Use target branch name in batch merge to allow multiple instances

A project can use multiple instances of marge bots assigned to
different target branches. This allows dealing with different
release branches in parallel.

However batches could not be used on multiple instances because
they conflicted on the name of the branch created for the batch.
By naming this branch after the target branch, then multiple
batch branches can exist.

marge/batch_job.py

@@ -20,16 +20,17 @@ def __init__(self, *, api, user, project, repo, options, merge_requests):
         super().__init__(api=api, user=user, project=project, repo=repo, options=options)
         self._merge_requests = merge_requests
 
-    def remove_batch_branch(self):
+    def remove_batch_branch(self, target_branch):
         log.info('Removing local batch branch')
         try:
-            self._repo.remove_branch(BatchMergeJob.BATCH_BRANCH_NAME)
+            self._repo.remove_branch(f'{BatchMergeJob.BATCH_BRANCH_NAME}/{target_branch}')
         except git.GitError:
             pass
 
-    def close_batch_mr(self):
+    def close_batch_mr(self, target_branch):
         log.info('Closing batch MRs')
         params = {
+            'target_branch': target_branch,
             'author_id': self._user.id,
             'labels': BatchMergeJob.BATCH_BRANCH_NAME,
             'state': 'opened',
@@ -48,7 +49,7 @@ def close_batch_mr(self):
     def create_batch_mr(self, target_branch):
         log.info('Creating batch MR')
         params = {
-            'source_branch': BatchMergeJob.BATCH_BRANCH_NAME,
+            'source_branch': f'{BatchMergeJob.BATCH_BRANCH_NAME}/{target_branch}',
             'target_branch': target_branch,
             'title': 'Marge Bot Batch MR - DO NOT TOUCH',
             'labels': BatchMergeJob.BATCH_BRANCH_NAME,
@@ -92,9 +93,9 @@ def get_mergeable_mrs(self, merge_requests):
                 mergeable_mrs.append(merge_request)
         return mergeable_mrs
 
-    def push_batch(self):
+    def push_batch(self, target_branch):
         log.info('Pushing batch branch')
-        self._repo.push(BatchMergeJob.BATCH_BRANCH_NAME, force=True)
+        self._repo.push(f'{BatchMergeJob.BATCH_BRANCH_NAME}/{target_branch}', force=True)
 
     def ensure_mr_not_changed(self, merge_request):
         log.info('Ensuring MR !%s did not change', merge_request.iid)
@@ -181,11 +182,13 @@ def accept_mr(
         return final_sha
 
     def execute(self):
-        # Cleanup previous batch work
-        self.remove_batch_branch()
-        self.close_batch_mr()
 
         target_branch = self._merge_requests[0].target_branch
+
+        # Cleanup previous batch work
+        self.remove_batch_branch(target_branch)
+        self.close_batch_mr(target_branch)
+
         merge_requests = self.get_mrs_with_common_target_branch(target_branch)
         merge_requests = self.get_mergeable_mrs(merge_requests)
 
@@ -201,7 +204,8 @@ def execute(self):
         remote_target_branch_sha = self._repo.get_commit_hash('origin/%s' % target_branch)
 
         self._repo.checkout_branch(target_branch, 'origin/%s' % target_branch)
-        self._repo.checkout_branch(BatchMergeJob.BATCH_BRANCH_NAME, 'origin/%s' % target_branch)
+        self._repo.checkout_branch(f'{BatchMergeJob.BATCH_BRANCH_NAME}/{target_branch}',
+                                   'origin/%s' % target_branch)
 
         working_merge_requests = []
 
@@ -241,7 +245,7 @@ def execute(self):
             raise CannotBatch('not enough ready merge requests')
         if self._project.only_allow_merge_if_pipeline_succeeds:
             # This switches git to <batch> branch
-            self.push_batch()
+            self.push_batch(target_branch)
             batch_mr = self.create_batch_mr(
                 target_branch=target_branch,
             )

tests/test_batch_job.py

@@ -49,9 +49,9 @@ def get_batch_merge_job(self, api, mocklab, **batch_merge_kwargs):
     def test_remove_batch_branch(self, api, mocklab):
         repo = create_autospec(marge.git.Repo, spec_set=True)
         batch_merge_job = self.get_batch_merge_job(api, mocklab, repo=repo)
-        batch_merge_job.remove_batch_branch()
+        batch_merge_job.remove_batch_branch('master')
         repo.remove_branch.assert_called_once_with(
-            BatchMergeJob.BATCH_BRANCH_NAME,
+            f'{BatchMergeJob.BATCH_BRANCH_NAME}/master',
         )
 
     def test_close_batch_mr(self, api, mocklab):
@@ -60,9 +60,10 @@ def test_close_batch_mr(self, api, mocklab):
             mr_class.search.return_value = [batch_mr]
 
             batch_merge_job = self.get_batch_merge_job(api, mocklab)
-            batch_merge_job.close_batch_mr()
+            batch_merge_job.close_batch_mr('master')
 
             params = {
+                'target_branch': 'master',
                 'author_id': batch_merge_job._user.id,
                 'labels': BatchMergeJob.BATCH_BRANCH_NAME,
                 'state': 'opened',
@@ -86,7 +87,7 @@ def test_create_batch_mr(self, api, mocklab):
             r_batch_mr = batch_merge_job.create_batch_mr(target_branch)
 
             params = {
-                'source_branch': BatchMergeJob.BATCH_BRANCH_NAME,
+                'source_branch': f'{BatchMergeJob.BATCH_BRANCH_NAME}/{target_branch}',
                 'target_branch': target_branch,
                 'title': 'Marge Bot Batch MR - DO NOT TOUCH',
                 'labels': BatchMergeJob.BATCH_BRANCH_NAME,
@@ -132,9 +133,9 @@ def test_ensure_mergeable_mr_ci_not_ok(self, bmj_get_mr_ci_status, api, mocklab)
 
     def test_push_batch(self, api, mocklab):
         batch_merge_job = self.get_batch_merge_job(api, mocklab)
-        batch_merge_job.push_batch()
+        batch_merge_job.push_batch('master')
         batch_merge_job._repo.push.assert_called_once_with(
-            BatchMergeJob.BATCH_BRANCH_NAME,
+            f'{BatchMergeJob.BATCH_BRANCH_NAME}/master',
             force=True,
         )