Re: Race conditions in logical decoding

On 2026-Sep-17, Antonin Houska wrote:

> > + for (int i = 0; i < nrunning; i++)
> > + {
> > + TransactionId running_xid = running->xids[i];
> > +
> > + if (bsearch(&running_xid, snap->xip, snap->xcnt,
> > + sizeof(TransactionId), xidComparator) != NULL)
> > + XactLockTableWait(running_xid, NULL, NULL, XLTW_None);
> > + }
> > + }
>
> I don't understand why you check all transactions in procarray, instead of
> only those in snap->xip.

Hmm, but he does: for all the transactions that are running, only those
that are found by bsearch() in the snap->xip array are waited for. Is
that not what we want?

I guess we could do it the other way around: iterate for each item on
snap->xip and search for those in running->xids. Is that what you
suggest?

We don't know offhand which array is largest; it would be better to
iterate on the smaller one and bsearch the largest. (Or maybe if both
are sorted, scan them simultaneously.) I don't find any reference to
say that running_xid is sorted.

I don't understand these two paragraphs:

* A subtransaction is covered by its top-level transaction, which is in
* snap->xip as well, or was purged from it because it is below xmin and
* thus finished long ago.
*
* Historic snapshots do not need this: between xmin and xmax they rely on
* xip alone, and transactions below xmin had left the procarray by the
* time the xl_running_xacts record that set xmin was written.

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