Restore_state helper to restore entity states from the DB on startup (#4614)

* Restore states

* feedback

* Remove component move into recorder

* space

* helper

* Address my own comments

* Improve test coverage

* Add test for light restore state

tests/common.py

@@ -22,7 +22,7 @@ from homeassistant.const import (
     STATE_ON, STATE_OFF, DEVICE_DEFAULT_NAME, EVENT_TIME_CHANGED,
     EVENT_STATE_CHANGED, EVENT_PLATFORM_DISCOVERED, ATTR_SERVICE,
     ATTR_DISCOVERED, SERVER_PORT)
-from homeassistant.components import sun, mqtt
+from homeassistant.components import sun, mqtt, recorder
 from homeassistant.components.http.auth import auth_middleware
 from homeassistant.components.http.const import (
     KEY_USE_X_FORWARDED_FOR, KEY_BANS_ENABLED, KEY_TRUSTED_NETWORKS)
@@ -452,3 +452,31 @@ def assert_setup_component(count, domain=None):
     res_len = 0 if res is None else len(res)
     assert res_len == count, 'setup_component failed, expected {} got {}: {}' \
         .format(count, res_len, res)
+
+
+def init_recorder_component(hass, add_config=None, db_ready_callback=None):
+    """Initialize the recorder."""
+    config = dict(add_config) if add_config else {}
+    config[recorder.CONF_DB_URL] = 'sqlite://'  # In memory DB
+
+    saved_recorder = recorder.Recorder
+
+    class Recorder2(saved_recorder):
+        """Recorder with a callback after db_ready."""
+
+        def _setup_connection(self):
+            """Setup the connection and run the callback."""
+            super(Recorder2, self)._setup_connection()
+            if db_ready_callback:
+                _LOGGER.debug('db_ready_callback start (db_ready not set,'
+                              'never use get_instance in the callback)')
+                db_ready_callback()
+                _LOGGER.debug('db_ready_callback completed')
+
+    with patch('homeassistant.components.recorder.Recorder',
+               side_effect=Recorder2):
+        assert setup_component(hass, recorder.DOMAIN,
+                               {recorder.DOMAIN: config})
+    assert recorder.DOMAIN in hass.config.components
+    recorder.get_instance().block_till_db_ready()
+    _LOGGER.info("In-memory recorder successfully started")