Re: Minimal logical decoding on standbys

Hi,

On 2019-04-10 12:11:21 +0530, tushar wrote:
> 
> On 03/13/2019 08:40 PM, tushar wrote:
> > Hi ,
> > 
> > I am getting a server crash on standby while executing
> > pg_logical_slot_get_changes function   , please refer this scenario
> > 
> > Master cluster( ./initdb -D master)
> > set wal_level='hot_standby in master/postgresql.conf file
> > start the server , connect to  psql terminal and create a physical
> > replication slot ( SELECT * from
> > pg_create_physical_replication_slot('p1');)
> > 
> > perform pg_basebackup using --slot 'p1'  (./pg_basebackup -D slave/ -R
> > --slot p1 -v))
> > set wal_level='logical' , hot_standby_feedback=on,
> > primary_slot_name='p1' in slave/postgresql.conf file
> > start the server , connect to psql terminal and create a logical
> > replication slot (  SELECT * from
> > pg_create_logical_replication_slot('t','test_decoding');)
> > 
> > run pgbench ( ./pgbench -i -s 10 postgres) on master and select
> > pg_logical_slot_get_changes on Slave database
> > 
> > postgres=# select * from pg_logical_slot_get_changes('t',null,null);
> > 2019-03-13 20:34:50.274 IST [26817] LOG:  starting logical decoding for
> > slot "t"
> > 2019-03-13 20:34:50.274 IST [26817] DETAIL:  Streaming transactions
> > committing after 0/6C000060, reading WAL from 0/6C000028.
> > 2019-03-13 20:34:50.274 IST [26817] STATEMENT:  select * from
> > pg_logical_slot_get_changes('t',null,null);
> > 2019-03-13 20:34:50.275 IST [26817] LOG:  logical decoding found
> > consistent point at 0/6C000028
> > 2019-03-13 20:34:50.275 IST [26817] DETAIL:  There are no running
> > transactions.
> > 2019-03-13 20:34:50.275 IST [26817] STATEMENT:  select * from
> > pg_logical_slot_get_changes('t',null,null);
> > TRAP: FailedAssertion("!(data == tupledata + tuplelen)", File:
> > "decode.c", Line: 977)
> > server closed the connection unexpectedly
> >     This probably means the server terminated abnormally
> >     before or while processing the request.
> > The connection to the server was lost. Attempting reset: 2019-03-13
> > 20:34:50.276 IST [26809] LOG:  server process (PID 26817) was terminated
> > by signal 6: Aborted
> > 
> Andres - Do you think - this is an issue which needs to  be fixed ?

Yes, it definitely needs to be fixed. I just haven't had sufficient time
to look into it. Have you reproduced this with Amit's latest version?

Amit, have you spent any time looking into it? I know that you're not
that deeply steeped into the internals of logical decoding, but perhaps
there's something obvious going on.

Greetings,

Andres Freund