[mpich-discuss] Behavioral change for MPI_Ibarrier(MPI_COMM_SELF, &r) in mpich-3.2

[Jed Brown]

In MPICH 3.2, MPI_Ibarrier(MPI_COMM_SELF,&r) returns r ==
MPI_REQUEST_NULL.  This is also the case for other communicators of size
1.  Previous versions of MPICH and also all versions of Open MPI return
a valid request not equal to MPI_REQUEST_NULL.  This bit me because it
was natural to compare to MPI_REQUEST_NULL in a conditional for an
Ibarrier/Issend/Iprobe algorithm.  It can be worked around with a
slightly more complicate conditional, but is this change intentional?

  The call returns a request handle, which must be passed to a
  completion call.
  -- MPI-3 §5.12

This does not say "unless the communicator has size 1".
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