default partition and concurrent attach partition

Hi,

Starting a new thread to discuss a bug related to $subject that Hao Wu
reported on thread titled "ALTER TABLE .. DETACH PARTITION
CONCURRENTLY" [1].  I have been able to reproduce the bug using steps
that Hao gave in that email:

create table tpart (i int, j int) partition by range(i);
create table tpart_1 (like tpart);
create table tpart_2 (like tpart);
create table tpart_default (like tpart);
alter table tpart attach partition tpart_1 for values from (0) to (100);
alter table tpart attach partition tpart_default default;
insert into tpart_2 values (110,110), (120,120), (150,150);

Session 1:

begin;
alter table tpart attach partition tpart_2 for values from (100) to (200);

Session 2:

begin;
insert into tpart values (110,110), (120,120), (150,150);
<blocks waiting for the concurrent attach to finish>

Session 1:

end;

Session 2:

select tableoid::regclass, * from tpart;
end;

The select will show that rows inserted by session 2 are inserted into
tpart_default, whereas after successfully attaching tpart_2, they do
not actually belong there.

The problem is that when session 2 inserts those rows into tpart, it
only knows about 2 partitions: tpart_1, tpart_default, of which it
selects tpart_default to insert those rows into.  When tpart_default
is locked to perform the insert, it waits for session 1 to release the
lock taken on tpart_default during the attach command.  When it is
unblocked, it proceeds to finish the insert without rechecking the
partition constraint which would have been updated as result of a new
partition having been added to the parent.

Note that we don't normally check the partition constraint when
inserting a row into a partition if the insert occurs via tuple
routing, which makes sense for non-default partitions whose partition
constraint cannot change due to concurrent activity.  But this test
case has shown that the assumption is not safe for a default partition
whose constraint is a function of other partitions that exist as of
when the insert occurs.

By the way, if you reverse the order of operations between session 1
and 2 such that the insert by session 2 occurs first and then the
attach by session 1, then you will correctly get this error from the
attach command:

ERROR:  updated partition constraint for default partition
"tpart_default" would be violated by some row

Attached is a patch to fix things on the insert side.

-- 
Amit Langote
EnterpriseDB: http://www.enterprisedb.com


[1] https://www.postgresql.org/message-id/DM5PR0501MB3910E97A9EDFB4C775CF3D75A42F0%40DM5PR0501MB3910.namprd05.prod.outlook.com