Thread: ERROR: tuple concurrently updated when modifying privileges

Hello, hackers

we witnessed this slightly misleading error in production and it took us a while to figure out what was taking place.
Below are reproduction steps:


-- setup
create table trun(cate int4);

-- session 1
begin;
truncate table trun;

-- session 2
grant insert on table trun to postgres;

-- session 1
end;

-- session 2:
ERROR: XX000: tuple concurrently updated
LOCATION: simple_heap_update, heapam.c:4474

Apparently the tuple in question is the pg_class entry of the table being truncated. I didn't look too deep into the
cause,but I'm certain the error message could be improved at least.
 

Regards,
Nick.

On Tue, May 14, 2019 at 08:08:05AM +0200, Chris Travers wrote:
> Having thought about this a bit, I think the best solution would be to have
> grant take out an access share lock to the tables granted.  This would
> prevent concurrent alter table operations from altering the schema
> underneath the grant as well, and thus possibly cause other race conditions.
>
> Any thoughts?

"tuple concurrently updated" is an error message which should never be
user-facing, and unfortunately there are many scenarios where it can
be triggered by playing with concurrent DDLs:
https://postgr.es/m/20171228063004.GB6181@paquier.xyz

If you have an idea of patch, could you write it?  Having an isolation
test case would be nice as well.
--
Michael