Ztorrent/internal/tracker/tracker.go

package tracker

import (
	"context"
	"errors"
	"fmt"
	"net"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"time"

	"github.com/jackpal/bencode-go"
	"github.com/veggiedefender/torrent-client/internal/torrentfile"
)

type TrackerResponse struct {
	FailureReason string `bencode:"failure reason"`
	Interval      int    `bencode:"interval"`
	Peers         string `bencode:"peers"`
}

type Peer struct {
	IP   net.IP
	Port uint16
}

type AnnounceOptions struct {
	PeerID     [20]byte
	Port       uint16
	Uploaded   int64
	Downloaded int64
	NumWant    int
	Timeout    time.Duration
}

type httpDoer interface {
	Do(req *http.Request) (*http.Response, error)
}

func GetPeers(ctx context.Context, tf *torrentfile.TorrentFile, opts AnnounceOptions) ([]Peer, error) {
	opts = normalizeOptions(opts)
	client := &http.Client{Timeout: opts.Timeout}
	return getPeersWithClient(ctx, client, tf, opts)
}

func getPeersWithClient(ctx context.Context, client httpDoer, tf *torrentfile.TorrentFile, opts AnnounceOptions) ([]Peer, error) {
	if tf == nil {
		return nil, errors.New("torrent metadata is nil")
	}

	opts = normalizeOptions(opts)

	if client == nil {
		client = &http.Client{Timeout: opts.Timeout}
	}

	announceURL, err := buildAnnounceURL(tf, opts)
	if err != nil {
		return nil, err
	}

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, announceURL, nil)
	if err != nil {
		return nil, err
	}

	resp, err := client.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("tracker returned HTTP %d", resp.StatusCode)
	}

	var tr TrackerResponse

	err = bencode.Unmarshal(resp.Body, &tr)
	if err != nil {
		return nil, err
	}

	if tr.FailureReason != "" {
		return nil, fmt.Errorf("tracker failure: %s", tr.FailureReason)
	}

	return parsePeers([]byte(tr.Peers))
}

func normalizeOptions(opts AnnounceOptions) AnnounceOptions {
	if opts.Port == 0 {
		opts.Port = 6881
	}
	if opts.NumWant <= 0 {
		opts.NumWant = 50
	}
	if opts.Timeout <= 0 {
		opts.Timeout = 10 * time.Second
	}
	return opts
}

func buildAnnounceURL(tf *torrentfile.TorrentFile, opts AnnounceOptions) (string, error) {
	if tf == nil {
		return "", errors.New("torrent metadata is nil")
	}

	baseURL, err := url.Parse(tf.Announce)
	if err != nil {
		return "", fmt.Errorf("invalid tracker URL %q: %w", tf.Announce, err)
	}
	if baseURL.Scheme != "http" && baseURL.Scheme != "https" {
		return "", fmt.Errorf("unsupported tracker scheme %q (only http/https are supported)", baseURL.Scheme)
	}
	if baseURL.Host == "" {
		return "", errors.New("tracker URL host is empty")
	}

	uploaded := opts.Uploaded
	if uploaded < 0 {
		uploaded = 0
	}

	downloaded := opts.Downloaded
	if downloaded < 0 {
		downloaded = 0
	}

	left := int64(tf.Length) - downloaded
	if left < 0 {
		left = 0
	}

	parts := make([]string, 0, 10)
	if baseURL.RawQuery != "" {
		parts = append(parts, baseURL.RawQuery)
	}

	parts = append(parts,
		"info_hash="+escapeBinary(tf.InfoHash[:]),
		"peer_id="+escapeBinary(opts.PeerID[:]),
		"port="+strconv.Itoa(int(opts.Port)),
		"uploaded="+strconv.FormatInt(uploaded, 10),
		"downloaded="+strconv.FormatInt(downloaded, 10),
		"left="+strconv.FormatInt(left, 10),
		"compact=1",
		"numwant="+strconv.Itoa(opts.NumWant),
	)

	baseURL.RawQuery = strings.Join(parts, "&")
	return baseURL.String(), nil
}

func escapeBinary(data []byte) string {
	const hex = "0123456789ABCDEF"

	var b strings.Builder
	for _, c := range data {
		if isURLUnreserved(c) {
			b.WriteByte(c)
			continue
		}
		b.WriteByte('%')
		b.WriteByte(hex[c>>4])
		b.WriteByte(hex[c&0x0F])
	}
	return b.String()
}

func isURLUnreserved(c byte) bool {
	switch {
	case c >= 'a' && c <= 'z':
		return true
	case c >= 'A' && c <= 'Z':
		return true
	case c >= '0' && c <= '9':
		return true
	case c == '-', c == '.', c == '_', c == '~':
		return true
	default:
		return false
	}
}

func parsePeers(data []byte) ([]Peer, error) {
	if len(data) == 0 {
		return []Peer{}, nil
	}
	if len(data)%6 != 0 {
		return nil, fmt.Errorf("invalid compact peers length %d", len(data))
	}

	var peers []Peer

	for i := 0; i < len(data); i += 6 {

		ip := net.IP(data[i : i+4])

		port := uint16(data[i+4])<<8 |
			uint16(data[i+5])

		peers = append(peers, Peer{
			IP:   ip,
			Port: port,
		})
	}

	return peers, nil
}