Issue #14059: Implement multiprocessing.Barrier

Doc/library/multiprocessing.rst

@@ -226,11 +226,11 @@ However, if you really do need to use some shared data then
    holds Python objects and allows other processes to manipulate them using
    proxies.
 
-   A manager returned by :func:`Manager` will support types :class:`list`,
-   :class:`dict`, :class:`Namespace`, :class:`Lock`, :class:`RLock`,
-   :class:`Semaphore`, :class:`BoundedSemaphore`, :class:`Condition`,
-   :class:`Event`, :class:`Queue`, :class:`Value` and :class:`Array`.  For
-   example, ::
+   A manager returned by :func:`Manager` will support types
+   :class:`list`, :class:`dict`, :class:`Namespace`, :class:`Lock`,
+   :class:`RLock`, :class:`Semaphore`, :class:`BoundedSemaphore`,
+   :class:`Condition`, :class:`Event`, :class:`Barrier`,
+   :class:`Queue`, :class:`Value` and :class:`Array`.  For example, ::
 
       from multiprocessing import Process, Manager
 
@@ -885,6 +885,12 @@ program as they are in a multithreaded program.  See the documentation for
 Note that one can also create synchronization primitives by using a manager
 object -- see :ref:`multiprocessing-managers`.
 
+.. class:: Barrier(parties[, action[, timeout]])
+
+   A barrier object: a clone of :class:`threading.Barrier`.
+
+   .. versionadded:: 3.3
+
 .. class:: BoundedSemaphore([value])
 
    A bounded semaphore object: a clone of :class:`threading.BoundedSemaphore`.
@@ -1280,6 +1286,13 @@ their parent process exits.  The manager classes are defined in the
 
    It also supports creation of shared lists and dictionaries.
 
+   .. method:: Barrier(parties[, action[, timeout]])
+
+      Create a shared :class:`threading.Barrier` object and return a
+      proxy for it.
+
+      .. versionadded:: 3.3
+
    .. method:: BoundedSemaphore([value])
 
       Create a shared :class:`threading.BoundedSemaphore` object and return a

Lib/multiprocessing/__init__.py

@@ -23,8 +23,8 @@ __all__ = [
     'Manager', 'Pipe', 'cpu_count', 'log_to_stderr', 'get_logger',
     'allow_connection_pickling', 'BufferTooShort', 'TimeoutError',
     'Lock', 'RLock', 'Semaphore', 'BoundedSemaphore', 'Condition',
-    'Event', 'Queue', 'SimpleQueue', 'JoinableQueue', 'Pool', 'Value', 'Array',
-    'RawValue', 'RawArray', 'SUBDEBUG', 'SUBWARNING',
+    'Event', 'Barrier', 'Queue', 'SimpleQueue', 'JoinableQueue', 'Pool',
+    'Value', 'Array', 'RawValue', 'RawArray', 'SUBDEBUG', 'SUBWARNING',
     ]
 
 __author__ = 'R. Oudkerk (r.m.oudkerk@gmail.com)'
@@ -186,6 +186,13 @@ def Event():
     from multiprocessing.synchronize import Event
     return Event()
 
+def Barrier(parties, action=None, timeout=None):
+    '''
+    Returns a barrier object
+    '''
+    from multiprocessing.synchronize import Barrier
+    return Barrier(parties, action, timeout)
+
 def Queue(maxsize=0):
     '''
     Returns a queue object

Lib/multiprocessing/dummy/__init__.py

@@ -35,7 +35,7 @@
 __all__ = [
     'Process', 'current_process', 'active_children', 'freeze_support',
     'Lock', 'RLock', 'Semaphore', 'BoundedSemaphore', 'Condition',
-    'Event', 'Queue', 'Manager', 'Pipe', 'Pool', 'JoinableQueue'
+    'Event', 'Barrier', 'Queue', 'Manager', 'Pipe', 'Pool', 'JoinableQueue'
     ]
 
 #
@@ -49,7 +49,7 @@ import array
 
 from multiprocessing.dummy.connection import Pipe
 from threading import Lock, RLock, Semaphore, BoundedSemaphore
-from threading import Event, Condition
+from threading import Event, Condition, Barrier
 from queue import Queue
 
 #

Lib/multiprocessing/managers.py

@@ -993,6 +993,26 @@ class EventProxy(BaseProxy):
     def wait(self, timeout=None):
         return self._callmethod('wait', (timeout,))
 
+
+class BarrierProxy(BaseProxy):
+    _exposed_ = ('__getattribute__', 'wait', 'abort', 'reset')
+    def wait(self, timeout=None):
+        return self._callmethod('wait', (timeout,))
+    def abort(self):
+        return self._callmethod('abort')
+    def reset(self):
+        return self._callmethod('reset')
+    @property
+    def parties(self):
+        return self._callmethod('__getattribute__', ('parties',))
+    @property
+    def n_waiting(self):
+        return self._callmethod('__getattribute__', ('n_waiting',))
+    @property
+    def broken(self):
+        return self._callmethod('__getattribute__', ('broken',))
+
+
 class NamespaceProxy(BaseProxy):
     _exposed_ = ('__getattribute__', '__setattr__', '__delattr__')
     def __getattr__(self, key):
@@ -1084,6 +1104,7 @@ SyncManager.register('Semaphore', threading.Semaphore, AcquirerProxy)
 SyncManager.register('BoundedSemaphore', threading.BoundedSemaphore,
                      AcquirerProxy)
 SyncManager.register('Condition', threading.Condition, ConditionProxy)
+SyncManager.register('Barrier', threading.Barrier, BarrierProxy)
 SyncManager.register('Pool', Pool, PoolProxy)
 SyncManager.register('list', list, ListProxy)
 SyncManager.register('dict', dict, DictProxy)

Lib/multiprocessing/synchronize.py

@@ -333,3 +333,43 @@ class Event(object):
             return False
         finally:
             self._cond.release()
+
+#
+# Barrier
+#
+
+class Barrier(threading.Barrier):
+
+    def __init__(self, parties, action=None, timeout=None):
+        import struct
+        from multiprocessing.heap import BufferWrapper
+        wrapper = BufferWrapper(struct.calcsize('i') * 2)
+        cond = Condition()
+        self.__setstate__((parties, action, timeout, cond, wrapper))
+        self._state = 0
+        self._count = 0
+
+    def __setstate__(self, state):
+        (self._parties, self._action, self._timeout,
+         self._cond, self._wrapper) = state
+        self._array = self._wrapper.create_memoryview().cast('i')
+
+    def __getstate__(self):
+        return (self._parties, self._action, self._timeout,
+                self._cond, self._wrapper)
+
+    @property
+    def _state(self):
+        return self._array[0]
+
+    @_state.setter
+    def _state(self, value):
+        self._array[0] = value
+
+    @property
+    def _count(self):
+        return self._array[1]
+
+    @_count.setter
+    def _count(self, value):
+        self._array[1] = value

Lib/test/test_multiprocessing.py

@@ -18,6 +18,7 @@ import array
 import socket
 import random
 import logging
+import struct
 import test.support
 
 
@@ -1056,6 +1057,336 @@ class _TestEvent(BaseTestCase):
         p.start()
         self.assertEqual(wait(), True)
 
+#
+# Tests for Barrier - adapted from tests in test/lock_tests.py
+#
+
+# Many of the tests for threading.Barrier use a list as an atomic
+# counter: a value is appended to increment the counter, and the
+# length of the list gives the value.  We use the class DummyList
+# for the same purpose.
+
+class _DummyList(object):
+
+    def __init__(self):
+        wrapper = multiprocessing.heap.BufferWrapper(struct.calcsize('i'))
+        lock = multiprocessing.Lock()
+        self.__setstate__((wrapper, lock))
+        self._lengthbuf[0] = 0
+
+    def __setstate__(self, state):
+        (self._wrapper, self._lock) = state
+        self._lengthbuf = self._wrapper.create_memoryview().cast('i')
+
+    def __getstate__(self):
+        return (self._wrapper, self._lock)
+
+    def append(self, _):
+        with self._lock:
+            self._lengthbuf[0] += 1
+
+    def __len__(self):
+        with self._lock:
+            return self._lengthbuf[0]
+
+def _wait():
+    # A crude wait/yield function not relying on synchronization primitives.
+    time.sleep(0.01)
+
+
+class Bunch(object):
+    """
+    A bunch of threads.
+    """
+    def __init__(self, namespace, f, args, n, wait_before_exit=False):
+        """
+        Construct a bunch of `n` threads running the same function `f`.
+        If `wait_before_exit` is True, the threads won't terminate until
+        do_finish() is called.
+        """
+        self.f = f
+        self.args = args
+        self.n = n
+        self.started = namespace.DummyList()
+        self.finished = namespace.DummyList()
+        self._can_exit = namespace.Value('i', not wait_before_exit)
+        for i in range(n):
+            namespace.Process(target=self.task).start()
+
+    def task(self):
+        pid = os.getpid()
+        self.started.append(pid)
+        try:
+            self.f(*self.args)
+        finally:
+            self.finished.append(pid)
+            while not self._can_exit.value:
+                _wait()
+
+    def wait_for_started(self):
+        while len(self.started) < self.n:
+            _wait()
+
+    def wait_for_finished(self):
+        while len(self.finished) < self.n:
+            _wait()
+
+    def do_finish(self):
+        self._can_exit.value = True
+
+
+class AppendTrue(object):
+    def __init__(self, obj):
+        self.obj = obj
+    def __call__(self):
+        self.obj.append(True)
+
+
+class _TestBarrier(BaseTestCase):
+    """
+    Tests for Barrier objects.
+    """
+    N = 5
+    defaultTimeout = 10.0  # XXX Slow Windows buildbots need generous timeout
+
+    def setUp(self):
+        self.barrier = self.Barrier(self.N, timeout=self.defaultTimeout)
+
+    def tearDown(self):
+        self.barrier.abort()
+        self.barrier = None
+
+    def DummyList(self):
+        if self.TYPE == 'threads':
+            return []
+        elif self.TYPE == 'manager':
+            return self.manager.list()
+        else:
+            return _DummyList()
+
+    def run_threads(self, f, args):
+        b = Bunch(self, f, args, self.N-1)
+        f(*args)
+        b.wait_for_finished()
+
+    @classmethod
+    def multipass(cls, barrier, results, n):
+        m = barrier.parties
+        assert m == cls.N
+        for i in range(n):
+            results[0].append(True)
+            assert len(results[1]) == i * m
+            barrier.wait()
+            results[1].append(True)
+            assert len(results[0]) == (i + 1) * m
+            barrier.wait()
+        try:
+            assert barrier.n_waiting == 0
+        except NotImplementedError:
+            pass
+        assert not barrier.broken
+
+    def test_barrier(self, passes=1):
+        """
+        Test that a barrier is passed in lockstep
+        """
+        results = [self.DummyList(), self.DummyList()]
+        self.run_threads(self.multipass, (self.barrier, results, passes))
+
+    def test_barrier_10(self):
+        """
+        Test that a barrier works for 10 consecutive runs
+        """
+        return self.test_barrier(10)
+
+    @classmethod
+    def _test_wait_return_f(cls, barrier, queue):
+        res = barrier.wait()
+        queue.put(res)
+
+    def test_wait_return(self):
+        """
+        test the return value from barrier.wait
+        """
+        queue = self.Queue()
+        self.run_threads(self._test_wait_return_f, (self.barrier, queue))
+        results = [queue.get() for i in range(self.N)]
+        self.assertEqual(results.count(0), 1)
+
+    @classmethod
+    def _test_action_f(cls, barrier, results):
+        barrier.wait()
+        if len(results) != 1:
+            raise RuntimeError
+
+    def test_action(self):
+        """
+        Test the 'action' callback
+        """
+        results = self.DummyList()
+        barrier = self.Barrier(self.N, action=AppendTrue(results))
+        self.run_threads(self._test_action_f, (barrier, results))
+        self.assertEqual(len(results), 1)
+
+    @classmethod
+    def _test_abort_f(cls, barrier, results1, results2):
+        try:
+            i = barrier.wait()
+            if i == cls.N//2:
+                raise RuntimeError
+            barrier.wait()
+            results1.append(True)
+        except threading.BrokenBarrierError:
+            results2.append(True)
+        except RuntimeError:
+            barrier.abort()
+
+    def test_abort(self):
+        """
+        Test that an abort will put the barrier in a broken state
+        """
+        results1 = self.DummyList()
+        results2 = self.DummyList()
+        self.run_threads(self._test_abort_f,
+                         (self.barrier, results1, results2))
+        self.assertEqual(len(results1), 0)
+        self.assertEqual(len(results2), self.N-1)
+        self.assertTrue(self.barrier.broken)
+
+    @classmethod
+    def _test_reset_f(cls, barrier, results1, results2, results3):
+        i = barrier.wait()
+        if i == cls.N//2:
+            # Wait until the other threads are all in the barrier.
+            while barrier.n_waiting < cls.N-1:
+                time.sleep(0.001)
+            barrier.reset()
+        else:
+            try:
+                barrier.wait()
+                results1.append(True)
+            except threading.BrokenBarrierError:
+                results2.append(True)
+        # Now, pass the barrier again
+        barrier.wait()
+        results3.append(True)
+
+    def test_reset(self):
+        """
+        Test that a 'reset' on a barrier frees the waiting threads
+        """
+        results1 = self.DummyList()
+        results2 = self.DummyList()
+        results3 = self.DummyList()
+        self.run_threads(self._test_reset_f,
+                         (self.barrier, results1, results2, results3))
+        self.assertEqual(len(results1), 0)
+        self.assertEqual(len(results2), self.N-1)
+        self.assertEqual(len(results3), self.N)
+
+    @classmethod
+    def _test_abort_and_reset_f(cls, barrier, barrier2,
+                                results1, results2, results3):
+        try:
+            i = barrier.wait()
+            if i == cls.N//2:
+                raise RuntimeError
+            barrier.wait()
+            results1.append(True)
+        except threading.BrokenBarrierError:
+            results2.append(True)
+        except RuntimeError:
+            barrier.abort()
+        # Synchronize and reset the barrier.  Must synchronize first so
+        # that everyone has left it when we reset, and after so that no
+        # one enters it before the reset.
+        if barrier2.wait() == cls.N//2:
+            barrier.reset()
+        barrier2.wait()
+        barrier.wait()
+        results3.append(True)
+
+    def test_abort_and_reset(self):
+        """
+        Test that a barrier can be reset after being broken.
+        """
+        results1 = self.DummyList()
+        results2 = self.DummyList()
+        results3 = self.DummyList()
+        barrier2 = self.Barrier(self.N)
+
+        self.run_threads(self._test_abort_and_reset_f,
+                         (self.barrier, barrier2, results1, results2, results3))
+        self.assertEqual(len(results1), 0)
+        self.assertEqual(len(results2), self.N-1)
+        self.assertEqual(len(results3), self.N)
+
+    @classmethod
+    def _test_timeout_f(cls, barrier, results):
+        i = barrier.wait(20)
+        if i == cls.N//2:
+            # One thread is late!
+            time.sleep(4.0)
+        try:
+            barrier.wait(0.5)
+        except threading.BrokenBarrierError:
+            results.append(True)
+
+    def test_timeout(self):
+        """
+        Test wait(timeout)
+        """
+        results = self.DummyList()
+        self.run_threads(self._test_timeout_f, (self.barrier, results))
+        self.assertEqual(len(results), self.barrier.parties)
+
+    @classmethod
+    def _test_default_timeout_f(cls, barrier, results):
+        i = barrier.wait(20)
+        if i == cls.N//2:
+            # One thread is later than the default timeout
+            time.sleep(4.0)
+        try:
+            barrier.wait()
+        except threading.BrokenBarrierError:
+            results.append(True)
+
+    def test_default_timeout(self):
+        """
+        Test the barrier's default timeout
+        """
+        barrier = self.Barrier(self.N, timeout=1.0)
+        results = self.DummyList()
+        self.run_threads(self._test_default_timeout_f, (barrier, results))
+        self.assertEqual(len(results), barrier.parties)
+
+    def test_single_thread(self):
+        b = self.Barrier(1)
+        b.wait()
+        b.wait()
+
+    @classmethod
+    def _test_thousand_f(cls, barrier, passes, conn, lock):
+        for i in range(passes):
+            barrier.wait()
+            with lock:
+                conn.send(i)
+
+    def test_thousand(self):
+        if self.TYPE == 'manager':
+            return
+        passes = 1000
+        lock = self.Lock()
+        conn, child_conn = self.Pipe(False)
+        for j in range(self.N):
+            p = self.Process(target=self._test_thousand_f,
+                           args=(self.barrier, passes, child_conn, lock))
+            p.start()
+
+        for i in range(passes):
+            for j in range(self.N):
+                self.assertEqual(conn.recv(), i)
+
 #
 #
 #
@@ -2532,7 +2863,7 @@ class ProcessesMixin(object):
     Process = multiprocessing.Process
     locals().update(get_attributes(multiprocessing, (
         'Queue', 'Lock', 'RLock', 'Semaphore', 'BoundedSemaphore',
-        'Condition', 'Event', 'Value', 'Array', 'RawValue',
+        'Condition', 'Event', 'Barrier', 'Value', 'Array', 'RawValue',
         'RawArray', 'current_process', 'active_children', 'Pipe',
         'connection', 'JoinableQueue', 'Pool'
         )))
@@ -2547,7 +2878,7 @@ class ManagerMixin(object):
     manager = object.__new__(multiprocessing.managers.SyncManager)
     locals().update(get_attributes(manager, (
         'Queue', 'Lock', 'RLock', 'Semaphore', 'BoundedSemaphore',
-       'Condition', 'Event', 'Value', 'Array', 'list', 'dict',
+        'Condition', 'Event', 'Barrier', 'Value', 'Array', 'list', 'dict',
         'Namespace', 'JoinableQueue', 'Pool'
         )))
 
@@ -2560,7 +2891,7 @@ class ThreadsMixin(object):
     Process = multiprocessing.dummy.Process
     locals().update(get_attributes(multiprocessing.dummy, (
         'Queue', 'Lock', 'RLock', 'Semaphore', 'BoundedSemaphore',
-        'Condition', 'Event', 'Value', 'Array', 'current_process',
+        'Condition', 'Event', 'Barrier', 'Value', 'Array', 'current_process',
         'active_children', 'Pipe', 'connection', 'dict', 'list',
         'Namespace', 'JoinableQueue', 'Pool'
         )))

Misc/NEWS

@@ -21,6 +21,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #14059: Implement multiprocessing.Barrier.
+
 - Issue #15061: The inappropriately named hmac.secure_compare has been
   renamed to hmac.compare_digest, restricted to operating on bytes inputs
   only and had its documentation updated to more accurately reflect both its