Fix failed merge detection in gitlab

gitlab is funny in that it can return status_code=200 with an error in
the body. This can happen when MR accept request fails (i.e. when
project requires squashing, but we have not done this). Ensure status of
the MR is "merged" otherwise return "merge_error" attribute as a failure
description.

Change-Id: I343fb08cdf58fdf6d2068e274a89a34a55adde5a

releasenotes/notes/detect-gitlab-merge-error-69816ea60127b363.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fix detection of the failed merge in gitlab driver.

tests/base.py

@@ -2156,7 +2156,7 @@ class FakeGitlabAPIClient(gitlabconnection.GitlabAPIClient):
             mr = self._get_mr(match)
             mr.mergeMergeRequest()
 
-        return {}, 200, "", "PUT"
+        return {'state': 'merged'}, 200, "", "PUT"
 
     def addProject(self, project):
         self.addProjectByName(project.name)

zuul/driver/gitlab/gitlabconnection.py

@@ -404,6 +404,10 @@ class GitlabAPIClient():
         resp = self.put(self.baseurl + path, zuul_event_id=zuul_event_id)
         try:
             self._manage_error(*resp, zuul_event_id=zuul_event_id)
+            if resp[0]['state'] != 'merged':
+                raise MergeFailure(
+                    "Merge request merge failed: %s" % resp.get('merge_error')
+                )
         except GitlabAPIClientException as e:
             raise MergeFailure('Merge request merge failed: %s' % e)
         return resp[0]