Add payload to the service response

This introduces payload to the ServiceResponse with
the merge ref HEAD commit data

3 files changed, 39 insertions, 14 deletions

diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 38b9b886e5f..cf63a7e79bd 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -1080,10 +1080,6 @@ class MergeRequest < ApplicationRecord
 
   # Returns the current merge-ref HEAD commit.
   #
-  # Consider calling mergeability_check method _before_ this if you need
-  # the latest possible version of it (it's already automatically updated
-  # along the merge_status).
-  #
   def merge_ref_head
     project.repository.commit(merge_ref_path)
   end
diff --git a/app/services/merge_requests/mergeability_check_service.rb b/app/services/merge_requests/mergeability_check_service.rb
index d277d38127c..ef833774e65 100644
--- a/app/services/merge_requests/mergeability_check_service.rb
+++ b/app/services/merge_requests/mergeability_check_service.rb
@@ -2,6 +2,8 @@
 
 module MergeRequests
   class MergeabilityCheckService < ::BaseService
+    include Gitlab::Utils::StrongMemoize
+
     delegate :project, to: :@merge_request
     delegate :repository, to: :project
 
@@ -16,8 +18,8 @@ module MergeRequests
     # and the merge-ref is synced. Success in case of being/becoming mergeable,
     # error otherwise.
     def execute
-      return ServiceResponse.error('Invalid argument') unless merge_request
-      return ServiceResponse.error('Unsupported operation') if Gitlab::Database.read_only?
+      return ServiceResponse.error(message: 'Invalid argument') unless merge_request
+      return ServiceResponse.error(message: 'Unsupported operation') if Gitlab::Database.read_only?
 
       update_merge_status
 
@@ -25,13 +27,39 @@ module MergeRequests
         return ServiceResponse.error(message: 'Merge request is not mergeable')
       end
 
-      ServiceResponse.success
+      unless payload.fetch(:merge_ref_head)
+        return ServiceResponse.error(message: 'Merge ref was not found')
+      end
+
+      ServiceResponse.success(payload: payload)
     end
 
     private
 
     attr_reader :merge_request
 
+    def payload
+      strong_memoize(:payload) do
+        {
+          merge_ref_head: merge_ref_head_payload
+        }
+      end
+    end
+
+    def merge_ref_head_payload
+      commit = merge_request.merge_ref_head
+
+      return unless commit
+
+      target_id, source_id = commit.parent_ids
+
+      {
+        commit_id: commit.id,
+        source_id: source_id,
+        target_id: target_id
+      }
+    end
+
     def update_merge_status
       return unless merge_request.recheck_merge_status?
 
diff --git a/app/services/service_response.rb b/app/services/service_response.rb
index 1de30e68d87..f3437ba16de 100644
--- a/app/services/service_response.rb
+++ b/app/services/service_response.rb
@@ -1,19 +1,20 @@
 # frozen_string_literal: true
 
 class ServiceResponse
-  def self.success(message: nil)
-    new(status: :success, message: message)
+  def self.success(message: nil, payload: {})
+    new(status: :success, message: message, payload: payload)
   end
 
-  def self.error(message:, http_status: nil)
-    new(status: :error, message: message, http_status: http_status)
+  def self.error(message:, payload: {}, http_status: nil)
+    new(status: :error, message: message, payload: payload, http_status: http_status)
   end
 
-  attr_reader :status, :message, :http_status
+  attr_reader :status, :message, :http_status, :payload
 
-  def initialize(status:, message: nil, http_status: nil)
+  def initialize(status:, message: nil, payload: {}, http_status: nil)
     self.status = status
     self.message = message
+    self.payload = payload
     self.http_status = http_status
   end
 
@@ -27,5 +28,5 @@ class ServiceResponse
 
   private
 
-  attr_writer :status, :message, :http_status
+  attr_writer :status, :message, :http_status, :payload
 end