Allow running airflow against sqlite in-memory DB for tests (#37144)

This makes no sense for production workloads, but it is useful for
writing tests of Airflow operators etc.

I also moved the warning from configure_vars to convigure_orm so that it
is checked after local settings have been applied in case that changes
anything.

(cherry picked from commit 7324400)

airflow/settings.py

@@ -192,13 +192,6 @@ 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"))
 
@@ -232,6 +225,20 @@ def configure_orm(disable_connection_pool=False, pool_class=None):
     """Configure ORM using SQLAlchemy."""
     from airflow.utils.log.secrets_masker import mask_secret
 
+    if (
+        SQL_ALCHEMY_CONN
+        and SQL_ALCHEMY_CONN.startswith("sqlite")
+        and not SQL_ALCHEMY_CONN.startswith("sqlite:////")
+        # In memory is not useful for production, but useful for writing tests against Airflow for extensions
+        and SQL_ALCHEMY_CONN != "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`."
+        )
+
     global Session
     global engine
     if os.environ.get("_AIRFLOW_SKIP_DB_TESTS") == "true":

tests/core/test_settings.py

@@ -17,11 +17,12 @@
 # under the License.
 from __future__ import annotations
 
+import contextlib
 import os
 import sys
 import tempfile
 from unittest import mock
-from unittest.mock import MagicMock, call, patch
+from unittest.mock import MagicMock, call
 
 import pytest
 
@@ -216,11 +217,20 @@ def test_uses_updated_session_timeout_config_by_default(self):
         assert session_lifetime_config == default_timeout_minutes
 
 
-def test_sqlite_relative_path():
+@pytest.mark.parametrize(
+    ["value", "expectation"],
+    [
+        (
+            "sqlite:///./relative_path.db",
+            pytest.raises(AirflowConfigException, match=r"Cannot use relative path:"),
+        ),
+        # Should not raise an exception
+        ("sqlite://", contextlib.nullcontext()),
+    ],
+)
+def test_sqlite_relative_path(monkeypatch, value, expectation):
     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)
+    monkeypatch.setattr(settings, "SQL_ALCHEMY_CONN", value)
+    with expectation:
+        settings.configure_orm()