HDF5File.write() should check function names in parallel

[Tormod Landet]

Updated: The following gives no error, but produces corrupt files due to different random function name order on different parallel processes. This can easily happen when using dictionaries to store functions to be output

import dolfin

mesh = dolfin.UnitSquareMesh(4, 4)
V = dolfin.FunctionSpace(mesh, 'CG', 1)

funcs = []
for i in range(10):
    name = 'u_%d' % i
    funcs.append((name, dolfin.Function(V)))

import random
random.shuffle(funcs)

with dolfin.HDF5File(mesh.mpi_comm(), 'test.h5', 'w') as h5:
    h5.write(mesh, '/mesh')
    for name, u in funcs:
        h5.write(u, '/%s' % name)

[Michal Habera]

Hi Tormod, I tried to run this with XDMFFile::write_checkpoint method and it failed on XDMFFile::check_function_name(std::string function_name). This check method was added to make sure that function name in parallel is the same on all processes, otherwise HDF paths are completly messed up. Maybe all we can do is add the similar check for HDFFile interface, any other thoughts?

[Tormod Landet]

That makes sense. Although, my "randomization" was just dictionary iteration, which should be deterministic on all processes since dictionary insertion should have been the same on all processes. But, I might have made a mistake somewhere

[Tormod Landet]

Updated description to make the real reason apparent. Thanks @michalhabera !

[Michal Habera]

Unlike in pyton2.7 in python3.3 dictionary iteration is by default not deterministic. So I think the line for name, u in funcs just iterates in different order on different processes. See https://stackoverflow.com/questions/14956313/why-is-dictionary-ordering-non-deterministic

[Tormod Landet]

Then I learned something today as well :-) I was sure it was somehow related to different function spaces being output after one another and some obscure bug related to internal state in HDF5File. Then, it was just my fault as normal ;-)