Bug#852514: libpetsc3.7-dev: leaves alternatives after purge: /usr/lib/petscdir/{3.7, 3.7-real}

[Drew Parsons]

> 
> 2m18.4s INFO: Warning: Package purging left files on system:
>   /etc/alternatives/petsc3.7 -> /usr/lib/petscdir/3.7.4/x86_64-linux-
gnu-real	 not owned
>   /etc/alternatives/petsc3.7-real -> /usr/lib/petscdir/3.7.4/x86_64-
linux-gnu-real	 not owned
>   /usr/lib/petscdir/	 owned by: libpetsc3.7.5-dev:amd64,
libpetsc3.7.4-dev:amd64
>   /usr/lib/petscdir/3.7 -> /etc/alternatives/petsc3.7	 not
owned
>   /usr/lib/petscdir/3.7-real -> /etc/alternatives/petsc3.7-real	
 not owned



The petsc3.7 and petsc3.7-real alternative is set up by the
libpetsc3.7-dev.  At each installation (or upgrade) of libpetsc3.7-dev, 
the alternative is updated to the latest.

What's happening with this bug is that  3.7.4+dfsg1-9 is installed
first, setting up the 3.7.4 alternative. It's then upgraded to
3.7.5+dfsg1-3, setting up the 3.7.5 alternative.  But the 3.7.4
alternative remains in place.

When libpetsc3.7-dev is removed, prerm removes the petsc3.7
alternative. But the prerm script only deals with it's own version. So
only the 3.7.5 alternative is removed.  The 3.7.4 alternative remains
in place.

Then libpetsc3.7.Z-dev is removed, taking the alternative targets with
it. So when libpetsc3.7.4-dev is removed, the petsc3.7 alternative for
3.7.4 remains (see previous paragraph), but the 3.7.4 directories that
it points to no longer exists.

Looks like either libpetsc3.7.Z-dev should handle the petsc3.7
alternative instead of libpetsc3.7-dev.  Or they should at least test
for it on removal and clear the hanging links.

Drew