"can't pickle _thread.lock" in ResumableDatasetLoop with SubprocessRunner in Python 3

[TaiSakuma]

at 8db62fc, a few commits after v0.24.2

Traceback (most recent call last):
  File "./bdphi-scripts/bdphiROC/twirl_mktbl_heppy.py", line 721, in <module>
    main()
  File "./bdphi-scripts/bdphiROC/twirl_mktbl_heppy.py", line 62, in main
    run(reader_collector_pairs)
  File "./bdphi-scripts/bdphiROC/twirl_mktbl_heppy.py", line 609, in run
    treeName='tree'
  File "./bdphi-scripts/external/atheppy/atheppy/fw.py", line 110, in run
    self._run(loop)
  File "./bdphi-scripts/external/atheppy/atheppy/fw.py", line 285, in _run
    loop()
  File "./bdphi-scripts/external/alphatwirl/alphatwirl/datasetloop/loop.py", line 60, in __call__
    pickle.dump(self.reader, f, protocol=pickle.HIGHEST_PROTOCOL)
TypeError: can't pickle _thread.lock objects

The error occures in ResumableDatasetLoop:

alphatwirl/alphatwirl/datasetloop/loop.py

Lines 52 to 61 in 8db62fc

	def __call__(self):
	self.reader.begin()
	for dataset in self.datasets:
	self.reader.read(dataset)
	path = os.path.join(self.workingarea.path, 'reader.p.gz')
	with gzip.open(path, 'wb') as f:
	pickle.dump(self.reader, f, protocol=pickle.HIGHEST_PROTOCOL)
	return self.reader.end()

• This happens only in Python 3, not in Python 2.7
• This happens only in the parrallel mode subprocess, not in htcondor.

The proc in SubprocessRunner might not be picklable in Python 3:

alphatwirl/alphatwirl/concurrently/SubprocessRunner.py

Lines 44 to 50 in 8db62fc

	proc = subprocess.Popen(
	args,
	stdout=subprocess.PIPE if self.pipe else None,
	stderr=subprocess.PIPE if self.pipe else None,
	cwd=taskdir
	)
	self.running_procs.append(proc)

• can try to pickle before self.reader.read(dataset) or self.reader.begin()

[TaiSakuma]

• now pickle before start reading events 2eea376
• this is not covered by unit tests