Message-ID: <d29e4458e7406b276c50197d9f3b31a5247fe915.camel@j-davis.com>
Subject: Re: Minimal logical decoding on standbys
From: Jeff Davis <pgsql@j-davis.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Andres Freund
	 <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, Thomas Munro
 <thomas.munro@gmail.com>,  Alvaro Herrera <alvherre@2ndquadrant.com>, Ibrar
 Ahmed <ibrar.ahmad@gmail.com>, Amit Khandekar <amitdkhan.pg@gmail.com>,
 fabriziomello@gmail.com, tushar <tushar.ahuja@enterprisedb.com>, Rahila
 Syed <rahila.syed@2ndquadrant.com>,  pgsql-hackers
 <pgsql-hackers@postgresql.org>
Date: Thu, 02 Mar 2023 11:45:51 -0800
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On Thu, 2023-03-02 at 10:20 +0100, Drouvot, Bertrand wrote:
> Right, but in our case, right after the wakeup (the one due to the CV
> broadcast,
> aka the one that will remove it from the wait queue) we'll exit the
> loop due to:
>=20
> "
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /* check whether we're d=
one */
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (loc <=3D RecentFlush=
Ptr)
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 =
break;
> "
>=20
> as the CV broadcast means that a flush/replay occurred.

But does it mean that the flush/replay advanced *enough* to be greater
than or equal to loc?

> - If it is awakened due to the CV broadcast, then we'll right after
> exit the loop (see above)

...

> I think that's not needed as we'd exit the loop right after we are
> awakened by a CV broadcast.

See the comment here:

 * If this process has been taken out of the wait list, then we know =20
 * that it has been signaled by ConditionVariableSignal (or          =20
 * ConditionVariableBroadcast), so we should return to the caller. But
 * that doesn't guarantee that the exit condition is met, only that we
 * ought to check it.

You seem to be arguing that in this case, it doesn't matter; that
walreceiver knows what walsender is waiting for, and will never wake it
up before it's ready. I don't think that's true, and even if it is, it
needs explanation.

>=20
> I agree that's a good idea and that it should/would work too. I just
> wanted to highlight that in this particular
> case that might not be necessary to build this new API.

In this case it looks easier to add the right API than to be sure about
whether it's needed or not.

Regards,
	Jeff Davis