BUG #19794: Assert in btvacuumcleanup() fails when page counted in pages_free twice

The following bug has been logged on the website:

Bug reference: 19794
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 19rc1
Operating system: Ubuntu 24.04
Description:

The following script:
echo "
CREATE TABLE t (id int);
CREATE INDEX t_i1 ON t (id);
CREATE INDEX t_i2 ON t (id);
INSERT INTO t SELECT g FROM generate_series(2000000, 1, -1) g;
DELETE FROM t WHERE id > 999000 AND id < 1000000;
VACUUM (INDEX_CLEANUP ON);
DELETE FROM t WHERE id > 1000000;
" | psql

echo "
SELECT 'SELECT txid_current();' FROM generate_series(1, 10000)
\gexec
" | psql >/dev/null &
psql -c 'VACUUM t'
wait

triggers:
TRAP: failed Assert("stats->pages_deleted >= stats->pages_free"), File:
"nbtree.c", Line: 1208, PID: 3843334
ExceptionalCondition at assert.c:51:13
btvacuumcleanup at nbtree.c:1209:22
index_vacuum_cleanup at indexam.c:698:1
vac_cleanup_one_index at vacuum.c:2743:10
parallel_vacuum_process_one_index at vacuumparallel.c:1171:16
parallel_vacuum_process_unsafe_indexes at vacuumparallel.c:1100:3
parallel_vacuum_process_all_indexes at vacuumparallel.c:981:2
parallel_vacuum_cleanup_all_indexes at vacuumparallel.c:636:1
lazy_cleanup_all_indexes at vacuumlazy.c:3019:2
lazy_scan_heap at vacuumlazy.c:1634:1
heap_vacuum_rel at vacuumlazy.c:878:31
table_relation_vacuum at tableam.h:1803:1
vacuum_rel at vacuum.c:2360:2
vacuum at vacuum.c:635:10
ExecVacuum at vacuum.c:470:2
standard_ProcessUtility at utility.c:861:4
ProcessUtility at utility.c:525:3
PortalRunUtility at pquery.c:1153:2
PortalRunMulti at pquery.c:1311:5
PortalRun at pquery.c:792:5
exec_simple_query at postgres.c:1304:11
PostgresMain at postgres.c:4950:27
BackendInitialize at backend_startup.c:142:1
postmaster_child_launch at launch_backend.c:269:3
BackendStartup at postmaster.c:3650:8
ServerLoop at postmaster.c:1730:10
PostmasterMain at postmaster.c:1414:11
main at main.c:232:2
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x8b)[0x7a3a2f82a28b]
postgres: postgres regression [local] VACUUM(_start+0x25)[0x5a37f1dd61b5]
2026-10-24 14:40:20.803 EEST [3843333:4] psql LOG: disconnection: session
time: 0:00:00.517 user=postgres database=regression host=[local]
2026-10-24 14:40:20.898 EEST [3843260:6] LOG: client backend (PID 3843334)
was terminated by signal 6: Aborted

With diagnostic log added in btvacuumcleanup(), I can see:
2026-10-24 14:33:48.916 EEST [3829584:7] psql LOG: !!!btvacuumcleanup|
stats->pages_deleted: 4927, stats->pages_free: 4928
2026-10-24 14:33:48.916 EEST [3829584:8] psql CONTEXT: while scanning
relation "public.t"
2026-10-24 14:33:48.916 EEST [3829584:9] psql STATEMENT: VACUUM t
TRAP: failed Assert("stats->pages_deleted >= stats->pages_free"), File:
"nbtree.c", Line: 1209, PID: 3829584

Reproduced starting from 0f0cfb494.