Re: Introduce XID age and inactive timeout based replication slot invalidation

On Fri, Mar 22, 2024 at 5:30 PM Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> On Fri, Mar 22, 2024 at 03:56:23PM +0530, Amit Kapila wrote:
> > On Fri, Mar 22, 2024 at 3:15 PM Bertrand Drouvot
> > <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Mar 22, 2024 at 01:45:01PM +0530, Bharath Rupireddy wrote:
> > >
> > > 1 ===
> > >
> > > @@ -691,6 +699,13 @@ ReplicationSlotRelease(void)
> > > ConditionVariableBroadcast(&slot->active_cv);
> > > }
> > >
> > > + if (slot->data.persistency == RS_PERSISTENT)
> > > + {
> > > + SpinLockAcquire(&slot->mutex);
> > > + slot->last_inactive_at = GetCurrentTimestamp();
> > > + SpinLockRelease(&slot->mutex);
> > > + }
> > >
> > > I'm not sure we should do system calls while we're holding a spinlock.
> > > Assign a variable before?
> > >
> > > 2 ===
> > >
> > > Also, what about moving this here?
> > >
> > > "
> > > if (slot->data.persistency == RS_PERSISTENT)
> > > {
> > > /*
> > > * Mark persistent slot inactive. We're not freeing it, just
> > > * disconnecting, but wake up others that may be waiting for it.
> > > */
> > > SpinLockAcquire(&slot->mutex);
> > > slot->active_pid = 0;
> > > SpinLockRelease(&slot->mutex);
> > > ConditionVariableBroadcast(&slot->active_cv);
> > > }
> > > "
> > >
> > > That would avoid testing twice "slot->data.persistency == RS_PERSISTENT".
> > >
> >
> > That sounds like a good idea. Also, don't we need to consider physical
> > slots where we don't reserve WAL during slot creation? I don't think
> > there is a need to set inactive_at for such slots.
>
> If the slot is not active, why shouldn't we set inactive_at? I can understand
> that such a slots do not present "any risks" but I think we should still set
> inactive_at (also to not give the false impression that the slot is active).
>

But OTOH, there is a chance that we will invalidate such slots even
though they have never reserved WAL in the first place which doesn't
appear to be a good thing.

> > > 5 ===
> > >
> > > Most of the fields that reflect a time (not duration) in the system views are
> > > xxxx_time, so I'm wondering if instead of "last_inactive_at" we should use
> > > something like "last_inactive_time"?
> > >
> >
> > How about naming it as last_active_time? This will indicate the time
> > at which the slot was last active.
>
> I thought about it too but I think it could be missleading as one could think that
> it should be updated each time WAL record decoding is happening.
>

Fair enough.

--
With Regards,
Amit Kapila.