Unfortunately, there is a catch: MPI_COMM_SELF is only 
relevant/available/valid in the World Process Model (WPM), i.e., if 
using `MPI_Init`/`MPI_Finalize`. In the Sessions process model, 
predefined communicators are not available.

The life-time of datatypes is a known quirk and I think it was 
discovered after Sessions became part of the standard. They are not 
bound to any other MPI object and can survive complete shutdown of all 
sessions / the WPM. IIRC in Open MPI (but have to check), datatypes 
retain a reference on the internal MPI instance and it is the 
application's responsibility to free all MPI objects before shutdown. 
Once the last datatype/session/wpm is gone we release the instance.

I don't like the state of things there and it is problematic. For 
starters, it prevents complete session isolation (and the benefits that 
come with it, such as different threading levels). It's not clear to me 
how that can be rectified and I think the Forum is not clear on that 
either, which is why we ended up with this weird zombie state. If 
someone wants to open a ticket to start a discussion on this I'm happy 
to participate.

For the problem at hand though (as I understand it), maybe it's 
sufficient to add attributes to datatypes? I don't see why that would be 
a problem and if it helps with language adoption we have a good argument 
for it.

Cheers
Joseph

On 4/25/25 17:24, Alfredo Correa via mpiwg-languages wrote:
> Hi Sayan, On Fri, Apr 25, 2025 at 2: 03 PM Ghosh, Sayan 
> <sayan. ghosh@ pnnl. gov> wrote: Consider finalize-delete-callback 
> (this is what Alfredo is alluding to perhaps w. r. t 
> datatype-attached-to-environment) – that seems to rely on MPI_COMM_SELF
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> Hi Sayan,
>
> On Fri, Apr 25, 2025 at 2:03 PM Ghosh, Sayan <sayan.ghosh@pnnl.gov> wrote:
>
>       * Consider finalize-delete-callback (this is what Alfredo is
>         alluding to perhaps w.r.t datatype-attached-to-environment) –
>         that seems to rely on MPI_COMM_SELF callback (freeing
>         comm-self triggers callback)
>
>
>
> That is a good point. At first glance, attaching things to 
> MPI_COMM_WORLD or MPI_COMM_SELF would have a similar effect to 
> attaching stuff to the environment.
> I didn't think about this because I was reluctant to modify (in any 
> way) either of these special communicators, in particular MPI_COMM_WORLD.
> But MPI_COMM_SELF might still be a good candidate; others can point 
> out if there is a catch.
>
> Thanks,
> Alfredo
> _
> _
>
>