,
                fmt.Sprintf("peer id: %+q", cn.PeerID),
                fmt.Sprintf("extensions: %v", cn.PeerExtensionBytes),
                fmt.Sprintf("pex: %s", cn.pexStatus()),
        }
}

// 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() (all, known bool) {
        if cn.peerSentHaveAll {
                return true, true
        }
        if !cn.t.haveInfo() {
                return false, false
        }
        return cn._peerPieces.GetCardinality() == uint64(cn.t.numPieces()), true
}

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

// 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) {
        cn._peerPieces.RemoveRange(bitmap.BitRange(num), bitmap.ToEnd)
        cn.peerPiecesChanged()
}

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

func (cn *PeerConn) choke(msg messageWriter) (more bool) {
        if cn.choking {
                return true
        }
        cn.choking = true
        more = msg(pp.Message{
                Type: pp.Choke,
        })
        if !cn.fastEnabled() {
                cn.deleteAllPeerRequests()
        }
        return
}

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

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

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

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

func (me *PeerConn) _cancel(r RequestIndex) bool {
        me.write(makeCancelMessage(me.t.requestIndexToRequest(r)))
        // Transmission does not send rejects for received cancels. See
        // https://github.com/transmission/transmission/pull/2275.
        return me.fastEnabled() && !me.remoteIsTransmission()
}

func (cn *PeerConn) fillWriteBuffer() {
        if cn.messageWriter.writeBuffer.Len() > writeBufferLowWaterLen {
                // 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.
        }
        cn.maybeUpdateActualRequestState()
        if cn.pex.IsEnabled() {
                if flow := cn.pex.Share(cn.write); !flow {
                        return
                }
        }
        cn.upload(cn.write)
}

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),
        })
        cn.sentHaves.Add(bitmap.BitIndex(piece))
}

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

func (cn *PeerConn) handleUpdateRequests() {
        // 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) error {
        if cn.t.haveInfo() && piece >= cn.t.numPieces() || piece < 0 {
                return errors.New("invalid piece")
        }
        if cn.peerHasPiece(piece) {
                return nil
        }
        cn.raisePeerMinPieces(piece + 1)
        if !cn.peerHasPiece(piece) {
                cn.t.incPieceAvailability(piece)
        }
        cn._peerPieces.Add(uint32(piece))
        if cn.t.wantPieceIndex(piece) {
                cn.updateRequests("have")
        }
        cn.peerPiecesChanged()
        return nil
}

func (cn *PeerConn) peerSentBitfield(bf []bool) error {
        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() && len(bf) > int(cn.t.numPieces()) {
                // Ignore known excess pieces.
                bf = bf[:cn.t.numPieces()]
        }
        bm := boolSliceToBitmap(bf)
        if cn.t.haveInfo() && pieceIndex(bm.GetCardinality()) == cn.t.numPieces() {
                cn.onPeerHasAllPieces()
                return nil
        }
        if !bm.IsEmpty() {
                cn.raisePeerMinPieces(pieceIndex(bm.Maximum()) + 1)
        }
        shouldUpdateRequests := false
        if cn.peerSentHaveAll {
                if !cn.t.deleteConnWithAllPieces(&cn.Peer) {
                        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)
                } else {
                        if !shouldUpdateRequests && cn.t.wantPieceIndex(pieceIndex(x)) {
                                shouldUpdateRequests = true
                        }
                        // We must be gaining this piece
                        cn.t.incPieceAvailability(pieceIndex(x))
                }
                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)
        if shouldUpdateRequests {
                cn.updateRequests("bitfield")
        }
        // We didn't guard this before, I see no reason to do it now.
        cn.peerPiecesChanged()
        return nil
}

func (cn *PeerConn) onPeerHasAllPieces() {
        t := cn.t
        if t.haveInfo() {
                cn._peerPieces.Iterate(func(x uint32) bool {
                        t.decPieceAvailability(pieceIndex(x))
                        return true
                })
        }
        t.addConnWithAllPieces(&cn.Peer)
        cn.peerSentHaveAll = true
        cn._peerPieces.Clear()
        if !cn.t._pendingPieces.IsEmpty() {
                cn.updateRequests("Peer.onPeerHasAllPieces")
        }
        cn.peerPiecesChanged()
}

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

func (cn *PeerConn) peerSentHaveNone() error {
        if !cn.peerSentHaveAll {
                cn.t.decPeerPieceAvailability(&cn.Peer)
        }
        cn._peerPieces.Clear()
        cn.peerSentHaveAll = false
        cn.peerPiecesChanged()
        return nil
}

func (c *PeerConn) requestPendingMetadata() {
        if c.t.haveInfo() {
                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(); index++ {
                if !c.t.haveMetadataPiece(index) && !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() bool {
        return c.PeerExtensionBytes.SupportsFast() && c.t.cl.config.Extensions.SupportsFast()
}

func (c *PeerConn) reject(r Request) {
        if !c.fastEnabled() {
                panic("fast not enabled")
        }
        c.write(r.ToMsg(pp.Reject))
        // It is possible to reject a request before it is added to peer requests due to being invalid.
        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) error {
        requestedChunkLengths.Add(strconv.FormatUint(r.Length.Uint64(), 10), 1)
        if _, ok := c.peerRequests[r]; ok {
                torrent.Add("duplicate requests received", 1)
                if c.fastEnabled() {
                        return errors.New("received duplicate request with fast enabled")
                }
                return nil
        }
        if c.choking {
                torrent.Add("requests received while choking", 1)
                if c.fastEnabled() {
                        torrent.Add("requests rejected while choking", 1)
                        c.reject(r)
                }
                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() {
                        c.reject(r)
                }
                // 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() {
                        c.reject(r)
                        return nil
                } else {
                        return err
                }
        }
        if !c.t.havePiece(pieceIndex(r.Index)) {
                // 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))
        // 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) {
        b, err := c.readPeerRequestData(r, prs)
        c.locker().Lock()
        defer c.locker().Unlock()
        if err != nil {
                c.peerRequestDataReadFailed(err, r)
        } else {
                if b == nil {
                        panic("data must be non-nil to trigger send")
                }
                torrent.Add("peer request data read successes", 1)
                prs.data = b
                // 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) {
        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.WithDefaultLevel(logLevel).Printf("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) {
                // 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)
        }
        // 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() {
                c.reject(r)
        } 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)
        }
}

func (c *PeerConn) readPeerRequestData(r Request, prs *peerRequestState) ([]byte, error) {
        // Should we depend on Torrent closure here? I think it's okay to get cancelled from elsewhere,
        // or fail to read and then cleanup.
        ctx := context.Background()
        err := prs.allocReservation.Wait(ctx)
        if err != nil {
                if ctx.Err() == nil {
                        // The error is from the reservation itself. Something is very broken, or we're not
                        // guarding against excessively large requests.
                        err = log.WithLevel(log.Critical, err)
                }
                err = fmt.Errorf("waiting for alloc limit reservation: %w", err)
                return nil, err
        }
        b := make([]byte, r.Length)
        p := c.t.info.Piece(int(r.Index))
        n, err := c.t.readAt(b, p.Offset()+int64(r.Begin))
        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
                func() {
                        cl.unlock()
                        defer cl.lock()
                        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.lastMessageReceived = time.Now()
                if msg.Keepalive {
                        receivedKeepalives.Add(1)
                        continue
                }
                messageTypesReceived.Add(msg.Type.String(), 1)
                if msg.Type.FastExtension() && !c.fastEnabled() {
                        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 c.peerChoking {
                                break
                        }
                        if !c.fastEnabled() {
                                c.deleteAllRequests("choked by non-fast PeerConn")
                        } 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.peerChoking = true
                        c.updateExpectingChunks()
                case pp.Unchoke:
                        if !c.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
                        }
                        c.peerChoking = false
                        preservedCount := 0
                        c.requestState.Requests.Iterate(func(x RequestIndex) bool {
                                if !c.peerAllowedFast.Contains(c.t.pieceIndexOfRequestIndex(x)) {
                                        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())

                                torrent.Add("requestsPreservedThroughChoking", int64(preservedCount))
                        }
                        if !c.t._pendingPieces.IsEmpty() {
                                c.updateRequests("unchoked")
                        }
                        c.updateExpectingChunks()
                case pp.Interested:
                        c.peerInterested = true
                        c.tickleWriter()
                case pp.NotInterested:
                        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))
                case pp.Bitfield:
                        err = c.peerSentBitfield(msg.Bitfield)
                case pp.Request:
                        r := newRequestFromMessage(&msg)
                        err = c.onReadRequest(r, true)
                        if err != nil {
                                err = fmt.Errorf("on reading request %v: %w", r, err)
                        }
                case pp.Piece:
                        c.doChunkReadStats(int64(len(msg.Piece)))
                        err = 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)
                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)
                        }
                        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")
                case pp.HaveAll:
                        err = c.onPeerSentHaveAll()
                case pp.HaveNone:
                        err = c.peerSentHaveNone()
                case pp.Reject:
                        req := newRequestFromMessage(&msg)
                        if !c.remoteRejectedRequest(c.t.requestIndexFromRequest(req)) {
                                c.logger.Printf("received invalid reject [request=%v, peer=%v]", req, c)
                                err = fmt.Errorf("received invalid reject [request=%v]", req)
                        }
                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")
                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:
                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.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
                }
                if d.MetadataSize != 0 {
                        if err = t.setMetadataSize(d.MetadataSize); err != nil {
                                return fmt.Errorf("setting metadata size to %d: %w", d.MetadataSize, err)
                        }
                }
                c.requestPendingMetadata()
                if !t.cl.config.DisablePEX {
                        t.pex.Add(c) // we learnt enough now
                        // This checks the extension is supported internally.
                        c.pex.Init(c)
                }
                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:
                if !c.pex.IsEnabled() {
                        return nil // or hang-up maybe?
                }
                err = c.pex.Recv(payload)
                if err != nil {
                        err = fmt.Errorf("receiving pex message: %w", err)
                }
                return
        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() {
                return true
        }
        if !c.peerHasWantedPieces() {
                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 {
        // 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) {
                        return false
                }
                for r, state := range c.peerRequests {
                        if state.data == nil {
                                continue
                        }
                        res := c.t.cl.config.UploadRateLimiter.ReserveN(time.Now(), int(r.Length))
                        if !res.OK() {
                                panic(fmt.Sprintf("upload rate limiter burst size < %d", r.Length))
                        }
                        delay := res.Delay()
                        if delay > 0 {
                                res.Cancel()
                                c.setRetryUploadTimer(delay)
                                // Hard to say what to return here.
                                return true
                        }
                        more := c.sendChunk(r, msg, state)
                        delete(c.peerRequests, r)
                        if !more {
                                return false
                        }
                        goto another
                }
                return true
        }
        return c.choke(msg)
}

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

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, state *peerRequestState) (more bool) {
        c.lastChunkSent = time.Now()
        state.allocReservation.Release()
        return msg(pp.Message{
                Type:  pp.Piece,
                Index: r.Index,
                Begin: r.Begin,
                Piece: state.data,
        })
}

func (c *PeerConn) setTorrent(t *Torrent) {
        if c.t != nil {
                panic("connection already associated with a torrent")
        }
        c.t = t
        c.logger.WithDefaultLevel(log.Debug).Printf("set torrent=%v", t)
        t.reconcileHandshakeStats(c)
}

func (c *PeerConn) pexPeerFlags() pp.PexPeerFlags {
        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() PeerRemoteAddr {
        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) (_ pexEvent, err error) {
        f := c.pexPeerFlags()
        dialAddr := c.dialAddr()
        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 (c *PeerConn) String() string {
        return fmt.Sprintf("%T %p [id=%q, exts=%v, v=%q]", c, c, c.PeerID, c.PeerExtensionBytes, c.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 the peer has sent HaveAll.
func (cn *PeerConn) PeerPieces() *roaring.Bitmap {
        cn.locker().RLock()
        defer cn.locker().RUnlock()
        return cn.newPeerPieces()
}

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

func (pc *PeerConn) remoteDialAddrPort() (netip.AddrPort, error) {
        dialAddr := pc.dialAddr()
        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() {
        cn.t.maybeDropMutuallyCompletePeer(cn)
}

// 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() bool {
        t := c.t
        if c.closed.IsSet() {
                return false
        }
        if !t.haveInfo() {
                return c.supportsExtension("ut_metadata")
        }
        if t.seeding() && c.peerInterested {
                return true
        }
        if c.peerHasWantedPieces() {
                return true
        }
        return false
}