add BoundedSemaphore (from stdlib's threading)

gevent/coros.py

@@ -152,6 +152,23 @@ class DummySemaphore(object):
         self.release()
 
 
+class BoundedSemaphore(Semaphore):
+    """A bounded semaphore checks to make sure its current value doesn’t exceed its initial value.
+    If it does, ``ValueError`` is raised. In most situations semaphores are used to guard resources
+    with limited capacity. If the semaphore is released too many times it’s a sign of a bug.
+
+    If not given, *value* defaults to 1."""
+
+    def __init__(self, value=1):
+        Semaphore.__init__(self, value)
+        self._initial_value = value
+
+    def release(self):
+        if self.counter >= self._initial_value:
+            raise ValueError, "Semaphore released too many times"
+        return _Semaphore.release(self)
+
+
 class RLock(object):
 
     def __init__(self):