issue #213: avoid service.Pool construction race

Ensure concurrent calls to service.Pool do not result in a duplicate
pool being constructed in a child.

mitogen/service.py

@@ -42,14 +42,20 @@ from mitogen.core import LOG
 
 DEFAULT_POOL_SIZE = 16
 _pool = None
+#: Serialize pool construction.
+_pool_lock = threading.Lock()
 
 
 @mitogen.core.takes_router
 def get_or_create_pool(size=None, router=None):
     global _pool
+    _pool_lock.acquire()
+    try:
         if _pool is None:
             _pool = Pool(router, [], size=size or DEFAULT_POOL_SIZE)
         return _pool
+    finally:
+        _pool_lock.release()
 
 
 @mitogen.core.takes_router