bpo-10978: Semaphores can release multiple threads at a time (GH-15588)
diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
index 7de12fb..0489421 100644
--- a/Doc/library/threading.rst
+++ b/Doc/library/threading.rst
@@ -802,11 +802,14 @@
.. versionchanged:: 3.2
The *timeout* parameter is new.
- .. method:: release()
+ .. method:: release(n=1)
- Release a semaphore, incrementing the internal counter by one. When it
- was zero on entry and another thread is waiting for it to become larger
- than zero again, wake up that thread.
+ Release a semaphore, incrementing the internal counter by *n*. When it
+ was zero on entry and other threads are waiting for it to become larger
+ than zero again, wake up *n* of those threads.
+
+ .. versionchanged:: 3.9
+ Added the *n* parameter to release multiple waiting threads at once.
.. class:: BoundedSemaphore(value=1)
diff --git a/Lib/test/lock_tests.py b/Lib/test/lock_tests.py
index 23a02e0..8885868 100644
--- a/Lib/test/lock_tests.py
+++ b/Lib/test/lock_tests.py
@@ -663,6 +663,38 @@
b.wait_for_finished()
self.assertEqual(sem_results, [True] * (6 + 7 + 6 + 1))
+ def test_multirelease(self):
+ sem = self.semtype(7)
+ sem.acquire()
+ results1 = []
+ results2 = []
+ phase_num = 0
+ def f():
+ sem.acquire()
+ results1.append(phase_num)
+ sem.acquire()
+ results2.append(phase_num)
+ b = Bunch(f, 10)
+ b.wait_for_started()
+ while len(results1) + len(results2) < 6:
+ _wait()
+ self.assertEqual(results1 + results2, [0] * 6)
+ phase_num = 1
+ sem.release(7)
+ while len(results1) + len(results2) < 13:
+ _wait()
+ self.assertEqual(sorted(results1 + results2), [0] * 6 + [1] * 7)
+ phase_num = 2
+ sem.release(6)
+ while len(results1) + len(results2) < 19:
+ _wait()
+ self.assertEqual(sorted(results1 + results2), [0] * 6 + [1] * 7 + [2] * 6)
+ # The semaphore is still locked
+ self.assertFalse(sem.acquire(False))
+ # Final release, to let the last thread finish
+ sem.release()
+ b.wait_for_finished()
+
def test_try_acquire(self):
sem = self.semtype(2)
self.assertTrue(sem.acquire(False))
diff --git a/Lib/threading.py b/Lib/threading.py
index 32a3d7c..c3bbb19 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -439,16 +439,19 @@
__enter__ = acquire
- def release(self):
- """Release a semaphore, incrementing the internal counter by one.
+ def release(self, n=1):
+ """Release a semaphore, incrementing the internal counter by one or more.
When the counter is zero on entry and another thread is waiting for it
to become larger than zero again, wake up that thread.
"""
+ if n < 1:
+ raise ValueError('n must be one or more')
with self._cond:
- self._value += 1
- self._cond.notify()
+ self._value += n
+ for i in range(n):
+ self._cond.notify()
def __exit__(self, t, v, tb):
self.release()
@@ -475,8 +478,8 @@
Semaphore.__init__(self, value)
self._initial_value = value
- def release(self):
- """Release a semaphore, incrementing the internal counter by one.
+ def release(self, n=1):
+ """Release a semaphore, incrementing the internal counter by one or more.
When the counter is zero on entry and another thread is waiting for it
to become larger than zero again, wake up that thread.
@@ -485,11 +488,14 @@
raise a ValueError.
"""
+ if n < 1:
+ raise ValueError('n must be one or more')
with self._cond:
- if self._value >= self._initial_value:
+ if self._value + n > self._initial_value:
raise ValueError("Semaphore released too many times")
- self._value += 1
- self._cond.notify()
+ self._value += n
+ for i in range(n):
+ self._cond.notify()
class Event:
diff --git a/Misc/NEWS.d/next/Library/2019-08-29-01-19-13.bpo-10978.J6FQYY.rst b/Misc/NEWS.d/next/Library/2019-08-29-01-19-13.bpo-10978.J6FQYY.rst
new file mode 100644
index 0000000..2b8f300
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-08-29-01-19-13.bpo-10978.J6FQYY.rst
@@ -0,0 +1,2 @@
+Semaphores and BoundedSemaphores can now release more than one waiting
+thread at a time.