Re: Adding a test for speculative insert abort case

Hi,

On 2019-04-30 17:15:55 -0700, Melanie Plageman wrote:
> Today, while poking around the table_complete_speculative code which Ashwin
> mentioned in [1], we were trying to understand when exactly we would
> complete a
> speculative insert by aborting.

(FWIW, it's on my todo queue to look at this)


> We added a logging message to heapam_tuple_complete_speculative before it
> calls
> heap_abort_speculative and ran the regression and isolation tests to see
> what
> test failed so we knew how to exercise this codepath.
> No tests failed, so we spent some time trying to understand when 'succeeded'
> would be true coming into heap_tuple_complete_speculative.
> 
> Eventually, we figured out that if one transaction speculatively inserts a
> tuple into a table with a unique index and then pauses before inserting the
> value into the index, and while it is paused, another transaction
> successfully
> inserts a value which would conflict with that value, it would result in an
> aborted speculative insertion.
> 
> t1(id,val)
> unique index t1(id)
> 
> s1: insert into t1 values(1, 'someval') on conflict(id) do update set val =
> 'someotherval';
> s1: pause in ExecInsert before calling ExecInsertIndexTuples
> s2: insert into t1 values(1, 'someval');
> s2: continue
> 
> We don't know of a way to add this scenario to the current isolation
> framework.

> Can anyone think of a good way to put this codepath under test?

Not easily so - that's why the ON CONFLICT patch didn't add code
coverage for it :(. I wonder if you could whip something up by having
another non-unique expression index, where the expression acquires a
advisory lock? If that advisory lock where previously acquired by
another session, that should allow to write a reliable isolation test?

Alternatively, as a fallback, there's a short pgbench test, I wonder if we
could just adapt that to use ON CONFLICT UPDATE?

Greetings,

Andres Freund