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Message-ID: <f1788cc0-253a-4a3a-aee0-1b8ab9538736@gmail.com>
Date: Fri, 28 Mar 2025 21:00:00 +0200
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Subject: Re: per backend WAL statistics
To: Michael Paquier <michael@paquier.xyz>,
 Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Xuneng Zhou <xunengzhou@gmail.com>,
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From: Alexander Lakhin <exclusion@gmail.com>
In-Reply-To: <Z89-gxOVxtEP_w9d@paquier.xyz>
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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)
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    <div class="moz-cite-prefix">Hello Michael,<br>
      <br>
      11.03.2025 02:06, Michael Paquier wrote:<span
      style="white-space: pre-wrap">
</span></div>
    <blockquote type="cite" cite="mid:Z89-gxOVxtEP_w9d@paquier.xyz">
      <pre wrap="" class="moz-quote-pre">And I guess that we're OK here, so applied.  That should be the last
one.
</pre>
    </blockquote>
    <br>
    Please try the following query:<br>
    BEGIN;<br>
    SET LOCAL stats_fetch_consistency = snapshot;<br>
    SELECT * FROM pg_stat_get_backend_wal(pg_backend_pid());<br>
    <br>
    with sanitizers (or under Valgrind). When I run it, I get:<br>
    2025-03-28 18:38:08.259 UTC [3415399] LOG:  statement: SELECT * FROM
    pg_stat_get_backend_wal(pg_backend_pid());<br>
    =================================================================<br>
    ==3415399==ERROR: AddressSanitizer: heap-use-after-free on address
    0x53100003c83c at pc 0x556e3d2d9967 bp 0x7ffda3cd2350 sp
    0x7ffda3cd2340<br>
    READ of size 4 at 0x53100003c83c thread T0<br>
        #0 0x556e3d2d9966 in pgstat_fetch_stat_backend_by_pid
    .../src/backend/utils/activity/pgstat_backend.c:136<br>
        #1 0x556e3d53b671 in pg_stat_get_backend_wal
    .../src/backend/utils/adt/pgstatfuncs.c:1673<br>
        #2 0x556e3cb14045 in ExecMakeTableFunctionResult
    .../src/backend/executor/execSRF.c:234<br>
        #3 0x556e3cb6c0fd in FunctionNext
    .../src/backend/executor/nodeFunctionscan.c:94<br>
        #4 0x556e3cb171d2 in ExecScanFetch
    ../../../src/include/executor/execScan.h:126<br>
        #5 0x556e3cb171d2 in ExecScanExtended
    ../../../src/include/executor/execScan.h:170<br>
        #6 0x556e3cb171d2 in ExecScan
    .../src/backend/executor/execScan.c:59<br>
        #7 0x556e3cb6bbf7 in ExecFunctionScan
    .../src/backend/executor/nodeFunctionscan.c:269<br>
        #8 0x556e3cb0aba9 in ExecProcNodeFirst
    .../src/backend/executor/execProcnode.c:469<br>
    ...<br>
    <br>
    Reproduced starting from 76def4cdd.<br>
    <br>
    Best regards,<br>
    Alexander Lakhin<br>
    Neon (<a class="moz-txt-link-freetext" href="https://neon.tech">https://neon.tech</a>)
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