Make db_session singleton

Please note that this patch does not change the use of database session
in MBS. So, in the frontend, the database session is still managed by
Flask-SQLAlchemy, that is the db.session. And the backend, running event
handlers, has its own database session created from SQLAclehmy session
API directly.

This patch aims to reduce the number of scoped_session created when call
original function make_db_session. For technical detailed information,
please refer to SQLAlchemy documentation Contextual/Thread-local
Sessions.

As a result, a global scoped_session is accessible from the
code running inside backend, both the event handlers and functions
called from handlers. The library code shared by frontend and backend,
like resolvers, has no change.

Similarly, db.session is only used to recreate database for every test.

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

module_build_service/utils/submit.py

@@ -17,6 +17,7 @@ from gi.repository import GLib
 
 import module_build_service.scm
 from module_build_service import conf, log, models, Modulemd
+from module_build_service.db_session import db_session
 from module_build_service.errors import ValidationError, UnprocessableEntity, Forbidden, Conflict
 from module_build_service.utils import (
     to_text_type, deps_to_dict, mmd_to_str, load_mmd, load_mmd_file,
@@ -24,29 +25,25 @@ from module_build_service.utils import (
 )
 
 
-def record_module_build_arches(mmd, build, db_session):
+def record_module_build_arches(mmd, build):
     """
     Finds out the list of build arches against which the ModuleBuld `build` should be built
     and records them to `build.arches`.
 
     :param Modulemd mmd: The MMD file associated with a ModuleBuild.
     :param ModuleBuild build: The ModuleBuild.
-    :param db_session: Database session.
     """
-    arches = get_build_arches(db_session, mmd, conf)
+    arches = get_build_arches(mmd, conf)
     for arch in arches:
         arch_obj = db_session.query(models.ModuleArch).filter_by(name=arch).first()
         if not arch_obj:
             arch_obj = models.ModuleArch(name=arch)
-            db_session.add(arch_obj)
-            db_session.commit()
+        build.arches.append(arch_obj)
 
-        if arch_obj not in build.arches:
-            build.arches.append(arch_obj)
-            db_session.add(build)
+    db_session.commit()
 
 
-def record_filtered_rpms(db_session, mmd):
+def record_filtered_rpms(mmd):
     """Record filtered RPMs that should not be installed into buildroot
 
     These RPMs are filtered:
@@ -54,7 +51,6 @@ def record_filtered_rpms(db_session, mmd):
     * Reads the mmd["xmd"]["buildrequires"] and extends it with "filtered_rpms"
       list containing the NVRs of filtered RPMs in a buildrequired module.
 
-    :param db_session: SQLAlchemy session object.
     :param Modulemd mmd: Modulemd that will be built next.
     :rtype: Modulemd.Module
     :return: Modulemd extended with the "filtered_rpms" in XMD section.
@@ -395,7 +391,7 @@ def get_module_srpm_overrides(module):
 
 
 def record_component_builds(
-    db_session, mmd, module, initial_batch=1, previous_buildorder=None, main_mmd=None
+    mmd, module, initial_batch=1, previous_buildorder=None, main_mmd=None
 ):
     # Imported here to allow import of utils in GenericBuilder.
     import module_build_service.builder
@@ -466,7 +462,7 @@ def record_component_builds(
             included_mmd = fetch_mmd(full_url, whitelist_url=True)[0]
             format_mmd(included_mmd, module.scmurl, module, db_session)
             batch = record_component_builds(
-                db_session, included_mmd, module, batch, previous_buildorder, main_mmd)
+                included_mmd, module, batch, previous_buildorder, main_mmd)
             continue
 
         package = component.get_name()
@@ -1120,12 +1116,11 @@ def fetch_mmd(url, branch=None, allow_local_url=False, whitelist_url=False, mand
     return mmd, scm
 
 
-def load_local_builds(db_session, local_build_nsvs):
+def load_local_builds(local_build_nsvs):
     """
     Loads previously finished local module builds from conf.mock_resultsdir
     and imports them to database.
 
-    :param db_session: SQLAlchemy session object.
     :param local_build_nsvs: List of NSV separated by ':' defining the modules
         to load from the mock_resultsdir.
     """