Merge branch '50824-fix-prepend-concern' into 'master'

CE: Properly implement prepending for Concern

See merge request gitlab-org/gitlab-ce!21444

2 files changed, 73 insertions, 4 deletions

diff --git a/lib/gitlab/patch/prependable.rb b/lib/gitlab/patch/prependable.rb
new file mode 100644
index 00000000000..a9f6cfb19cb
--- /dev/null
+++ b/lib/gitlab/patch/prependable.rb
@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+# We're patching `ActiveSupport::Concern` in
+# config/initializers/0_as_concern.rb
+#
+# We want to patch `ActiveSupport::Concern` for two reasons:
+# 1. Allow defining class methods via: `class_methods` method
+# 2. Allow `prepended do; end` work like `included do; end`
+# If we don't need anything above, we don't need this patch nor the concern!
+
+# 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
+
+      def prepend_features(base)
+        return false if prepended?(base)
+
+        super
+
+        if const_defined?(:ClassMethods)
+          klass_methods = const_get(:ClassMethods)
+          base.singleton_class.prepend klass_methods
+          base.instance_variable_set(:@_prepended_class_methods, klass_methods)
+        end
+
+        if instance_variable_defined?(:@_prepended_block)
+          base.class_eval(&@_prepended_block)
+        end
+
+        true
+      end
+
+      def class_methods
+        super
+
+        if instance_variable_defined?(:@_prepended_class_methods)
+          const_get(:ClassMethods).prepend @_prepended_class_methods
+        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)
+
+        base.ancestors[0...index].index(self)
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/utils/override.rb b/lib/gitlab/utils/override.rb
index 7b2a62fed48..d00921e6cdc 100644
--- a/lib/gitlab/utils/override.rb
+++ b/lib/gitlab/utils/override.rb
@@ -89,15 +89,19 @@ module Gitlab
       def included(base = nil)
         super
 
-        queue_verification(base)
+        queue_verification(base) if base
       end
 
-      alias_method :prepended, :included
+      def prepended(base = nil)
+        super
+
+        queue_verification(base) if base
+      end
 
-      def extended(mod)
+      def extended(mod = nil)
         super
 
-        queue_verification(mod.singleton_class)
+        queue_verification(mod.singleton_class) if mod
       end
 
       def queue_verification(base)