Add warning on Python 39 for macs

Closes sgkit-dev#209

bio2zarr/core.py

@@ -7,8 +7,10 @@
 import multiprocessing
 import os
 import os.path
+import sys
 import threading
 import time
+import warnings
 
 import humanfriendly
 import numcodecs
@@ -87,6 +89,11 @@ def du(path):
 
 
 class SynchronousExecutor(cf.Executor):
+    # Arguably we should use workers=0 as the default and use this
+    # executor implementation. However, the docs are fairly explicit
+    # about saying we shouldn't instantiate Future objects directly,
+    # so it's best to keep this as a semi-secret debugging interface
+    # for now.
     def submit(self, fn, /, *args, **kwargs):
         future = cf.Future()
         future.set_result(fn(*args, **kwargs))
@@ -215,6 +222,22 @@ def setup_progress_counter(counter):
     _progress_counter = counter
 
 
+def warn_py39_mac():
+    if sys.platform == "darwin" and sys.version_info[:2] == (3, 9):
+        warnings.warn(
+            "There is a known issue with bio2zarr on MacOS Python 3.9 "
+            "in which OS-level named semaphores are leaked. "
+            "You will also probably see warnings like 'There appear to be N "
+            "leaked semaphore objects at shutdown'. "
+            "While this is likely harmless for a few runs, it could lead to "
+            "issues if you do a lot of conversion. To get prevent this issue "
+            "either: (1) use --worker-processes=0 or (2) upgrade to a newer "
+            "Python version. See https://github.com/sgkit-dev/bio2zarr/issues/209 "
+            "for more details.",
+            stacklevel=2,
+        )
+
+
 class ParallelWorkManager(contextlib.AbstractContextManager):
     def __init__(self, worker_processes=1, progress_config=None):
         # Need to specify this explicitly to suppport Macs and
@@ -223,9 +246,11 @@ def __init__(self, worker_processes=1, progress_config=None):
         global _progress_counter
         _progress_counter = ctx.Value("Q", 0)
         if worker_processes <= 0:
-            # NOTE: this is only for testing, not for production use!
+            # NOTE: this is only for testing and debugging, not for
+            # production. See note on the SynchronousExecutor class.
             self.executor = SynchronousExecutor()
         else:
+            warn_py39_mac()
             self.executor = cf.ProcessPoolExecutor(
                 max_workers=worker_processes,
                 mp_context=ctx,

tests/test_core.py

@@ -104,6 +104,17 @@ def test_error_in_workers_on_exit(self, total, workers):
                 # Raises a TypeError:
                 pwm.submit(frozenset, j)
 
+    @pytest.mark.parametrize("workers", [0, 1, 2])
+    def test_warn_mac_os(self, recwarn, workers):
+        with core.ParallelWorkManager(workers):
+            pass
+        if sys.platform == "darwin" and sys.version_info[:2] == (3, 9):
+            assert len(recwarn) >= 1
+            w = recwarn.pop(UserWarning)
+            assert str(w.message).startswith("There is a known issue")
+        else:
+            assert len(recwarn) == 0
+
 
 class TestChunkAlignedSlices:
     @pytest.mark.parametrize(