[bep52|bep47] Fix file ordering sanity check and padding for empty files

src/MonoTorrent.Client/MonoTorrent/Torrent.cs

@@ -629,7 +629,7 @@ void LoadInternal (BEncodedDictionary torrentInformation, RawInfoHashes infoHash
 
                         var merkleTrees = dict.ToDictionary (
                             key => MerkleRoot.FromMemory (key.Key.AsMemory ()),
-                            kvp => ReadOnlyMerkleLayers.FromLayer (PieceLength, MerkleRoot.FromMemory (kvp.Key.AsMemory ()), ((BEncodedString)kvp.Value).Span) ?? throw new TorrentException ($"Invalid merkle tree. A layer not produce the expected root hash.")
+                            kvp => ReadOnlyMerkleLayers.FromLayer (PieceLength, MerkleRoot.FromMemory (kvp.Key.AsMemory ()), ((BEncodedString) kvp.Value).Span) ?? throw new TorrentException ($"Invalid merkle tree. A layer not produce the expected root hash.")
                         );
 
                         hashesV2 = LoadHashesV2 (Files, merkleTrees, PieceLength);
@@ -778,7 +778,7 @@ static void LoadTorrentFilesV2 (string key, BEncodedDictionary data, List<ITorre
             if (key == "") {
                 var length = ((BEncodedNumber) data["length"]).Number;
                 if (length == 0) {
-                    files.Add (new TorrentFile (path, length, 0, 0, 0, TorrentFileAttributes.None, 0));
+                    files.Insert (0, new TorrentFile (path, length, 0, 0, 0, TorrentFileAttributes.None, 0));
                 } else {
                     totalPieces++;
                     var offsetInTorrent = (files.LastOrDefault ()?.OffsetInTorrent ?? 0) + (files.LastOrDefault ()?.Length ?? 0) + (files.LastOrDefault ()?.Padding ?? 0);

src/MonoTorrent.Client/MonoTorrent/TorrentCreator.cs

@@ -265,8 +265,10 @@ internal async Task<BEncodedDictionary> CreateAsync (string name, ITorrentFileSo
             if (Type.HasV2 ())
                 rawFiles = rawFiles.OrderBy (t => t.Destination, StringComparer.Ordinal).ToArray ();
 
-            // The last file never has padding bytes
-            rawFiles[rawFiles.Length - 1].padding = 0;
+            // The last non-empty file never has padding bytes
+            var last = rawFiles.Where (t => t.length != 0).Last ();
+            var index = Array.IndexOf (rawFiles, last);
+            rawFiles[index].padding = 0;
 
             var files = TorrentFileInfo.Create (PieceLength, rawFiles);
             var manager = new TorrentManagerInfo (files,

src/MonoTorrent.Client/MonoTorrent/TorrentFile.cs

@@ -156,6 +156,8 @@ internal static ITorrentFile[] Create (int pieceLength, TorrentFileTuple[] files
                         // Do not try to handle this, it would be very messy.
                         throw new ArgumentException ("Can not handle torrent that starts with padding");
                     } else {
+                        if (files[real].length == 0)
+                            throw new InvalidOperationException ("Attempted to add padding bytes to a file which should not have padding.");
                         // add the count to it will also work in case of consecutive padding files, also slightly edge-case-y
                         files[real].padding += files[t].length;
                     }

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

@@ -78,7 +78,7 @@ private static async Task<Torrent> CreateTestTorrent (TorrentType type)
         [TestCase(TorrentType.V1OnlyWithPaddingFiles)]
         [TestCase(TorrentType.V1V2Hybrid)]
         [TestCase(TorrentType.V2Only)]
-        public async Task CreateHybridWithUnusualFilenames (TorrentType type)
+        public async Task CreateWithUnusualFilenames (TorrentType type)
         {
             var files = new Source {
                 TorrentName = "asd",
@@ -92,6 +92,45 @@ public async Task CreateHybridWithUnusualFilenames (TorrentType type)
             Assert.DoesNotThrow (() => Torrent.Load (torrent));
         }
 
+        [Test]
+        [TestCase (TorrentType.V1Only)]
+        [TestCase (TorrentType.V1OnlyWithPaddingFiles)]
+        [TestCase (TorrentType.V1V2Hybrid)]
+        [TestCase (TorrentType.V2Only)]
+        public async Task CreateWithManyEmptyFiles (TorrentType type)
+        {
+            var files = new Source {
+                TorrentName = "asd",
+                Files = new[] {
+                    new FileMapping("a", "a", 0),
+                    new FileMapping("b", "b", PieceLength),
+                    new FileMapping("c", "c", 0),
+                    new FileMapping("d1", "d1", PieceLength / 2),
+                    new FileMapping("d2", "d2", PieceLength / 2),
+                    new FileMapping("e", "e", 0),
+                    new FileMapping("f", "f", 0),
+                    new FileMapping("g", "g", PieceLength + 1),
+                    new FileMapping("h", "h", 0),
+                }
+            };
+
+            var torrent = Torrent.Load (await CreateTestBenc (type, files));
+            foreach(var emptyFile in files.Files.Where (t => t.Length == 0)) {
+                var file = torrent.Files.Single (t => t.Path == emptyFile.Destination);
+                Assert.AreEqual (0, file.Length);
+                Assert.AreEqual (0, file.OffsetInTorrent);
+                Assert.AreEqual (0, file.StartPieceIndex);
+                Assert.AreEqual (0, file.EndPieceIndex);
+                Assert.AreEqual (0, file.Padding);
+                Assert.IsTrue (file.PiecesRoot.IsEmpty);
+            }
+            var expectedPadding = type == TorrentType.V1OnlyWithPaddingFiles || type == TorrentType.V1V2Hybrid ? PieceLength / 2 : 0;
+            Assert.AreEqual (0, torrent.Files.Single (t => t.Path == "b").Padding);
+            Assert.AreEqual (expectedPadding, torrent.Files.Single (t => t.Path == "d1").Padding);
+            Assert.AreEqual (expectedPadding, torrent.Files.Single (t => t.Path == "d2").Padding);
+            Assert.AreEqual (0, torrent.Files.Single (t => t.Path == "g").Padding);
+        }
+
         [Test]
         public async Task FileLengthSameAsPieceLength ([Values (TorrentType.V1Only, TorrentType.V1V2Hybrid)] TorrentType type)
         {