Re: per backend WAL statistics

Hi,

On Sat, Mar 29, 2025 at 07:14:16AM +0900, Michael Paquier wrote:
> On Fri, Mar 28, 2025 at 09:00:00PM +0200, Alexander Lakhin wrote:
> > Please try the following query:
> > BEGIN;
> > SET LOCAL stats_fetch_consistency = snapshot;
> > SELECT * FROM pg_stat_get_backend_wal(pg_backend_pid());

Thanks for the report! I'm able to reproduce it on my side. The issue can
also be triggered with pg_stat_get_backend_io().

The issue is that in pgstat_fetch_stat_backend_by_pid() (and with
stats_fetch_consistency set to snapshot) a call to
pgstat_clear_backend_activity_snapshot() is done:

#0  __memset_evex_unaligned_erms () at ../sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S:250
#1  0x0000000001833bf2 in wipe_mem (ptr=0x632000018800, size=80800) at ../../../../src/include/utils/memdebug.h:42
#2  0x0000000001834c51 in AllocSetReset (context=0x619000003c80) at aset.c:586
#3  0x000000000184f32d in MemoryContextResetOnly (context=0x619000003c80) at mcxt.c:419
#4  0x0000000001834ede in AllocSetDelete (context=0x619000003c80) at aset.c:636
#5  0x000000000184f79b in MemoryContextDeleteOnly (context=0x619000003c80) at mcxt.c:528
#6  0x000000000184f5a9 in MemoryContextDelete (context=0x619000003c80) at mcxt.c:482
#7  0x0000000001361e84 in pgstat_clear_backend_activity_snapshot () at backend_status.c:541
#8  0x0000000001367f08 in pgstat_clear_snapshot () at pgstat.c:943
#9  0x0000000001368ac3 in pgstat_prep_snapshot () at pgstat.c:1121
#10 0x00000000013680b9 in pgstat_fetch_entry (kind=6, dboid=0, objid=0) at pgstat.c:961
#11 0x000000000136dd05 in pgstat_fetch_stat_backend (procNumber=0) at pgstat_backend.c:94
#12 0x000000000136de7d in pgstat_fetch_stat_backend_by_pid (pid=3294022, bktype=0x0) at pgstat_backend.c:136

*before* we check for "beentry->st_procpid != pid".

I think we can simply move the pgstat_fetch_stat_backend() call at the end
of pgstat_fetch_stat_backend_by_pid(), like in the attached. With this in place
the issue is fixed on my side.

Thoughts?

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com