Do not update pool if already created

We want to only update the pool if previous do not exist
or is shutdown. This commit adds additional validation to
add_or_renew_pool to make sure this condition is met.

Fixes: scylladb#317

cassandra/cluster.py

@@ -1980,7 +1980,7 @@ def on_up(self, host):
             futures_results = []
             callback = partial(self._on_up_future_completed, host, futures, futures_results, futures_lock)
             for session in tuple(self.sessions):
-                future = session.add_or_renew_pool(host, is_host_addition=False)
+                future = session.add_or_renew_pool(host, is_host_addition=False, require_previous_shutdown_or_none=False)
                 if future is not None:
                     have_future = True
                     future.add_done_callback(callback)
@@ -2119,7 +2119,7 @@ def future_completed(future):
 
         have_future = False
         for session in tuple(self.sessions):
-            future = session.add_or_renew_pool(host, is_host_addition=True)
+            future = session.add_or_renew_pool(host, is_host_addition=True, require_previous_shutdown_or_none=False)
             if future is not None:
                 have_future = True
                 futures.add(future)
@@ -2650,7 +2650,7 @@ def __init__(self, cluster, hosts, keyspace=None):
         # create connection pools in parallel
         self._initial_connect_futures = set()
         for host in hosts:
-            future = self.add_or_renew_pool(host, is_host_addition=False)
+            future = self.add_or_renew_pool(host, is_host_addition=False, require_previous_shutdown_or_none=False)
             if future:
                 self._initial_connect_futures.add(future)
 
@@ -3273,7 +3273,7 @@ def __del__(self):
             # when cluster.shutdown() is called explicitly.
             pass
 
-    def add_or_renew_pool(self, host, is_host_addition):
+    def add_or_renew_pool(self, host, is_host_addition, require_previous_shutdown_or_none):
         """
         For internal use only.
         """
@@ -3321,7 +3321,11 @@ def callback(pool, errors):
                         self._lock.acquire()
                         return False
                     self._lock.acquire()
-                self._pools[host] = new_pool
+                if require_previous_shutdown_or_none and (previous and not previous.is_shutdown):
+                    new_pool.shutdown()
+                    return True
+                else:
+                    self._pools[host] = new_pool
 
             log.debug("Added pool for host %s to session", host)
             if previous:
@@ -3361,7 +3365,7 @@ def update_created_pools(self):
                 # to allow us to attempt connections to hosts that have gone from ignored to something
                 # else.
                 if distance != HostDistance.IGNORED and host.is_up in (True, None):
-                    future = self.add_or_renew_pool(host, False)
+                    future = self.add_or_renew_pool(host, False, require_previous_shutdown_or_none=True)
             elif distance != pool.host_distance:
                 # the distance has changed
                 if distance == HostDistance.IGNORED: