[izakusilov]

What does the pull request do?

Fixes concurrent IStorageProvider calls (TryGetFolderFromPathAsync, TryGetFileFromPathAsync, etc.) failing on Linux when the first storage provider in the fallback chain is unavailable, e.g. in a Dev Container without an XDG desktop portal.

What is the current behavior?

On X11 and Wayland, IStorageProvider is a FallbackStorageProvider that lazily tries a list of provider factories (DBus portal, GTK, managed). It tracks the next factory to try in _nextProviderFactory, but only increments it after the awaited factory completes.

When several calls run concurrently, they all await the same factory. If that factory returns null asynchronously (the DBus portal does when no portal is running), each caller increments the index when it resumes. The first caller moves on to the next factory, while the others find the index already past the end of the list and return null. As a result, one call succeeds and the rest fail:

await Task.WhenAll(
    storageProvider.TryGetFolderFromPathAsync(folderPath1),
    storageProvider.TryGetFolderFromPathAsync(folderPath2));

When the first factory does return a provider, concurrent calls add it to the list more than once and skip the factories after it.

What is the updated/expected behavior with this PR?

All concurrent calls see the same providers and succeed.

To test: on Linux without an XDG desktop portal (e.g. the Dev Container from the issue), call TryGetFolderFromPathAsync several times in parallel. All calls return the folder.

How was the solution implemented (if it's not obvious)?

• FallbackStorageProvider now caches the task returned by each factory, so each factory is only invoked once and concurrent callers await the same task instead of advancing a shared index.
• Access to the cache is synchronized, since callers can resume on different threads (e.g. when called from a background thread).
• If a factory fails, its cached task is discarded so that the next call retries it, as before.
• Reset replaces the factories and the cache together; an enumeration in progress keeps using the arrays it started with.

Added FallbackStorageProviderTests; the two concurrency tests fail without the fix. Also verified with an X11 app on Xvfb with a DBus session but no desktop portal: without the fix one of three parallel TryGetFolderFromPathAsync calls fails, with the fix all three succeed.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #22559

[cla-avalonia]

• All contributors have signed the CLA.

[izakusilov]

@cla-avalonia agree

[github-actions]

NuGet packages

The packages for this pull request have been published (workflow run):

• Version: 12.2.999-cibuild0100716-alpha
• Feed URL: https://nuget-feed-all.avaloniaui.net/v3/index.json