Date: Tue, 28 Apr 2020 20:47:10 -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
Message-ID: <20200429004710.GA4742@alvherre.pgsql>
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I pushed this one.  Some closing remarks:

On 2020-Apr-28, Alvaro Herrera wrote:

> On 2020-Apr-28, Kyotaro Horiguchi wrote:

> > Agreed to describe what is failed rather than the cause.  However,
> > logical replications slots are always "previously reserved" at
> > creation.
> 
> Bah, of course.  I was thinking in making the equivalent messages all
> identical in all callsites, but maybe they should be different when
> slots are logical.  I'll go over them again.

I changed the ones that can only be logical slots so that they no longer
say "previously reserved WAL".  The one in
pg_replication_slot_advance still uses that wording, because I didn't
think it was worth creating two separate error paths.

> > ERROR:  replication slot "repl" is not usable to get changes
> 
> That wording seems okay, but my specific point for this error message is
> that we were trying to use a physical slot to get logical changes; so
> the fact that the slot has been invalidated is secondary and we should
> complain about the *type* of slot rather than the restart_lsn.

I moved the check for validity to after CreateDecodingContext, so the
other errors are reported preferently. I also chose a different wording:

		/*
		 * After the sanity checks in CreateDecodingContext, make sure the
		 * restart_lsn is valid.  Avoid "cannot get changes" wording in this
		 * errmsg because that'd be confusingly ambiguous about no changes
		 * being available.
		 */
		if (XLogRecPtrIsInvalid(MyReplicationSlot->data.restart_lsn))
			ereport(ERROR,
					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
					 errmsg("can no longer get changes from replication slot \"%s\"",
							NameStr(*name)),
					 errdetail("This slot has never previously reserved WAL, or has been invalidated.")));

I hope this is sufficiently clear, but if not, feel free to nudge me and
we can discuss it further.

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