Issue #21362: concurrent.futures does not validate that max_workers is proper

brianquinlan · brianquinlan · commit 20efceb75790 · 2014-05-17T13:51:10.000-07:00
diff --git a/Doc/library/concurrent.futures.rst b/Doc/library/concurrent.futures.rst
@@ -175,6 +175,8 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
    An :class:`Executor` subclass that executes calls asynchronously using a pool
    of at most *max_workers* processes.  If *max_workers* is ``None`` or not
    given, it will default to the number of processors on the machine.
+   If *max_workers* is lower or equal to ``0``, then a :exc:`ValueError`
+   will be raised.
 
    .. versionchanged:: 3.3
       When one of the worker processes terminates abruptly, a
diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
@@ -334,6 +334,9 @@ def __init__(self, max_workers=None):
         if max_workers is None:
             self._max_workers = os.cpu_count() or 1
         else:
+            if max_workers <= 0:
+                raise ValueError("max_workers must be greater than 0")
+
             self._max_workers = max_workers
 
         # Make the call queue slightly larger than the number of processes to
diff --git a/Lib/concurrent/futures/thread.py b/Lib/concurrent/futures/thread.py
@@ -87,6 +87,9 @@ def __init__(self, max_workers):
             max_workers: The maximum number of threads that can be used to
                 execute the given calls.
         """
+        if max_workers <= 0:
+            raise ValueError("max_workers must be greater than 0")
+
         self._max_workers = max_workers
         self._work_queue = queue.Queue()
         self._threads = set()
diff --git a/Lib/test/test_concurrent_futures.py b/Lib/test/test_concurrent_futures.py
@@ -425,6 +425,13 @@ def test_no_stale_references(self):
         self.assertTrue(collected,
                         "Stale reference not collected within timeout.")
 
+    def test_max_workers_negative(self):
+        for number in (0, -1):
+            with self.assertRaisesRegexp(ValueError,
+                                         "max_workers must be greater "
+                                         "than 0"):
+                self.executor_type(max_workers=number)
+
 
 class ThreadPoolExecutorTest(ThreadPoolMixin, ExecutorTest, unittest.TestCase):
     def test_map_submits_without_iteration(self):
