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Re: 7198e7c71fc: MDEV-15458 Segfault in heap_scan() upon UPDATE after ADD SYSTEM VERSIONING



On Tue, Dec 25, 2018 at 1:27 PM Sergei Golubchik <serg@xxxxxxxxxxx> wrote:
> Hi, Aleksey!
> On Dec 23, Aleksey Midenkov wrote:
> > revision-id: 7198e7c71fc (versioning-1.0.6-82-g7198e7c71fc)
> > parent(s): 6be155757b7
> > author: Aleksey Midenkov <midenok@xxxxxxxxx>
> > committer: Aleksey Midenkov <midenok@xxxxxxxxx>
> > timestamp: 2018-12-20 13:25:00 +0300
> > message:
> >
> > MDEV-15458 Segfault in heap_scan() upon UPDATE after ADD SYSTEM VERSIONING
> >
> > * Versioning tests support
> As far as I can see, there are two approaches to fixing this bug.
> And if we go the way of HA_EXTRA_REMEMBER_POS/HA_EXTRA_RESTORE_POS then
> your fix is good.
> But I'd still like to understand whether write_row is supposed to
> disturb the scan. Why should heap_write() overwrite
> info->current_ptr=pos and info->current_hash_ptr? Inserting a row
> doesn't happen at the "current position", so I don't quite understand
> why it modifies current_ptr at all. There is no functionality like
> "continue the scan from the last inserted row", right? It just doesn't
> make sense to me.

I don't know why it works like this, maybe to keep consistency at API
level: we insert record and then we able to access it without search.
HEAP storage was copypasted from MyISAM where `lastpos` analogy also
is updated at mi_write(). Without updating `current_ptr` and `lastpos`
tests don't fail (main, funcs_1, heap). But I can't say whether the
performance is affected and how it will go in production.

> Regards,
> Sergei
> Chief Architect MariaDB
> and security@xxxxxxxxxxx

All the best,

Aleksey Midenkov

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