Horton

Re: per backend WAL statistics

Alexander Law <exclusion@gmail.com>

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From: Alexander Lakhin <exclusion@gmail.com>
To: Michael Paquier <michael@paquier.xyz>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Xuneng Zhou <xunengzhou@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-03-28T19:00:00Z
Lists: pgsql-hackers 
Hello Michael,

11.03.2025 02:06, Michael Paquier wrote:
> And I guess that we're OK here, so applied.  That should be the last
> one.

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.

Best regards,
Alexander Lakhin
Neon (https://neon.tech)

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