Make db_pool close connections after time-out as documented,

and fix get-after-close race condition that occurs when
using TpooledConnectionPool.

eventlet/db_pool.py

@@ -4,7 +4,9 @@ import time
 
 from eventlet.pools import Pool
 from eventlet import timeout
-from eventlet import greenthread
+from eventlet import hubs
+from eventlet.hubs.timer import Timer
+from eventlet.greenthread import GreenThread
 
 
 class ConnectTimeout(Exception):
@@ -88,8 +90,9 @@ class BaseConnectionPool(Pool):
 
         if next_delay > 0:
             # set up a continuous self-calling loop
-            self._expiration_timer = greenthread.spawn_after(next_delay,
-                                                    self._schedule_expiration)
+            self._expiration_timer = Timer(next_delay, GreenThread(hubs.get_hub().greenlet).switch,
+                                           self._schedule_expiration, [], {})
+            self._expiration_timer.schedule()
 
     def _expire_old_connections(self, now):
         """ Iterates through the open connections contained in the pool, closing
@@ -103,8 +106,6 @@ class BaseConnectionPool(Pool):
             conn
             for last_used, created_at, conn in self.free_items
             if self._is_expired(now, last_used, created_at)]
-        for conn in expired:
-            self._safe_close(conn, quiet=True)
 
         new_free = [
             (last_used, created_at, conn)
@@ -117,6 +118,9 @@ class BaseConnectionPool(Pool):
         # connections
         self.current_size -= original_count - len(self.free_items)
 
+        for conn in expired:
+            self._safe_close(conn, quiet=True)
+
     def _is_expired(self, now, last_used, created_at):
         """ Returns true and closes the connection if it's expired."""
         if ( self.max_idle <= 0