Better accounting of current_size in pools.Pool, thanks to Brett Hoerner for reporting #91.

eventlet/pools.py

@@ -113,10 +113,15 @@ class Pool(object):
         """
         if self.free_items:
             return self.free_items.popleft()
-        if self.current_size < self.max_size:
-            created = self.create()
-            self.current_size += 1
+        self.current_size += 1
+        if self.current_size <= self.max_size:
+            try:
+                created = self.create()
+            except:
+                self.current_size -= 1
+                raise                
             return created
+        self.current_size -= 1 # did not create
         return self.channel.get()
 
     if item_impl is not None:

tests/pools_test.py

@@ -137,6 +137,26 @@ class TestIntPool(TestCase):
         # resize larger and assert that there are more free items
         pool.resize(2)
         self.assertEquals(pool.free(), 2)
+        
+    def test_create_contention(self):
+        creates = [0]
+        def sleep_create():
+            creates[0] += 1
+            eventlet.sleep()
+            return "slept"
+            
+        p = pools.Pool(max_size=4, create=sleep_create)
+
+        def do_get():
+            x = p.get()
+            self.assertEquals(x, "slept")
+            p.put(x)
+
+        gp = eventlet.GreenPool()
+        for i in xrange(100):
+            gp.spawn_n(do_get)
+        gp.waitall()
+        self.assertEquals(creates[0], 4)
 
 
 class TestAbstract(TestCase):