Synchronization issue when recording

[nvitucci]

Hi all,

I've tried running the P300 notebook with a Muse 2 using Python 3 and muse-lsl 2.0.1 on a Linux machine. Besides a couple fixes on imports that I can add with a PR, I couldn't get the stimulus.start() + recording.start() section to work.
The Muse is correctly working and I can view the stream, but when I get to that section it either fails with a "Cannot find EEG stream" message or, if I invert the order of the calls, with an exception related to an empty list (sorry, I don't have the full exception handy at the moment, but I can attach it later).
The interesting bit is that, if I start the stimulus and press "Esc" within a couple seconds, the EEG stream will actually be found; the markers will not be found anyway, but the recording will go on. Any ideas on what it could be?

[nvitucci]

For completeness, when recording.start() is called before stimulus.start(), the output is as follows:

Looking for a EEG stream...
Started acquiring data.
Looking for a Markers stream...

Process Process-3:
Traceback (most recent call last):
  File "/usr/lib64/python3.6/multiprocessing/process.py", line 249, in _bootstrap
    self.run()
  File "/usr/lib64/python3.6/multiprocessing/process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "[...]/eeg-notebooks/venv/lib64/python3.6/site-packages/muselsl/record.py", line 43, in record
    inlet_marker = StreamInlet(marker_streams[0])
  File "[...]/eeg-notebooks/venv/lib64/python3.6/site-packages/pylsl/pylsl.py", line 641, in __init__
    self.sample = self.sample_type()
TypeError: 'list' object is not callable

[nvitucci]

I just found another interesting behaviour. If I skip the notebook part where the viewer is started (Step 2), and then I run the experiment launching recording.start() before stimulus.start(), both the EEG stream and the Markers stream are successfully recorded. It doesn't appear to be a problem if, instead, I launch the four processes manually from the command line in this sequence:

1. muselsl stream
2. muselsl view -b Qt5Agg
3. muselsl record
4. python stimulus_presentation/visual_p300_stripes.py

(In both cases I am using PyQt5 as a backend.)

Maybe some weird interactions among threads/processes when multiprocessing is used?