I use Windows and had weird issue with Radiant CMS, which uses rack-cache gem.
The following code causes the issue:
@root = File.expand_path(root)
FileUtils.mkdir_p(root, :mode => 0755)
You see, FileUtils.mkdir_p( '/tmp/foo/bar' ) creates C:/tmp/foo/bar on Windows (If my current drive is C).
Of course that’s not possible on hosting environments. There are no permissions.
Generally on shared-hosting environment you have access to application folder only. It would be nice to change that behaviour so that folders are created inside Rails app root.
Please let me know if you need any help. I could submit a patch.