[GROW-3134] release already acquired connections, when get_connections raised an exception (redis#3)

* [GROW-3134] release already acquired connections, when get_connections raised an exception

* fix style

(cherry picked from commit 94bb915)

Author: zach-iee

redis/cluster.py

@@ -1995,6 +1995,8 @@ def _send_cluster_commands(
                     try:
                         connection = get_connection(redis_node, c.args)
                     except (ConnectionError, TimeoutError) as e:
+                        for n in nodes.values():
+                            n.connection_pool.release(n.connection)
                         if self.retry and isinstance(e, self.retry._supported_errors):
                             backoff = self.retry._backoff.compute(attempts_count)
                             if backoff > 0:

tests/test_cluster.py

@@ -7,6 +7,7 @@
 
 import pytest
 
+import redis.cluster
 from redis import Redis
 from redis.backoff import (
     ConstantBackoff,
@@ -2893,6 +2894,33 @@ def raise_ask_error():
             assert ask_node.redis_connection.connection.read_response.called
             assert res == ["MOCK_OK"]
 
+    @pytest.mark.parametrize("error", [ConnectionError, TimeoutError])
+    def test_return_previous_acquired_connections(self, r, error):
+        # 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 error("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