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

[Raymond Hettinger]

https://github.com/python/cpython/commit/35f6301d68bdb0517be284421782d64407dfe72c
commit: 35f6301d68bdb0517be284421782d64407dfe72c
branch: master
author: Raymond Hettinger <rhettinger at users.noreply.github.com>
committer: GitHub <noreply at github.com>
date: 2019-08-29T01:45:19-07:00
summary:

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

files:
A Misc/NEWS.d/next/Library/2019-08-29-01-19-13.bpo-10978.J6FQYY.rst
M Doc/library/threading.rst
M Lib/test/lock_tests.py
M Lib/threading.py

diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
index 7de12fbf9dcd..0489421cdf60 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 23a02e0b4eb2..888586840f2c 100644
--- a/Lib/test/lock_tests.py
+++ b/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))
diff --git a/Lib/threading.py b/Lib/threading.py
index 32a3d7c30336..c3bbb19acbe2 100644
--- a/Lib/threading.py
+++ b/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:
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 000000000000..2b8f300fc280
--- /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.