fix: make STA Word queue lifetime safe

DocumentAutomation/src/DocxWorkbench.WordInterop/Properties/AssemblyInfo.cs

@@ -0,0 +1,3 @@
+using System.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("DocxWorkbench.Tests")]

DocumentAutomation/src/DocxWorkbench.WordInterop/StaWordQueue.cs

@@ -6,6 +6,9 @@ internal sealed class StaWordQueue : IDisposable
 {
     private readonly BlockingCollection<Action> _jobs = new();
     private readonly Thread _thread;
+    private readonly ManualResetEventSlim _stopped = new(false);
+    private readonly object _lifetimeGate = new();
+    private bool _accepting = true;
 
     public StaWordQueue()
     {
@@ -16,25 +19,42 @@ internal sealed class StaWordQueue : IDisposable
 
     public Task<T> Enqueue<T>(Func<T> operation, CancellationToken cancellationToken)
     {
+        if (operation is null) throw new ArgumentNullException(nameof(operation));
+        if (cancellationToken.IsCancellationRequested) return Task.FromCanceled<T>(cancellationToken);
         var completion = new TaskCompletionSource<T>(TaskCreationOptions.RunContinuationsAsynchronously);
-        _jobs.Add(() =>
+        lock (_lifetimeGate)
         {
-            try { cancellationToken.ThrowIfCancellationRequested(); completion.SetResult(operation()); }
-            catch (OperationCanceledException ex) { completion.SetCanceled(ex.CancellationToken); }
-            catch (Exception ex) { completion.SetException(ex); }
-        }, cancellationToken);
+            if (!_accepting) throw new ObjectDisposedException(nameof(StaWordQueue));
+            try { _jobs.Add(() => Execute(operation, completion, cancellationToken), cancellationToken); }
+            catch (InvalidOperationException) { throw new ObjectDisposedException(nameof(StaWordQueue)); }
+            catch (OperationCanceledException) { completion.TrySetCanceled(cancellationToken); }
+        }
         return completion.Task;
     }
 
+    private static void Execute<T>(Func<T> operation, TaskCompletionSource<T> completion, CancellationToken cancellationToken)
+    {
+        try { cancellationToken.ThrowIfCancellationRequested(); completion.TrySetResult(operation()); }
+        catch (OperationCanceledException ex) { completion.TrySetCanceled(ex.CancellationToken); }
+        catch (Exception ex) { completion.TrySetException(ex); }
+    }
+
     private void Run()
     {
-        foreach (var job in _jobs.GetConsumingEnumerable()) job();
+        try { foreach (var job in _jobs.GetConsumingEnumerable()) job(); }
+        finally { _stopped.Set(); }
     }
 
     public void Dispose()
     {
-        _jobs.CompleteAdding();
-        if (Thread.CurrentThread != _thread) _thread.Join(TimeSpan.FromSeconds(5));
-        _jobs.Dispose();
+        lock (_lifetimeGate)
+        {
+            if (!_accepting) return;
+            _accepting = false;
+            _jobs.CompleteAdding();
+        }
+        if (Thread.CurrentThread != _thread) _stopped.Wait(TimeSpan.FromSeconds(30));
+        // Do not dispose the collection or stop event while the STA worker may
+        // still be draining it. The worker owns the final enumeration lifetime.
     }
 }

DocumentAutomation/tests/DocxWorkbench.Tests/StaWordQueueTests.cs

@@ -0,0 +1,22 @@
+using DocxWorkbench.WordInterop;
+
+namespace DocxWorkbench.Tests;
+
+public sealed class StaWordQueueTests
+{
+    [Fact]
+    public async Task Dispose_DrainsActiveJobAndIsIdempotent()
+    {
+        using var queue = new StaWordQueue(); using var started = new ManualResetEventSlim(false); using var release = new ManualResetEventSlim(false);
+        var job = queue.Enqueue(() => { started.Set(); Assert.Equal(ApartmentState.STA, Thread.CurrentThread.GetApartmentState()); release.Wait(); return 42; }, CancellationToken.None); Assert.True(started.Wait(TimeSpan.FromSeconds(2)));
+        var dispose = Task.Run(queue.Dispose); var completed = await Task.WhenAny(dispose, Task.Delay(100)); Assert.NotSame(dispose, completed); release.Set(); await dispose; Assert.Equal(42, await job); queue.Dispose(); Assert.Throws<ObjectDisposedException>(() => EnqueueAfterDispose(queue));
+    }
+
+    [Fact]
+    public async Task Enqueue_CancellationCompletesCanceledTask()
+    {
+        using var queue = new StaWordQueue(); using var cancellation = new CancellationTokenSource(); cancellation.Cancel(); await Assert.ThrowsAnyAsync<OperationCanceledException>(() => queue.Enqueue(() => 1, cancellation.Token));
+    }
+
+    private static void EnqueueAfterDispose(StaWordQueue queue) => queue.Enqueue(() => 1, CancellationToken.None);
+}