fix: parallelize and bound OS shutdown disk saves

src/ManagedDrive.App/App.xaml.cs

@@ -11,6 +11,11 @@ public partial class App
 {
     private const string SingleInstanceMutexName = "Global\\ManagedDrive-4A7C2E1B-9F3D-4B8A-A1C5-3E6D2F0B8C9A";
 
+    /// <summary>
+    /// Upper bound on how long <see cref="OnSessionEnding"/> waits for all disk saves to finish.
+    /// </summary>
+    private static readonly TimeSpan SessionEndingSaveTimeout = TimeSpan.FromSeconds(10);
+
     private readonly DispatcherTimer _timerPollCursor = new();
     private readonly DispatcherTimer _timerShowTrayInfoPopup = new();
     private readonly DispatcherTimer _timerTooltipCooldown = new();
@@ -406,9 +411,14 @@ private void OnDiskSaveFailed(object? sender, Exception ex)
     /// <summary>
     /// Fires when Windows is logging off, shutting down, or restarting. WPF's own
     /// <c>Exit</c> event does not fire in this case, and the OS may kill the process shortly
-    /// after this callback returns, so save every mounted disk's image synchronously and
-    /// without unmounting (unmounting is unnecessary here and would risk exceeding the
-    /// shutdown time budget).
+    /// after this callback returns, so save every mounted disk's image and without unmounting
+    /// (unmounting is unnecessary here and would risk exceeding the shutdown time budget).
+    /// Disks are saved in parallel rather than sequentially — each disk's save is independent
+    /// file I/O guarded by its own <c>RamDisk</c> lock, so running them concurrently lets the
+    /// total time fit the shutdown budget even with several large/compressed/encrypted disks.
+    /// This method blocks synchronously until all saves finish or <see cref="SessionEndingSaveTimeout"/>
+    /// elapses, whichever comes first — a bound is needed so a single stuck save (e.g. a backing
+    /// path that has become unreachable) can't hang this callback indefinitely.
     /// </summary>
     private void OnSessionEnding(object sender, SessionEndingEventArgs e)
     {
@@ -417,17 +427,21 @@ private void OnSessionEnding(object sender, SessionEndingEventArgs e)
             return;
         }
 
-        foreach (var disk in _mountManager.GetAll())
-        {
-            try
+        var saveTasks = _mountManager.GetAll()
+            .Select(disk => Task.Run(() =>
             {
-                disk.SaveToImageSafe();
-            }
-            catch
-            {
-                // Best-effort, matches RamDisk.Dispose()/TryAutoSave() swallow pattern.
-            }
-        }
+                try
+                {
+                    disk.SaveToImageSafe();
+                }
+                catch
+                {
+                    // Best-effort, matches RamDisk.Dispose()/TryAutoSave() swallow pattern.
+                }
+            }))
+            .ToArray();
+
+        Task.WaitAll(saveTasks, SessionEndingSaveTimeout);
     }
 
     private void PositionTrayPopup()

src/ManagedDrive.Core/RamDisk.cs

@@ -275,7 +275,7 @@ public void Dispose()
                 {
                     try
                     {
-                        if (_fs.IsDirty || Options.PersistImagePath != _lastSavedImagePath)
+                        if (NeedsSave())
                         {
                             SaveToImage();
                         }
@@ -370,13 +370,19 @@ public void SaveToImage()
     /// Saves the disk image while holding <see cref="_autoSaveLock"/>, without unmounting.
     /// Intended for external shutdown-notification callers (e.g. Windows session-ending) that
     /// need a quick, safe save that can't race the periodic auto-save tick. Does nothing if
-    /// <see cref="DiskOptions.PersistImagePath"/> is <c>null</c>.
+    /// <see cref="DiskOptions.PersistImagePath"/> is <c>null</c>, or if the disk's content hasn't
+    /// changed since the last successful save and the configured image path hasn't changed
+    /// either (same skip condition as <see cref="TryAutoSave"/>) — this keeps unmodified disks
+    /// out of the OS shutdown time budget.
     /// </summary>
     public void SaveToImageSafe()
     {
         lock (_autoSaveLock)
         {
-            SaveToImage();
+            if (NeedsSave())
+            {
+                SaveToImage();
+            }
         }
     }
 
@@ -599,6 +605,14 @@ private void ConfigureAutoSaveTimer()
         }
     }
 
+    /// <summary>
+    /// Whether a save would actually write anything: the disk has unsaved changes, or the
+    /// configured persist path has changed since the last successful save. Shared by
+    /// <see cref="SaveToImageSafe"/> and <see cref="TryAutoSave"/> so both skip saving under the
+    /// same condition.
+    /// </summary>
+    private bool NeedsSave() => _fs.IsDirty || Options.PersistImagePath != _lastSavedImagePath;
+
     /// <summary>
     /// Saves the disk image on the periodic timer tick, swallowing any exception so a failed
     /// save does not affect the mounted disk or crash the timer thread. If the previous tick's
@@ -615,7 +629,7 @@ private void TryAutoSave()
 
         try
         {
-            if (_fs.IsDirty || Options.PersistImagePath != _lastSavedImagePath)
+            if (NeedsSave())
             {
                 SaveToImage();
                 TryWriteSnapshot();