Transfers: stop a transfer queued as the last one finishes from waiting for ever

tests/DodoSSH.Client.Transfer.Tests/FileTransferQueueTests.cs

@@ -304,6 +304,36 @@ public sealed class FileTransferQueueTests : IDisposable
         finished.Failure.ShouldBe("the host is not reachable");
     }
 
+    /// <remarks>
+    /// <para>
+    /// The pump decides the queue is empty under the lock and only finishes its task after letting go, so a
+    /// transfer queued in that gap used to find a pump that still looked busy, start none, and sit queued for
+    /// ever. Queuing each transfer the moment the last one reports finishing is what lands in that gap.
+    /// </para>
+    /// <para>
+    /// A race, so it is run many times rather than arranged once; on the code that had it, a few hundred
+    /// rounds strand a transfer every time. The session factory refuses at once, which keeps each round to
+    /// the queue's own bookkeeping — the part the race is in.
+    /// </para>
+    /// </remarks>
+    [Fact]
+    public async Task ATransferQueuedAsTheLastOneFinishes_StillRuns()
+    {
+        await using var queue = new FileTransferQueue(
+            _ => Task.FromException<IRemoteFileStore>(new IOException("refused at once")),
+            TimeProvider.System);
+
+        for (var round = 0; round < 500; round++)
+        {
+            var finished = await RunToCompletionAsync(
+                queue, TransferDirection.Download, Local($"round-{round}"), Remote($"round-{round}"), 1);
+
+            finished.State.ShouldBe(TransferState.Failed, $"round {round}");
+        }
+
+        queue.IsBusy.ShouldBeFalse();
+    }
+
     private FileTransferQueue NewQueue() =>
         new(_ => Task.FromResult<IRemoteFileStore>(host), TimeProvider.System);
 
@@ -371,7 +401,7 @@ public sealed class FileTransferQueueTests : IDisposable
         {
             start();
 
-            return await finished.Task.WaitAsync(TimeSpan.FromSeconds(30), Token);
+            return await finished.Task.WaitAsync(TimeSpan.FromSeconds(10), Token);
         }
         finally
         {