Use sqlite's WAL mode to avoid `database is locked` errors (#6519)

* Use sqlite's WAL mode to avoid `database is locked` errors

- Relevant issue: https://github.com/home-assistant/home-assistant/issues/4780

Code:

- http://stackoverflow.com/a/23661501/1588795
- http://docs.sqlalchemy.org/en/rel_0_9/dialects/sqlite.html#foreign-key-support
- https://github.com/g2p/bedup/pull/86/files

* Only set WAL if using sqlite

* Reorder imports

* Fix pylint warnings

homeassistant/components/recorder/__init__.py

@@ -287,13 +287,27 @@ class Recorder(threading.Thread):
 
     def _setup_connection(self):
         """Ensure database is ready to fly."""
-        from sqlalchemy import create_engine
+        from sqlalchemy import create_engine, event
+        from sqlalchemy.engine import Engine
         from sqlalchemy.orm import scoped_session
         from sqlalchemy.orm import sessionmaker
+
         from . import models
 
         kwargs = {}
 
+        # pylint: disable=unused-variable
+        @event.listens_for(Engine, "connect")
+        def set_sqlite_pragma(dbapi_connection, connection_record):
+            """Set sqlite's WAL mode."""
+            if self.db_url.startswith("sqlite://"):
+                old_isolation = dbapi_connection.isolation_level
+                dbapi_connection.isolation_level = None
+                cursor = dbapi_connection.cursor()
+                cursor.execute("PRAGMA journal_mode=WAL")
+                cursor.close()
+                dbapi_connection.isolation_level = old_isolation
+
         if self.db_url == 'sqlite://' or ':memory:' in self.db_url:
             from sqlalchemy.pool import StaticPool