AFAICT this is working as it should.

If any file is missing the torrent will not be marked complete. Missing
zero length files don't impact partial seeding status as no-one would
try downloading them.

src/MonoTorrent.Client/MonoTorrent.Client.Modes/DownloadMode.cs

@@ -88,8 +88,6 @@ public override void Tick (int counter)
 
         internal async Task UpdateSeedingDownloadingState ()
         {
-            // FIXME: Ensure this properly handles the case where files are missing.
-
             //If download is fully complete, set state to 'Seeding' and send an announce to the tracker.
             if (Manager.Complete && state == TorrentState.Downloading) {
                 state = TorrentState.Seeding;

src/MonoTorrent.Client/MonoTorrent.Client/Managers/TorrentManager.cs

@@ -103,20 +103,28 @@ public async Task SetFilePriorityAsync (ITorrentManagerFile file, Priority prior
             if (oldPriority == Priority.DoNotDownload && !(await Engine.DiskManager.CheckFileExistsAsync (file))) {
                 // Always create the file the user requested to download
                 await Engine.DiskManager.CreateAsync (file, Engine.Settings.FileCreationOptions);
+
+                if (file.Length == 0)
+                    ((TorrentFileInfo) file).BitField[0] = await Engine.DiskManager.CheckFileExistsAsync (file);
             }
 
+            // Update the priority for the file itself now that we've successfully created it!
+            ((TorrentFileInfo) file).Priority = priority;
+
             if (oldPriority == Priority.DoNotDownload && file.Length > 0) {
                 // Look for any file which are still marked DoNotDownload but also overlap this file.
                 // We need to create those ones too because if there are three 400kB files and the
                 // piece length is 512kB, and the first file is set to 'DoNotDownload', then we still
                 // need to create it as we'll download the first 512kB under bittorrent v1.
-                foreach (var maybeCreateFile in Files.Where (t => t.Priority == Priority.DoNotDownload && t.Length > 0 && t != file)) {
+                foreach (var maybeCreateFile in Files.Where (t => t.Priority == Priority.DoNotDownload && t.Length > 0)) {
                     // If this file overlaps, create it!
                     if (maybeCreateFile.Overlaps(file) && !(await Engine.DiskManager.CheckFileExistsAsync (maybeCreateFile)))
                         await Engine.DiskManager.CreateAsync (maybeCreateFile, Engine.Settings.FileCreationOptions);
                 }
             }
+;
 
+            // With the new priority, calculate which files we're actively downloading!
             if (needsToUpdateSelector) {
                 // If we change the priority of a file we need to figure out which files are marked
                 // as 'DoNotDownload' and which ones are downloadable.
@@ -130,7 +138,6 @@ public async Task SetFilePriorityAsync (ITorrentManagerFile file, Priority prior
                 }
             }
 
-            ((TorrentFileInfo) file).Priority = priority;
             Mode.HandleFilePriorityChanged (file, oldPriority);
         }
 
@@ -1119,7 +1126,7 @@ internal async Task LoadFastResumeAsync (FastResume data, bool skipStateCheckFor
         internal async ReusableTask RefreshAllFilesCorrectLengthAsync ()
         {
             var allFilesCorrectLength = true;
-            foreach (TorrentFileInfo file in Files.Where (t => t.Priority != Priority.DoNotDownload)) {
+            foreach (TorrentFileInfo file in Files) {
                 var maybeLength = await Engine!.DiskManager.GetLengthAsync (file);
 
                 // Empty files aren't stored in fast resume data because it's as easy to just check if they exist on disk.
@@ -1129,7 +1136,7 @@ internal async ReusableTask RefreshAllFilesCorrectLengthAsync ()
                 // If any file doesn't exist, or any file is too large, indicate that something is wrong.
                 // If files exist but are too short, then we can assume everything is fine and the torrent just
                 // needs to be downloaded.
-                if (!maybeLength.HasValue || maybeLength > file.Length)
+                if (file.Priority != Priority.DoNotDownload && (!maybeLength.HasValue || maybeLength > file.Length))
                     allFilesCorrectLength = false;
             }
             AllFilesCorrectLength = allFilesCorrectLength;

src/Tests/Tests.MonoTorrent.Client/MonoTorrent.Client.Modes/HashingModeTests.cs

@@ -271,6 +271,9 @@ public async Task SaveLoadFastResume ()
         [Test]
         public async Task DoNotDownload_All ()
         {
+            // No files exist!
+            Assert.AreEqual (0, PieceWriter.FilesWithLength.Count);
+
             var bf = new BitField (Manager.Bitfield).SetAll (true);
             var unhashed = new BitField (bf).SetAll (false);
             await Manager.LoadFastResumeAsync (new FastResume (Manager.InfoHashes, bf, unhashed));
@@ -292,6 +295,8 @@ public async Task DoNotDownload_All ()
             // No piece should be marked as available, and no pieces should actually be hashchecked.
             Assert.IsTrue (Manager.Bitfield.AllFalse, "#2");
             Assert.AreEqual (Manager.UnhashedPieces.TrueCount, Manager.UnhashedPieces.Length, "#3");
+
+            // Empty files will always have their bitfield set to true if they exist on disk. None should exist though
             foreach (var f in Manager.Files)
                 Assert.IsTrue (f.BitField.AllFalse, "#4." + f.Path);
         }

src/Tests/Tests.MonoTorrent.Client/MonoTorrent.Client/ClientEngineTests.cs

@@ -573,6 +573,7 @@ public async Task StartAsync_SetPriorityCreatesAllImplicatedFiles ()
 
             await rig.Manager.SetFilePriorityAsync (rig.Manager.Files[0], Priority.Normal);
             Assert.IsTrue (await writer.ExistsAsync (rig.Manager.Files[0]));
+            Assert.IsTrue (rig.Manager.Files[0].BitField.AllTrue);
             Assert.IsFalse (await writer.ExistsAsync (rig.Manager.Files[1]));
 
             await rig.Manager.SetFilePriorityAsync (rig.Manager.Files[1], Priority.Normal);