Piece hash check fails for last piece of some torrent files

spaceface777 commented 5 months ago

I'm currently working on a custom torrent client and storage backend for the library, and I created some test torrent files in order to test it (using qBittorrent 4.6.5, in case that helps), but I've noticed that one of them fails to download because the last piece's hash check fails (so the peer gets banned). At first I assumed this was a bug in either my code or the torrent file I created, but after further testing I experienced the same issue with a few other public torrent files, so I decided to look into it further, and discovered that the bug still occurs when using the sample client in this repo.

Here is the same test.torrent.zip that I used, in case it helps.

I wrote the following patch for this library, which does work around the issue for me, but I'm not familiar enough with the codebase or the bittorrent protocol in general to know if this is a proper fix.

diff --git a/torrent.go b/torrent.go
diff --git a/cmd/torrent/download.go b/cmd/torrent/download.go
index 60482018..8c1aa386 100644
--- a/cmd/torrent/download.go
+++ b/cmd/torrent/download.go
@@ -146,6 +146,9 @@ func addTorrents(
                if flags.Progress {
                        torrentBar(t, flags.PieceStates)
                }
+               // download the last piece of the torrent first to make reproduction quicker
+               t.Piece(t.NumPieces() - 1).SetPriority(torrent.PiecePriorityNow)
+
                t.AddPeers(testPeers)
                wg.Add(1)
                go func() {
diff --git a/torrent.go b/torrent.go
index 3a5d358c..3b350747 100644
--- a/torrent.go
+++ b/torrent.go
@@ -1175,6 +1175,19 @@ func (t *Torrent) hashPiece(piece pieceIndex) (
                var sum [20]byte
                sumExactly(sum[:], h.Sum)
                correct = sum == *p.hash
+
+               if !correct && piece == t.NumPieces()-1 {
+                       // try to pad the last piece with zeroes
+                       paddingLen := t.info.PieceLength - p.Info().Length()
+                       io.CopyN(h, zeroReader, paddingLen)
+                       sumExactly(sum[:], h.Sum)
+                       correct = sum == *p.hash
+
+                       if correct {
+                               log.Printf("patch worked")
+                       }
+               }
        } else if p.hashV2.Ok {
                h := merkle.NewHash()
                differingPeers, err = t.hashPieceWithSpecificHash(piece, h)