Stop "Purge verified files" from killing the tray

src/DiffEngineTray.Tests/FilePurgerTest.cs

@@ -1,5 +1,39 @@
 public class FilePurgerTest
 {
+    /// <summary>
+    /// The dialog lets the user pick a profile folder or a drive root, and both hold something
+    /// the scan cannot read. SearchOption.AllDirectories threw on the first one, out of a bare
+    /// Thread with nothing catching it, which took the whole tray down.
+    /// </summary>
+    [Test]
+    public async Task Find_SkipsInaccessibleDirectories()
+    {
+        var root = Directory.CreateDirectory(
+            Path.Combine(Path.GetTempPath(), $"FilePurgerTest_{Guid.NewGuid()}"));
+        var denied = root.CreateSubdirectory("denied");
+        await File.WriteAllTextAsync(Path.Combine(root.FullName, "a.verified.txt"), "content");
+
+        var security = denied.GetAccessControl();
+        var rule = new FileSystemAccessRule(
+            WindowsIdentity.GetCurrent().User!,
+            FileSystemRights.ListDirectory,
+            AccessControlType.Deny);
+        security.AddAccessRule(rule);
+        denied.SetAccessControl(security);
+        try
+        {
+            var found = FilePurger.Find(root.FullName);
+
+            await Assert.That(found).HasSingleItem();
+            await Assert.That(Path.GetFileName(found[0])).IsEqualTo("a.verified.txt");
+        }
+        finally
+        {
+            security.RemoveAccessRule(rule);
+            denied.SetAccessControl(security);
+            root.Delete(true);
+        }
+    }
     [Test]
     public async Task DeleteSucceeds_WhenFileNotLocked()
     {

src/DiffEngineTray.Tests/GlobalUsings.cs

@@ -1,6 +1,8 @@
 global using System.Diagnostics;
 global using System.Net;
 global using System.Net.Sockets;
+global using System.Security.AccessControl;
+global using System.Security.Principal;
 global using EmptyFiles;
 
 [assembly: ParallelLimiter<SingleThreadedLimit>]

src/DiffEngineTray/FilePurger.cs

@@ -2,11 +2,28 @@ static class FilePurger
 {
     public static void Launch()
     {
-        var thread = new Thread(Inner);
+        var thread = new Thread(Run);
         thread.SetApartmentState(ApartmentState.STA);
         thread.Start();
     }
 
+    /// <summary>
+    /// Nothing above this catches. An unhandled exception on a bare Thread takes the process down,
+    /// and the process is the tray - so a purge that threw took every pending move, delete and, if
+    /// this tray owns the queue, every pending inline snapshot with it.
+    /// </summary>
+    static void Run()
+    {
+        try
+        {
+            Inner();
+        }
+        catch (Exception exception)
+        {
+            ExceptionHandler.Handle("Failed to purge verified files.", exception);
+        }
+    }
+
     static void Inner()
     {
         using var dialog = new FolderBrowserDialog();
@@ -19,10 +36,7 @@ static void Inner()
             return;
         }
 
-        var verifiedFiles = Directory.GetFiles(path, "*.verified.*", SearchOption.AllDirectories);
-        var receivedFiles = Directory.GetFiles(path, "*.received.*", SearchOption.AllDirectories);
-
-        var files = verifiedFiles.Concat(receivedFiles).ToArray();
+        var files = Find(path);
 
         if (files.Length == 0)
         {
@@ -36,6 +50,25 @@ static void Inner()
         }
     }
 
+    /// <summary>
+    /// SearchOption.AllDirectories is the compatibility enumeration: it does not ignore what it
+    /// cannot read, and it follows reparse points. The dialog lets the user pick a profile folder
+    /// or a drive root, either of which holds a deny-ACL junction - Application Data,
+    /// $Recycle.Bin - so the scan reliably threw UnauthorizedAccessException before it had looked
+    /// at a single file.
+    /// </summary>
+    static readonly EnumerationOptions enumeration = new()
+    {
+        RecurseSubdirectories = true,
+        IgnoreInaccessible = true,
+        AttributesToSkip = FileAttributes.ReparsePoint
+    };
+
+    internal static string[] Find(string path) =>
+        Directory.GetFiles(path, "*.verified.*", enumeration)
+            .Concat(Directory.GetFiles(path, "*.received.*", enumeration))
+            .ToArray();
+
     static bool Confirm(string[] files)
     {
         var result = AskQuestion(