add basic back-off test

.gitignore

@@ -46,3 +46,6 @@ _build
 .idea
 .vscode
 *~
+
+# tox local cache
+.tox

tests/backoff_test.py

@@ -0,0 +1,59 @@
+# SPDX-FileCopyrightText: 2023 Vladimír Kotal
+#
+# SPDX-License-Identifier: Unlicense
+
+"""exponential back-off tests"""
+
+import socket
+import ssl
+import time
+from unittest import TestCase, main
+from unittest.mock import call, patch
+
+import adafruit_minimqtt.adafruit_minimqtt as MQTT
+
+
+class ExpBackOff(TestCase):
+    """basic exponential back-off test"""
+
+    connect_times = []
+
+    # pylint: disable=unused-argument
+    def fake_connect(self, arg):
+        """connect() replacement that records the call times and always raises OSError"""
+        self.connect_times.append(time.monotonic())
+        raise OSError("this connect failed")
+
+    def test_failing_connect(self) -> None:
+        """test that exponential back-off is used when connect() always raises OSError"""
+        # use RFC 1918 address to avoid dealing with IPv6 in the call list below
+        host = "172.40.0.3"
+        port = 1883
+
+        with patch.object(socket.socket, "connect") as mock_method:
+            mock_method.side_effect = self.fake_connect
+
+            connect_retries = 3
+            mqtt_client = MQTT.MQTT(
+                broker=host,
+                port=port,
+                socket_pool=socket,
+                ssl_context=ssl.create_default_context(),
+                connect_retries=connect_retries,
+            )
+            print("connecting")
+            with self.assertRaises(MQTT.MMQTTException) as context:
+                mqtt_client.connect()
+                self.assertTrue("Repeated connect failures" in str(context.exception))
+
+            mock_method.assert_called()
+            calls = [call((host, port)) for _ in range(0, connect_retries)]
+            mock_method.assert_has_calls(calls)
+
+            print(f"connect() call times: {self.connect_times}")
+            for i in range(1, connect_retries):
+                assert self.connect_times[i] >= 2**i
+
+
+if __name__ == "__main__":
+    main()