Fix races around Reader and Torrent.Drop when Torrent is closed

client.go

@@ -1352,9 +1352,6 @@ func (cl *Client) dropTorrent(infoHash metainfo.Hash, wg *sync.WaitGroup) (err e
 		return
 	}
 	err = t.close(wg)
-	if err != nil {
-		panic(err)
-	}
 	delete(cl.torrents, infoHash)
 	return
 }

reader.go

@@ -245,12 +245,20 @@ func (r *reader) readOnceAt(ctx context.Context, b []byte, pos int64) (n int, er
 			err = nil
 			return
 		}
+		if r.t.closed.IsSet() {
+			err = fmt.Errorf("reading from closed torrent: %w", err)
+			return
+		}
 		r.t.cl.lock()
 		// I think there's a panic here caused by the Client being closed before obtaining this
 		// lock. TestDropTorrentWithMmapStorageWhileHashing seems to tickle occasionally in CI.
 		func() {
 			// Just add exceptions already.
 			defer r.t.cl.unlock()
+			if r.t.closed.IsSet() {
+				// Can't update because Torrent's piece order is removed from Client.
+				return
+			}
 			// TODO: Just reset pieces in the readahead window. This might help
 			// prevent thrashing with small caches and file and piece priorities.
 			r.log(log.Fstr("error reading torrent %s piece %d offset %d, %d bytes: %v",

torrent.go

@@ -850,7 +850,10 @@ func (t *Torrent) numPiecesCompleted() (num pieceIndex) {
 }
 
 func (t *Torrent) close(wg *sync.WaitGroup) (err error) {
-	t.closed.Set()
+	if !t.closed.Set() {
+		err = errors.New("already closed")
+		return
+	}
 	if t.storage != nil {
 		wg.Add(1)
 		go func() {