1 files changed, 60 insertions, 0 deletions
diff --git a/lib/gitlab/merge_requests/merge_commit_message.rb b/lib/gitlab/merge_requests/merge_commit_message.rb
new file mode 100644
index 00000000000..2a6a7859b33
--- /dev/null
+++ b/lib/gitlab/merge_requests/merge_commit_message.rb
@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+module Gitlab
+  module MergeRequests
+    class MergeCommitMessage
+      def initialize(merge_request:)
+        @merge_request = merge_request
+      end
+
+      def message
+        return unless @merge_request.target_project.merge_commit_template.present?
+
+        message = @merge_request.target_project.merge_commit_template
+        message = message.delete("\r")
+
+        # Remove placeholders that correspond to empty values and are the last word in the line
+        # along with all whitespace characters preceding them.
+        # This allows us to recreate previous default merge commit message behaviour - we skipped new line character
+        # before empty description and before closed issues when none were present.
+        PLACEHOLDERS.each do |key, value|
+          unless value.call(merge_request).present?
+            message = message.gsub(BLANK_PLACEHOLDERS_REGEXES[key], '')
+          end
+        end
+
+        Gitlab::StringPlaceholderReplacer
+          .replace_string_placeholders(message, PLACEHOLDERS_REGEX) do |key|
+          PLACEHOLDERS[key].call(merge_request)
+        end
+      end
+
+      private
+
+      attr_reader :merge_request
+
+      PLACEHOLDERS = {
+        'source_branch' => ->(merge_request) { merge_request.source_branch.to_s },
+        'target_branch' => ->(merge_request) { merge_request.target_branch.to_s },
+        'title' => ->(merge_request) { merge_request.title },
+        'issues' => ->(merge_request) do
+          return "" if merge_request.visible_closing_issues_for.blank?
+
+          closes_issues_references = merge_request.visible_closing_issues_for.map do |issue|
+            issue.to_reference(merge_request.target_project)
+          end
+          "Closes #{closes_issues_references.to_sentence}"
+        end,
+        'description' => ->(merge_request) { merge_request.description.presence || '' },
+        'reference' => ->(merge_request) { merge_request.to_reference(full: true) }
+      }.freeze
+
+      PLACEHOLDERS_REGEX = Regexp.union(PLACEHOLDERS.keys.map do |key|
+        Regexp.new(Regexp.escape(key))
+      end).freeze
+
+      BLANK_PLACEHOLDERS_REGEXES = (PLACEHOLDERS.map do |key, value|
+        [key, Regexp.new("[\n\r]+%{#{Regexp.escape(key)}}$")]
+      end).to_h.freeze
+    end
+  end
+end