BUG #17874: Incorrect memory access at gistBuildCallback

The following bug has been logged on the website:

Bug reference:      17874
Logged by:          Egor Chindyaskin
Email address:      kyzevan23@mail.ru
PostgreSQL version: 15.2
Operating system:   Ubuntu 22.04
Description:

With a server compiled with flag -DUSE_VALGRIND the following query:

create table test(a int4, b point);

create index test_index on test using gist(b) with (buffering = on,
fillfactor=50);
drop index test_index;
insert into test (a, b) select g, point(g*10, g*10) from generate_series(1,
10000) g;
vacuum analyze test;

create index test_index on test using gist(b) with (buffering = on,
fillfactor=50);
drop index test_index;
insert into test (a, b) select g, point(g*10, g*10) from generate_series(1,
10000) g;
vacuum analyze test;

create index test_index on test using gist(b) with (buffering = on,
fillfactor=50);
drop index test_index;
insert into test (a, b) select g, point(g*10, g*10) from generate_series(1,
10000) g;
vacuum analyze test;

create index test_index on test using gist(b) with (buffering = on,
fillfactor=50);
drop index test_index;
insert into test (a, b) select g, point(g*10, g*10) from generate_series(1,
10000) g;
vacuum analyze test;

causes an incorrect memory access:

==00:00:00:20.465 256552== Invalid read of size 2
==00:00:00:20.465 256552==    at 0x20EE4F: gistBuildCallback
(gistbuild.c:921)
==00:00:00:20.465 256552==    by 0x241838: heapam_index_build_range_scan
(heapam_handler.c:1794)
==00:00:00:20.465 256552==    by 0x20F308: table_index_build_scan
(tableam.h:1791)
==00:00:00:20.465 256552==    by 0x20F308: gistbuild (gistbuild.c:317)
==00:00:00:20.465 256552==    by 0x2DC8A9: index_build (index.c:3032)
==00:00:00:20.465 256552==    by 0x2DE3EC: index_create (index.c:1264)
==00:00:00:20.465 256552==    by 0x3946F5: DefineIndex (indexcmds.c:1164)
==00:00:00:20.465 256552==    by 0x5E6565: ProcessUtilitySlow
(utility.c:1552)
==00:00:00:20.465 256552==    by 0x5E593C: standard_ProcessUtility
(utility.c:1077)
==00:00:00:20.465 256552==    by 0x5E5A01: ProcessUtility (utility.c:530)
==00:00:00:20.465 256552==    by 0x5E2DDE: PortalRunUtility
(pquery.c:1158)
==00:00:00:20.465 256552==    by 0x5E30DB: PortalRunMulti (pquery.c:1315)
==00:00:00:20.465 256552==    by 0x5E34DD: PortalRun (pquery.c:791)
==00:00:00:20.465 256552==  Address 0xeab8f1e is 382 bytes inside a block of
size 8,192 alloc'd
==00:00:00:20.465 256552==    at 0x4848899: malloc (in
/usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==00:00:00:20.465 256552==    by 0x74FFAD: AllocSetContextCreateInternal
(aset.c:446)
==00:00:00:20.465 256552==    by 0x7383F2: hash_create (dynahash.c:385)
==00:00:00:20.465 256552==    by 0x20F852: gistInitBuildBuffers
(gistbuildbuffers.c:82)
==00:00:00:20.465 256552==    by 0x20D5AE: gistInitBuffering
(gistbuild.c:833)
==00:00:00:20.465 256552==    by 0x20EF09: gistBuildCallback
(gistbuild.c:955)
==00:00:00:20.465 256552==    by 0x241838: heapam_index_build_range_scan
(heapam_handler.c:1794)
==00:00:00:20.465 256552==    by 0x20F308: table_index_build_scan
(tableam.h:1791)
==00:00:00:20.465 256552==    by 0x20F308: gistbuild (gistbuild.c:317)
==00:00:00:20.465 256552==    by 0x2DC8A9: index_build (index.c:3032)
==00:00:00:20.465 256552==    by 0x2DE3EC: index_create (index.c:1264)
==00:00:00:20.465 256552==    by 0x3946F5: DefineIndex (indexcmds.c:1164)
==00:00:00:20.465 256552==    by 0x5E6565: ProcessUtilitySlow
(utility.c:1552)
==00:00:00:20.465 256552== 
{
   <insert_a_suppression_name_here>
   Memcheck:Addr2
   fun:gistBuildCallback
   fun:heapam_index_build_range_scan
   fun:table_index_build_scan
   fun:gistbuild
   fun:index_build
   fun:index_create
   fun:DefineIndex
   fun:ProcessUtilitySlow
   fun:standard_ProcessUtility
   fun:ProcessUtility
   fun:PortalRunUtility
   fun:PortalRunMulti
   fun:PortalRun
}
==00:00:00:20.465 256552== 
==00:00:00:20.465 256552== Exit program on first error
(--exit-on-first-error=yes)

This issue reproduces on all supported branches.