Horton

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

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Alexander Lakhin <exclusion@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-06-13T04:04:00Z
Lists: pgsql-hackers 
On Wed, Jun 7, 2023 at 6:02 PM Tomas Vondra
<tomas.vondra@enterprisedb.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.

Commits

  1. Fix executing invalidation messages generated by subtransactions during decoding.

  2. Fix assertion failures while processing NEW_CID record in logical decoding.

  3. Immediately WAL-log subtransaction and top-level XID association.

  4. Stop demanding that top xact must be seen before subxact in decoding.