From: Chao Li <li.evan.chao@gmail.com>
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Subject: repack: fix uninitialized DecodingWorkerShared.initialized
Message-Id: <9AFE5694-9420-40C3-842C-4A4473E1D02B@gmail.com>
Date: Wed, 15 Apr 2026 18:22:44 +0800
To: Postgres hackers <pgsql-hackers@lists.postgresql.org>
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Hi,

I have not reviewed the repack-related patches before. Recently, I =
started trying to understand how repack works and trace through the =
code.

While tracing start_repack_decoding_worker(), I noticed something =
suspicious:

```
	seg =3D dsm_create(size, 0);
	shared =3D (DecodingWorkerShared *) dsm_segment_address(seg);
	shared->lsn_upto =3D InvalidXLogRecPtr;
	shared->done =3D false;
	SharedFileSetInit(&shared->sfs, seg);
	shared->last_exported =3D -1;
	SpinLockInit(&shared->mutex);
	shared->dbid =3D MyDatabaseId;
```

Here, the code creates a shared-memory segment and lets =E2=80=9Cshared" =
point to that memory. It then initializes some fields of =E2=80=9Cshared".=
 However, later code reads shared->initialized, but this field was not =
initialized:
```
	for (;;)
	{
		bool		initialized;

		SpinLockAcquire(&shared->mutex);
		initialized =3D shared->initialized;
		SpinLockRelease(&shared->mutex);

		if (initialized)
			break;

		ConditionVariableSleep(&shared->cv, =
WAIT_EVENT_REPACK_WORKER_EXPORT);
	}
```

I checked the code of dsm_create(), and I did not see anything showing =
that the created shared-memory segment is zeroed.

This is actually just an eyeball finding. =46rom my tracing on my =
MacBook, after shared =3D (DecodingWorkerShared *) =
dsm_segment_address(seg);, the memory always seemed to be zeroed, so I =
may have missed something. But given that the code explicitly =
initializes several other fields, it seems better not to rely on that =
implicitly.

For the fix, since start_repack_decoding_worker() is not on a hot path, =
I think it is fine to zero the whole shared struct explicitly, and then =
initialize the non-zero fields afterwards.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/


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	filename=v1-0001-repack-zero-initialize-DecodingWorkerShared.patch
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=46rom=20b9c22b95492bff66a511159ceffd37d6394cc5b1=20Mon=20Sep=2017=20=
00:00:00=202001=0AFrom:=20"Chao=20Li=20(Evan)"=20<lic@highgo.com>=0A=
Date:=20Wed,=2015=20Apr=202026=2016:46:20=20+0800=0ASubject:=20[PATCH=20=
v1]=20repack:=20zero-initialize=20DecodingWorkerShared=0A=0AAuthor:=20=
Chao=20Li=20<lic@highgo.com>=0AReviewed-by:=0ADiscussion:=20=
https://postgr.es/m/=0A---=0A=20src/backend/commands/repack.c=20|=203=20=
+--=0A=201=20file=20changed,=201=20insertion(+),=202=20deletions(-)=0A=0A=
diff=20--git=20a/src/backend/commands/repack.c=20=
b/src/backend/commands/repack.c=0Aindex=2058e3867246f..058b39a67ad=20=
100644=0A---=20a/src/backend/commands/repack.c=0A+++=20=
b/src/backend/commands/repack.c=0A@@=20-3311,8=20+3311,7=20@@=20=
start_repack_decoding_worker(Oid=20relid)=0A=20=09=09=
BUFFERALIGN(REPACK_ERROR_QUEUE_SIZE);=0A=20=09seg=20=3D=20=
dsm_create(size,=200);=0A=20=09shared=20=3D=20(DecodingWorkerShared=20*)=20=
dsm_segment_address(seg);=0A-=09shared->lsn_upto=20=3D=20=
InvalidXLogRecPtr;=0A-=09shared->done=20=3D=20false;=0A+=09=
memset(shared,=200,=20sizeof(DecodingWorkerShared));=0A=20=09=
SharedFileSetInit(&shared->sfs,=20seg);=0A=20=09shared->last_exported=20=
=3D=20-1;=0A=20=09SpinLockInit(&shared->mutex);=0A--=20=0A2.50.1=20=
(Apple=20Git-155)=0A=0A=

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