The flatpak portal allows apps to expose files and folders from within
the sandbox to a side-sandbox using flatpak-spawn. So far it has used
the --filesystem option to mount those files and folders, but it takes a
path. Paths are inherently racy and they allow the app to swap out any
component of the path with a symlink after handing it off. If they win
the race, flatpak will mount a completely different directory.
This adds a new way to mount files and directories based on O_PATH
file descriptor that needs to provided when execing the flatpak binary.
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