Properly implement prepending for Concern

1 files changed, 71 insertions, 0 deletions

diff --git a/lib/gitlab/patch/prependable.rb b/lib/gitlab/patch/prependable.rb
new file mode 100644
index 00000000000..45fac0ed4a7
--- /dev/null
+++ b/lib/gitlab/patch/prependable.rb
@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+# rubocop:disable Gitlab/ModuleWithInstanceVariables
+module Gitlab
+  module Patch
+    module Prependable
+      class MultiplePrependedBlocks < StandardError
+        def initialize
+          super "Cannot define multiple 'prepended' blocks for a Concern"
+        end
+      end
+
+      module MetaConcern
+        def extended(base)
+          super
+          base.instance_variable_set(:@_prepend_dependencies, [])
+        end
+      end
+
+      def self.prepended(base)
+        super
+        base.singleton_class.prepend MetaConcern
+      end
+
+      def append_features(base)
+        super
+
+        prepend_features(base)
+      end
+
+      def prepend_features(base)
+        if base.instance_variable_defined?(:@_prepend_dependencies)
+          base.instance_variable_get(:@_prepend_dependencies) << self
+          false
+        else
+          return false if prepended?(base)
+
+          @_prepend_dependencies.each { |dep| base.prepend(dep) }
+
+          super
+
+          if const_defined?(:ClassMethods)
+            base.singleton_class.prepend const_get(:ClassMethods)
+          end
+
+          if instance_variable_defined?(:@_prepended_block)
+            base.class_eval(&@_prepended_block)
+          end
+        end
+      end
+
+      def prepended(base = nil, &block)
+        if base.nil?
+          raise MultiplePrependedBlocks if
+            instance_variable_defined?(:@_prepended_block)
+
+          @_prepended_block = block
+        else
+          super
+        end
+      end
+
+      def prepended?(base)
+        index = base.ancestors.index(base)
+
+        @_prepend_dependencies.index(self) ||
+          base.ancestors[0...index].index(self)
+      end
+    end
+  end
+end