RE: Parallel scan with SubTransGetTopmostTransaction assert coredump

Hi Andres,
   Reproduce steps.

1, Modify and adjust NUM_SUBTRANS_BUFFERS to 128 from 32 in the file  "src/include/access/subtrans.h" line number 15.
2, configure with enable assert and build it.
3, init a new database cluster.
4, modify  postgres.conf  and add some parameters as below. As the coredump from parallel scan, so we adjust parallel setting, make it easy to reproduce. 

  max_connections = 2000

  parallel_setup_cost=0
  parallel_tuple_cost=0
  min_parallel_table_scan_size=0
  max_parallel_workers_per_gather=8
  max_parallel_workers = 32

5, start the database cluster.
6, use the script init_test.sql  in attachment to create tables. 
7, use pgbench with script sub_120.sql in attachment to test it. Try it sometimes, you should get the coredump file.
    pgbench  -d postgres -p 33550  -n -r -f sub_120.sql   -c 200 -j 200 -T 120

Thanks
Pengcheng


-----Original Message-----
From: Andres Freund <andres@anarazel.de> 
Sent: 2021年5月7日 11:55
To: Pengchengliu <pengchengliu@tju.edu.cn>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Parallel scan with SubTransGetTopmostTransaction assert coredump

Hi,

On 2021-05-07 11:32:57 +0800, Pengchengliu wrote:
> Hi Hackers，
> 
> Last email, format error, missing some information, so I resend this email.  
> 
>  With PG 13.2(3fb4c75e857adee3da4386e947ba58a75f3e74b7), I tested subtransaction with parallel scan, I got a subtransaction coredump as below:


> So the root cause is the Parallel Workers process set the TransactionXmin with later transcation snapshot. When parallel scan, Parallel Workers process use the older active snapshot.
> 
> It leads to subtrans assert coredump. I don't know how to fix it. Is there any ideas?

Do you have steps to reliably reproduce this?

Greetings,

Andres Freund