Issue 19158: a rare race in BoundedSemaphore could allow .release() too often.

Author: Tim Peters

Lib/test/test_threading.py

@@ -468,6 +468,24 @@ def test_is_alive_after_fork(self):
                 self.assertEqual(0, status)
 
 
+    def test_BoundedSemaphore_limit(self):
+        # BoundedSemaphore should raise ValueError if released too often.
+        for limit in range(1, 10):
+            bs = threading.BoundedSemaphore(limit)
+            threads = [threading.Thread(target=bs.acquire)
+                       for _ in range(limit)]
+            for t in threads:
+                t.start()
+            for t in threads:
+                t.join()
+            threads = [threading.Thread(target=bs.release)
+                       for _ in range(limit)]
+            for t in threads:
+                t.start()
+            for t in threads:
+                t.join()
+            self.assertRaises(ValueError, bs.release)
+
 class ThreadJoinOnShutdown(BaseTestCase):
 
     # Between fork() and exec(), only async-safe functions are allowed (issues

Lib/threading.py

@@ -283,9 +283,11 @@ def __init__(self, value=1):
         self._initial_value = value
 
     def release(self):
-        if self._value >= self._initial_value:
-            raise ValueError("Semaphore released too many times")
-        return Semaphore.release(self)
+        with self._cond:
+            if self._value >= self._initial_value:
+                raise ValueError("Semaphore released too many times")
+            self._value += 1
+            self._cond.notify()
 
 
 class Event: