PsqlDosBackend: ensure sqla sessions are garbage-collected on `clos…

…e` (#5728)

When calling `Manager.unload_profile`, the `close` method is called on
the `StorageBackend`. For the `PsqlDosBackend` this means cleaning up
all SqlAlchemy connections and sessions. This was implemented, however,
SqlAlchemy would often still be holding on to at least one session in
the `WeakrefValueDictionary` of `sqlalchemy.orm.sessions._sessions`. It
turns out however that the reference was already cleared in fact, as
calling the garbage collector explicitly cleans the session references.

aiida/storage/psql_dos/backend.py

@@ -11,6 +11,7 @@
 # pylint: disable=missing-function-docstring
 from contextlib import contextmanager, nullcontext
 import functools
+import gc
 import pathlib
 from typing import TYPE_CHECKING, Iterator, List, Optional, Sequence, Set, Union
 
@@ -144,6 +145,10 @@ def close(self) -> None:
         self._session_factory.close()
         self._session_factory = None
 
+        # Without this, sqlalchemy keeps a weakref to a session
+        # in sqlalchemy.orm.session._sessions
+        gc.collect()
+
     def _clear(self, recreate_user: bool = True) -> None:
         from aiida.storage.psql_dos.models.settings import DbSetting
         from aiida.storage.psql_dos.models.user import DbUser

tests/storage/psql_dos/test_backend.py

@@ -144,3 +144,24 @@ def mock_get_info(self, detailed=False, **kwargs):  # pylint: disable=unused-arg
     assert 'extra_value' in repository_info_out
     assert repository_info_out['value'] == 42
     assert repository_info_out['extra_value'] == 0
+
+
+@pytest.mark.usefixtures('aiida_profile_clean')
+def test_unload_profile():
+    """Test that unloading the profile closes all sqla sessions.
+
+    This is a regression test for #5506.
+    """
+    from sqlalchemy.orm.session import _sessions  # pylint: disable=import-outside-toplevel
+
+    # Just running the test suite itself should have opened at least one session
+    assert len(_sessions) != 0, str(_sessions)
+
+    manager = get_manager()
+    profile_name = manager.get_profile().name
+
+    try:
+        manager.unload_profile()
+        assert len(_sessions) == 0, str(_sessions)
+    finally:
+        manager.load_profile(profile_name)