Check branches in supercedent pipelines

Supercedent was designed for a post pipeline and predates common
use of a promote pipeline.  It therefore only consulted the 'ref'
attribute of a change (so that it would work on tags as well as
branches).  However, it should work just fine as a manager for
promote pipelines too, so have it check the branch attribute for
de-duplication first, and only use the ref attribute if it isn't
present.

Change-Id: I1393c828b86a577fba568a183efee98cf5d030e7

tests/fixtures/layouts/supercedent-promote.yaml

@@ -0,0 +1,20 @@
+- pipeline:
+    name: promote
+    manager: supercedent
+    trigger:
+      gerrit:
+        - event: change-merged
+
+- job:
+    name: base
+    parent: null
+    run: playbooks/base.yaml
+
+- job:
+    name: promote-job
+
+- project:
+    name: org/project
+    promote:
+      jobs:
+        - promote-job

tests/unit/test_supercedent.py

@@ -81,3 +81,36 @@ class TestSupercedent(ZuulTestCase):
             dict(name='post-job', result='SUCCESS', newrev=arev),
             dict(name='post-job', result='SUCCESS', newrev=brev),
         ], ordered=False)
+
+    @simple_layout('layouts/supercedent-promote.yaml')
+    def test_supercedent_promote(self):
+        self.executor_server.hold_jobs_in_build = True
+        A = self.fake_gerrit.addFakeChange('org/project', 'master', 'A')
+        A.setMerged()
+        self.fake_gerrit.addEvent(A.getChangeMergedEvent())
+        self.waitUntilSettled()
+
+        # We should never run jobs for more than one change at a time
+        self.assertEqual(len(self.builds), 1)
+
+        # This change should be superceded by the next
+        B = self.fake_gerrit.addFakeChange('org/project', 'master', 'B')
+        B.setMerged()
+        self.fake_gerrit.addEvent(B.getChangeMergedEvent())
+        self.waitUntilSettled()
+
+        self.assertEqual(len(self.builds), 1)
+
+        C = self.fake_gerrit.addFakeChange('org/project', 'master', 'C')
+        C.setMerged()
+        self.fake_gerrit.addEvent(C.getChangeMergedEvent())
+        self.waitUntilSettled()
+
+        self.assertEqual(len(self.builds), 1)
+
+        self.executor_server.hold_jobs_in_build = True
+        self.orderedRelease()
+        self.assertHistory([
+            dict(name='promote-job', result='SUCCESS', changes='1,1'),
+            dict(name='promote-job', result='SUCCESS', changes='3,1'),
+        ], ordered=False)

zuul/manager/supercedent.py

@@ -26,10 +26,13 @@ class SupercedentPipelineManager(PipelineManager):
             return DynamicChangeQueueContextManager(existing)
 
         # Don't use Pipeline.getQueue to find an existing queue
-        # because we're matching project and ref.
+        # because we're matching project and (branch or ref).
         for queue in self.pipeline.queues:
             if (queue.queue[-1].change.project == change.project and
-                queue.queue[-1].change.ref == change.ref):
+                ((hasattr(change, 'branch') and
+                  hasattr(queue.queue[-1].change, 'branch') and
+                  queue.queue[-1].change.branch == change.branch) or
+                queue.queue[-1].change.ref == change.ref)):
                 self.log.debug("Found existing queue %s", queue)
                 return DynamicChangeQueueContextManager(queue)
         change_queue = model.ChangeQueue(