Verify plugin replacement cancellation commit point

src/ui/Logic/Plugins/PluginDownloadService.cs

@@ -74,27 +74,10 @@ await Task.Run(() =>
                     targetName = SanitizeFolderName(entry.Name);
                 }
 
-                // Remove any previously installed copy of the same plugin.
                 var existing = _pluginCatalog.GetPlugins()
                     .FirstOrDefault(p => p.Manifest.Name.Equals(entry.Name, StringComparison.OrdinalIgnoreCase));
-                if (existing != null && Directory.Exists(existing.FolderPath))
-                {
-                    Directory.Delete(existing.FolderPath, recursive: true);
-                }
-
-                cancellationToken.ThrowIfCancellationRequested();
-
                 var targetPath = Path.Combine(Se.PluginsFolder, targetName);
-                if (Directory.Exists(targetPath))
-                {
-                    Directory.Delete(targetPath, recursive: true);
-                }
-
-                // Last chance to abort before the move that publishes the new
-                // plugin. Cancelling between the deletes above and the move
-                // would leave the user with no plugin at all.
-                cancellationToken.ThrowIfCancellationRequested();
-                Directory.Move(source, targetPath);
+                PublishPlugin(source, targetPath, existing?.FolderPath, cancellationToken);
             }, cancellationToken);
         }
         finally
@@ -113,6 +96,30 @@ await Task.Run(() =>
         }
     }
 
+    internal static void PublishPlugin(
+        string source,
+        string targetPath,
+        string? existingPluginPath,
+        CancellationToken cancellationToken)
+    {
+        // This is the commit point for the replacement. Honour cancellation before any
+        // destructive operation; once an installed copy is removed, finish publishing the
+        // already-downloaded replacement instead of leaving the user with no plugin at all.
+        cancellationToken.ThrowIfCancellationRequested();
+
+        if (!string.IsNullOrEmpty(existingPluginPath) && Directory.Exists(existingPluginPath))
+        {
+            Directory.Delete(existingPluginPath, recursive: true);
+        }
+
+        if (Directory.Exists(targetPath))
+        {
+            Directory.Delete(targetPath, recursive: true);
+        }
+
+        Directory.Move(source, targetPath);
+    }
+
     private static string SanitizeFolderName(string name)
     {
         var invalid = Path.GetInvalidFileNameChars();

tests/UI/Logic/Plugins/PluginDownloadServiceTests.cs

@@ -0,0 +1,60 @@
+using Nikse.SubtitleEdit.Logic.Plugins;
+using Xunit;
+
+namespace UITests.Logic.Plugins;
+
+public class PluginDownloadServiceTests : IDisposable
+{
+    private readonly string _tempRoot;
+
+    public PluginDownloadServiceTests()
+    {
+        _tempRoot = Path.Combine(Path.GetTempPath(), "PluginPublish_" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(_tempRoot);
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_tempRoot))
+        {
+            Directory.Delete(_tempRoot, recursive: true);
+        }
+    }
+
+    [Fact]
+    public void PublishPlugin_CancelledBeforeCommitPoint_PreservesInstalledPlugin()
+    {
+        var source = Path.Combine(_tempRoot, "new-plugin");
+        var target = Path.Combine(_tempRoot, "installed-plugin");
+        Directory.CreateDirectory(source);
+        Directory.CreateDirectory(target);
+        File.WriteAllText(Path.Combine(source, "new.txt"), "new");
+        File.WriteAllText(Path.Combine(target, "old.txt"), "old");
+
+        using var cts = new CancellationTokenSource();
+        cts.Cancel();
+
+        Assert.Throws<OperationCanceledException>(() =>
+            PluginDownloadService.PublishPlugin(source, target, target, cts.Token));
+
+        Assert.True(File.Exists(Path.Combine(target, "old.txt")));
+        Assert.True(File.Exists(Path.Combine(source, "new.txt")));
+    }
+
+    [Fact]
+    public void PublishPlugin_NotCancelled_ReplacesInstalledPlugin()
+    {
+        var source = Path.Combine(_tempRoot, "new-plugin");
+        var target = Path.Combine(_tempRoot, "installed-plugin");
+        Directory.CreateDirectory(source);
+        Directory.CreateDirectory(target);
+        File.WriteAllText(Path.Combine(source, "new.txt"), "new");
+        File.WriteAllText(Path.Combine(target, "old.txt"), "old");
+
+        PluginDownloadService.PublishPlugin(source, target, target, CancellationToken.None);
+
+        Assert.False(Directory.Exists(source));
+        Assert.True(File.Exists(Path.Combine(target, "new.txt")));
+        Assert.False(File.Exists(Path.Combine(target, "old.txt")));
+    }
+}