Horton

Re: per backend WAL statistics

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Michael Paquier <michael@paquier.xyz>, Andres Freund <andres@anarazel.de>, Xuneng Zhou <xunengzhou@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-03-12T05:37:11Z
Lists: pgsql-hackers

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Hi,

On Tue, Mar 11, 2025 at 11:14:24PM -0400, Tom Lane wrote:
> Michael Paquier <michael@paquier.xyz> writes:
> > And I guess that we're OK here, so applied.  That should be the last
> > one.
> 
> Quite a few buildfarm members are not happy about the initialization
> that 9a8dd2c5a added to PendingBackendStats.  For instance [1]:
> 
> gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O2 -I. -I. -I../../../../src/include  -D_GNU_SOURCE -I/usr/include/libxml2   -c -o pgstat_backend.o pgstat_backend.c
> pgstat_backend.c:39:1: warning: missing braces around initializer [-Wmissing-braces]
>  static PgStat_BackendPending PendingBackendStats = {0};
>  ^
> pgstat_backend.c:39:1: warning: (near initialization for \342\200\230PendingBackendStats.pending_io\342\200\231) [-Wmissing-braces]
> 
> I guess that more than one level of braces is needed for this to
> be fully correct?

Thanks for the report! I think that it's better to remove the PendingBackendStats
initializer (instead of adding extra braces). The reason is that I'm concerned
about padding bytes (that could be added to the struct in the future) not being
zeroed (see [1]).

Done that way in the attached.

[1]: https://www.postgresql.org/message-id/Z8/W73%2BHVo%2B/pKHZ%40ip-10-97-1-34.eu-west-3.compute.internal

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