Add osx+py38 case for avoid multiprocessing issue (#2800)

* add osx+py38 case for avoid multiprocessing issue

* add comment, fix warning

* extend comment

Co-Authored-By: Radim Řehůřek <me@radimrehurek.com>

* Update gensim/utils.py

* Update gensim/utils.py

Co-Authored-By: Michael Penkov <m@penkov.dev>

Co-authored-by: Radim Řehůřek <me@radimrehurek.com>
Co-authored-by: Michael Penkov <m@penkov.dev>

gensim/utils.py

@@ -1238,7 +1238,11 @@ def run(self):
             self.q.put(wrapped_chunk.pop(), block=True)
 
 
-if os.name == 'nt':
+# Multiprocessing on Windows (and on OSX with python3.8+) uses "spawn" mode, which
+# causes issues with pickling.
+# So for these two platforms, use simpler serial processing in `chunkize`.
+# See https://github.com/RaRe-Technologies/gensim/pull/2800#discussion_r410890171
+if os.name == 'nt' or (sys.platform == "darwin" and sys.version_info >= (3, 8)):
     def chunkize(corpus, chunksize, maxsize=0, as_numpy=False):
         """Split `corpus` into fixed-sized chunks, using :func:`~gensim.utils.chunkize_serial`.
 
@@ -1260,7 +1264,8 @@ def chunkize(corpus, chunksize, maxsize=0, as_numpy=False):
 
         """
         if maxsize > 0:
-            warnings.warn("detected Windows; aliasing chunkize to chunkize_serial")
+            entity = "Windows" if os.name == 'nt' else "OSX with python3.8+"
+            warnings.warn("detected %s; aliasing chunkize to chunkize_serial" % entity)
         for chunk in chunkize_serial(corpus, chunksize, as_numpy=as_numpy):
             yield chunk
 else: