Обсуждение: Tighten asserts on ParallelWorkerNumber a little bit

Hi,

While hacking on something I happened to notice a couple asserts on
ParallelWorkerNumber when collecting shared instrumentation:

  Assert(ParallelWorkerNumber <= node->shared_info->num_workers);

This is not quire right, because num_workers is used to size arrays
indexed by ParallelWorkerNumber. And the comment in parallel.c also
claims (ParallelWorkerNumber < num_workers).

So AFAICS the assert(s) should be

  Assert(ParallelWorkerNumber < node->shared_info->num_workers);

I don't think we had issues with this not catching a bug. But it may be
a bit misleading, so worth fixing and (probably) backpatching.


regards

-- 
Tomas Vondra

Hi,

On Thu, Mar 12, 2026 at 07:59:43PM +0100, Tomas Vondra wrote:
> Hi,
> 
> While hacking on something I happened to notice a couple asserts on
> ParallelWorkerNumber when collecting shared instrumentation:
> 
>   Assert(ParallelWorkerNumber <= node->shared_info->num_workers);
> 
> This is not quire right, because num_workers is used to size arrays
> indexed by ParallelWorkerNumber. And the comment in parallel.c also
> claims (ParallelWorkerNumber < num_workers).
> 
> So AFAICS the assert(s) should be
> 
>   Assert(ParallelWorkerNumber < node->shared_info->num_workers);

I think that you're right. It looks like the first one has been introduced
by bf11e7ee2e3 and then the others are probably copy/paste.

> I don't think we had issues with this not catching a bug. But it may be
> a bit misleading, so worth fixing and (probably) backpatching.

I do agree.

With that patch applied, the remaining one:

$ git grep "<=.*num_workers"
src/backend/executor/nodeGatherMerge.c: Assert(nreaders <= castNode(GatherMerge, gm_state->ps.plan)->num_workers);

does not need to be fixed, so I think that your patch does not miss any and
LGTM.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

On 3/13/26 11:53, Bertrand Drouvot wrote:
> ...
>
>> So AFAICS the assert(s) should be
>>
>>   Assert(ParallelWorkerNumber < node->shared_info->num_workers);
> 
> I think that you're right. It looks like the first one has been introduced
> by bf11e7ee2e3 and then the others are probably copy/paste.
> 
>> I don't think we had issues with this not catching a bug. But it may be
>> a bit misleading, so worth fixing and (probably) backpatching.
> 
> I do agree.
> 
> With that patch applied, the remaining one:
> 
> $ git grep "<=.*num_workers"
> src/backend/executor/nodeGatherMerge.c: Assert(nreaders <= castNode(GatherMerge, gm_state->ps.plan)->num_workers);
> 
> does not need to be fixed, so I think that your patch does not miss any and
> LGTM.
> 

Thanks. Pushed and backpatched.

-- 
Tomas Vondra