Don't error on wrong MR IID in changelog for cherry-pick MRs

Signed-off-by: Rémy Coutable <remy@rymai.me>

danger/changelog/Dangerfile

@@ -35,7 +35,11 @@ def check_changelog_yaml(path)
   fail "`title` should be set, in #{gitlab.html_link(path)}! #{SEE_DOC}" if yaml["title"].nil?
   fail "`type` should be set, in #{gitlab.html_link(path)}! #{SEE_DOC}" if yaml["type"].nil?
 
-  if yaml["merge_request"].nil? && !helper.security_mr?
+  return if helper.security_mr?
+
+  cherry_pick_against_stable_branch = helper.cherry_pick_mr? && helper.stable_branch?
+
+  if yaml["merge_request"].nil?
     mr_line = raw_file.lines.find_index("merge_request:\n")
 
     if mr_line
@@ -43,7 +47,7 @@ def check_changelog_yaml(path)
     else
       message "Consider setting `merge_request` to #{gitlab.mr_json["iid"]} in #{gitlab.html_link(path)}. #{SEE_DOC}"
     end
-  elsif yaml["merge_request"] != gitlab.mr_json["iid"] && !helper.security_mr?
+  elsif yaml["merge_request"] != gitlab.mr_json["iid"] && !cherry_pick_against_stable_branch
     fail "Merge request ID was not set to #{gitlab.mr_json["iid"]}! #{SEE_DOC}"
   end
 rescue Psych::SyntaxError, Psych::DisallowedClass, Psych::BadAlias

lib/gitlab/danger/helper.rb

@@ -193,6 +193,18 @@ module Gitlab
         gitlab_helper.mr_json['web_url'].include?('/gitlab-org/security/')
       end
 
+      def cherry_pick_mr?
+        return false unless gitlab_helper
+
+        /cherry[\s-]*pick/i.match?(gitlab_helper.mr_json['title'])
+      end
+
+      def stable_branch?
+        return false unless gitlab_helper
+
+        /\A\d+-\d+-stable-ee/i.match?(gitlab_helper.mr_json['target_branch'])
+      end
+
       def mr_has_labels?(*labels)
         return false unless gitlab_helper
 

spec/lib/gitlab/danger/helper_spec.rb

@@ -366,6 +366,69 @@ RSpec.describe Gitlab::Danger::Helper do
     end
   end
 
+  describe '#cherry_pick_mr?' do
+    it 'returns false when `gitlab_helper` is unavailable' do
+      expect(helper).to receive(:gitlab_helper).and_return(nil)
+
+      expect(helper).not_to be_cherry_pick_mr
+    end
+
+    context 'when MR title does not mention a cherry-pick' do
+      it 'returns false' do
+        expect(fake_gitlab).to receive(:mr_json)
+          .and_return('title' => 'Add feature xyz')
+
+        expect(helper).not_to be_cherry_pick_mr
+      end
+    end
+
+    context 'when MR title mentions a cherry-pick' do
+      [
+        'Cherry Pick !1234',
+        'cherry-pick !1234',
+        'CherryPick !1234'
+      ].each do |mr_title|
+        it 'returns true' do
+          expect(fake_gitlab).to receive(:mr_json)
+            .and_return('title' => mr_title)
+
+          expect(helper).to be_cherry_pick_mr
+        end
+      end
+    end
+  end
+
+  describe '#stable_branch?' do
+    it 'returns false when `gitlab_helper` is unavailable' do
+      expect(helper).to receive(:gitlab_helper).and_return(nil)
+
+      expect(helper).not_to be_stable_branch
+    end
+
+    context 'when MR target branch is not a stable branch' do
+      it 'returns false' do
+        expect(fake_gitlab).to receive(:mr_json)
+          .and_return('target_branch' => 'my-feature-branch')
+
+        expect(helper).not_to be_stable_branch
+      end
+    end
+
+    context 'when MR target branch is a stable branch' do
+      %w[
+        13-1-stable-ee
+        13-1-stable-ee-patch-1
+      ].each do |target_branch|
+        it 'returns true' do
+          expect(fake_gitlab).to receive(:mr_json)
+            .and_return('target_branch' => target_branch)
+
+          expect(helper).to be_stable_branch
+        end
+      end
+    end
+  end
+
   describe '#mr_has_label?' do
     it 'returns false when `gitlab_helper` is unavailable' do
       expect(helper).to receive(:gitlab_helper).and_return(nil)