Fix for the issue #3646.

[dmitry-ganyushin]

It looks like the default for macos and python 3.8+ "spawn" method does not work on macos, swith to "fork" for all situations. https://bugs.python.org/issue33725. Rebased on the current master.

[eisenhauer]

"Fix" is a strong word here. This change avoids a problem via an unknown mechanism, but I haven't seen anything that indicates that it repairs it. The docs for the python multiprocessing package say:
On macOS, the *spawn* start method is now the default. The *fork* start method should be considered unsafe as it can lead to crashes of the subprocess as macOS system libraries may start threads. See :issue:33725.

[dmitry-ganyushin]

That test shows that the manual is only partly correct: the "spawn" method does not work universally as it is expected by Python principles. It yields the following traceback for the test:
Traceback (most recent call last):
File "/Users/runner/miniforge3/conda-bld/adios2-split_1685656764632/work/testing/adios2/engine/dataman/TestDataMan1D.py", line 34, in test_run
s.start()
File "/Users/runner/miniforge3/conda-bld/adios2-split_1685656764632/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placeh/lib/python3.10/multiprocessing/process.py", line 121, in start
self._popen = self._Popen(self)
File "/Users/runner/miniforge3/conda-bld/adios2-split_1685656764632/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placeh/lib/python3.10/multiprocessing/context.py", line 224, in _Popen
return _default_context.get_context().Process._Popen(process_obj)
File "/Users/runner/miniforge3/conda-bld/adios2-split_1685656764632/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placeh/lib/python3.10/multiprocessing/context.py", line 288, in _Popen
return Popen(process_obj)
File "/Users/runner/miniforge3/conda-bld/adios2-split_1685656764632/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placeh/lib/python3.10/multiprocessing/popen_spawn_posix.py", line 32, in init
super().init(process_obj)
File "/Users/runner/miniforge3/conda-bld/adios2-split_1685656764632/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placeh/lib/python3.10/multiprocessing/popen_fork.py", line 19, in init
self._launch(process_obj)
File "/Users/runner/miniforge3/conda-bld/adios2-split_1685656764632/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placeh/lib/python3.10/multiprocessing/popen_spawn_posix.py", line 47, in _launch
reduction.dump(process_obj, fp)
File "/Users/runner/miniforge3/conda-bld/adios2-split_1685656764632/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placeh/lib/python3.10/multiprocessing/reduction.py", line 60, in dump
ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle '_io.TextIOWrapper' object

[eisenhauer]

I'm familiar with the details, and if there was an open Python issue or a PR for changing it back to fork in a future version, I'd be more inclined to think that this is the right thing to do. I just haven't seen anything like that.

But, you know, it doesn't matter. This is a minor test, duplicating a problem a user had. Lets just merge.

[ax3l]

cc @vicentebolea for release_29?