Clean-up memcache connection sockets

The ConnectionPool currently opens sockets, but never closes them. As a
result, any client using oslo.cache memcache pool leaves sockets in a
CLOSE_WAIT state, with the source port not being re-usable.

In our production system, were we have A LOT of activity, this creates
a storm of non-reusable ports: all source ports are in use, making the
node unuseable for other things.

This patch adds a __del__ destructor closing inactive connections,
fixing the issue.

Closes-Bug: #2043121
Change-Id: I09d632346c76d1aff7c534f0d040162d1985f548

oslo_cache/_memcache_pool.py

@@ -93,6 +93,27 @@ class ConnectionPool(queue.Queue):
         self._connection_get_timeout = conn_get_timeout
         self._acquired = 0
 
+    def __del__(self):
+        """Delete the connection pool.
+
+        Destory all connections left in the queue.
+        """
+        while True:
+            # As per https://docs.python.org/3/library/collections.html
+            # self.queue.pop() will raise IndexError when no elements are
+            # present, ending the while True: loop.
+            # The logic loops over all connections in the queue but it does
+            # not retry for a single one in case a connection closure fails
+            # then it leaves that one and process the next.
+            try:
+                conn = self.queue.pop().connection
+                self._destroy_connection(conn)
+            except IndexError:
+                break
+            except Exception as e:
+                self._do_log(
+                    LOG.warning, "Unable to cleanup a connection: %s", e)
+
     def _create_connection(self):
         """Returns a connection instance.