Horton

Re: pg_receivewal starting position

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: Ronan Dunklau <ronan.dunklau@aiven.io>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>
Date: 2021-10-26T01:15:40Z
Lists: pgsql-hackers 
On Mon, Oct 25, 2021 at 05:46:57PM +0530, Bharath Rupireddy wrote:
> StreamLog() isn't reached for create and drop slot cases, see [1]. I
> suggest to remove replication_slot != NULL and have Assert(slot_name)
> in GetSlotInformation():
>         /*
>          * Try to get the starting point from the slot.  This is supported in
>          * PostgreSQL 15 and up.
>          */
>         if (PQserverVersion(conn) >= 150000)
>         {
>             if (!GetSlotInformation(conn, replication_slot, &stream.startpos,
>                                     &stream.timeline))
>             {
>                 /* Error is logged by GetSlotInformation() */
>                 return;
>             }
>         }

Please note that it is possible to use pg_receivewal without a slot,
which is the default case, so we cannot do what you are suggesting
here.  An assertion on slot_name in GetSlotInformation() is not that
helpful either in my opinion, as we would just crash a couple of lines
down the road.

I have changed the patch per Ronan's suggestion to have the version
check out of GetSlotInformation(), addressed what you have reported,
and the result looked good.  So I have applied this part.

What remains on this thread is the addition of new tests to make sure
that pg_receivewal is able to follow a timeline switch.  Now that we
can restart from a slot that should be a bit easier to implemented as
a test by creating a slot on a standby.  Ronan, are you planning to
send a new patch for this part?
--
Michael

Commits

  1. Add TAP test for pg_receivewal with timeline switch

  2. Speed up TAP tests of pg_receivewal

  3. Allow pg_receivewal to stream from a slot's restart LSN

  4. Add replication command READ_REPLICATION_SLOT

  5. doc: Describe calculation method of streaming start for pg_receivewal

  6. Add PostgresNode::command_fails_like()