Issue 6433: multiprocessing.pool.map hangs on empty list

Lib/multiprocessing/pool.py

@@ -207,6 +207,8 @@ class Pool(object):
             chunksize, extra = divmod(len(iterable), len(self._pool) * 4)
             if extra:
                 chunksize += 1
+        if len(iterable) == 0:
+            chunksize = 0
 
         task_batches = Pool._get_tasks(func, iterable, chunksize)
         result = MapResult(self._cache, chunksize, len(iterable), callback)

Lib/test/test_multiprocessing.py

@@ -990,6 +990,12 @@ class _TestPool(BaseTestCase):
         self.assertEqual(pmap(sqr, range(100), chunksize=20),
                          map(sqr, range(100)))
 
+    def test_map_chunksize(self):
+        try:
+            self.pool.map_async(sqr, [], chunksize=1).get(timeout=TIMEOUT1)
+        except multiprocessing.TimeoutError:
+            self.fail("pool.map_async with chunksize stalled on null list")
+
     def test_async(self):
         res = self.pool.apply_async(sqr, (7, TIMEOUT1,))
         get = TimingWrapper(res.get)

Misc/ACKS

@@ -199,6 +199,7 @@ Walter Dörwald
 Hans Eckardt
 Grant Edwards
 John Ehresman
+Eric Eisner
 Andrew Eland
 Lance Ellinghaus
 David Ely

Misc/NEWS

@@ -352,6 +352,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #6433: fixed issues with multiprocessing.pool.map hanging on empty list
+
 - Issue #6314: logging: Extra checks on the "level" argument in more places.
 
 - Issue #2622: Fixed an ImportError when importing email.messsage from a