Potential improvements to mount-flatpak

[Demi Marie Obenour <demiobenour@gmail.com>]

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I looked at the mount-flatpak codebase and noticed some potential
improvements for the future:

- The code uses std::fs::write to write to files in the target
  directory.  This is safe because the VM isn't running, but it might
  be better to use libpathrs.

- The code doesn't validate the user-provided app ID, resulting in
  worse error messages than otherwise possible.  App IDs are always
  D-Bus well-known names, so there is no risk of the rules changing
  in future versions of Flatpak.

- Passing something that isn't a flatpak repository as a parameter
  will result in a confusing "no such file or directory" error.

- Various checks for corrupt repositories could be added, such as:
  - Wrong number of slashes in "current" symlink.
  - Wrong number of slashes in runtime path.
  - Bad "active" symlink.

- The code that resolves the "active" symlink is duplicated.

- The target mount point is not made "noexec".

- mount_setattr() is called on a directory FD.  I suspect this
  changes parameters of the source mount.  It might be better to use
  open_tree_attr() with OPEN_TREE_CLONE instead.

Which, if any, of these ideas would make sense to include?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)


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