Deadlock risk while inserting directly into partition? - Mailing list pgsql-hackers

I noticed that while inserting directly into a partition table we
compute the PartitionCheckExpr by traversing all the parent partitions
via ExecPartitionCheck()->RelationGetPartitionQual()->generate_partition_qual().
We take AccessShareLock on parent tables while generating qual.

Now, on the other hand, while dropping constraint on a partitioned
table, we take the lock from parent to all the child tables.

I think taking locks in opposite directions can lead to deadlock in
these operations.

I have tried with the below example on HEAD.

Setup
=======
create or replace function func_dummy(price integer) returns integer as
$$
    begin
        raise notice 'hello from func_dummy';
        return price;
    end;
$$ language plpgsql immutable parallel unsafe;


CREATE TABLE pt_test (a int, c char(1000)) PARTITION BY range (a);
CREATE TABLE pt_test1 PARTITION OF pt_test FOR VALUES FROM (0) TO (100000);
CREATE TABLE pt_test2 PARTITION OF pt_test FOR VALUES FROM (100000) TO (400000);

ALTER TABLE pt_test ADD CONSTRAINT check_cons CHECK(func_dummy(a) == a);

Actual test
=============
Session-1
--------------
Add breakpoint in generate_partition_qual(). Perform below statement.
insert into pt_test2 values(100001, 'aaaa');

Now, stop in the debugger just before taking AccessShareLock on the
parent table.

Session-2
=========
 ALTER TABLE pt_test DROP CONSTRAINT check_cons;

You will see that session-2 is waiting to get a lock on pt_test2.
Then, continue debugging in session-1 which will lead to a deadlock.

Is this expected, if so why?

-- 
With Regards,
Amit Kapila.