Re: Make relfile tombstone files conditional on WAL level

On Wed, Sep 29, 2021 at 4:07 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> Hmm, on closer inspection, isn't the lack of real interlocking with
> checkpoints a bit suspect already?  What stops bgwriter from writing
> to the previous relfilenode generation's fd, if a relfilenode is
> recycled while BgBufferSync() is running?  Not sinval, and not the
> above code that only runs between BgBufferSync() invocations.

I managed to produce a case where live data is written to an unlinked
file and lost, with a couple of tweaks to get the right timing and
simulate OID wraparound.  See attached.  If you run the following
commands repeatedly with shared_buffers=256kB and
bgwriter_lru_multiplier=10, you should see a number lower than 10,000
from the last query in some runs, depending on timing.

create extension if not exists chaos;
create extension if not exists pg_prewarm;

drop table if exists t1, t2;
checkpoint;
vacuum pg_class;

select clobber_next_oid(200000);
create table t1 as select 42 i from generate_series(1, 10000);
select pg_prewarm('t1'); -- fill buffer pool with t1
update t1 set i = i; -- dirty t1 buffers so bgwriter writes some
select pg_sleep(2); -- give bgwriter some time

drop table t1;
checkpoint;
vacuum pg_class;

select clobber_next_oid(200000);
create table t2 as select 0 i from generate_series(1, 10000);
select pg_prewarm('t2'); -- fill buffer pool with t2
update t2 set i = 1 where i = 0; -- dirty t2 buffers so bgwriter writes some
select pg_sleep(2); -- give bgwriter some time

select pg_prewarm('pg_attribute'); -- evict all clean t2 buffers
select sum(i) as t2_sum_should_be_10000 from t2; -- have any updates been lost?