Explicitly use spawn multiprocessing context

Closes #175
Closes #201

Author: jeromekelleher

bio2zarr/core.py

@@ -187,7 +187,7 @@ class ProgressConfig:
 # progressable thing happening per source process. This is
 # probably fine in practise, but there could be corner cases
 # where it's not. Something to watch out for.
-_progress_counter = multiprocessing.Value("Q", 0)
+_progress_counter = None
 
 
 def update_progress(inc):
@@ -206,18 +206,30 @@ def set_progress(value):
         _progress_counter.value = value
 
 
+def setup_progress_counter(counter):
+    global _progress_counter
+    _progress_counter = counter
+
+
 class ParallelWorkManager(contextlib.AbstractContextManager):
     def __init__(self, worker_processes=1, progress_config=None):
+        # Need to specify this explicitly to suppport Macs and
+        # for future proofing.
+        ctx = multiprocessing.get_context("spawn")
+        global _progress_counter
+        _progress_counter = ctx.Value("Q", 0)
         if worker_processes <= 0:
             # NOTE: this is only for testing, not for production use!
             self.executor = SynchronousExecutor()
         else:
             self.executor = cf.ProcessPoolExecutor(
                 max_workers=worker_processes,
+                mp_context=ctx,
+                initializer=setup_progress_counter,
+                initargs=(_progress_counter,),
             )
         self.futures = set()
 
-        set_progress(0)
         if progress_config is None:
             progress_config = ProgressConfig()
         self.progress_config = progress_config

tests/test_core.py

@@ -56,7 +56,6 @@ def test_bad_min_max(self, min_value, max_value):
             core.min_int_dtype(min_value, max_value)
 
 
-@pytest.mark.skipif(sys.platform == "darwin", reason="Issue #75")
 class TestParallelWorkManager:
     @pytest.mark.parametrize("total", [1, 10, 2**63])
     @pytest.mark.parametrize("workers", [0, 1])