Add Hass.io discovery to MQTT (#16962)

* Add Hass.io discovery to MQTT

* Update en.json

* Update __init__.py

* Update config_flow.py

* Update strings.json

* Update test_config_flow.py

* Lint

* Ensure we don't send bad data in config entry

tests/components/mqtt/test_config_flow.py

@@ -5,7 +5,7 @@ import pytest
 
 from homeassistant.setup import async_setup_component
 
-from tests.common import mock_coro
+from tests.common import mock_coro, MockConfigEntry
 
 
 @pytest.fixture(autouse=True)
@@ -88,3 +88,67 @@ async def test_manual_config_set(hass, mock_try_connection,
     result = await hass.config_entries.flow.async_init(
         'mqtt', context={'source': 'user'})
     assert result['type'] == 'abort'
+
+
+async def test_user_single_instance(hass):
+    """Test we only allow a single config flow."""
+    MockConfigEntry(domain='mqtt').add_to_hass(hass)
+
+    result = await hass.config_entries.flow.async_init(
+        'mqtt', context={'source': 'user'})
+    assert result['type'] == 'abort'
+    assert result['reason'] == 'single_instance_allowed'
+
+
+async def test_hassio_single_instance(hass):
+    """Test we only allow a single config flow."""
+    MockConfigEntry(domain='mqtt').add_to_hass(hass)
+
+    result = await hass.config_entries.flow.async_init(
+        'mqtt', context={'source': 'hassio'})
+    assert result['type'] == 'abort'
+    assert result['reason'] == 'single_instance_allowed'
+
+
+async def test_hassio_confirm(hass, mock_try_connection,
+                              mock_finish_setup):
+    """Test we can finish a config flow."""
+    mock_try_connection.return_value = True
+
+    result = await hass.config_entries.flow.async_init(
+        'mqtt',
+        data={
+            'addon': 'Mock Addon',
+            'broker': 'mock-broker',
+            'port': 1883,
+            'username': 'mock-user',
+            'password': 'mock-pass',
+            'protocol': '3.1.1'
+        },
+        context={'source': 'hassio'}
+    )
+    assert result['type'] == 'form'
+    assert result['step_id'] == 'hassio_confirm'
+    assert result['description_placeholders'] == {
+        'addon': 'Mock Addon',
+    }
+
+    result = await hass.config_entries.flow.async_configure(
+        result['flow_id'], {
+            'discovery': True,
+        }
+    )
+
+    assert result['type'] == 'create_entry'
+    assert result['result'].data == {
+        'broker': 'mock-broker',
+        'port': 1883,
+        'username': 'mock-user',
+        'password': 'mock-pass',
+        'protocol': '3.1.1',
+        'discovery': True,
+    }
+    # Check we tried the connection
+    assert len(mock_try_connection.mock_calls) == 1
+    # Check config entry got setup
+    assert len(mock_finish_setup.mock_calls) == 1