testing.CrossValidation & co.: use forkserver process start method on…

… OS X

Orange/evaluation/testing.py

@@ -1,3 +1,4 @@
+import sys
 import multiprocessing as mp
 from itertools import product
 from threading import Thread
@@ -19,10 +20,14 @@
 __all__ = ["Results", "CrossValidation", "LeaveOneOut", "TestOnTrainingData",
            "ShuffleSplit", "TestOnTestData", "sample"]
 
+
+# Workaround for locks on Macintosh
+# https://pythonhosted.org/joblib/parallel.html#bad-interaction-of-multiprocessing-and-third-party-libraries
+MP_CONTEXT = mp.get_context('forkserver' if sys.platform == 'darwin' else None)
+
 mp_result = namedtuple('mp_result', ('fold_idx', 'learner_idx', 'model',
                                      'failed', 'n_values', 'values', 'probs'))
 
-
 def _identity(x):
     return x
 
@@ -349,7 +354,7 @@ def _callback_percent(n_steps, queue):
                 self._callback(percent)
 
         results = []
-        with joblib.Parallel(n_jobs=n_jobs) as parallel:
+        with joblib.Parallel(n_jobs=n_jobs, backend=MP_CONTEXT) as parallel:
             tasks = (joblib.delayed(_mp_worker)(*args) for args in args_iter)
             thread = Thread(target=lambda: results.append(parallel(tasks)))
             thread.start()