Date: Tue, 7 Apr 2020 18:45:22 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: jgdr@dalibo.com, andres@anarazel.de, michael@paquier.xyz,
	sawada.mshk@gmail.com, peter.eisentraut@2ndquadrant.com,
	pgsql-hackers@lists.postgresql.org, thomas.munro@enterprisedb.com,
	sk@zsrv.org, michael.paquier@gmail.com
Subject: Re: [HACKERS] Restricting maximum keep segments by repslots
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On 2020-Apr-07, Kyotaro Horiguchi wrote:

> > Mmm. Couldn't we have a new member 'invalidated' in ReplicationSlot?
> 
> I did that in the attached. The invalidated is shared-but-not-saved
> member of a slot and initialized to false then irreversibly changed to
> true when the slot loses required segment.
> 
> It is checked by the new function CheckReplicationSlotInvalidated() at
> acquireing a slot and at updating slot by standby reply message. This
> change stops walsender without explicitly killing but I didn't remove
> that code.

This change didn't work well with my proposed change to make
checkpointer acquire slots before marking them invalid.  When I
incorporated your patch in the last version I posted yesterday, there
was a problem that when checkpointer attempted to acquire the slot, it
would fail with "the slot is invalidated"; also if you try to drop the
slot, it would obviously fail.  I think it would work to remove the
SlotIsInvalidated check from the Acquire routine, and instead move it to
the routines that need it (ie. not the InvalidateObsolete one, and also
not the routine to drop slots).

I pushed version 26, with a few further adjustments.

I think what we have now is sufficient, but if you want to attempt this
"invalidated" flag on top of what I pushed, be my guest.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services