Feedback-ID: ie3de48e3:Fastmail
Date: Fri, 18 Sep 2026 14:28:55 +0200
From: Alvaro Herrera <alvherre@kurilemu.de>
To: Antonin Houska <ah@cybertec.at>
Cc: Rui Zhao <zhaorui126@gmail.com>, Andres Freund <andres@anarazel.de>,
	pgsql-hackers@lists.postgresql.org,
 Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Subject: Re: Race conditions in logical decoding
Message-ID: <aq0sA8aF8q6kTmJM@alvherre.pgsql>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <8247.1789634255@localhost>
Archived-At: 
 <https://www.postgresql.org/message-id/aq0sA8aF8q6kTmJM%40alvherre.pgsql>
Precedence: bulk

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.


-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"After a quick R of TFM, all I can say is HOLY CR** THAT IS COOL! PostgreSQL was
amazing when I first started using it at 7.2, and I'm continually astounded by
learning new features and techniques made available by the continuing work of
the development team."
Berend Tober, http://archives.postgresql.org/pgsql-hackers/2007-08/msg01009.php