Make fake test Gerrit merger more realistic

The current Gerrit simulated merge operation simply resets the
target branch to the specified change's commit -- it doesn't actually
perform a git merge operation as gerrit (typically) would.  This
makes some test situations difficult, such as when we want to rely on
a post-merge pipeline preparing git repos on top of previously merged
changes.

This change actually performs git merge operations when gerrit changes
are merged in tests, so the result should be far more realistic.

One test relied on a side effect of the old behavior:
test_zuul_trigger_project_change_merged

This test merges change A and then performed a merge check on change B,
which conflicts, and then reports Verified -1.  It then repeats the test
by merging change D which conflicts with change E which should also
receive Verified -1.  However, the test inadvertently ran a second merge
check on change B when change D merged, but because it tested D as written
instead of merged on top of A after A was merged, the conflict of B against
A was not seen (because B does not conflict with D) and therefore not
rported.  With this patch, the conflict of B against the merged change A
is seen and the test failed because it reported B's merge conflict twice.
To correct this, we make the test even more realistic by adding a pipeline
requirement that we have not already reported Verified -1 in order to
enqueue a change for merge-check.  In order for this to work, we need to
make sure Zuul sees the comment-added events for the Verified -1 votes
(so that the Gerrit driver updates its view of the votes on the change), so
those are added to the test.

Change-Id: Ia5b4b83096de3b8719c8b6d256e8e33749e7a5b8

4 files changed, 16 insertions, 10 deletions

diff --git a/tests/base.py b/tests/base.py
index cd8c0c780..4a4e536a3 100644
--- a/tests/base.py
+++ b/tests/base.py
@@ -638,8 +638,11 @@ class FakeGerritChange(object):
 
         path = os.path.join(self.upstream_root, self.project)
         repo = git.Repo(path)
-        repo.heads[self.branch].commit = \
-            repo.commit(self.patchsets[-1]['revision'])
+
+        repo.head.reference = self.branch
+        zuul.merger.merger.reset_repo_to_head(repo)
+        repo.git.merge('-s', 'resolve', self.patchsets[-1]['ref'])
+        repo.heads[self.branch].commit = repo.head.commit
 
     def setReported(self):
         self.reported += 1
diff --git a/tests/fixtures/config/zuultrigger/project-change-merged/git/common-config/zuul.yaml b/tests/fixtures/config/zuultrigger/project-change-merged/git/common-config/zuul.yaml
index 045e0a988..77a96c25b 100644
--- a/tests/fixtures/config/zuultrigger/project-change-merged/git/common-config/zuul.yaml
+++ b/tests/fixtures/config/zuultrigger/project-change-merged/git/common-config/zuul.yaml
@@ -36,6 +36,10 @@
     name: merge-check
     manager: independent
     ignore-dependencies: true
+    reject:
+      gerrit:
+        approval:
+          - Verified: -1
     trigger:
       zuul:
         - event: project-change-merged
diff --git a/tests/unit/test_serial.py b/tests/unit/test_serial.py
index 458d860db..f91bb4eda 100644
--- a/tests/unit/test_serial.py
+++ b/tests/unit/test_serial.py
@@ -26,14 +26,7 @@ class TestSerial(ZuulTestCase):
         A.setMerged()
         self.fake_gerrit.addEvent(A.getChangeMergedEvent())
         self.waitUntilSettled()
-        # The gerrit upstream repo simulation isn't perfect -- when
-        # change A is merged above, the master ref is updated to point
-        # to that change, it doesn't actually "merge" it.  The same is
-        # true for B, so if it didn't have A in its git history, then
-        # A would not appear in the jobs run for B.  We simulate the
-        # correct situation by setting A as the git parent of B.
-        B = self.fake_gerrit.addFakeChange('org/project', 'master', 'B',
-                                           parent='refs/changes/1/1/1')
+        B = self.fake_gerrit.addFakeChange('org/project', 'master', 'B')
         B.setMerged()
         self.fake_gerrit.addEvent(B.getChangeMergedEvent())
         self.waitUntilSettled()
diff --git a/tests/unit/test_zuultrigger.py b/tests/unit/test_zuultrigger.py
index 80cb7387c..aeda12433 100644
--- a/tests/unit/test_zuultrigger.py
+++ b/tests/unit/test_zuultrigger.py
@@ -220,6 +220,12 @@ class TestZuulTriggerProjectChangeMerged(ZuulTestCase):
         self.assertTrue("project:{org/project} status:open" in
                         self.fake_gerrit.queries)
 
+        # Ensure the gerrit driver has updated its cache after the
+        # previous comments were left:
+        self.fake_gerrit.addEvent(A.getChangeCommentEvent(2))
+        self.fake_gerrit.addEvent(B.getChangeCommentEvent(2))
+        self.waitUntilSettled()
+
         # Reconfigure and run the test again.  This is a regression
         # check to make sure that we don't end up with a stale trigger
         # cache that has references to projects from the old