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

4 files changed, 56 insertions, 13 deletions

diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
index 7de12fbf9d..0489421cdf 100644
--- a/Doc/library/threading.rst
+++ b/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 *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.
 
-      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.
+      .. 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 23a02e0b4e..888586840f 100644
--- a/Lib/test/lock_tests.py
+++ b/Lib/test/lock_tests.py
@@ -663,6 +663,38 @@ class BaseSemaphoreTests(BaseTestCase):
         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 32a3d7c303..c3bbb19acb 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -439,16 +439,19 @@ class Semaphore:
 
     __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 @@ class BoundedSemaphore(Semaphore):
         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 @@ class BoundedSemaphore(Semaphore):
         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 0000000000..2b8f300fc2
--- /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.