Abstract auto merge processes

We have one auto merge strategy today - Merge When Pipeline
Succeeds.

In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.

Removed arguments

Fix spec

1 files changed, 54 insertions, 0 deletions

diff --git a/app/services/auto_merge/merge_when_pipeline_succeeds_service.rb b/app/services/auto_merge/merge_when_pipeline_succeeds_service.rb
new file mode 100644
index 00000000000..d0586468859
--- /dev/null
+++ b/app/services/auto_merge/merge_when_pipeline_succeeds_service.rb
@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+module AutoMerge
+  class MergeWhenPipelineSucceedsService < BaseService
+    def execute(merge_request)
+      return :failed unless merge_request.actual_head_pipeline
+
+      if merge_request.actual_head_pipeline.active?
+        merge_request.merge_params.merge!(params)
+
+        unless merge_request.auto_merge_enabled?
+          merge_request.auto_merge_enabled = true
+          merge_request.merge_user = @current_user
+          merge_request.auto_merge_strategy = AutoMergeService::STRATEGY_MERGE_WHEN_PIPELINE_SUCCEEDS
+
+          SystemNoteService.merge_when_pipeline_succeeds(merge_request, @project, @current_user, merge_request.diff_head_commit)
+        end
+
+        return :failed unless merge_request.save
+
+        :merge_when_pipeline_succeeds
+      elsif merge_request.actual_head_pipeline.success?
+        # This can be triggered when a user clicks the auto merge button while
+        # the tests finish at about the same time
+        merge_request.merge_async(current_user.id, merge_params)
+
+        :success
+      else
+        :failed
+      end
+    end
+
+    def process(merge_request)
+      return unless merge_request.actual_head_pipeline&.success?
+      return unless merge_request.mergeable?
+
+      merge_request.merge_async(merge_request.merge_user_id, merge_request.merge_params)
+    end
+
+    def cancel(merge_request)
+      if merge_request.reset_auto_merge
+        SystemNoteService.cancel_merge_when_pipeline_succeeds(merge_request, @project, @current_user)
+
+        success
+      else
+        error("Can't cancel the automatic merge", 406)
+      end
+    end
+
+    def available_for?(merge_request)
+      merge_request.actual_head_pipeline&.active?
+    end
+  end
+end