Re: Synchronizing slots from primary to standby

On Mon, Feb 26, 2024 at 12:59 PM Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> On Mon, Feb 26, 2024 at 02:18:58AM +0000, Zhijie Hou (Fujitsu) wrote:
> > On Friday, February 23, 2024 6:12 PM Bertrand Drouvot <bertranddrouvot.pg@gmail.com> wrote:
> > > +       if (!ok)
> > > +               GUC_check_errdetail("List syntax is invalid.");
> > > +
> > > +       /*
> > > +        * If there is a syntax error in the name or if the replication slots'
> > > +        * data is not initialized yet (i.e., we are in the startup process), skip
> > > +        * the slot verification.
> > > +        */
> > > +       if (!ok || !ReplicationSlotCtl)
> > > +       {
> > > +               pfree(rawname);
> > > +               list_free(elemlist);
> > > +               return ok;
> > > +       }
> > >
> > > we are testing the "ok" value twice, what about using if...else if... instead and
> > > test it once? If so, it might be worth to put the:
> > >
> > > "
> > > +       pfree(rawname);
> > > +       list_free(elemlist);
> > > +       return ok;
> > > "
> > >
> > > in a "goto".
> >
> > There were comments to remove the 'goto' statement and avoid
> > duplicate free code, so I prefer the current style.
>
> The duplicate free code would come from the if...else if... rewrite but then
> the "goto" would remove it, so I'm not sure to understand your point.
>

I think Hou-San wants to say that there was previously a comment to
remove goto and now you are saying to introduce it. But, I think we
can avoid both code duplication and goto, if the first thing we check
in the function is ReplicationSlotCtl and return false if the same is
not set. Won't that be better?

>
> > >
> > > 10 ===
> > >
> > > +                       if (slot->data.invalidated != RS_INVAL_NONE)
> > > +                       {
> > > +                               /*
> > > +                                * Specified physical slot have been invalidated,
> > > so no point
> > > +                                * in waiting for it.
> > >
> > > We discovered in [1], that if the wal_status is "unreserved" then the slot is still
> > > serving the standby. I think we should handle this case differently, thoughts?
> >
> > I think the 'invalidated' slot can still be used is a separate bug.
> > Because
> > once the slot is invalidated, it can neither protect WALs or ROWs from being
> > removed even if the restart_lsn of the slot can be moved forward after being invalidated.
> >
> > If the standby can move restart_lsn forward for invalidated slots, then
> > it should also set the 'invalidated' flag back to NONE, otherwise the slot
> > cannot serve its purpose anymore. I also reported similar bug before[1].
>
...
>
> My point is that I think we should behave like it's not a bug and then adapt the
> code accordingly here (until the bug gets fixed).
>

oh, I think this doesn't sound like a good idea to me. We should fix
that bug independently rather than adding code in new features to
consider the bug as a valid behavior. It will add the burden on us to
remember and remove the additional new check(s).

--
With Regards,
Amit Kapila.