Tests: Make PsqlDosStorage profile unload test more robust

[sphuber]

The test_unload_profile test verifies that if a loaded profile is unloaded, it properly relinquishes of the session that is maintained by sqlalchemy. It did so by checking that after unloading, there were no sessions being referenced. However, this would fail sometimes, because another session may still be held on to, even though that session had nothing to do with the test.

A more robust test is simply to check that after unloading, there is exactly one less session being held on to, after having called the garbage collector at the beginning of the test to clear any lingering unrelated sessions.

[unkcpz]

Interesting, looks all good! How do you know adding a gc will fix it?

[sphuber]

Interesting, looks all good! How do you know adding a gc will fix it?

Because that was what was causing the test to fail. The test checked that after unloading, the WeakValueDictionary session container of sqlalchemy was empty. But this was often not the case because there were still other references to a session in there. But these were not related to the test. They probably correspond to sessions created in other tests that had been closed, but the garbage collector had not yet had the time to run. Values in a WeakValueDictionary only dissappear once they no longer have a ref and the garbage collector is run.