Merge pull request #688 from alanmcgovern/improve-bep47-compatibility

Improve compatibility (arguably incorrect) BEP47 torrents

src/MonoTorrent.Client/MonoTorrent/Torrent.cs

@@ -484,8 +484,26 @@ void ProcessInfo (BEncodedDictionary dictionary, ref PieceHashesV1? hashesV1, re
                     if (v1File.Length != v2File.Length)
                         throw new TorrentException ("Inconsistent hybrid torrent, file length mismatch.");
 
-                    if (v1File.Padding != v2File.Padding)
-                        throw new TorrentException ("Inconsistent hybrid torrent, file padding mismatch.");
+                    if (v1File.Padding != v2File.Padding) {
+                        // BEP47 says padding is there so the *subsequent* file aligns with a piece start boundary.
+                        // By a literal reading, and in line with the rest of the bittorrent spec, the last file
+                        // can and should be considered the 'end' of the torrent (obviously :p) and so does not
+                        // have a subsequent file, and so does not need padding. Similar to how blocks are requested
+                        // in 16kB chunks, except for the final block which is just wahtever bytes are left over.
+                        //
+                        // Requested a clarification on the BEP. However both variants will need to be supported
+                        // regardless of what the spec says because both are in the wild.
+                        // Issue: https://github.com/bittorrent/bittorrent.org/issues/160
+                        //
+                        // If padding is mandatory for the last file, then remove the code which strips it out
+                        // inside 'LoadTorrentFilesV2'.
+                        if (v1File == v1Files.Last () && v2File == v2Files.Last ()) {
+                            var mutableFiles = v2Files.ToList ();
+                            mutableFiles[v2Files.Count - 1] = new TorrentFile (v2File.Path, v2File.Length, v2File.StartPieceIndex, v2File.EndPieceIndex, v2File.OffsetInTorrent, v2File.PiecesRoot, TorrentFileAttributes.None, v1File.Padding);
+                            v2Files = Array.AsReadOnly (mutableFiles.ToArray ());
+                        } else
+                            throw new TorrentException ("Inconsistent hybrid torrent, file padding mismatch.");
+                    }
                 }
 
                 Files = v2Files;

src/Tests/Tests.MonoTorrent.Client/MonoTorrent/TorrentCreatorBep47Tests.cs

@@ -296,6 +296,74 @@ public async Task HybridTorrentWithPadding ()
             Assert.IsTrue (sha256.AsSpan ().SequenceEqual (torrent.CreatePieceHashes ().GetHash (1).V2Hash.Span));
         }
 
+        [Test]
+        public void HybridTorrent_FinalFileHasUnexpectedPadding ([Values(true, false)] bool hasFinalFilePadding)
+        {
+            // Test validating both variants of torrent can be loaded
+            //
+            // https://github.com/bittorrent/bittorrent.org/issues/160
+            //
+            var v1Files = new BEncodedList {
+                new BEncodedDictionary {
+                    { "length", (BEncodedNumber)9 },
+                    { "path", new BEncodedList{ (BEncodedString)"file1.txt" } },
+                },
+                new BEncodedDictionary {
+                    { "attr", (BEncodedString) "p"},
+                    { "length", (BEncodedNumber)32759 },
+                    { "path", new BEncodedList{ (BEncodedString)".pad32759" } },
+                },
+
+                new BEncodedDictionary {
+                    { "length", (BEncodedNumber) 14 },
+                    { "path", new BEncodedList{ (BEncodedString)"file2.txt" } },
+                }
+            };
+
+            if (hasFinalFilePadding)
+                v1Files.Add (new BEncodedDictionary {
+                    { "attr", (BEncodedString) "p" },
+                    { "length", (BEncodedNumber)32754 },
+                    { "path", new BEncodedList{ (BEncodedString)".pad32754" } },
+                });
+
+            var v2Files = new BEncodedDictionary {
+                { "file1.txt", new BEncodedDictionary {
+                    {"", new BEncodedDictionary {
+                        { "length" , (BEncodedNumber) 9 },
+                        { "pieces root", (BEncodedString) Enumerable.Repeat<byte>(0, 32).ToArray () }
+                    } }
+                } },
+
+                { "file2.txt", new BEncodedDictionary {
+                    {"", new BEncodedDictionary {
+                        { "length", (BEncodedNumber) 14 },
+                        { "pieces root", (BEncodedString) Enumerable.Repeat<byte>(1, 32).ToArray () }
+                    } }
+                } },
+            };
+
+            var infoDict = new BEncodedDictionary {
+                {"files", v1Files },
+                {"file tree", v2Files },
+                { "meta version", (BEncodedNumber) 2 },
+                { "name", (BEncodedString) "padding test"},
+                { "piece length", (BEncodedNumber) 32768},
+                { "pieces", (BEncodedString) new byte[40] }
+            };
+
+            var dict = new BEncodedDictionary {
+                { "info", infoDict }
+            };
+
+            var torrent = Torrent.Load (dict);
+            Assert.AreEqual (2, torrent.Files.Count);
+            Assert.AreEqual (9, torrent.Files[0].Length);
+            Assert.AreEqual (32768 - 9, torrent.Files[0].Padding);
+            Assert.AreEqual (14, torrent.Files[1].Length);
+            Assert.AreEqual (hasFinalFilePadding ? 32768 - 14 : 0, torrent.Files[1].Padding);
+        }
+
         static BEncodedString SHA1SumZeros (long length)
         {
             using var hasher = IncrementalHash.CreateHash (HashAlgorithmName.SHA1);