Store extended traceback for gather errors

When a gathering thread raises an exception, it gets forwarded to the
main thread and re-raised there. However, during this transition it
loses details about exact location of the problem.

This patch creates an extended traceback in the worker, which should
make it easier to track the problem down later.

JIRA: RHELCMP-4259
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_compose.py

@@ -777,3 +777,33 @@ class DumpContainerMetadataTest(unittest.TestCase):
     def test_dump_empty_metadata(self, ThreadPool):
         self.compose.dump_containers_metadata()
         self.assertFalse(os.path.isfile(self.tmp_dir + "/compose/metadata/osbs.json"))
+
+
+class TracebackTest(unittest.TestCase):
+    def setUp(self):
+        self.tmp_dir = tempfile.mkdtemp()
+        with mock.patch("pungi.compose.ComposeInfo"):
+            self.compose = Compose({}, self.tmp_dir)
+        self.patcher = mock.patch("kobo.tback.Traceback")
+        self.Traceback = self.patcher.start()
+        self.Traceback.return_value.get_traceback.return_value = b"traceback"
+
+    def tearDown(self):
+        shutil.rmtree(self.tmp_dir)
+        self.patcher.stop()
+
+    def assertTraceback(self, filename):
+        self.assertTrue(
+            os.path.isfile("%s/logs/global/%s.global.log" % (self.tmp_dir, filename))
+        )
+        self.assertEqual(
+            self.Traceback.mock_calls, [mock.call(), mock.call().get_traceback()]
+        )
+
+    def test_traceback_default(self):
+        self.compose.traceback()
+        self.assertTraceback("traceback")
+
+    def test_with_detail(self):
+        self.compose.traceback("extra-info")
+        self.assertTraceback("traceback-extra-info")