minstrel/internal/release/torrent.go

package release

import (
	"bytes"
	"crypto/sha1"
	"encoding/hex"
	"errors"
	"fmt"
	"strconv"
	"strings"
)

// Reading a .torrent file. A .torrent
// is the metadata: its info dictionary names every file, and its SHA-1 is the
// infohash. So a release offered as a .torrent link can be judged on what it
// holds before any of it is downloaded. Bencode is four shapes; walking them
// here costs less than a dependency.

// TorrentMeta is what a .torrent says about itself.
type TorrentMeta struct {
	// InfoHash is the v1 infohash: the SHA-1 of the info dictionary's own
	// bytes, lowercase hex.
	InfoHash string
	Name     string
	Files    []File
}

// ErrNotTorrent is bytes that are not a .torrent.
var ErrNotTorrent = errors.New("not a .torrent file")

// maxTorrentBytes bounds what is read, as Exchange bounds what it accepts.
const maxTorrentBytes = 10 << 20

// ParseTorrent reads a .torrent's info dictionary.
func ParseTorrent(b []byte) (TorrentMeta, error) {
	if len(b) == 0 || len(b) > maxTorrentBytes || b[0] != 'd' {
		return TorrentMeta{}, ErrNotTorrent
	}
	d := &bdecoder{b: b}
	root, err := d.value(0)
	if err != nil {
		return TorrentMeta{}, fmt.Errorf("%w: %v", ErrNotTorrent, err)
	}
	top, ok := root.(map[string]any)
	if !ok || d.infoEnd == 0 {
		return TorrentMeta{}, fmt.Errorf("%w: no info dictionary", ErrNotTorrent)
	}
	info, ok := top["info"].(map[string]any)
	if !ok {
		return TorrentMeta{}, fmt.Errorf("%w: no info dictionary", ErrNotTorrent)
	}
	sum := sha1.Sum(b[d.infoStart:d.infoEnd])
	out := TorrentMeta{InfoHash: hex.EncodeToString(sum[:])}
	out.Name, _ = info["name"].(string)

	files, multi := info["files"].([]any)
	if !multi {
		// A single-file torrent: the name IS the file.
		length, _ := info["length"].(int64)
		out.Files = []File{{Path: out.Name, Size: length}}
		return out, nil
	}
	for index, f := range files {
		entry, ok := f.(map[string]any)
		if !ok {
			continue
		}
		parts, _ := entry["path"].([]any)
		segs := []string{out.Name}
		for _, p := range parts {
			if s, ok := p.(string); ok {
				segs = append(segs, s)
			}
		}
		length, _ := entry["length"].(int64)
		// BEP 47 padding files carry no content.
		if attr, _ := entry["attr"].(string); strings.Contains(attr, "p") {
			continue
		}
		out.Files = append(out.Files, File{Index: index, Path: strings.Join(segs, "/"), Size: length})
	}
	return out, nil
}

// bdecoder walks bencode, and remembers where the top-level info dictionary
// sits so its raw bytes can be hashed exactly as published.
type bdecoder struct {
	b                  []byte
	i                  int
	infoStart, infoEnd int
}

// maxDepth: a real torrent nests four deep. Anything past this is hostile.
const maxDepth = 32

func (d *bdecoder) value(depth int) (any, error) {
	if depth > maxDepth {
		return nil, errors.New("nested too deep")
	}
	if d.i >= len(d.b) {
		return nil, errors.New("truncated")
	}
	switch c := d.b[d.i]; {
	case c == 'i':
		end := bytes.IndexByte(d.b[d.i:], 'e')
		if end < 0 {
			return nil, errors.New("unterminated integer")
		}
		n, err := strconv.ParseInt(string(d.b[d.i+1:d.i+end]), 10, 64)
		if err != nil {
			return nil, err
		}
		d.i += end + 1
		return n, nil
	case c == 'l':
		d.i++
		var out []any
		for d.i < len(d.b) && d.b[d.i] != 'e' {
			v, err := d.value(depth + 1)
			if err != nil {
				return nil, err
			}
			out = append(out, v)
		}
		if d.i >= len(d.b) {
			return nil, errors.New("unterminated list")
		}
		d.i++
		return out, nil
	case c == 'd':
		d.i++
		out := map[string]any{}
		for d.i < len(d.b) && d.b[d.i] != 'e' {
			k, err := d.str()
			if err != nil {
				return nil, err
			}
			start := d.i
			v, err := d.value(depth + 1)
			if err != nil {
				return nil, err
			}
			if depth == 0 && k == "info" {
				d.infoStart, d.infoEnd = start, d.i
			}
			// Pieces are 20 bytes a piece and never read: not kept.
			if k != "pieces" {
				out[k] = v
			}
		}
		if d.i >= len(d.b) {
			return nil, errors.New("unterminated dictionary")
		}
		d.i++
		return out, nil
	case c >= '0' && c <= '9':
		return d.str()
	default:
		return nil, fmt.Errorf("unexpected %q", c)
	}
}

func (d *bdecoder) str() (string, error) {
	colon := bytes.IndexByte(d.b[d.i:], ':')
	if colon < 0 {
		return "", errors.New("unterminated string length")
	}
	n, err := strconv.Atoi(string(d.b[d.i : d.i+colon]))
	if err != nil || n < 0 {
		return "", errors.New("bad string length")
	}
	start := d.i + colon + 1
	if start+n > len(d.b) {
		return "", errors.New("string runs past the end")
	}
	d.i = start + n
	return string(d.b[start : start+n]), nil
}