python3: set 'fork' start method

In Python 3 start method 'spawn' in multiprocessing module becomes
default on Mac OS [1].

The 'spawn' method causes re-execution of some code, which is already
executed in the main process. At least it is seen on the lib/__init__.py
code, which removes the 'var' directory. Some other code may have side
effects too, it requires investigation.

The method also requires object serialization that doesn't work when
objects use lambdas, whose for example used in class TestSuite
(lib/test_suite.py).

The latter problem is easy to fix, but the former looks more
fundamental. So we stick to the 'fork' method now.

The new start method is available on Python 3 only:

Traceback (most recent call last):
  File "../../test/test-run.py", line 227, in <module>
    multiprocessing.set_start_method('fork')
AttributeError: 'module' object has no attribute 'set_start_method'

1. https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods

Fixes #265
Part of #20

Co-authored-by: Alexander Turenko <[email protected]>

Author: ligurio

test-run.py

@@ -56,6 +56,7 @@
 from lib import Options
 from lib.colorer import color_stdout
 from lib.utils import print_tail_n
+from lib.utils import PY3
 from lib.worker import get_task_groups
 from lib.worker import get_reproduce_file
 from lib.worker import reproduce_task_groups
@@ -218,6 +219,23 @@ def main_consistent():
 
 
 if __name__ == "__main__":
+    # In Python 3 start method 'spawn' in multiprocessing module becomes
+    # default on Mac OS.
+    #
+    # The 'spawn' method causes re-execution of some code, which is already
+    # executed in the main process. At least it is seen on the
+    # lib/__init__.py code, which removes the 'var' directory. Some other
+    # code may have side effects too, it requires investigation.
+    #
+    # The method also requires object serialization that doesn't work when
+    # objects use lambdas, whose for example used in class TestSuite
+    # (lib/test_suite.py).
+    #
+    # The latter problem is easy to fix, but the former looks more
+    # fundamental. So we stick to the 'fork' method now.
+    if PY3:
+        multiprocessing.set_start_method('fork')
+
     # don't sure why, but it values 1 or 2 gives 1.5x speedup for parallel
     # test-run (and almost doesn't affect consistent test-run)
     os.environ['OMP_NUM_THREADS'] = '2'