merger: Handle merges with cherry-pick merge-mode

Merges cannot be cherry-picked in git, so if a change is a merge, do a
`git merge` instead of a cherry-pick to match how Gerrit will merge the
change.

Change-Id: I9bc7025d2371913b63f0a6723aff480e7e63d8a3
Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>

releasenotes/notes/cherry-pick-merges-9c78fd914b682671.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    The `cherry-pick` merger mode can now handle merges between branches by
+    performing a `git merge` instead of `git cherry-pick` if the change has
+    multiple parents. Previously, this would fail because git doesn't allow
+    a merge to be cherry-picked.

tests/base.py

@@ -494,7 +494,8 @@ class FakeGerritChange(object):
         repo.git.clean('-x', '-f', '-d')
 
         repo.index.merge_tree(parents[1])
-        r = repo.index.commit(msg)
+        parent_commits = [repo.commit(p) for p in parents]
+        r = repo.index.commit(msg, parent_commits=parent_commits)
 
         repo.head.reference = 'master'
         repo.head.reset(working_tree=True)

tests/unit/test_scheduler.py

@@ -7062,6 +7062,51 @@ class TestSchedulerMerges(ZuulTestCase):
         result = self._test_project_merge_mode('cherry-pick')
         self.assertEqual(result, expected_messages)
 
+    def test_project_merge_mode_cherrypick_branch_merge(self):
+        "Test that branches can be merged together in cherry-pick mode"
+        self.create_branch('org/project-merge-branches', 'mp')
+        self.fake_gerrit.addEvent(
+            self.fake_gerrit.getFakeBranchCreatedEvent(
+                'org/project-merge-branches', 'mp'))
+        self.waitUntilSettled()
+
+        path = os.path.join(self.upstream_root, 'org/project-merge-branches')
+        repo = git.Repo(path)
+        master_sha = repo.heads.master.commit.hexsha
+        mp_sha = repo.heads.mp.commit.hexsha
+
+        self.executor_server.hold_jobs_in_build = True
+        M = self.fake_gerrit.addFakeChange(
+            'org/project-merge-branches', 'master', 'M',
+            merge_parents=[
+                master_sha,
+                mp_sha,
+            ])
+        M.addApproval('Code-Review', 2)
+        self.fake_gerrit.addEvent(M.addApproval('Approved', 1))
+        self.waitUntilSettled()
+
+        self.executor_server.release('.*-merge')
+        self.waitUntilSettled()
+
+        build = self.builds[-1]
+        self.assertEqual(build.parameters['zuul']['branch'], 'master')
+        path = os.path.join(build.jobdir.src_root, 'review.example.com',
+                            "org/project-merge-branches")
+        repo = git.Repo(path)
+        repo_messages = [c.message.strip() for c in repo.iter_commits()]
+        repo_messages.reverse()
+        correct_messages = [
+            'initial commit',
+            'add content from fixture',
+            'mp commit',
+            'M-1']
+        self.assertEqual(repo_messages, correct_messages)
+
+        self.executor_server.hold_jobs_in_build = False
+        self.executor_server.release()
+        self.waitUntilSettled()
+
     def test_merge_branch(self):
         "Test that the right commits are on alternate branches"
         self.create_branch('org/project-merge-branches', 'mp')

zuul/merger/merger.py

@@ -555,9 +555,15 @@ class Repo(object):
     def cherryPick(self, ref, zuul_event_id=None):
         log = get_annotated_logger(self.log, zuul_event_id)
         repo = self.createRepoObject(zuul_event_id)
-        log.debug("Cherry-picking %s", ref)
         self.fetch(ref, zuul_event_id=zuul_event_id)
-        repo.git.cherry_pick("FETCH_HEAD")
+        if len(repo.commit("FETCH_HEAD").parents) > 1:
+            args = ["-s", "resolve", "FETCH_HEAD"]
+            log.debug("Merging %s with args %s instead of cherry-picking",
+                      ref, args)
+            repo.git.merge(*args)
+        else:
+            log.debug("Cherry-picking %s", ref)
+            repo.git.cherry_pick("FETCH_HEAD")
         return repo.head.commit
 
     def merge(self, ref, strategy=None, zuul_event_id=None):