DatabaseResource.make() sends creation to a non-global facade

[Mike Bayer]

Trying to actually use a GeneratesSchema facade with a project that has its own TransactionContextManager, which is now the standard approach. more research is needed here but when provision.DatabaseResource.make() generates a new TCM, that's not the one which the calling application knows about. schema is created on the wrong engine and the whole thing fails. this is of course local to the SQLite case where we are using in-memory DBs, for a server/file DB the two facades would talk to the same URL. But the design here doesn't work, it seems like it should be replacing the factory inside the TCM but leaving the TCM in place. Short attempts to make this happen didn't work so this needs some more effort. Also need to re-understand how this works at all since I'm not following how the existing test in test_fixtures.py works, it might be dependent on the fact that this is the global manager.

[Mike Bayer (zzzeek)]

OK, research has been done. The problem is that hooks like GeneratesSchema.generate_schema_create_all are called before we apply the ReplaceEngineFacadeFixture to patch the local enginefacade into the one given to us by the application. Since it's common that tests want to use their normal enginefacade-enabled application code to set up test data, this causes things to not work. As a workaround, a fixture can do this:

    def generate_schema_create_all(self, engine):
        migration.create_schema(engine)

        _reset_facade = myapp.context_manager.patch_engine(
            engine)
        self.addCleanup(_reset_facade)

        # do enginefacade-specific setup

this is what I've done in placement for now.

[Ben Nemec (bnemec)]

So do we need to add a test fixture for this?

[Mike Bayer (zzzeek)]

We will, yes. I was planning to work on it at some point. for the review where I had the problem see https://review.openstack.org/#/c/621304/.