`preprocess` fails for `n_jobs`>1

Running the latest master I run into issues with the parallelised preprocessing (from #277):

from braindecode.preprocessing import Preprocessor, preprocess, filterbank
from braindecode.datasets.moabb import MOABBDataset

ds = MOABBDataset(dataset_name='Schirrmeister2017',
                           subject_ids=[1], )
splits = ds.split([[i] for i in range(len(ds.datasets))])

fbank = [
            Preprocessor(
                apply_on_array=False,
                fn=filterbank,
                frequency_bands=[(0, 10)],
                verbose=False,
                n_jobs=1
            ),
        ]

for split_i, split_ds in splits.items():
    preprocess(concat_ds=split_ds, preprocessors=fbank, n_jobs=2)

gives the following output:

Creating RawArray with float64 data, n_channels=128, n_times=1225545
    Range : 0 ... 1225544 =      0.000 ...  2451.088 secs
Ready.
Creating RawArray with float64 data, n_channels=128, n_times=616535
    Range : 0 ... 616534 =      0.000 ...  1233.068 secs
Ready.
320 events found
Event IDs: [1 2 3 4]
160 events found
Event IDs: [1 2 3 4]
joblib.externals.loky.process_executor._RemoteTraceback: 
"""
Traceback (most recent call last):
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/joblib/externals/loky/process_executor.py", line 431, in _process_worker
    r = call_item()
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/joblib/externals/loky/process_executor.py", line 285, in __call__
    return self.fn(*self.args, **self.kwargs)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/joblib/_parallel_backends.py", line 595, in __call__
    return self.func(*args, **kwargs)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/joblib/parallel.py", line 262, in __call__
    return [func(*args, **kwargs)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/joblib/parallel.py", line 262, in <listcomp>
    return [func(*args, **kwargs)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/braindecode/preprocessing/preprocess.py", line 223, in _preprocess
    _preprocess_raw_or_epochs(ds.raw, preprocessors)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/braindecode/preprocessing/preprocess.py", line 220, in _preprocess_raw_or_epochs
    preproc.apply(raw_or_epochs)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/braindecode/preprocessing/preprocess.py", line 62, in apply
    self._try_apply(raw_or_epochs)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/braindecode/preprocessing/preprocess.py", line 74, in _try_apply
    self.fn(raw_or_epochs, **self.kwargs)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/braindecode/preprocessing/preprocess.py", line 473, in filterbank
    raw.add_channels(all_filtered)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/mne/channels/channels.py", line 1051, in add_channels
    self._data.base.resize(n_bytes)
TypeError: mmap can't resize a readonly or copy-on-write memory map.
"""
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/IPython/core/interactiveshell.py", line 3437, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-3-afd28816663e>", line 19, in <module>
    preprocess(concat_ds=split_ds, preprocessors=fbank, n_jobs=2)
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/braindecode/preprocessing/preprocess.py", line 158, in preprocess
    list_of_ds = Parallel(n_jobs=n_jobs)(
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/joblib/parallel.py", line 1054, in __call__
    self.retrieve()
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/joblib/parallel.py", line 933, in retrieve
    self._output.extend(job.get(timeout=self.timeout))
  File "/home/dan/Documents/AGBALL/RiemNetEnv/lib/python3.8/site-packages/joblib/_parallel_backends.py", line 542, in wrap_future_result
    return future.result(timeout=timeout)
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 444, in result
    return self.__get_result()
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 389, in __get_result
    raise self._exception
TypeError: mmap can't resize a readonly or copy-on-write memory map.

[hubertjb]

I was just able to reproduce the issue on a server, I’ll look into it @dcwil

[dcwil]

Hi @bruAristimunha

Looks like I can still reproduce the error in 0.6, using the same snippet