Uses a shared semaphore to spawn new processes when needed.

This more effectively utilizes as many cores as you tell it. It should
now spawn a new process whenever an old branch of the recursive tree
finishes, to always use as many processes as you specify.

world.py

@@ -349,9 +349,10 @@ def generate_quadtree(chunkmap, colstart, colend, rowstart, rowend, prefix, proc
 
     # procs is -1 here since the main process always runs as well, only spawn
     # procs-1 /new/ processes
-    quadtree_recurse(chunkmap, colstart, colend, rowstart, rowend, prefix, "base", procs-1)
+    sem = multiprocessing.BoundedSemaphore(procs-1)
+    quadtree_recurse(chunkmap, colstart, colend, rowstart, rowend, prefix, "base", sem)
 
-def quadtree_recurse(chunkmap, colstart, colend, rowstart, rowend, prefix, quadrant, procs):
+def quadtree_recurse(chunkmap, colstart, colend, rowstart, rowend, prefix, quadrant, sem):
     """Recursive method that generates a quadtree.
     A single call generates, saves, and returns an image with the range
     specified by colstart,colend,rowstart, and rowend.
@@ -386,6 +387,13 @@ def quadtree_recurse(chunkmap, colstart, colend, rowstart, rowend, prefix, quadr
 
     Each tile outputted is always 384 by 384 pixels.
 
+    The last parameter, sem, should be a multiprocessing.Semaphore or
+    BoundedSemaphore object. Before each recursive call, the semaphore is
+    acquired without blocking. If the acquire is successful, the recursive call
+    will spawn a new process. If it is not successful, the recursive call is
+    run in the same thread. The semaphore is passed to each recursive call, so
+    any call could spawn new processes if another one exits at some point.
+
     The return from this function is (path, hash) where path is the path to the
     file saved, and hash is a byte string that depends on the tile's contents.
     If the tile is blank, path will be None, but hash will still be valid.
@@ -481,34 +489,31 @@ def quadtree_recurse(chunkmap, colstart, colend, rowstart, rowend, prefix, quadr
 
         # Recurse to generate each quadrant of images
         # Quadrent 1:
-        if procs > 0:
+        if sem.acquire(False):
             Procobj = ReturnableProcess
-            procs -= 1
         else:
             Procobj = FakeProcess
 
-        quad0result = Procobj(target=quadtree_recurse,
+        quad0result = Procobj(sem, target=quadtree_recurse,
-                args=(chunkmap, colstart, colmid, rowstart, rowmid, newprefix, "0", 0)
+                args=(chunkmap, colstart, colmid, rowstart, rowmid, newprefix, "0", sem)
                 )
         quad0result.start()
 
-        if procs > 0:
+        if sem.acquire(False):
             Procobj = ReturnableProcess
-            procs -= 1
         else:
             Procobj = FakeProcess
-        quad1result = Procobj(target=quadtree_recurse,
+        quad1result = Procobj(sem, target=quadtree_recurse,
-                args=(chunkmap, colmid, colend, rowstart, rowmid, newprefix, "1", 0)
+                args=(chunkmap, colmid, colend, rowstart, rowmid, newprefix, "1", sem)
                 )
         quad1result.start()
 
-        if procs > 0:
+        if sem.acquire(False):
             Procobj = ReturnableProcess
-            procs -= 1
         else:
             Procobj = FakeProcess
-        quad2result = Procobj(target=quadtree_recurse,
+        quad2result = Procobj(sem, target=quadtree_recurse,
-                args=(chunkmap, colstart, colmid, rowmid, rowend, newprefix, "2", 0)
+                args=(chunkmap, colstart, colmid, rowmid, rowend, newprefix, "2", sem)
                 )
         quad2result.start()
 
@@ -516,7 +521,7 @@ def quadtree_recurse(chunkmap, colstart, colend, rowstart, rowend, prefix, quadr
         # since we're just going to turn around and wait for it.
         quad3file, hash3 = quadtree_recurse(chunkmap, 
                 colmid, colend, rowmid, rowend,
-                newprefix, "3", 0)
+                newprefix, "3", sem)
 
         quad0file, hash0 = quad0result.get()
         quad1file, hash1 = quad1result.get()
@@ -603,10 +608,17 @@ def remove_tile(prefix, quadrent):
 
 class ReturnableProcess(multiprocessing.Process):
     """Like the standard multiprocessing.Process class, but the return value of
-    the target method is available by calling get()"""
+    the target method is available by calling get().
+    
+    The given semaphore is released when the target finishes running"""
+    def __init__(self, semaphore, *args, **kwargs):
+        self.__sem = semaphore
+        multiprocessing.Process.__init__(self, *args, **kwargs)
+
     def run(self):
         results = self._target(*self._args, **self._kwargs)
         self._respipe_in.send(results)
+        self.__sem.release()
 
     def get(self):
         self.join()
@@ -621,11 +633,11 @@ class FakeProcess(object):
     Used to make the code simpler in quadtree_recurse
 
     """
-    def __init__(self, target, args=None, kwargs=None):
+    def __init__(self, semaphore, target, args=None, kwargs=None):
         self._target = target
         self._args = args if args else ()
         self._kwargs = kwargs if kwargs else {}
     def start(self):
-        return
+        self.ret = self._target(*self._args, **self._kwargs)
     def get(self):
-        return self._target(*self._args, **self._kwargs)
+        return self.ret