metainfo: Add Magnet.Params for more open handling

Addresses #310.

metainfo/magnet.go

@@ -3,6 +3,7 @@ package metainfo
 import (
 	"encoding/base32"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"net/url"
 	"strings"
@@ -13,65 +14,98 @@ type Magnet struct {
 	InfoHash    Hash
 	Trackers    []string
 	DisplayName string
+	Params      url.Values
 }
 
 const xtPrefix = "urn:btih:"
 
 func (m Magnet) String() string {
-	// net.URL likes to assume //, and encodes ':' on us, so we do most of
-	// this manually.
-	ret := "magnet:?xt="
-	ret += xtPrefix + hex.EncodeToString(m.InfoHash[:])
-	if m.DisplayName != "" {
-		ret += "&dn=" + url.QueryEscape(m.DisplayName)
+	// Deep-copy m.Params
+	vs := make(url.Values, len(m.Params)+len(m.Trackers)+2)
+	for k, v := range m.Params {
+		vs[k] = append([]string(nil), v...)
 	}
+
+	vs.Add("xt", xtPrefix+m.InfoHash.HexString())
 	for _, tr := range m.Trackers {
-		ret += "&tr=" + url.QueryEscape(tr)
+		vs.Add("tr", tr)
 	}
-	return ret
+	if m.DisplayName != "" {
+		vs.Add("dn", m.DisplayName)
+	}
+
+	return (&url.URL{
+		Scheme:   "magnet",
+		RawQuery: vs.Encode(),
+	}).String()
 }
 
 // ParseMagnetURI parses Magnet-formatted URIs into a Magnet instance
 func ParseMagnetURI(uri string) (m Magnet, err error) {
 	u, err := url.Parse(uri)
 	if err != nil {
-		err = fmt.Errorf("error parsing uri: %s", err)
+		err = fmt.Errorf("error parsing uri: %w", err)
 		return
 	}
 	if u.Scheme != "magnet" {
-		err = fmt.Errorf("unexpected scheme: %q", u.Scheme)
+		err = fmt.Errorf("unexpected scheme %q", u.Scheme)
 		return
 	}
-	xt := u.Query().Get("xt")
-	if !strings.HasPrefix(xt, xtPrefix) {
-		err = fmt.Errorf("bad xt parameter")
-		return
-	}
-	infoHash := xt[len(xtPrefix):]
-
-	// BTIH hash can be in HEX or BASE32 encoding
-	// will assign appropriate func judging from symbol length
-	var decode func(dst, src []byte) (int, error)
-	switch len(infoHash) {
-	case 40:
-		decode = hex.Decode
-	case 32:
-		decode = base32.StdEncoding.Decode
-	}
-
-	if decode == nil {
-		err = fmt.Errorf("unhandled xt parameter encoding: encoded length %d", len(infoHash))
-		return
-	}
-	n, err := decode(m.InfoHash[:], []byte(infoHash))
+	q := u.Query()
+	xt := q.Get("xt")
+	m.InfoHash, err = parseInfohash(q.Get("xt"))
 	if err != nil {
-		err = fmt.Errorf("error decoding xt: %s", err)
+		err = fmt.Errorf("error parsing infohash %q: %w", xt, err)
+		return
+	}
+	dropFirst(q, "xt")
+	m.DisplayName = q.Get("dn")
+	dropFirst(q, "dn")
+	m.Trackers = q["tr"]
+	delete(q, "tr")
+	if len(q) == 0 {
+		q = nil
+	}
+	m.Params = q
+	return
+}
+
+func parseInfohash(xt string) (ih Hash, err error) {
+	if !strings.HasPrefix(xt, xtPrefix) {
+		err = errors.New("bad xt parameter prefix")
+		return
+	}
+	encoded := xt[len(xtPrefix):]
+	decode := func() func(dst, src []byte) (int, error) {
+		switch len(encoded) {
+		case 40:
+			return hex.Decode
+		case 32:
+			return base32.StdEncoding.Decode
+		}
+		return nil
+	}()
+	if decode == nil {
+		err = fmt.Errorf("unhandled xt parameter encoding (encoded length %d)", len(encoded))
+		return
+	}
+	n, err := decode(ih[:], []byte(encoded))
+	if err != nil {
+		err = fmt.Errorf("error decoding xt: %w", err)
 		return
 	}
 	if n != 20 {
 		panic(n)
 	}
-	m.DisplayName = u.Query().Get("dn")
-	m.Trackers = u.Query()["tr"]
 	return
 }
+
+func dropFirst(vs url.Values, key string) {
+	sl := vs[key]
+	switch len(sl) {
+	case 0, 1:
+		vs.Del(key)
+	default:
+		vs[key] = sl[1:]
+	}
+}