Add a multiprocess Barrier class to use for testing parallel code.

lib/spack/spack/util/multiproc.py

@@ -27,9 +27,11 @@
 than multiprocessing.Pool.apply() can.  For example, apply() will fail
 to pickle functions if they're passed indirectly as parameters.
 """
-from multiprocessing import Process, Pipe
+from multiprocessing import Process, Pipe, Semaphore, Value
 from itertools import izip
 
+__all__ = ['spawn', 'parmap', 'Barrier']
+
 def spawn(f):
     def fun(pipe,x):
         pipe.send(f(x))
@@ -43,3 +45,49 @@ def parmap(f,X):
     [p.join() for p in proc]
     return [p.recv() for (p,c) in pipe]
 
+
+class Barrier:
+    """Simple reusable semaphore barrier.
+
+    Python 2.6 doesn't have multiprocessing barriers so we implement this.
+
+    See http://greenteapress.com/semaphores/downey08semaphores.pdf, p. 41.
+    """
+    def __init__(self, n, timeout=None):
+        self.n = n
+        self.to = timeout
+        self.count = Value('i', 0)
+        self.mutex = Semaphore(1)
+        self.turnstile1 = Semaphore(0)
+        self.turnstile2 = Semaphore(1)
+
+
+    def wait(self):
+        if not self.mutex.acquire(timeout=self.to):
+            raise BarrierTimeoutError()
+        self.count.value += 1
+        if self.count.value == self.n:
+            if not self.turnstile2.acquire(timeout=self.to):
+                raise BarrierTimeoutError()
+            self.turnstile1.release()
+        self.mutex.release()
+
+        if not self.turnstile1.acquire(timeout=self.to):
+            raise BarrierTimeoutError()
+        self.turnstile1.release()
+
+        if not self.mutex.acquire(timeout=self.to):
+            raise BarrierTimeoutError()
+        self.count.value -= 1
+        if self.count.value == 0:
+            if not self.turnstile1.acquire(timeout=self.to):
+                raise BarrierTimeoutError()
+            self.turnstile2.release()
+        self.mutex.release()
+
+        if not self.turnstile2.acquire(timeout=self.to):
+            raise BarrierTimeoutError()
+        self.turnstile2.release()
+
+
+class BarrierTimeoutError: pass