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

3 files changed, 13 insertions, 0 deletions

diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
index 07b5225..1299390 100644
--- a/Lib/concurrent/futures/process.py
+++ b/Lib/concurrent/futures/process.py
@@ -334,6 +334,9 @@ class ProcessPoolExecutor(_base.Executor):
         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
index f9beb0f..8d6081c 100644
--- a/Lib/concurrent/futures/thread.py
+++ b/Lib/concurrent/futures/thread.py
@@ -87,6 +87,9 @@ class ThreadPoolExecutor(_base.Executor):
             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
index c74b2ca..55254b5 100644
--- a/Lib/test/test_concurrent_futures.py
+++ b/Lib/test/test_concurrent_futures.py
@@ -425,6 +425,13 @@ class ExecutorTest:
         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):