Re: BUG #17255: Server crashes in index_delete_sort_cmp() due to race condition with vacuum

Hi,

On 2021-11-10 14:18:01 -0800, Peter Geoghegan wrote:
> On Wed, Nov 10, 2021 at 11:20 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > I hit a crash once in 13 with a slightly evolved version of the test (many
> > connections creating / dropping the partitions as in the original scenario,
> > using :client_id to target different tables). It's possible that my
> > instrumentation was the cause of that. Unfortunately it took quite a few hours
> > to hit the problem in 13...
>
> Have you thought about the case where a transaction does a HOT update
> of the same row twice, and then aborts?

Yes. I don't think it's problematic right now, because the redirect would, I
think, in all cases have to point to the chain element before those tuples,
because the preceding value would just have to be DELETE_IN_PROGRESS, which we
we don't follow in heap_prune_chain().

> I'm asking because I notice that the fragile "We need this primarily
> to handle aborted HOT updates" precheck for
> HeapTupleHeaderIsHeapOnly() doesn't just check if the heap-only tuple
> is DEAD before deciding to mark it LP_UNUSED. It also checks
> HeapTupleHeaderIsHotUpdated() against the target tuple -- that's
> another condition of the tuple being marked unused. Of course, whether
> or not a given tuple is considered HeapTupleHeaderIsHotUpdated() can
> change from true to false when an updater concurrently aborts. Could
> that have race conditions?

I wondered about that too, but I couldn't *quite* come up with a problematic
scenario, because I don't think any of the cases that can change
HeapTupleHeaderIsHotUpdated() would have allowed to set the redirect to a
subsequent chain element.

> In other words: what if the aforementioned "aborted HOT updates"
> precheck code doesn't deal with a DEAD tuple, imagining that it's not
> a relevant tuple, while at the same time the later HOT-chain-chasing
> code *also* doesn't get to the tuple? What if they each assume that
> the other will/has taken care of it, due to a race?

Then we'd just end up not pruning the tuple, I think. Which should be fine, as
it could only happen for fairly new tuples.

Greetings,

Andres Freund