Gracefully fall back to th next storage provider (#15929)

src/Avalonia.Base/Platform/Storage/FallbackStorageProvider.cs

@@ -0,0 +1,95 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+
+namespace Avalonia.Platform.Storage;
+#pragma warning disable CA1823
+
+internal class FallbackStorageProvider : IStorageProvider
+{
+    private readonly Func<Task<IStorageProvider?>>[] _factories;
+    private readonly List<IStorageProvider> _providers = new();
+    private int _nextProviderFactory = 0;
+
+    public FallbackStorageProvider(Func<Task<IStorageProvider?>>[] factories)
+    {
+        _factories = factories;
+    }
+
+    async IAsyncEnumerable<IStorageProvider> GetProviders()
+    {
+        foreach (var p in _providers)
+            yield return p;
+        for (;_nextProviderFactory < _factories.Length;)
+        {
+            var p = await _factories[_nextProviderFactory]();
+            _nextProviderFactory++;
+            if (p != null)
+            {
+                _providers.Add(p);
+                yield return p;
+            }
+        }
+    }
+
+    async Task<IStorageProvider> GetFor(Func<IStorageProvider, bool> filter)
+    {
+        await foreach (var p in GetProviders())
+            if (filter(p))
+                return p;
+        throw new IOException("Unable to select a suitable storage provider");
+    }
+
+
+    // Those should _really_ have been asynchronous,
+    // but this class is expected to fall back to the managed implementation anyway
+    public bool CanOpen => true;
+    public bool CanSave => true;
+    public bool CanPickFolder => true;
+
+    public async Task<IReadOnlyList<IStorageFile>> OpenFilePickerAsync(FilePickerOpenOptions options)
+    {
+        return await (await GetFor(p => p.CanOpen)).OpenFilePickerAsync(options);
+    }
+
+    public async Task<IStorageFile?> SaveFilePickerAsync(FilePickerSaveOptions options)
+    {
+        return await (await GetFor(p => p.CanSave)).SaveFilePickerAsync(options);
+    }
+
+
+    public async Task<IReadOnlyList<IStorageFolder>> OpenFolderPickerAsync(FolderPickerOpenOptions options)
+    {
+        return await (await GetFor(p => p.CanPickFolder)).OpenFolderPickerAsync(options);
+    }
+
+    async Task<TResult?> FirstNotNull<TArg, TResult>(TArg arg, Func<IStorageProvider, TArg, Task<TResult?>> cb)
+        where TResult : class
+    {
+        await foreach (var p in GetProviders())
+        {
+            var res = await cb(p, arg);
+            if (res != null)
+                return res;
+        }
+
+        return null;
+    }
+
+    public Task<IStorageBookmarkFile?> OpenFileBookmarkAsync(string bookmark) => 
+        FirstNotNull(bookmark, (p, a) => p.OpenFileBookmarkAsync(a));
+
+    public Task<IStorageBookmarkFolder?> OpenFolderBookmarkAsync(string bookmark) =>
+        FirstNotNull(bookmark, (p, a) => p.OpenFolderBookmarkAsync(a));
+
+    public Task<IStorageFile?> TryGetFileFromPathAsync(Uri filePath) =>
+        FirstNotNull(filePath, (p, a) => p.TryGetFileFromPathAsync(filePath));
+
+    public Task<IStorageFolder?> TryGetFolderFromPathAsync(Uri folderPath)
+        => FirstNotNull(folderPath, (p, a) => p.TryGetFolderFromPathAsync(a));
+
+    public Task<IStorageFolder?> TryGetWellKnownFolderAsync(WellKnownFolder wellKnownFolder) =>
+        FirstNotNull(wellKnownFolder, (p, a) => p.TryGetWellKnownFolderAsync(a));
+
+}

src/Avalonia.Dialogs/Avalonia.Dialogs.csproj

@@ -16,7 +16,8 @@
 
   <ItemGroup>
     <!-- For managed dialogs dev testing -->
-    <InternalsVisibleTo Include="ControlCatalog, PublicKey=$(AvaloniaPublicKey)" />    
+    <InternalsVisibleTo Include="ControlCatalog, PublicKey=$(AvaloniaPublicKey)" />
+    <InternalsVisibleTo Include="Avalonia.X11, PublicKey=$(AvaloniaPublicKey)" />
   </ItemGroup>
 
   <Import Project="..\..\build\DevAnalyzers.props" />

src/Avalonia.X11/X11Window.cs

@@ -24,6 +24,7 @@
 using static Avalonia.X11.XLib;
 using Avalonia.Input.Platform;
 using System.Runtime.InteropServices;
+using Avalonia.Dialogs;
 using Avalonia.Platform.Storage.FileIO;
 
 // ReSharper disable IdentifierTypo
@@ -237,10 +238,15 @@ public X11Window(AvaloniaX11Platform platform, IWindowImpl? popupParent, bool ov
                     _x11.Atoms.XA_CARDINAL, 32, PropertyMode.Replace, ref _xSyncCounter, 1);
             }
 
-            _storageProvider = new CompositeStorageProvider(new[]
+            _storageProvider = new FallbackStorageProvider(new[]
             {
-                () => _platform.Options.UseDBusFilePicker ? DBusSystemDialog.TryCreateAsync(Handle) : Task.FromResult<IStorageProvider?>(null),
-                () => GtkSystemDialog.TryCreate(this)
+                () => _platform.Options.UseDBusFilePicker
+                    ? DBusSystemDialog.TryCreateAsync(Handle)
+                    : Task.FromResult<IStorageProvider?>(null),
+                () => GtkSystemDialog.TryCreate(this),
+                () => Task.FromResult(InputRoot is TopLevel tl
+                    ? (IStorageProvider?)new ManagedStorageProvider(tl)
+                    : null)
             });
 
             platform.X11Screens.Changed += OnScreensChanged;