Better error message when sqlite URL uses relative path (#36774)

When sqlite URL uses relative path, the error printed is quite
cryptic:

```
sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) unable to open database file
```

This might easily happen for example when you are in a hurry and put
relative value in your AIRFLOW_HOME.

This PR checks if sql is relative and throws more appropriate and
explicit message what is wrong.

airflow/settings.py

@@ -191,6 +191,14 @@ def configure_vars():
     global PLUGINS_FOLDER
     global DONOT_MODIFY_HANDLERS
     SQL_ALCHEMY_CONN = conf.get("database", "SQL_ALCHEMY_CONN")
+    if SQL_ALCHEMY_CONN.startswith("sqlite") and not SQL_ALCHEMY_CONN.startswith("sqlite:////"):
+        from airflow.exceptions import AirflowConfigException
+
+        raise AirflowConfigException(
+            f"Cannot use relative path: `{SQL_ALCHEMY_CONN}` to connect to sqlite. "
+            "Please use absolute path such as `sqlite:////tmp/airflow.db`."
+        )
+
     DAGS_FOLDER = os.path.expanduser(conf.get("core", "DAGS_FOLDER"))
 
     PLUGINS_FOLDER = conf.get("core", "plugins_folder", fallback=os.path.join(AIRFLOW_HOME, "plugins"))

tests/core/test_settings.py

@@ -21,11 +21,11 @@
 import sys
 import tempfile
 from unittest import mock
-from unittest.mock import MagicMock, call
+from unittest.mock import MagicMock, call, patch
 
 import pytest
 
-from airflow.exceptions import AirflowClusterPolicyViolation
+from airflow.exceptions import AirflowClusterPolicyViolation, AirflowConfigException
 from tests.test_utils.config import conf_vars
 
 SETTINGS_FILE_POLICY = """
@@ -214,3 +214,13 @@ def test_uses_updated_session_timeout_config_by_default(self):
         session_lifetime_config = settings.get_session_lifetime_config()
         default_timeout_minutes = 30 * 24 * 60
         assert session_lifetime_config == default_timeout_minutes
+
+
+def test_sqlite_relative_path():
+    from airflow import settings
+
+    with patch("airflow.settings.conf.get") as conf_get_mock:
+        conf_get_mock.return_value = "sqlite:///./relative_path.db"
+        with pytest.raises(AirflowConfigException) as exc:
+            settings.configure_vars()
+        assert "Cannot use relative path:" in str(exc.value)