Feedback-ID: ia2694551:Fastmail
Date: Tue, 7 Apr 2026 10:40:25 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>, Noah Misch <noah@leadboat.com>,
	vignesh C <vignesh21@gmail.com>, Antonin Houska <ah@cybertec.at>,
	Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
 Amit Kapila <amit.kapila16@gmail.com>,
	Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>,
	Pg Hackers <pgsql-hackers@lists.postgresql.org>,
 Robert Treat <rob@xzilla.net>
Subject: Re: Adding REPACK [concurrently]
Message-ID: <202604070821.zkcnhnor3oto@alvherre.pgsql>
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On 2026-Apr-07, Tom Lane wrote:

> Maybe you saw this already, but BF member skink is failing on
> src/test/modules/injection_points/specs/repack.spec:
> 
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2026-04-06%2022%3A50%3A41

Thanks, I hadn't, and I cannot reproduce it locally.  Anyway, the
valgrind report is

==1617044== VALGRINDERROR-BEGIN
==1617044== Syscall param pwrite64(buf) points to uninitialised byte(s)
==1617044==    at 0x6704003: pwrite (pwrite64.c:25)
==1617044==    by 0x44AAC72: pg_pwritev (pg_iovec.h:101)
==1617044==    by 0x44AC6B5: FileWriteV (fd.c:2280)
==1617044==    by 0x44A8EC4: FileWrite (fd.h:245)
==1617044==    by 0x44A8EC4: BufFileDumpBuffer (buffile.c:538)
==1617044==    by 0x44A9034: BufFileFlush (buffile.c:724)
==1617044==    by 0x44A9661: BufFileClose (buffile.c:418)
==1617044==    by 0x426C4DE: export_initial_snapshot (repack_worker.c:346)
==1617044==    by 0x426CA17: RepackWorkerMain (repack_worker.c:145)
==1617044==    by 0x441D3AD: BackgroundWorkerMain (bgworker.c:865)
==1617044==    by 0x44219D9: postmaster_child_launch (launch_backend.c:265)
==1617044==  Address 0x12d745e2 is 106 bytes inside a block of size 8,272 client-defined
==1617044==    at 0x4661CFE: palloc (mcxt.c:1411)
==1617044==    by 0x44A8C54: makeBufFileCommon (buffile.c:121)
==1617044==    by 0x44A933F: BufFileCreateFileSet (buffile.c:272)
==1617044==    by 0x426C4A5: export_initial_snapshot (repack_worker.c:341)
==1617044==    by 0x426CA17: RepackWorkerMain (repack_worker.c:145)
==1617044==    by 0x441D3AD: BackgroundWorkerMain (bgworker.c:865)
==1617044==    by 0x44219D9: postmaster_child_launch (launch_backend.c:265)
==1617044==    by 0x4423B6D: StartBackgroundWorker (postmaster.c:4197)
==1617044==    by 0x4423DB8: maybe_start_bgworkers (postmaster.c:4362)
==1617044==    by 0x4425119: LaunchMissingBackgroundProcesses (postmaster.c:3437)
==1617044==    by 0x4426A75: ServerLoop (postmaster.c:1737)
==1617044==    by 0x44280DC: PostmasterMain (postmaster.c:1412)
==1617044==  Uninitialised value was created by a stack allocation
==1617044==    at 0x4674D39: SerializeSnapshot (snapmgr.c:1737)
==1617044== 
==1617044== VALGRINDERROR-END

and obviously BufFileCreateFileSet() is being called by existing code
already and it doesn't fail, so I *think* the problem might be that
SerializedSnapshotData has some padding bytes that are being written.
Maybe using palloc0() is enough?  I'll try that.

If that doesn't silence skink, I guess my next step is to reproduce
skink's environment more precisely ...

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Uno puede defenderse de los ataques; contra los elogios se esta indefenso"