Can't use num_workers > 0 on Mac

[lewfish]

Raster Vision can be installed and used directly on a Mac (using macOS 12.1 and Python 3.9). Everything seems to work except I cannot use num_workers > 0. This has something to do with the fact that as of Python 3.8 on Mac (but not Linux), the default multiprocessing backend has changed from fork to spawn. See this issue.

This problem can be replicated using the following command and setting the num_workers argument in SemanticSegmentationGeoDataConfig in https://github.com/azavea/raster-vision/blob/master/integration_tests/semantic_segmentation/config.py.

python -m rastervision.pipeline.cli run inprocess integration_tests/semantic_segmentation/config.py \
  -a root_uri /Users/lewfish/data/int-test/ -a data_uri ./integration_tests/semantic_segmentation/

The error:

  File "/Users/lewfish/projects/raster-vision/rastervision_pytorch_backend/rastervision/pytorch_backend/pytorch_learner_backend.py", line 120, in train
    learner.main()
  File "/Users/lewfish/projects/raster-vision/rastervision_pytorch_learner/rastervision/pytorch_learner/learner.py", line 264, in main
    self.plot_dataloaders(self.cfg.data.preview_batch_limit)
  File "/Users/lewfish/projects/raster-vision/rastervision_pytorch_learner/rastervision/pytorch_learner/learner.py", line 895, in plot_dataloaders
    self.plot_dataloader(
  File "/Users/lewfish/projects/raster-vision/rastervision_pytorch_learner/rastervision/pytorch_learner/learner.py", line 886, in plot_dataloader
    x, y = next(iter(dl))
  File "/opt/homebrew/lib/python3.9/site-packages/torch/utils/data/dataloader.py", line 444, in __iter__
    return self._get_iterator()
  File "/opt/homebrew/lib/python3.9/site-packages/torch/utils/data/dataloader.py", line 390, in _get_iterator
    return _MultiProcessingDataLoaderIter(self)
  File "/opt/homebrew/lib/python3.9/site-packages/torch/utils/data/dataloader.py", line 1077, in __init__
    w.start()
  File "/opt/homebrew/Cellar/[email protected]/3.9.12_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/opt/homebrew/Cellar/[email protected]/3.9.12_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/opt/homebrew/Cellar/[email protected]/3.9.12_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/opt/homebrew/Cellar/[email protected]/3.9.12_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/opt/homebrew/Cellar/[email protected]/3.9.12_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/opt/homebrew/Cellar/[email protected]/3.9.12_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/opt/homebrew/Cellar/[email protected]/3.9.12_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
  File "stringsource", line 2, in rasterio._io.DatasetReaderBase.__reduce_cython__
TypeError: self._hds cannot be converted to a Python object for pickling