r21577 - gnucash/trunk/src/backend/xml - Fix xml backend to load and save invoice kvp values.

[Christian Stimming]

Am Freitag, 18. November 2011, 22:59:05 schrieb Geert Janssens:
> > It
> > means you cannot use invoice KVP as long as the XML file claims it can
> > be
> > read by 2.4.x without loosing information. If this is true but we still
> > want to add a new data field to invoices, we would better change the XML
> > structure directly so that an older gnucash will refuse to load the file
> > altogether. That's still better than silently loosing data upon loading
> > in 2.4.x...
> 
> Yes that's probably true. Quite unfortunate though...
> 
> Would it be a reasonable compromise to implement the invoice kvp parts in
> 2.4 xml backend instead ?

No, that's not sufficient. If there isn't any release out there that contains 
code that is able to read it, you should implement the xml part so that all 
existing releases will refuse to load this file, if these data parts are being 
used. Hence, in this particular situation your new data field should be a 
normal XML element, and you need to add the xml reading code to the 2.4 branch 
right now. Then, an upcoming 2.4.9 will contain this and correctly read and 
write this field.

However, any earlier version (2.4.x or 2.2. or earlier) will refuse to load a 
file that contains this data field. In this particular case, we can't do 
anythin better because if it were a xml kvp, the earlier versions read the 
file but suffer silent data loss (due to the erroneously missing invoice kvp 
parser).

> I know that would only move the problem to 2.2.x,
> but I don't think we support backward compatibility over past two major
> releases.

Right. Backward compatibility in the sense that old versions should be able to 
read newer files needs to be maintained only over the last major release.

> The kvp I'm introducing is only relevant if you really do use
> credit notes. If you stick to invoices only, it doesn't matter if the kvp
> is present or not, or even if it's lost in a save by an older version of
> gnucash.

That's right, but nevertheless the old existing releases still don't know 
about this data field. They must be forced to refuse those files that contain 
this field.

> So:
> - if no credit note is in the data file:
>   2.2 can work on the file, but will lose any invoice kvp potentially
> created by 2.6

Yes, and this data loss we cannot allow. So: 2.2 must refuse to load it if the 
credit note field is in there.

>   2.4 can work on the file, and will keep any invoice kvp already present,
> but won't create new ones.

You must distinguish here between <= 2.4.8 and >= 2.4.9. The former is in the 
same situation as 2.2. The latter is fine with the newly introduced data 
field.

Regards,

Christian