On 12/07/18 19:06, Andrey Borodin wrote:
11 июля 2018 г., в 0:07, Heikki Linnakangas <hlinn...@iki.fi>

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

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

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