Date: Thu, 4 Jul 2019 13:35:12 +0900
From: Michael Paquier <michael@paquier.xyz>
To: marko@joh.to, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #15888: Bogus "idle in transaction" state for logical
 decoding client after creating a slot
Message-ID: <20190704043512.GK3084@paquier.xyz>
References: <15888-52d02086ac2085e8@postgresql.org>
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On Wed, Jul 03, 2019 at 08:03:24AM +0000, PG Bug reporting form wrote:
> Run:
>=20
>   pg_recvlogical -S foo -f /dev/null -d dbname --create-slot --start
>=20
> and pg_stat_activity will show something like this until that session
> disconnects:
>=20
> which is quite obviously bogus.

I think that's the transaction which is started when exporting the
snapshot at logical slot creation.  Or in short:
#2  0x000055b5f8e5720f in StartTransactionCommand () at xact.c:2699
#3  0x000055b5f90fab4e in SnapBuildExportSnapshot
(builder=3D0x55b5fa3427b0) at snapbuild.c:575
#4  0x000055b5f90fe5df in CreateReplicationSlot (cmd=3D0x55b5fa2914e8)
at walsender.c:843

SnapBuildExportSnapshot() needs to keep a transaction context opened
to keep the wanted xmin around.  In PG >=3D 10, this problem gets
"solved" by using NOEXPORT_SNAPSHOT in the CREATE_REPLICATION_SLOT
command.  One idea would be to enforce a commit in this case from
CreateReplicationSlot/streamutil.c?  We know that pg_recvlogical is
not going to use this snapshot anyway...
--
Michael

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