release already acquired connections on ClusterPipeline, when get_connection raises an exception

Author: zakaf

redis/cluster.py

@@ -1999,6 +1999,8 @@ def _send_cluster_commands(
                     try:
                         connection = get_connection(redis_node, c.args)
                     except ConnectionError:
+                        for n in nodes.values():
+                            n.connection_pool.release(n.connection)
                         # Connection retries are being handled in the node's
                         # Retry object. Reinitialize the node -> slot table.
                         self.nodes_manager.initialize()

tests/test_cluster.py

@@ -11,6 +11,7 @@
 
 import pytest
 
+import redis
 from redis import Redis
 from redis.backoff import ExponentialBackoff, NoBackoff, default_backoff
 from redis.cluster import (
@@ -3041,6 +3042,31 @@ def raise_ask_error():
             assert ask_node.redis_connection.connection.read_response.called
             assert res == ["MOCK_OK"]
 
+    def test_return_previously_acquired_connections(self, r):
+        # in order to ensure that a pipeline will make use of connections
+        #   from different nodes
+        assert r.keyslot('a') != r.keyslot('b')
+
+        orig_func = redis.cluster.get_connection
+        with patch("redis.cluster.get_connection") as get_connection:
+            def raise_error(target_node, *args, **kwargs):
+                if get_connection.call_count == 2:
+                    raise ConnectionError("mocked error")
+                else:
+                    return orig_func(target_node, *args, **kwargs)
+
+            get_connection.side_effect = raise_error
+
+            r.pipeline().get('a').get('b').execute()
+
+        # there should have been two get_connections per execution and
+        #   two executions due to exception raised in the first execution
+        assert get_connection.call_count == 4
+        for cluster_node in r.nodes_manager.nodes_cache.values():
+            connection_pool = cluster_node.redis_connection.connection_pool
+            num_of_conns = len(connection_pool._available_connections)
+            assert num_of_conns == connection_pool._created_connections
+
     def test_empty_stack(self, r):
         """
         If pipeline is executed with no commands it should