Re: TRAP: FailedAssertion("prev_first_lsn < cur_txn->first_lsn", File: "reorderbuffer.c", Line: 927, PID: 568639)

On Wed, Jun 7, 2023 at 6:02 PM Tomas Vondra
<tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
>
> Well, I think the issue is pretty clear - we end up with an initial
> snapshot that's in between the ASSIGNMENT and NEW_CID, and because
> NEW_CID has both xact and subxact XID it fails because we add two TXNs
> with the same LSN, not realizing one of them is subxact.
>
> That's obviously wrong, although somewhat benign in production because
> it only fails because of hitting an assert.
>

Doesn't this indicate that we can end up decoding a partial
transaction when we restore a snapshot? Won't that be a problem even
for production?

> Regular builds are likely to
> just ignore it, although I haven't checked if the COMMIT cleanup (I
> wonder if we remove the subxact from the toplevel list on commit).
>
> I think the problem is we just grab an existing snapshot, before all
> running xacts complete. Maybe we should fix that, and leave the
> needs_full_snapshot alone.
>

It is not clear what exactly you have in mind to fix this because if
there is no running xact, we don't even need to restore the snapshot
because of a prior check "if (running->oldestRunningXid ==
running->nextXid)". I think the main problem is that we started
decoding immediately from the point where we restored a snapshot as at
that point we could have some partial running xacts.

--
With Regards,
Amit Kapila.