1 files changed, 46 insertions, 0 deletions
diff --git a/lib/gitlab/ci/config/yaml/tags/reference.rb b/lib/gitlab/ci/config/yaml/tags/reference.rb
new file mode 100644
index 00000000000..22822614b67
--- /dev/null
+++ b/lib/gitlab/ci/config/yaml/tags/reference.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Ci
+    class Config
+      module Yaml
+        module Tags
+          class Reference < Base
+            MissingReferenceError = Class.new(Tags::TagError)
+
+            def self.tag
+              '!reference'
+            end
+
+            override :valid?
+            def valid?
+              data[:seq].is_a?(Array) &&
+                !data[:seq].empty? &&
+                data[:seq].all? { |identifier| identifier.is_a?(String) }
+            end
+
+            private
+
+            def location
+              data[:seq].to_a.map(&:to_sym)
+            end
+
+            override :_resolve
+            def _resolve(resolver)
+              object = resolver.config.dig(*location)
+              value = resolver.deep_resolve(object)
+
+              raise MissingReferenceError, missing_ref_error_message unless value
+
+              value
+            end
+
+            def missing_ref_error_message
+              "#{data[:tag]} #{data[:seq].inspect} could not be found"
+            end
+          end
+        end
+      end
+    end
+  end
+end