Re: Minimal logical decoding on standbys

On Wed, 2023-03-08 at 11:25 +0100, Drouvot, Bertrand wrote:
> - Maybe ConditionVariableEventSleep() should take care of the
> “WaitEventSetWait returns 1 and cvEvent.event == WL_POSTMASTER_DEATH”
> case?

Thank you, done. I think the nearby line was also wrong, returning true
when there was no timeout. I combined the lines and got rid of the
early return so it can check the list and timeout condition like
normal. Attached.

> - Maybe ConditionVariableEventSleep() could accept and deal with the
> CV being NULL?
> I used it in the POC attached to handle logical decoding on the
> primary server case.
> One option should be to create a dedicated CV for that case though.

I don't think it's a good idea to have a CV-based API that doesn't need
a CV. Wouldn't that just be a normal WaitEventSet?

> - In the POC attached I had to add this extra condition “(cv &&
> !RecoveryInProgress())” to avoid waiting on the timeout when there is
> a promotion.
> That makes me think that we may want to add 2 extra parameters (as 2
> functions returning a bool?) to ConditionVariableEventSleep()
> to check whether or not we still want to test the socket or the CV
> wake up in each loop iteration.

That seems like a complex API. Would it work to signal the CV during
promotion instead?

> Also 3 additional remarks:
>
> 1) About InitializeConditionVariableWaitSet() and
> ConditionVariableWaitSetCreate(): I'm not sure about the naming as
> there is no CV yet (they "just" deal with WaitEventSet).

It's a WaitEventSet that contains the conditions always required for
any CV, and allows you to add in more.

> 3)
>
> I wonder if there is no race conditions: ConditionVariableWaitSet is
> being initialized with PGINVALID_SOCKET
> as WL_LATCH_SET and might be also (if IsUnderPostmaster) be
> initialized with PGINVALID_SOCKET as WL_EXIT_ON_PM_DEATH.
>
> So IIUC, the patch is introducing 2 new possible source of wake up.

Those should be the same conditions already required by
ConditionVariableTimedSleep() in master, right?


Regards,
    Jeff Davis