Bug in amcheck?

Hi hackers.

We see the following error reported by amcheck (I have added dump of
opaque) when it interleaves with autovacuum and cancel pt:

ERROR:  mismatch between parent key and child high key in index
"pg_attribute_relid_attnam_index"
DETAIL:  Target block=274, target opaque->flags=0, child block=427,
child opaque=11, target page lsn=1/484A8FC8.
CONTEXT:  SQL statement "SELECT bt_index_parent_check(indexrelid, true,
true) from pg_index"

So child has BTP_HALF_DEAD bit set.
Autovacuum is interrupted in this place in _bt_pagedel:

        /*
         * Check here, as calling loops will have locks held, preventing
         * interrupts from being processed.
         */
        CHECK_FOR_INTERRUPTS();

Reproducing it is not so easy.
First of all I added sleep here:

        /*
         * Check here, as calling loops will have locks held, preventing
         * interrupts from being processed.
         */
        pg_usleep(10000);
        CHECK_FOR_INTERRUPTS();

Then I create two procedures:

create or replace procedure create_tables(tables integer, partitions
integer) as $$
declare
    i integer;
    j integer;
begin
    for i in 1..tables
    loop
        execute 'DROP TABLE IF EXISTS t_' || i;
        execute 'CREATE TABLE t_' || i || '(pk integer) partition by
range (pk)';
        for j in 1..partitions
        loop
            execute 'create table p_'||i||'_'||j||' partition of
t_'||i||' for values from ('||j||') to ('||(j + 1)||')';
        end loop;
        execute 'insert into t_'||i||' values
(generate_series(1,'||partitions||'))';
    end loop;
end;
$$ language plpgsql;

and

create or replace procedure run_amcheck() as $$
begin
    loop
        if (select count(*) from pg_stat_activity where
backend_type='autovacuum worker') > 0
        then
            raise notice 'Run amcheck!';
            perform bt_index_parent_check(indexrelid, true, true) from
pg_index;
        end if;
        perform pg_sleep(1);
    end loop;
end;
$$ language plpgsql;

Then I run concurrently run_amcheck()
and the following script for pgbench:

call create_tables(2,1000);
select pg_sleep(2);

If the problem is not reproduced, then cancel run_amcheck()  and restart
it once again.

Backtrace (pg16) is the following:

  * frame #0: 0x00000001017b6aac
amcheck.dylib`bt_child_highkey_check(state=0x000000010c846318,
target_downlinkoffnum=37, loaded_child="\U00000001", target_level=1) at
verify_nbtree.c:2146:23
    frame #1: 0x00000001017b7fd8
amcheck.dylib`bt_child_check(state=0x000000010c846318,
targetkey=0x000000013c01c448, downlinkoffnum=37) at verify_nbtree.c:2262:2
    frame #2: 0x00000001017b5f4c
amcheck.dylib`bt_target_page_check(state=0x000000010c846318) at
verify_nbtree.c:1623:4
    frame #3: 0x00000001017b3908
amcheck.dylib`bt_check_level_from_leftmost(state=0x000000010c846318,
level=(level = 1, leftmost = 3, istruerootlevel = false)) at
verify_nbtree.c:859:3
    frame #4: 0x00000001017b24e8
amcheck.dylib`bt_check_every_level(rel=0x0000000140074f18,
heaprel=0x0000000130070148, heapkeyspace=true, readonly=true,
heapallindexed=true, rootdescend=true) at verify_nbtree.c:603:13
    frame #5: 0x00000001017b198c
amcheck.dylib`bt_index_check_internal(indrelid=2674, parentcheck=true,
heapallindexed=true, rootdescend=true) at verify_nbtree.c:362:3
    frame #6: 0x00000001017b1a78
amcheck.dylib`bt_index_parent_check(fcinfo=0x000000010c83b040) at
verify_nbtree.c:242:2

I wonder if we should add P_ISHALFDEAD(opaque) for child page?