Add support for gitlab squash merge

Add support for allowing to squash merge request in Gitlab. It is
possible to enforce in Gitlab requiring squashing. With this change we
can use `merge-mode: squash-merge` on the project to be able to merge.
This sets parameter `squash: True` for the API request.

Change-Id: I2ce9f7f9fc570809ee0250970546e457568a45b8

doc/source/reference/project_def.rst

@@ -122,24 +122,24 @@ pipeline.
       .. value:: merge
 
          Uses the default git merge strategy (recursive). This maps to
-         the merge mode ``merge`` in Github.
+         the merge mode ``merge`` in Github and Gitlab.
 
       .. value:: merge-resolve
 
          Uses the resolve git merge strategy.  This is a very
          conservative merge strategy which most closely matches the
          behavior of Gerrit. This maps to the merge mode ``merge`` in
-         Github.
+         Github and Gitlab.
 
       .. value:: cherry-pick
 
          Cherry-picks each change onto the branch rather than
-         performing any merges. This is not supported by Github.
+         performing any merges. This is not supported by Github and Gitlab.
 
       .. value:: squash-merge
 
          Squash merges each change onto the branch. This maps to the
-         merge mode ``squash`` in Github.
+         merge mode ``squash`` in Github and Gitlab.
 
    .. attr:: vars
       :default: None

releasenotes/notes/add-gitlab-squash-mode-3e981204342ac6f9.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    Add support for squashing MR with gitlab.

zuul/driver/gitlab/gitlabconnection.py

@@ -398,9 +398,13 @@ class GitlabAPIClient():
 
     # https://docs.gitlab.com/ee/api/merge_requests.html#accept-mr
     def merge_mr(self, project_name, number,
+                 method,
                  zuul_event_id=None):
         path = "/projects/%s/merge_requests/%s/merge" % (
             quote_plus(project_name), number)
+        params = {}
+        if method == "squash":
+            params['squash'] = True
         resp = self.put(self.baseurl + path, zuul_event_id=zuul_event_id)
         try:
             self._manage_error(*resp, zuul_event_id=zuul_event_id)
@@ -653,9 +657,10 @@ class GitlabConnection(ZKChangeCacheMixin, CachedBranchConnection):
         # Will be done in a folloup commit
         return []
 
-    def mergeMR(self, project_name, number, event=None):
+    def mergeMR(self, project_name, number, method='merge', event=None):
         log = get_annotated_logger(self.log, event)
-        self.gl_client.merge_mr(project_name, number, zuul_event_id=event)
+        self.gl_client.merge_mr(
+            project_name, number, method, zuul_event_id=event)
         log.info("Merged MR %s#%s", project_name, number)
 
 

zuul/driver/gitlab/gitlabreporter.py

@@ -17,6 +17,8 @@ import logging
 import voluptuous as v
 
 from zuul.reporter import BaseReporter
+from zuul.model import MERGER_MERGE_RESOLVE, MERGER_MERGE, MERGER_MAP, \
+    MERGER_SQUASH_MERGE
 from zuul.lib.logutil import get_annotated_logger
 from zuul.driver.gitlab.gitlabsource import GitlabSource
 from zuul.exceptions import MergeFailure
@@ -28,6 +30,13 @@ class GitlabReporter(BaseReporter):
     name = 'gitlab'
     log = logging.getLogger("zuul.GitlabReporter")
 
+    # Merge modes supported by gitlab
+    merge_modes = {
+        MERGER_MERGE: 'merge',
+        MERGER_MERGE_RESOLVE: 'merge',
+        MERGER_SQUASH_MERGE: 'squash'
+    }
+
     def __init__(self, driver, connection, pipeline, config=None):
         super(GitlabReporter, self).__init__(driver, connection, config)
         self._create_comment = self.config.get('comment', True)
@@ -78,9 +87,18 @@ class GitlabReporter(BaseReporter):
         project = item.change.project.name
         mr_number = item.change.number
 
+        merge_mode = item.current_build_set.getMergeMode()
+
+        if merge_mode not in self.merge_modes:
+            mode = [x[0] for x in MERGER_MAP.items() if x[1] == merge_mode][0]
+            self.log.warning('Merge mode %s not supported by Gitlab', mode)
+            raise MergeFailure('Merge mode %s not supported by Gitlab' % mode)
+
+        merge_mode = self.merge_modes[merge_mode]
+
         for i in [1, 2]:
             try:
-                self.connection.mergeMR(project, mr_number)
+                self.connection.mergeMR(project, mr_number, merge_mode)
                 item.change.is_merged = True
                 return
             except MergeFailure: