Separate use of database sessions

This patch separates the use of database session in different MBS components
and do not mix them together.

In general, MBS components could be separated as the REST API (implemented
based on Flask) and non-REST API including the backend build workflow
(implemented as a fedmsg consumer on top of fedmsg-hub and running
independently) and library shared by them. As a result, there are two kind of
database session used in MBS, one is created and managed by Flask-SQLAlchemy,
and another one is created from SQLAclhemy Session API directly. The goal of
this patch is to make ensure session object is used properly in the right
place.

All the changes follow these rules:

* REST API related code uses the session object db.session created and
  managed by Flask-SQLAlchemy.
* Non-REST API related code uses the session object created with SQLAlchemy
  Session API. Function make_db_session does that.
* Shared code does not created a new session object as much as possible.
  Instead, it accepts an argument db_session.

The first two rules are applicable to tests as well.

Major changes:

* Switch tests back to run with a file-based SQLite database.
* make_session is renamed to make_db_session and SQLAlchemy connection pool
  options are applied for PostgreSQL backend.
* Frontend Flask related code uses db.session
* Shared code by REST API and backend build workflow accepts SQLAlchemy session
  object as an argument. For example, resolver class is constructed with a
  database session, and some functions accepts an argument for database session.
* Build workflow related code use session object returned from make_db_session
  and ensure db.session is not used.
* Only tests for views use db.session, and other tests use db_session fixture
  to access database.
* All argument name session, that is for database access, are renamed to
  db_session.
* Functions model_tests_init_data, reuse_component_init_data and
  reuse_shared_userspace_init_data, which creates fixture data for
  tests, are converted into pytest fixtures from original function
  called inside setup_method or a test method. The reason of this
  conversion is to use fixture ``db_session`` rather than create a
  new one. That would also benefit the whole test suite to reduce the
  number of SQLAlchemy session objects.

Signed-off-by: Chenxiong Qi <cqi@redhat.com>

tests/test_builder/test_base.py

@@ -26,7 +26,7 @@ import module_build_service.models
 import module_build_service.builder
 import module_build_service.resolver
 
-from tests import init_data, db
+from tests import init_data
 
 from module_build_service.builder import GenericBuilder
 from mock import patch
@@ -35,11 +35,10 @@ from mock import patch
 class TestGenericBuilder:
     def setup_method(self, test_method):
         init_data(1)
-        self.module = module_build_service.models.ModuleBuild.query.filter_by(id=1).one()
 
     @patch("module_build_service.resolver.DBResolver")
-    @patch("module_build_service.resolver.GenericResolver")
-    def test_default_buildroot_groups_cache(self, generic_resolver, resolver):
+    @patch("module_build_service.builder.base.GenericResolver")
+    def test_default_buildroot_groups_cache(self, generic_resolver, resolver, db_session):
         mbs_groups = {"buildroot": [], "srpm-buildroot": []}
 
         resolver = mock.MagicMock()
@@ -48,27 +47,29 @@ class TestGenericBuilder:
 
         expected_groups = {"build": [], "srpm-build": []}
 
-        with patch.object(module_build_service.resolver, "system_resolver", new=resolver):
-            # Call default_buildroot_groups, the result should be cached.
-            ret = GenericBuilder.default_buildroot_groups(db.session, self.module)
-            assert ret == expected_groups
-            resolver.resolve_profiles.assert_called_once()
-            resolver.resolve_profiles.reset_mock()
+        module = module_build_service.models.ModuleBuild.get_by_id(db_session, 1)
+
+        generic_resolver.create.return_value = resolver
+        # Call default_buildroot_groups, the result should be cached.
+        ret = GenericBuilder.default_buildroot_groups(db_session, module)
+        assert ret == expected_groups
+        resolver.resolve_profiles.assert_called_once()
+        resolver.resolve_profiles.reset_mock()
 
         # Now try calling it again to verify resolve_profiles is not called,
         # because it is cached.
-        with patch.object(module_build_service.resolver, "system_resolver", new=resolver):
-            ret = GenericBuilder.default_buildroot_groups(db.session, self.module)
-            assert ret == expected_groups
-            resolver.resolve_profiles.assert_not_called()
-            resolver.resolve_profiles.reset_mock()
+        generic_resolver.create.return_value = resolver
+        ret = GenericBuilder.default_buildroot_groups(db_session, module)
+        assert ret == expected_groups
+        resolver.resolve_profiles.assert_not_called()
+        resolver.resolve_profiles.reset_mock()
 
         # And now try clearing the cache and call it again.
-        with patch.object(module_build_service.resolver, "system_resolver", new=resolver):
-            GenericBuilder.clear_cache(self.module)
-            ret = GenericBuilder.default_buildroot_groups(db.session, self.module)
-            assert ret == expected_groups
-            resolver.resolve_profiles.assert_called_once()
+        generic_resolver.create.return_value = resolver
+        GenericBuilder.clear_cache(module)
+        ret = GenericBuilder.default_buildroot_groups(db_session, module)
+        assert ret == expected_groups
+        resolver.resolve_profiles.assert_called_once()
 
     def test_get_build_weights(self):
         weights = GenericBuilder.get_build_weights(["httpd", "apr"])