If XDG_RUNTIME_DIR is under app control, as it will be with #4120, we
don't want to be mounting pieces of filesystem directly into it, because
that will mean that the app could create a symlink that will cause us
to create a mount point for it at the target of the symlink, potentially
elsewhere in the host filesystem.
Instead, we mount them in /run/flatpak, which is a per-instance
directory entirely controlled by Flatpak; and then create (relative)
symlinks in XDG_RUNTIME_DIR, pointing into /run/flatpak.
In this commit, we still know that the XDG_RUNTIME_DIR is a
per-instance tmpfs, so we can safely create the symlinks using
the --symlink option. In a subsequent commit this will change to
creating them in a shared XDG_RUNTIME_DIR, if any.
Signed-off-by: Simon McVittie <smcv@collabora.com>
40510e8ae8