Best way to overlay a filesystem for a Wine app?

[Francois Gouget]

On Fri, 27 Jan 2012, Scott Ritchie wrote:
[...]
> To me, this sounds an awful lot like an overlay file system.  Would
> unionfs-fuse be the correct solution here?  The .desktop file that sets
> the Wine prefix and also launches the app could mount the FUSE
> filesystem, and the user-space Wine prefix's files could take priority
> over those in /opt.  Stuff like user-modding and update/repair systems
> could work properly in that context as well.

The main issue with copy-on-write overlay filesystems is that they 
cannot deal with registry files for two reasons:
 * First, once a file has been written to and thus copied to the 
   user's wineprefix, it will never be updated when you upgrade the 
   application's package. This means after an upgrade the user's 
   system.reg and user.reg files (the latter containing the dll 
   overrides) will be unchanged which may cause the application to be 
   broken.
 * The second issue is that the registry really contains both 
   application data and user data. The former should be upgraded while 
   the latter should be preserved. So neither a copying the new registry 
   files nor preserving the old registry files policies are appropriate 
   (and these policies are the only two available to overlay 
   filesystems).

Besides that there are other issues, most of which Dan already 
mentioned:
 * As far as I know most overlay filesystems don't deal well with 
   changes in the underlying filesystem. This pretty much breaks the 
   upgrade scenario.
 * They are not available on all Linux distributions (not to talk about 
   Mac OS X or FreeBSD). But I guess this is fine for a 
   latest-Ubuntu-only approach (like for menus and the trash!).

Another issue is applications that require a license key to be entered 
during installation. This causes trouble for both the overlay and 
symlink approaches since both have the packager install the application 
and then ship the installed image... along with its single license key. 
So for these applications a way to enter the license key on first run 
has to be found.

-- 
Francois Gouget <fgouget at free.fr>              http://fgouget.free.fr/
           Cahn's Axiom: When all else fails, read the instructions.