Disclaimer: this should be considered as an engeneering, explanatory patch.
Rational: for undo work, we are now manipulating IDs from potentially many different 'memory realms' (in contrast to current undo system, where, since we always reallocate the whole main database at each undo step, we only ever have to deal with two, the old one from the memfile, and the new one from re-read data from that memfile).
So far, new undo system has used ID names-based search to remap old to new pointers, but this is slower, less safe, and forces us to do full undo barrier e.g. when renaming an ID.
Idea of this patch is instead to ensure that within a given editing session, we never ever allocate a data-block at the same address twice. That way, we can consider ID pointers as valid UIDs withing that scope, and use their values to handle remapping in udo steps even in case an ID has had several memory addresses in the history of the editing session.
This patches does two things:
- Add a way to ensure a newly allocated ID gets a memory address never used before for any ID.
- Store the history of all memory addresses ever used by a given ID.
Note that point 1 above is currently using a fairly simple and naive solution, but also quiet innefficient (just allocating again and again until we get a 'free' address), we can imagine a much more efficient solution if we go deeper in MEM_ allocator system itself. Not sure if it is worth it though, as with new undo system actual re-allocation of a same ID at a different address should be fairly rare (i.e. history of used/forbidden addresses should remain quiet short).