lvconvert: provide useful error when snapshot-merge target missing

On Wed, Oct 13 2010 at  3:06pm -0400,
Milan Broz <[email protected]> wrote:

> On 10/13/2010 08:53 PM, Mike Snitzer wrote:
> > Convey need for snapshot-merge target in lvconvert error message and man
> > page.
> 
> > --- a/lib/metadata/segtype.h
> > +++ b/lib/metadata/segtype.h
> > @@ -66,6 +66,7 @@ struct segment_type {
> >  
> >  struct segtype_handler {
> >  	const char *(*name) (const struct lv_segment * seg);
> > +	const char *(*target_name) (const struct lv_segment * seg);
> >  	void (*display) (const struct lv_segment * seg);
> >  	int (*text_export) (const struct lv_segment * seg,
> >  			    struct formatter * f);
> 
> I think this will break shared (external) segment module ABI.
> Nobody uses that but worth to mention it.
> (See e.g. lib/mirror and --with-mirrors=shared)

OK, I'm not familiar with that ABI concern.  Any ideas on how I avoid
this breakage?  Add function pointer to end of struct?  Or any change
to struct will be taken as breakage?

Mike