[mpiwg-hybridpm] First draft of Continuations section

[Joseph Schuchart]

Dear all,

I have written and attached a first draft of the continuations section 
that I would like to discuss in one of the upcoming meetings. Right now 
it is embedded at the end Section 3 (Point-to-Point following test and 
wait etc) where I felt it would fit well, I'm open to other suggestions 
though.

There are still some open TODOs that I'd like to discuss. In particular, 
I'm grappling with the handling of statuses. As suggested during 
previous discussions, I removed the flag argument to MPI_Continue such 
that the MPI implementation is allowed to invoke the continuations 
directly (can be controlled through an info key though). That means that 
after the call to MPI_Continue there is no good reason to inspect the 
statuses anymore. However, we may still want the user to provide the 
status object(s) in order to avoid exposing implementation-internal 
memory in the callback. But then I am worried that this is a source of 
error, with users providing pointers to stack memory that goes out of 
scope before the continuation is invoked.

Also, I am unsure whether and how this whole mechanism works with 
Fortran. The python generator happily generates the interface but I have 
no idea whether that is correct...

Looking forward to discussing this on the call!

Cheers
Joseph
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