Do not report a move that succeeded as failed

src/DiffEngineViewer.Tests/MoveSweepTests.cs

@@ -0,0 +1,94 @@
+/// <summary>
+/// Accepting a tracked move also removes the directory the received file sat in, when nothing is
+/// left in it. That is a tidy-up after the fact, and the move it follows has already happened.
+/// </summary>
+public class MoveSweepTests :
+    IDisposable
+{
+    /// <summary>
+    /// The caller reads a throw from here as the move having failed, so it re-tracks the entry -
+    /// and the retry then fails with file not found, the temp file having been moved by the
+    /// attempt that "failed". Only IOException was caught, and a directory the parent will not
+    /// let go of raises UnauthorizedAccessException.
+    /// </summary>
+    [Test]
+    [RunOn(TUnit.Core.Enums.OS.Linux | TUnit.Core.Enums.OS.MacOs)]
+    public async Task A_directory_that_cannot_be_removed_does_not_fail_the_move()
+    {
+        // The received file is two deep, and it is the middle directory that cannot be removed.
+        // Everything the move itself touches - taking the file out of "received", and writing it
+        // into a directory of its own - stays permitted, so the only thing denied is the tidy-up
+        var locked = Path.Combine(root, "locked");
+        var received = Path.Combine(locked, "received");
+        Directory.CreateDirectory(received);
+        var temp = Path.Combine(received, "sample.received.txt");
+        File.WriteAllText(temp, "the snapshot");
+        var target = Path.Combine(root, "target");
+        Directory.CreateDirectory(target);
+        var verified = Path.Combine(target, "sample.verified.txt");
+        File.SetUnixFileMode(locked, UnixFileMode.UserRead | UnixFileMode.UserExecute);
+
+        ViewerActions.Real.MoveFile(temp, verified);
+
+        await Assert.That(File.Exists(verified)).IsTrue();
+        // Still there, which is the point: it could not be removed and that did not matter
+        await Assert.That(Directory.Exists(received)).IsTrue();
+    }
+
+    [Test]
+    public async Task An_emptied_directory_is_removed()
+    {
+        var received = Path.Combine(root, "received");
+        Directory.CreateDirectory(received);
+        var temp = Path.Combine(received, "sample.received.txt");
+        File.WriteAllText(temp, "the snapshot");
+        var target = Path.Combine(root, "sample.verified.txt");
+
+        ViewerActions.Real.MoveFile(temp, target);
+
+        await Assert.That(File.Exists(target)).IsTrue();
+        await Assert.That(Directory.Exists(received)).IsFalse();
+    }
+
+    /// <summary>
+    /// And one still holding something is left alone.
+    /// </summary>
+    [Test]
+    public async Task A_directory_with_anything_left_in_it_stays()
+    {
+        var received = Path.Combine(root, "received");
+        Directory.CreateDirectory(received);
+        var temp = Path.Combine(received, "sample.received.txt");
+        File.WriteAllText(temp, "the snapshot");
+        File.WriteAllText(Path.Combine(received, "other.received.txt"), "another");
+        var target = Path.Combine(root, "sample.verified.txt");
+
+        ViewerActions.Real.MoveFile(temp, target);
+
+        await Assert.That(Directory.Exists(received)).IsTrue();
+    }
+
+    public MoveSweepTests()
+    {
+        root = Path.Combine(Path.GetTempPath(), $"MoveSweepTests_{Guid.NewGuid()}");
+        Directory.CreateDirectory(root);
+    }
+
+    public void Dispose()
+    {
+        // Whatever the test denied itself, given back, or the tree cannot be removed here either
+        if (!OperatingSystem.IsWindows())
+        {
+            foreach (var directory in Directory.EnumerateDirectories(root, "*", SearchOption.AllDirectories))
+            {
+                File.SetUnixFileMode(
+                    directory,
+                    UnixFileMode.UserRead | UnixFileMode.UserWrite | UnixFileMode.UserExecute);
+            }
+        }
+
+        Directory.Delete(root, true);
+    }
+
+    readonly string root;
+}

src/DiffEngineViewer/ViewerActions.cs

@@ -58,22 +58,39 @@ static void Missing(string temp, string target) =>
     static void Move(string temp, string target)
     {
         File.Move(temp, target, true);
+        Sweep(Path.GetDirectoryName(temp));
+    }
 
-        var directory = Path.GetDirectoryName(temp);
-        if (directory is null ||
-            Directory.EnumerateFileSystemEntries(directory).Any())
+    /// <summary>
+    /// The directory the received file sat in, if it is now empty.
+    /// <para>
+    /// Nothing in here can fail the move. It is already done, and the caller reads a throw as the
+    /// move having failed: the entry goes back on the queue, and the retry fails with file not
+    /// found on a temp file that is no longer there. Only <see cref="IOException" /> was covered,
+    /// so a directory whose parent will not have it removed - or whose permissions are not this
+    /// process's to change - reported a move that had succeeded as a failure.
+    /// </para>
+    /// </summary>
+    static void Sweep(string? directory)
+    {
+        if (directory is null)
         {
             return;
         }
 
         try
         {
+            if (Directory.EnumerateFileSystemEntries(directory).Any())
+            {
+                return;
+            }
+
             Directory.Delete(directory);
         }
-        catch (IOException)
+        catch (Exception exception)
+            when (exception is IOException or UnauthorizedAccessException)
         {
-            // Raced by something writing into it. The move itself succeeded, which is what the
-            // caller is reporting on.
+            // Raced by something writing into it, or not ours to remove.
         }
     }
 }