Re: per backend WAL statistics

Michael Paquier <michael@paquier.xyz>

From: Michael Paquier <michael@paquier.xyz>

To: Alexander Lakhin <exclusion@gmail.com>

Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Andres Freund <andres@anarazel.de>, Xuneng Zhou <xunengzhou@gmail.com>, pgsql-hackers@lists.postgresql.org

Date: 2025-03-28T22:14:16Z

Lists: pgsql-hackers

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());
> 
> with sanitizers (or under Valgrind). When I run it, I get:
> 2025-03-28 18:38:08.259 UTC [3415399] LOG:  statement: SELECT * FROM pg_stat_get_backend_wal(pg_backend_pid());
> =================================================================
> ==3415399==ERROR: AddressSanitizer: heap-use-after-free on address
> 0x53100003c83c at pc 0x556e3d2d9967 bp 0x7ffda3cd2350 sp 0x7ffda3cd2340
> READ of size 4 at 0x53100003c83c thread T0
>     #0 0x556e3d2d9966 in pgstat_fetch_stat_backend_by_pid .../src/backend/utils/activity/pgstat_backend.c:136
>     #1 0x556e3d53b671 in pg_stat_get_backend_wal .../src/backend/utils/adt/pgstatfuncs.c:1673
>     #2 0x556e3cb14045 in ExecMakeTableFunctionResult .../src/backend/executor/execSRF.c:234
>     #3 0x556e3cb6c0fd in FunctionNext .../src/backend/executor/nodeFunctionscan.c:94
>     #4 0x556e3cb171d2 in ExecScanFetch ../../../src/include/executor/execScan.h:126
>     #5 0x556e3cb171d2 in ExecScanExtended ../../../src/include/executor/execScan.h:170
>     #6 0x556e3cb171d2 in ExecScan .../src/backend/executor/execScan.c:59
>     #7 0x556e3cb6bbf7 in ExecFunctionScan .../src/backend/executor/nodeFunctionscan.c:269
>     #8 0x556e3cb0aba9 in ExecProcNodeFirst .../src/backend/executor/execProcnode.c:469
> ...
> 
> Reproduced starting from 76def4cdd.

Thanks for the report.  I have added an open item to not lose track of
this issue, and will get back to it when I can.
--
Michael

Commits

Fix use-after-free in pgstat_fetch_stat_backend_by_pid()
- 3191a593d6de 18.0 landed
Remove initialization from PendingBackendStats
- f554a95379a9 18.0 landed
Add WAL data to backend statistics
- 76def4cdd7c2 18.0 landed
Improve check for detection of pending data in backend statistics
- 9a8dd2c5a6d9 18.0 landed
Fix some gaps in pg_stat_io with WAL receiver and WAL summarizer
- f4694e0f35b2 18.0 landed
Handle auxiliary processes in SQL functions of backend statistics
- 3f1db99bfabb 18.0 landed
Invent pgstat_fetch_stat_backend_by_pid()
- c2a50ac678eb 18.0 landed
Refactor code of pg_stat_get_wal() building result tuple
- 495864a4cf16 18.0 landed
Adding new PgStat_WalCounters structure in pgstat.h
- 0e42d31b0b22 18.0 landed
Remove pgstat_flush_wal()
- d7cbeaf261da 18.0 landed
Refactor some code related to backend statistics
- 2c14037bb57c 18.0 landed
Add backend-level statistics to pgstats
- 9aea73fc61d4 18.0 cited