Add magnet-metainfo, tool to download magnet to torrent file

client.go

@@ -1083,6 +1083,21 @@ func (cl *Client) setMetaData(t *torrent, md metainfo.Info, bytes []byte) (err e
 	}
 
 	cl.downloadStrategy.TorrentStarted(t)
+	select {
+	case t.gotMetainfo <- &metainfo.MetaInfo{
+		Info: metainfo.InfoEx{
+			Info: md,
+		},
+		CreationDate: time.Now().Unix(),
+		Comment:      "metadata set in client",
+		CreatedBy:    "go.torrent",
+		// TODO(anacrolix): Expose trackers given when torrent added.
+	}:
+	default:
+		panic("shouldn't block")
+	}
+	close(t.gotMetainfo)
+	t.gotMetainfo = nil
 	return
 }
 
@@ -1096,7 +1111,10 @@ func newTorrent(ih InfoHash, announceList [][]string) (t *torrent, err error) {
 
 		closing:           make(chan struct{}),
 		ceasingNetworking: make(chan struct{}),
+
+		gotMetainfo: make(chan *metainfo.MetaInfo, 1),
 	}
+	t.GotMetainfo = t.gotMetainfo
 	t.Trackers = make([][]tracker.Client, len(announceList))
 	for tierIndex := range announceList {
 		tier := t.Trackers[tierIndex]
@@ -1120,12 +1138,12 @@ func newTorrent(ih InfoHash, announceList [][]string) (t *torrent, err error) {
 	return
 }
 
-func (cl *Client) AddMagnet(uri string) (err error) {
+func (cl *Client) AddMagnet(uri string) (t *torrent, err error) {
 	m, err := ParseMagnetURI(uri)
 	if err != nil {
 		return
 	}
-	t, err := newTorrent(m.InfoHash, [][]string{m.Trackers})
+	t, err = newTorrent(m.InfoHash, [][]string{m.Trackers})
 	if err != nil {
 		return
 	}

cmd/magnet-metainfo/main.go

@@ -0,0 +1,48 @@
+// Converts magnet URIs and info hashes into torrent metainfo files.
+package main
+
+import (
+	"flag"
+	"log"
+	"os"
+	"sync"
+
+	"github.com/anacrolix/libtorgo/bencode"
+
+	"bitbucket.org/anacrolix/go.torrent"
+)
+
+func main() {
+	flag.Parse()
+	cl, err := torrent.NewClient(nil)
+	if err != nil {
+		log.Fatalf("error creating client: %s", err)
+	}
+	wg := sync.WaitGroup{}
+	for _, arg := range flag.Args() {
+		t, err := cl.AddMagnet(arg)
+		if err != nil {
+			log.Fatalf("error adding magnet to client: %s", err)
+		}
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			mi, ok := <-t.GotMetainfo
+			t.Close()
+			if !ok {
+				log.Printf("no metainfo for %q", t.Name())
+				return
+			}
+			f, err := os.Create(mi.Info.Name + ".torrent")
+			if err != nil {
+				log.Fatalf("error creating torrent metainfo file: %s", err)
+			}
+			defer f.Close()
+			err = bencode.NewEncoder(f).Encode(mi)
+			if err != nil {
+				log.Fatalf("error writing torrent metainfo file: %s", err)
+			}
+		}()
+	}
+	wg.Wait()
+}

cmd/torrent/main.go

@@ -56,7 +56,7 @@ func main() {
 				log.Fatalf("error parsing magnet uri: %s", err)
 			}
 			ih = m.InfoHash
-			err = client.AddMagnet(arg)
+			_, err = client.AddMagnet(arg)
 			if err != nil {
 				log.Fatalf("error adding magnet: %s", err)
 			}

cmd/torrentfs/main.go

@@ -130,7 +130,7 @@ func main() {
 						log.Printf("error adding torrent to client: %s", err)
 					}
 				} else if ev.MagnetURI != "" {
-					err := client.AddMagnet(ev.MagnetURI)
+					_, err := client.AddMagnet(ev.MagnetURI)
 					if err != nil {
 						log.Printf("error adding magnet: %s", err)
 					}

fs/torrentfs_test.go

@@ -142,7 +142,7 @@ func TestDownloadOnDemand(t *testing.T) {
 		seeder.WriteStatus(w)
 	})
 	defer seeder.Stop()
-	err = seeder.AddMagnet(fmt.Sprintf("magnet:?xt=urn:btih:%x", layout.Metainfo.Info.Hash))
+	_, err = seeder.AddMagnet(fmt.Sprintf("magnet:?xt=urn:btih:%x", layout.Metainfo.Info.Hash))
 	if err != nil {
 		t.Fatal(err)
 	}

torrent.go

@@ -67,6 +67,9 @@ type torrent struct {
 	DisplayName  string
 	MetaData     []byte
 	metadataHave []bool
+
+	gotMetainfo chan *metainfo.MetaInfo
+	GotMetainfo <-chan *metainfo.MetaInfo
 }
 
 func (t *torrent) worstConnsHeap() (wcs *worstConns) {