Experiment with only transferring data in background thread to work with bad sources.

src/NTwain/TwainAppSession.Callbacks.cs

@@ -86,8 +86,6 @@ namespace NTwain
       return (ushort)TWRC.SUCCESS;
     }
 
-    bool _closeDsRequested;
-    bool _inTransfer;
 
     private void HandleSourceMsg(MSG msg, [CallerMemberName] string? caller = null)
     {
@@ -95,7 +93,7 @@ namespace NTwain
 
       // the reason we post these to the background is
       // if they're coming from UI message loop then
-      // this needs to return asap (?)
+      // this needs to return asap
 
       switch (msg)
       {
@@ -104,18 +102,33 @@ namespace NTwain
           if (!_inTransfer)
           {
             _inTransfer = true;
-            _bgPendingMsgs.Add(msg);
+            _xferReady.Set();
+            //_bgPendingMsgs.Add(msg);
           }
           break;
-        case MSG.DEVICEEVENT:
         case MSG.CLOSEDSOK:
-          _bgPendingMsgs.Add(msg);
-          break;
         case MSG.CLOSEDSREQ:
           _closeDsRequested = true;
           if (!_inTransfer)
           {
-            _bgPendingMsgs.Add(msg);
+            // this should be done on ui thread (or same one that enabled the ds)
+            _uiThreadMarshaller.BeginInvoke(() =>
+            {
+              DisableSource();
+            });
+          }
+          break;
+        case MSG.DEVICEEVENT:
+          if (DeviceEvent != null && DGControl.DeviceEvent.Get(ref _appIdentity, ref _currentDS, out TW_DEVICEEVENT de) == TWRC.SUCCESS)
+          {
+            _uiThreadMarshaller.BeginInvoke(() =>
+            {
+              try
+              {
+                DeviceEvent.Invoke(this, de);
+              }
+              catch { }
+            });
           }
           break;
       }

src/NTwain/TwainAppSession.Sources.cs

@@ -137,6 +137,7 @@ namespace NTwain
     /// <returns></returns>
     public STS DisableSource()
     {
+      _closeDsRequested = true;
       var rc = DGControl.UserInterface.DisableDS(ref _appIdentity, ref _currentDS, ref _userInterface);
       if (rc == TWRC.SUCCESS)
       {

src/NTwain/TwainAppSession.Windows.cs

@@ -87,12 +87,15 @@ namespace NTwain
           _procEvent.pEvent = Marshal.AllocHGlobal(Marshal.SizeOf(winMsg));
         Marshal.StructureToPtr(winMsg, _procEvent.pEvent, true);
 
-        var rc = DGControl.Event.ProcessEvent(ref _appIdentity, ref _currentDS, ref _procEvent);
+        if (!_closeDsRequested)
-        handled = rc == TWRC.DSEVENT;
-        if (_procEvent.TWMessage != 0 && (handled || rc == TWRC.NOTDSEVENT))
         {
-          //Debug.WriteLine($"[thread {Environment.CurrentManagedThreadId}] CheckIfTwainMessage at state {State} with MSG={_procEvent.TWMessage}.");
+          var rc = DGControl.Event.ProcessEvent(ref _appIdentity, ref _currentDS, ref _procEvent);
-          HandleSourceMsg((MSG)_procEvent.TWMessage);
+          handled = rc == TWRC.DSEVENT;
+          if (_procEvent.TWMessage != 0 && (handled || rc == TWRC.NOTDSEVENT))
+          {
+            //Debug.WriteLine($"[thread {Environment.CurrentManagedThreadId}] CheckIfTwainMessage at state {State} with MSG={_procEvent.TWMessage}.");
+            HandleSourceMsg((MSG)_procEvent.TWMessage);
+          }
         }
       }
       return handled;

src/NTwain/TwainAppSession.Xfers.cs

@@ -171,14 +171,14 @@ namespace NTwain
       }
 
       HandleXferCode(sts);
-      _inTransfer = false;
       if (State >= STATE.S5)
       {
-        _uiThreadMarshaller.BeginInvoke(() =>
+        _uiThreadMarshaller.Invoke(() =>
         {
           DisableSource();
         });
       }
+      _inTransfer = false;
     }
 
     private void HandleXferCode(STS sts)

src/NTwain/TwainAppSession.cs

@@ -6,6 +6,7 @@ using System.Diagnostics;
 using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading;
+using System.Xml;
 
 namespace NTwain
 {
@@ -93,7 +94,7 @@ namespace NTwain
       _osxCallbackDelegate = OSXCallbackHandler;
 
       _uiThreadMarshaller = uiThreadMarshaller;
-      StartBackgroundThread();
+      StartTransferThread();
     }
 
     internal IntPtr _hwnd;
@@ -102,13 +103,16 @@ namespace NTwain
     TW_EVENT _procEvent; // kept here so the alloc/free only happens once
 #endif
     // test threads a bit
-    readonly BlockingCollection<MSG> _bgPendingMsgs = new();
+    //readonly BlockingCollection<MSG> _bgPendingMsgs = new();
     private readonly IThreadMarshaller _uiThreadMarshaller;
+    bool _closeDsRequested;
+    bool _inTransfer;
+    readonly AutoResetEvent _xferReady = new(false);
     private bool disposedValue;
 
-    void StartBackgroundThread()
+    void StartTransferThread()
     {
-      Thread t = new(BackgroundThreadLoop)
+      Thread t = new(TransferLoopLoop)
       {
         IsBackground = true
       };
@@ -118,44 +122,21 @@ namespace NTwain
       t.Start();
     }
 
-    private void BackgroundThreadLoop(object? obj)
+    private void TransferLoopLoop(object? obj)
     {
-      foreach (var msg in _bgPendingMsgs.GetConsumingEnumerable())
+      while (!disposedValue)
       {
-        switch (msg)
+        try
         {
-          case MSG.CLOSEDSOK:
+          _xferReady.WaitOne();
-          case MSG.CLOSEDSREQ:
-            // this should be done on ui thread (or same one that enabled the ds)
-            _uiThreadMarshaller.BeginInvoke(() =>
-            {
-              DisableSource();
-            });
-            break;
-          case MSG.DEVICEEVENT:
-            if (DeviceEvent != null && DGControl.DeviceEvent.Get(ref _appIdentity, ref _currentDS, out TW_DEVICEEVENT de) == TWRC.SUCCESS)
-            {
-              _uiThreadMarshaller.BeginInvoke(() =>
-              {
-                try
-                {
-                  DeviceEvent.Invoke(this, de);
-                }
-                catch { }
-              });
-            }
-            break;
-          case MSG.XFERREADY:
-            //_uiThreadMarshaller.Invoke(() =>
-            //{
-            State = STATE.S6;
-            //});
-            EnterTransferRoutine();
-            break;
         }
-
+        catch (ObjectDisposedException) { break; }
+        try
+        {
+          EnterTransferRoutine();
+        }
+        catch { }
       }
-      Debug.WriteLine("Ending BackgroundThreadLoop...");
     }
 
     protected virtual void Dispose(bool disposing)
@@ -165,7 +146,9 @@ namespace NTwain
         if (disposing)
         {
           // this will end the bg thread
-          _bgPendingMsgs.CompleteAdding();
+          _xferReady.Dispose();
+
+          //_bgPendingMsgs.CompleteAdding();
         }
 #if WINDOWS || NETFRAMEWORK
         if (_procEvent.pEvent != IntPtr.Zero) Marshal.FreeHGlobal(_procEvent.pEvent);