Re: GiST VACUUM

On 12/07/18 19:06, Andrey Borodin wrote:
>> 11 июля 2018 г., в 0:07, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
>> написал(а):
>>
>> This seems misplaced. This code deals with internal pages, and as
>> far as I can see, this patch never marks internal pages as deleted,
>> only leaf pages. However, we should have something like this in the
>> leaf-page branch, to deal with the case that an insertion lands on
>> a page that was concurrently deleted. Did you have any tests, where
>> an insertion runs concurrently with vacuum, that would exercise
>> this?
>
> That bug could manifest only in case of crash between removing
> downlinks and marking pages deleted.

Hmm. The downlink is removed first, so I don't think you can see that
situation after a crash. After a crash, you might have some empty,
orphaned, pages that have already been unlinked from the parent, but a
search/insert should never encounter them.

Actually, now that I think about it more, I'm not happy with leaving
orphaned pages like that behind. Let's WAL-log the removal of the
downlink, and marking the leaf pages as deleted, in one WAL record, to
avoid that.

But the situation in gistdoinsert(), where you encounter a deleted leaf
page, could happen during normal operation, if vacuum runs concurrently
with an insert. Insertion locks only one page at a time, as it descends
the tree, so after it has released the lock on the parent, but before it
has locked the child, vacuum might have deleted the page. In the latest
patch, you're checking for that just before swapping the shared lock for
an exclusive one, but I think that's wrong; you need to check for that
after swapping the lock, because otherwise vacuum might delete the page
while you're not holding the lock.

> I do not know how to test this
> reliably. Internal pages are locked before leafs and locks are
> coupled. No cuncurrent backend can see downlinks to pages being
> deleted, unless crash happens.

Are you sure? At a quick glance, I don't think the locks are coupled.

We do need some way of testing this..

- Heikki