Skip second-stage upgrade testing if UPGRADE_OUTPUT option is absent or empty

This is useful to speed up britney in dry run mode, if we are only interested
in builds and package tests.

britney.py

@@ -3335,6 +3335,13 @@ class Britney(object):
         else:
             self.upgrade_me = self.options.actions.split()
 
+        # skip upgrade testing if UPGRADE_OUTPUT option is absent or empty
+        try:
+            if not self.options.upgrade_output:
+                return
+        except AttributeError:
+            return
+
         ensuredir(os.path.dirname(self.options.upgrade_output))
         with open(self.options.upgrade_output, 'w', encoding='utf-8') as f:
             self.__output = f

tests/test_autopkgtest.py

@@ -169,6 +169,11 @@ class T(TestBase):
         self.assertEqual(self.pending_requests, {})
         self.assertEqual(self.amqp_requests, set())
 
+        with open(os.path.join(self.data.path, 'output', 'series', 'output.txt')) as f:
+            upgrade_out = f.read()
+        self.assertNotIn('accepted:', upgrade_out)
+        self.assertIn('SUCCESS (0/0)', upgrade_out)
+
     def test_no_wait_for_always_failed_test(self):
         '''We do not need to wait for results for tests which have always failed'''
 
@@ -202,6 +207,11 @@ class T(TestBase):
             set(['debci-series-amd64:darkgreen {"triggers": ["darkgreen/2"]}',
                  'debci-series-i386:darkgreen {"triggers": ["darkgreen/2"]}']))
 
+        with open(os.path.join(self.data.path, 'output', 'series', 'output.txt')) as f:
+            upgrade_out = f.read()
+        self.assertIn('accepted: darkgreen', upgrade_out)
+        self.assertIn('SUCCESS (1/0)', upgrade_out)
+
     def test_multi_rdepends_with_tests_all_running(self):
         '''Multiple reverse dependencies with tests (all running)'''
 
@@ -1578,6 +1588,19 @@ class T(TestBase):
         self.assertEqual(self.amqp_requests, set())
         self.assertEqual(self.pending_requests, {})
 
+    def test_disable_upgrade_tester(self):
+        '''Run without second stage upgrade tester'''
+
+        for line in fileinput.input(self.britney_conf, inplace=True):
+            if not line.startswith('UPGRADE_OUTPUT'):
+                sys.stdout.write(line)
+
+        self.do_test(
+            [('libgreen1', {'Version': '2', 'Source': 'green', 'Depends': 'libc6'}, 'autopkgtest')],
+            {})[1]
+
+        self.assertFalse(os.path.exists(os.path.join(self.data.path, 'output', 'series', 'output.txt')))
+
 
 if __name__ == '__main__':
     unittest.main()