Deadlock with custom MPI communicators in JIT

[Patrick Farrell]

The following code used to work with SWIG, but deadlocks since SWIG was removed:

from dolfin import *
comm = mpi_comm_self()
world = mpi_comm_world()

if world.rank == 0:
    mesh = UnitIntervalMesh(comm, 2)
    V = FunctionSpace(mesh, "CG", 1)
    bc = DirichletBC(V, 0.0, "on_boundary")

With SWIG, I get

[pefarrell@aoibheann:/tmp]$ mpiexec -n 2 python demo2.py 
[pefarrell@aoibheann:/tmp]$ 

Since SWIG's removal, I get

[pefarrell@aoibheann:/tmp]$ mpiexec -n 2 python3 demo2.py 
Calling FFC just-in-time (JIT) compiler, this may take some time.

and it hangs.

[Tormod Landet]

The problem is that this if statment branch does not pass the MPI Comm to CompiledSubDomain (and CompiledSubDomain must be extended to be able to receive a comm)

[Tormod Landet]

@chris_richardson : This is probably the reason why the test_mpi_dependent_jiting unit test fails

[Patrick Farrell]

Allow CompiledSubDomain to take in an MPI communicator.

In DirichletBC, pass the communicator from the mesh to the CompiledSubDomain (if you have one).

Fixes issue #977.

→ <<cset 4b1cad4e9d97>>