Horton

Re: per backend WAL statistics

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

View thread View on postgr.es
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Andres Freund <andres@anarazel.de>, Xuneng Zhou <xunengzhou@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-03-08T07:53:04Z
Lists: pgsql-hackers 
Hi,

On Sat, Mar 08, 2025 at 10:57:38AM +0900, Michael Paquier wrote:
> On Fri, Mar 07, 2025 at 08:33:04AM +0000, Bertrand Drouvot wrote:
> > But when it's time to flush, then pgstat_backend_have_pending_cb() checks
> > for zeros in PendingBackendStats *without* any check on the backend type.
> > 
> > I think the issue is "masked" on HEAD because PendingBackendStats is
> > probably automatically initialized with zeros (as being a static variable at
> > file scope).
> 
> If this weren't true, we would have a lot of problems in more places
> than this one.

Yeah I fully agree and I think that was fine. I just added "probably" as 
cautious wording, as the "We assume this initializes to zeroes" comments
that have been removed in 07e9e28b56d and 88f5ebbbee3 for example.

> It does not hurt to add an initialization at the top
> of pgstat_backend.c for PendingBackendStats, to document the
> intention, while we're on it.

-static PgStat_BackendPending PendingBackendStats;
+static PgStat_BackendPending PendingBackendStats = {0};

Not sure about this change: I think that that would not always work should
PgStat_BackendPending contains padding. I mean there is no guarantee that would
initialize padding bytes to zeroes (if any).

That would not be an issue should we only access the struct
fields in the code, but that's not the case as we're making use of
pg_memory_is_all_zeros() on it.

Regards,

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

Commits

  1. Fix use-after-free in pgstat_fetch_stat_backend_by_pid()

  2. Remove initialization from PendingBackendStats

  3. Add WAL data to backend statistics

  4. Improve check for detection of pending data in backend statistics

  5. Fix some gaps in pg_stat_io with WAL receiver and WAL summarizer

  6. Handle auxiliary processes in SQL functions of backend statistics

  7. Invent pgstat_fetch_stat_backend_by_pid()

  8. Refactor code of pg_stat_get_wal() building result tuple

  9. Adding new PgStat_WalCounters structure in pgstat.h

  10. Remove pgstat_flush_wal()

  11. Refactor some code related to backend statistics

  12. Add backend-level statistics to pgstats