[Bug]: Can not access file from protal filechooser if XDG_RUNTIME_DIR is in /tmp

[cedk]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a bug that matches the one I want to file, without success.
• If this is an issue with a particular app, I have tried filing it in the appropriate issue tracker for the app (e.g. under https://github.com/flathub/) and determined that it is an issue with Flatpak itself.
• This issue is not a report of a security vulnerability (see here if you need to report a security issue).

Flatpak version

1.14.4

What Linux distribution are you using?

Gentoo Linux

Linux distribution version

6.1.57-gentoo-dist

What architecture are you using?

x86_64

How to reproduce

1. Set XDG_RUNTIME_DIR to /tmp/${USER}/runtime
2. Run a flatpack application with file chooser
3. Read the selected file

Expected Behavior

The file can be read.

Actual Behavior

The application does not find the file.

Additional Information

Changing the XDG_RUNTIME_DIR to /tmp/user/${UID} works.

[algor512]

It seems I'm stuck with the same problem. My host's XDG_RUNTIME_DIR is set to /tmp/1000-runtime-dir.gcO/, whereas inside the sandbox, it is /run/user/1000/. When attempting to save a file, such as test.txt, it appears in /tmp/1000-runtime-dir.gcO/doc/c25fa616/ on both my host system and inside the sandbox. However, the folder /run/user/1000/doc/c25fa616/ inside the sandbox remains empty, even though it exists.

It seems that the file chooser placed a file at $HOST_XDG_RUNTIME_DIR/doc/(some hash), while an
application inside the sandbox (in my case, Firefox) expects to find it at
$SANDBOX_XDG_RUNTIME_DIR/doc/(some hash).

I hope it will be helpful.

UPD. I use Flatpak 1.15.6 on Void Linux (glibc version).