do not wait hash check before starting next piece download

torrent/messagehandler.go

@@ -64,6 +64,9 @@ func (t *torrent) handlePieceMessage(pm peer.PieceMessage) {
 	t.closePieceDownloader(pd)
 	pe.StopSnubTimer()
 
+	// Request next piece while writing the completed piece, being optimistic about hash check.
+	t.startPieceDownloaderFor(pe)
+
 	if piece.Writing {
 		panic("piece is already writing")
 	}

torrent/run.go

@@ -131,27 +131,21 @@ func (t *torrent) run() {
 				break
 			}
 
-			// Copy requested peers
-			requestedPeers := t.piecePicker.RequestedPeers(pw.Piece.Index)
-			peers := make([]*peer.Peer, len(requestedPeers))
-			for i := range requestedPeers {
-				peers[i] = requestedPeers[i]
+			pw.Piece.Done = true
+			if t.bitfield.Test(pw.Piece.Index) {
+				panic("already have the piece")
 			}
+			t.bitfield.Set(pw.Piece.Index)
 
 			if t.piecePicker != nil {
-				for _, pe := range peers {
+				for _, pe := range t.piecePicker.RequestedPeers(pw.Piece.Index) {
 					pd2 := t.pieceDownloaders[pe]
 					t.closePieceDownloader(pd2)
 					pd2.CancelPending()
+					t.startPieceDownloaderFor(pe)
 				}
 			}
 
-			pw.Piece.Done = true
-			if t.bitfield.Test(pw.Piece.Index) {
-				panic("already have the piece")
-			}
-			t.bitfield.Set(pw.Piece.Index)
-
 			// Tell everyone that we have this piece
 			for pe := range t.peers {
 				t.updateInterestedState(pe)
@@ -163,12 +157,6 @@ func (t *torrent) run() {
 				pe.SendMessage(msg)
 			}
 
-			// Start download on current peer first, then canceled peers
-			t.startPieceDownloaderFor(pw.Peer)
-			for _, p := range peers {
-				t.startPieceDownloaderFor(p)
-			}
-
 			completed := t.checkCompletion()
 			if t.resume != nil {
 				if completed {