Add LIFO for connection pooling

Added new "lifo" mode to :class:`.QueuePool`, typically enabled by setting
the flag :paramref:`.create_engine.pool_use_lifo` to True.   "lifo" mode
means the same connection just checked in will be the first to be checked
out again, allowing excess connections to be cleaned up from the server
side during periods of the pool being only partially utilized.  Pull request
courtesy Taem Park.

Change-Id: Idb5e299c5082b3e6b547bd03022acf65fdc34f35
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/467

1 files changed, 11 insertions, 2 deletions

diff --git a/lib/sqlalchemy/util/queue.py b/lib/sqlalchemy/util/queue.py
index 1958702c7..640f70ea9 100644
--- a/lib/sqlalchemy/util/queue.py
+++ b/lib/sqlalchemy/util/queue.py
@@ -39,10 +39,12 @@ class Full(Exception):
 
 
 class Queue:
-    def __init__(self, maxsize=0):
+    def __init__(self, maxsize=0, use_lifo=False):
         """Initialize a queue object with a given maximum size.
 
         If `maxsize` is <= 0, the queue size is infinite.
+
+        If `use_lifo` is True, this Queue acts like a Stack (LIFO).
         """
 
         self._init(maxsize)
@@ -57,6 +59,8 @@ class Queue:
         # Notify not_full whenever an item is removed from the queue;
         # a thread waiting to put is notified then.
         self.not_full = threading.Condition(self.mutex)
+        # If this queue uses LIFO or FIFO
+        self.use_lifo = use_lifo
 
     def qsize(self):
         """Return the approximate size of the queue (not reliable!)."""
@@ -196,4 +200,9 @@ class Queue:
 
     # Get an item from the queue
     def _get(self):
-        return self.queue.popleft()
+        if self.use_lifo:
+            # LIFO
+            return self.queue.pop()
+        else:
+            # FIFO
+            return self.queue.popleft()