bpo-10978: Semaphores can release multiple threads at a time (GH-15588)

Doc/library/threading.rst

@@ -802,11 +802,14 @@ Semaphores also support the :ref:`context management protocol <with-locks>`.
       .. 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)

Lib/test/lock_tests.py

@@ -663,6 +663,38 @@ def f():
         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))

Lib/threading.py

@@ -439,16 +439,19 @@ def acquire(self, blocking=True, timeout=None):
 
     __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 @@ def __init__(self, value=1):
         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 @@ def release(self):
         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:

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.