batches.py: Don't pass SQLAlchemy objects between threads

SQLAlchemy objects can't be used from multiple threads - so when starting
threads for builds, pass the ComponentBuild id rather than the object.
(Note that despite the comment that the threads were sharing a session,
they weren't - what was passed to the thread was a scoped_session that
acts as a separate thread-local session per-thread.)

BUILD_COMPONENT_DB_SESSION_LOCK - a threading.Lock() object that was used
in a few places - but not nearly enough places to effectively lock usage
of a shared session - is removed.

tests/test_scheduler/test_batches.py

@@ -172,6 +172,7 @@ class TestBatches:
         start_next_batch_build(conf, module_build, builder)
 
         # Batch number should increase.
+        module_build = models.ModuleBuild.get_by_id(db_session, 3)
         assert module_build.batch == 2
 
         # Make sure we only have one message returned for the one reused component
@@ -218,6 +219,8 @@ class TestBatches:
         builder.recover_orphaned_artifact.return_value = []
         start_next_batch_build(conf, module_build, builder)
 
+        module_build = models.ModuleBuild.get_by_id(db_session, 3)
+
         # Batch number should increase.
         assert module_build.batch == 2
         # No component reuse messages should be returned
@@ -233,7 +236,15 @@ class TestBatches:
         builder.build.side_effect = Exception("Something have gone terribly wrong")
         component = mock.MagicMock()
 
-        start_build_component(db_session, builder, component)
+        component = models.ComponentBuild.from_component_name(
+            db_session, "perl-List-Compare", 3)
+
+        assert component.state != koji.BUILD_STATES["FAILED"]
+
+        start_build_component(db_session, builder, component.id)
+
+        component = models.ComponentBuild.from_component_name(
+            db_session, "perl-List-Compare", 3)
 
         assert component.state == koji.BUILD_STATES["FAILED"]
 
@@ -264,6 +275,8 @@ class TestBatches:
         builder.recover_orphaned_artifact.return_value = []
         start_next_batch_build(conf, module_build, builder)
 
+        module_build = models.ModuleBuild.get_by_id(db_session, 3)
+
         # Batch number should increase
         assert module_build.batch == 2
 
@@ -292,6 +305,8 @@ class TestBatches:
         mock_sbc.reset_mock()
 
         # Complete the build
+        plc_component = models.ComponentBuild.from_component_name(
+            db_session, "perl-List-Compare", 3)
         plc_component.state = koji.BUILD_STATES["COMPLETE"]
         pt_component = models.ComponentBuild.from_component_name(
             db_session, "perl-Tangerine", 3)
@@ -301,6 +316,9 @@ class TestBatches:
 
         # Start the next build batch
         start_next_batch_build(conf, module_build, builder)
+
+        module_build = models.ModuleBuild.get_by_id(db_session, 3)
+
         # Batch number should increase
         assert module_build.batch == 3
         # Verify that tangerine was reused even though perl-Tangerine was rebuilt in the previous
@@ -344,6 +362,8 @@ class TestBatches:
         builder.recover_orphaned_artifact.return_value = []
         start_next_batch_build(conf, module_build, builder)
 
+        module_build = models.ModuleBuild.get_by_id(db_session, 3)
+
         # Batch number should increase.
         assert module_build.batch == 2
 
@@ -357,8 +377,8 @@ class TestBatches:
 
         # Test the order of the scheduling
         expected_calls = [
-            mock.call(db_session, builder, plc_component),
-            mock.call(db_session, builder, pt_component)
+            mock.call(db_session, builder, plc_component.id),
+            mock.call(db_session, builder, pt_component.id)
         ]
         assert mock_sbc.mock_calls == expected_calls
 
@@ -379,6 +399,8 @@ class TestBatches:
         builder = mock.MagicMock()
         start_next_batch_build(conf, module_build, builder)
 
+        module_build = models.ModuleBuild.get_by_id(db_session, 3)
+
         # Batch number should not increase.
         assert module_build.batch == 2
         # Make sure start build was called for the second component which wasn't reused
@@ -399,5 +421,7 @@ class TestBatches:
         builder = mock.MagicMock()
         builder.buildroot_ready.return_value = False
 
+        module_build = models.ModuleBuild.get_by_id(db_session, 3)
+
         # Batch number should not increase.
         assert module_build.batch == 1