issue 8777

Add threading.Barrier

Lib/threading.py

@@ -392,6 +392,178 @@ class _Event(_Verbose):
         finally:
             self._cond.release()
 
+
+# A barrier class.  Inspired in part by the pthread_barrier_* api and
+# the CyclicBarrier class from Java.  See
+# http://sourceware.org/pthreads-win32/manual/pthread_barrier_init.html and
+# http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/
+#        CyclicBarrier.html
+# for information.
+# We maintain two main states, 'filling' and 'draining' enabling the barrier
+# to be cyclic.  Threads are not allowed into it until it has fully drained
+# since the previous cycle.  In addition, a 'resetting' state exists which is
+# similar to 'draining' except that threads leave with a BrokenBarrierError,
+# and a 'broken' state in which all threads get get the exception.
+class Barrier(_Verbose):
+    """
+    Barrier.  Useful for synchronizing a fixed number of threads
+    at known synchronization points.  Threads block on 'wait()' and are
+    simultaneously once they have all made that call.
+    """
+    def __init__(self, parties, action=None, timeout=None, verbose=None):
+        """
+        Create a barrier, initialised to 'parties' threads.
+        'action' is a callable which, when supplied, will be called
+        by one of the threads after they have all entered the
+        barrier and just prior to releasing them all.
+        If a 'timeout' is provided, it is uses as the default for
+        all subsequent 'wait()' calls.
+        """
+        _Verbose.__init__(self, verbose)
+        self._cond = Condition(Lock())
+        self._action = action
+        self._timeout = timeout
+        self._parties = parties
+        self._state = 0 #0 filling, 1, draining, -1 resetting, -2 broken
+        self._count = 0
+
+    def wait(self, timeout=None):
+        """
+        Wait for the barrier.  When the specified number of threads have
+        started waiting, they are all simultaneously awoken. If an 'action'
+        was provided for the barrier, one of the threads will have executed
+        that callback prior to returning.
+        Returns an individual index number from 0 to 'parties-1'.
+        """
+        if timeout is None:
+            timeout = self._timeout
+        with self._cond:
+            self._enter() # Block while the barrier drains.
+            index = self._count
+            self._count += 1
+            try:
+                if index + 1 == self._parties:
+                    # We release the barrier
+                    self._release()
+                else:
+                    # We wait until someone releases us
+                    self._wait(timeout)
+                return index
+            finally:
+                self._count -= 1
+                # Wake up any threads waiting for barrier to drain.
+                self._exit()
+
+    # Block until the barrier is ready for us, or raise an exception
+    # if it is broken.
+    def _enter(self):
+        while self._state in (-1, 1):
+            # It is draining or resetting, wait until done
+            self._cond.wait()
+        #see if the barrier is in a broken state
+        if self._state < 0:
+            raise BrokenBarrierError
+        assert self._state == 0
+
+    # Optionally run the 'action' and release the threads waiting
+    # in the barrier.
+    def _release(self):
+        try:
+            if self._action:
+                self._action()
+            # enter draining state
+            self._state = 1
+            self._cond.notify_all()
+        except:
+            #an exception during the _action handler.  Break and reraise
+            self._break()
+            raise
+
+    # Wait in the barrier until we are relased.  Raise an exception
+    # if the barrier is reset or broken.
+    def _wait(self, timeout):
+        while self._state == 0:
+            if self._cond.wait(timeout) is False:
+                #timed out.  Break the barrier
+                self._break()
+                raise BrokenBarrierError
+            if self._state < 0:
+                raise BrokenBarrierError
+        assert self._state == 1
+
+    # If we are the last thread to exit the barrier, signal any threads
+    # waiting for the barrier to drain.
+    def _exit(self):
+        if self._count == 0:
+            if self._state in (-1, 1):
+                #resetting or draining
+                self._state = 0
+                self._cond.notify_all()
+
+    def reset(self):
+        """
+        Reset the barrier to the initial state.
+        Any threads currently waiting will get the BrokenBarrier exception
+        raised.
+        """
+        with self._cond:
+            if self._count > 0:
+                if self._state == 0:
+                    #reset the barrier, waking up threads
+                    self._state = -1
+                elif self._state == -2:
+                    #was broken, set it to reset state
+                    #which clears when the last thread exits
+                    self._state = -1
+            else:
+                self._state = 0
+            self._cond.notify_all()
+
+    def abort(self):
+        """
+        Place the barrier into a 'broken' state.
+        Useful in case of error.  Any currently waiting threads and
+        threads attempting to 'wait()' will have BrokenBarrierError
+        raised.
+        """
+        with self._cond:
+            self._break()
+
+    def _break(self):
+        # An internal error was detected.  The barrier is set to
+        # a broken state all parties awakened.
+        self._state = -2
+        self._cond.notify_all()
+
+    @property
+    def parties(self):
+        """
+        Return the number of threads required to trip the barrier.
+        """
+        return self._parties
+
+    @property
+    def n_waiting(self):
+        """
+        Return the number of threads that are currently waiting at the barrier.
+        """
+        # We don't need synchronization here since this is an ephemeral result
+        # anyway.  It returns the correct value in the steady state.
+        if self._state == 0:
+            return self._count
+        return 0
+
+    @property
+    def broken(self):
+        """
+        Return True if the barrier is in a broken state
+        """
+        return self._state == -2
+
+#exception raised by the Barrier class
+class BrokenBarrierError(RuntimeError): pass
+
+
 # Helper to generate new thread names
 _counter = 0
 def _newname(template="Thread-%d"):