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, 50 insertions, 0 deletions

diff --git a/app/services/auto_merge_service.rb b/app/services/auto_merge_service.rb
new file mode 100644
index 00000000000..a3a780ff388
--- /dev/null
+++ b/app/services/auto_merge_service.rb
@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+class AutoMergeService < BaseService
+  STRATEGY_MERGE_WHEN_PIPELINE_SUCCEEDS = 'merge_when_pipeline_succeeds'.freeze
+  STRATEGIES = [STRATEGY_MERGE_WHEN_PIPELINE_SUCCEEDS].freeze
+
+  class << self
+    def all_strategies
+      STRATEGIES
+    end
+
+    def get_service_class(strategy)
+      return unless all_strategies.include?(strategy)
+
+      "::AutoMerge::#{strategy.camelize}Service".constantize
+    end
+  end
+
+  def execute(merge_request, strategy)
+    service = get_service_instance(strategy)
+
+    return :failed unless service&.available_for?(merge_request)
+
+    service.execute(merge_request)
+  end
+
+  def process(merge_request)
+    return unless merge_request.auto_merge_enabled?
+
+    get_service_instance(merge_request.auto_merge_strategy).process(merge_request)
+  end
+
+  def cancel(merge_request)
+    return error("Can't cancel the automatic merge", 406) unless merge_request.auto_merge_enabled?
+
+    get_service_instance(merge_request.auto_merge_strategy).cancel(merge_request)
+  end
+
+  def available_strategies(merge_request)
+    self.class.all_strategies.select do |strategy|
+      get_service_instance(strategy).available_for?(merge_request)
+    end
+  end
+
+  private
+
+  def get_service_instance(strategy)
+    self.class.get_service_class(strategy)&.new(project, current_user, params)
+  end
+end