api: make connection close idempotent

After this patch, it is possible to call `conn.close()` method multiple
times.

Part of #250

Author: DifferentialOrange

Diff for: CHANGELOG.md

@@ -149,6 +149,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Package build (#238).
 - Allow any MessagePack supported type as a request key (#240).
 - Puting test files in pip package (#238).
+- Make connection close idempotent (#250).
 
 ## 0.9.0 - 2022-06-20
 

Diff for: tarantool/connection.py

@@ -520,10 +520,11 @@ def __init__(self, host, port,
 
     def close(self):
         """
-        Close a connection to the server.
+        Close a connection to the server. The method is idempotent.
         """
 
-        self._socket.close()
+        if self._socket is not None:
+            self._socket.close()
         self._socket = None
 
     def is_closed(self):

Diff for: test/suites/test_dml.py

@@ -309,6 +309,13 @@ def test_13_unix_socket_connect(self):
         self.sock_con = tarantool.connect(self.sock_srv.host, self.sock_srv.args['primary'])
         self.assertEqual(self.sock_con.ping(notime=True), "Success")
 
+    def test_14_idempotent_close(self):
+        con = tarantool.connect(self.srv.host, self.srv.args['primary'])
+        con.close()
+        self.assertEqual(con.is_closed(), True)
+        con.close()
+        self.assertEqual(con.is_closed(), True)
+
     @classmethod
     def tearDownClass(self):
         self.con.close()