From: Antonin Houska <ah@cybertec.at>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
cc: Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
    Pg Hackers <pgsql-hackers@lists.postgresql.org>,
    Robert Treat <rob@xzilla.net>
Subject: Re: Adding REPACK [concurrently]
In-reply-to: <202603101915.kitboh3blier@alvherre.pgsql>
References: <202603101915.kitboh3blier@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@alvh.no-ip.org>
   message dated "Tue, 10 Mar 2026 20:24:25 +0100."
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Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> I have just pushed 0001 with some additional changes.

Thanks!

> Here's a rebase of the next ones; no changes other than fixing the
> conflicts.
>=20
> I'm seeing this warning caused by 0004, which I think is also being
> reported in CI
> https://cirrus-ci.com/task/6606871575920640
>=20
> [281/1134] Compiling C object src/backend/postgres_lib.a.p/commands_clust=
er.c.o
> In file included from ../../source/repack/src/include/access/htup_details=
.h:22,
>                  from ../../source/repack/src/include/access/relscan.h:17,
>                  from ../../source/repack/src/include/access/heapam.h:19,
>                  from ../../source/repack/src/backend/commands/cluster.c:=
37:
> In function =E2=80=98VARSIZE_ANY=E2=80=99,
>     inlined from =E2=80=98restore_tuple=E2=80=99 at ../../source/repack/s=
rc/backend/commands/cluster.c:3129:18,
>     inlined from =E2=80=98apply_concurrent_changes=E2=80=99 at ../../sour=
ce/repack/src/backend/commands/cluster.c:2915:9,
>     inlined from =E2=80=98process_concurrent_changes=E2=80=99 at ../../so=
urce/repack/src/backend/commands/cluster.c:3386:2:
> ../../source/repack/src/include/varatt.h:243:51: warning: array subscript=
 =E2=80=98varattrib_4b[0]=E2=80=99 is partly outside array bounds of =E2=80=
=98varlena[1]=E2=80=99 [-Warray-bounds=3D]
>   243 |         ((((const varattrib_4b *) (PTR))->va_4byte.va_header >> 2=
) & 0x3FFFFFFF)
>       |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
> ../../source/repack/src/include/varatt.h:467:24: note: in expansion of ma=
cro =E2=80=98VARSIZE_4B=E2=80=99
>   467 |                 return VARSIZE_4B(PTR);
>       |                        ^~~~~~~~~~
> ../../source/repack/src/backend/commands/cluster.c: In function =E2=80=98=
process_concurrent_changes=E2=80=99:
> ../../source/repack/src/backend/commands/cluster.c:3121:33: note: object =
=E2=80=98varhdr=E2=80=99 of size 4
>  3121 |                 varlena         varhdr;
>       |                                 ^~~~~~

I'm not sure it can be fixed nicely in the REPACK (CONCURRENTLY) patch. I
think the problem is that, in the current tree, VARSIZE_ANY() is used in su=
ch
a way that the compiler cannot check the "array bounds". The restore_tuple()
function is special in that it uses VARSIZE_ANY() to check a variable
allocated from the stack, so the compiler can check the size.

I'm trying to fix that in a new diff 0002 - the point is that VARSIZE_ANY()
should not need to dereference a pointer to varattrib_4b, since the size
information is always located at the beginning of the structure. Maybe you
have better idea.


Besides that, I've done a related change in 0003 (now 0004, due to the new
diff):

diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c
index 77e301b7c63..8b5571374d0 100644
--- a/src/backend/commands/cluster.c
+++ b/src/backend/commands/cluster.c
@@ -3118,7 +3118,7 @@ restore_tuple(BufFile *file, Relation relation, Memor=
yContext cxt)
 	BufFileReadExact(file, &natt_ext, sizeof(natt_ext));
 	for (int i =3D 0; i < natt_ext; i++)
 	{
-		varlena		varhdr;
+		alignas(uint32) varlena		varhdr;
 		char	   *ext_val;
 		Size		ext_val_size;
=20


And also this one in the same file, to suppress another compiler warning
(occuring when configured w/o --enable-cassert):

diff --git a/src/backend/replication/pgoutput_repack/pgoutput_repack.c b/sr=
c/backend/replication/pgoutput_repack/pgoutput_repack.c
index 707940c1127..90f3a8975b9 100644
--- a/src/backend/replication/pgoutput_repack/pgoutput_repack.c
+++ b/src/backend/replication/pgoutput_repack/pgoutput_repack.c
@@ -93,12 +93,9 @@ static void
 plugin_change(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 			  Relation relation, ReorderBufferChange *change)
 {
-	RepackDecodingState *dstate;
-
-	dstate =3D (RepackDecodingState *) ctx->output_writer_private;
-
 	/* Changes of other relation should not have been decoded. */
-	Assert(RelationGetRelid(relation) =3D=3D dstate->relid);
+	Assert(RelationGetRelid(relation) =3D=3D
+		   ((RepackDecodingState *) ctx->output_writer_private)->relid);
=20
 	/* Decode entry depending on its type */
 	switch (change->action)


--=20
Antonin Houska
Web: https://www.cybertec-postgresql.com


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