error message output in Jupyter notebook

[oskooi]

The following example which intentionally involves a bug related to a missing direction parameter in the FluxRegion object causes a Jupyter notebook kernel failure with the message: The kernel appears to have died. It will restart automatically..

import meep as mp

cell = mp.Vector3(16,16,0)
geometry = [mp.Block(mp.Vector3(12,1,mp.inf),
                     center=mp.Vector3(-2.5,-3.5),
                     material=mp.Medium(epsilon=12)),
            mp.Block(mp.Vector3(1,12,mp.inf),
                     center=mp.Vector3(3.5,2),
                     material=mp.Medium(epsilon=12))]
pml_layers = [mp.PML(1.0)]
resolution = 10

sources = [mp.Source(mp.ContinuousSource(wavelength=2*(11**0.5), width=20),
                     component=mp.Ez,
                     center=mp.Vector3(-6,-3.5),
                     size=mp.Vector3(1,1.2))]

sim = mp.Simulation(cell_size=cell,
                    boundary_layers=pml_layers,
                    geometry=geometry,
                    sources=sources,
                    resolution=resolution)

### error: missing 'direction' parameter in FluxRegion
tran = sim.add_flux(1/2*(11**0.5), 0, 1, mp.FluxRegion(center=mp.Vector3(3.5,2), size=mp.Vector3(2,1.2)))

sim.run(until=200)

The error message from Meep which describes the cause does not appear in the notebook but rather in the terminal which launched the notebook:

meep: Could not determine normal direction for given grid_volume.
application called MPI_Abort(MPI_COMM_WORLD, 1) - process 0

It would be useful, for debugging purposes, etc., to have all error messages appear directly within the notebook.

[stevengj]

For MPI jobs, I think we may need to continue calling MPI_Abort to make sure that all the processes stop working on an error. But for single-process functions I think it may be as simple as changing the meep::abort function to throw an exception, and adding an %except handler in SWIG to make sure this is caught as a Python exception.

That is, edit https://github.com/NanoComp/meep/blob/master/src/mympi.cpp#L121-L132 — if count_processors() == 1, then throw an exception rather than calling abort/exit.