1 files changed, 105 insertions, 0 deletions
diff --git a/lib/gitlab/utils/delegator_override/validator.rb b/lib/gitlab/utils/delegator_override/validator.rb
new file mode 100644
index 00000000000..402154b41c2
--- /dev/null
+++ b/lib/gitlab/utils/delegator_override/validator.rb
@@ -0,0 +1,105 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Utils
+    module DelegatorOverride
+      class Validator
+        UnexpectedDelegatorOverrideError = Class.new(StandardError)
+
+        attr_reader :delegator_class, :target_classes
+
+        OVERRIDE_ERROR_MESSAGE = <<~EOS
+          We've detected that the delegator is overriding a specific method(s) on the target class.
+          Please make sure if it's intentional and handle this error accordingly.
+          See https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/presenters/README.md#validate-accidental-overrides for more information.
+        EOS
+
+        def initialize(delegator_class)
+          @delegator_class = delegator_class
+          @target_classes = []
+        end
+
+        def add_allowlist(names)
+          allowed_method_names.concat(names)
+        end
+
+        def allowed_method_names
+          @allowed_method_names ||= []
+        end
+
+        def add_target(target_class)
+          @target_classes << target_class if target_class
+        end
+
+        # This will make sure allowlist we put into ancestors are all included
+        def expand_on_ancestors(validators)
+          delegator_class.ancestors.each do |ancestor|
+            next if delegator_class == ancestor # ancestor includes itself
+
+            validator_ancestor = validators[ancestor]
+
+            next unless validator_ancestor
+
+            add_allowlist(validator_ancestor.allowed_method_names)
+          end
+        end
+
+        def validate_overrides!
+          return if target_classes.empty?
+
+          errors = []
+
+          # Workaround to fully load the instance methods in the target class.
+          # See https://gitlab.com/gitlab-org/gitlab/-/merge_requests/69823#note_678887402
+          begin
+            target_classes.map(&:new)
+          rescue ArgumentError
+            # Some models might raise ArgumentError here, but it's fine in this case,
+            # because this is enough to force ActiveRecord to generate the methods we
+            # need to verify, so it's safe to ignore it.
+          end
+
+          (delegator_class.instance_methods - allowlist).each do |method_name|
+            target_classes.each do |target_class|
+              next unless target_class.instance_methods.include?(method_name)
+
+              errors << generate_error(method_name, target_class, delegator_class)
+            end
+          end
+
+          return if errors.empty?
+
+          details = errors.map { |error| "- #{error}" }.join("\n")
+
+          raise UnexpectedDelegatorOverrideError,
+            <<~TEXT
+              #{OVERRIDE_ERROR_MESSAGE}
+              Here are the conflict details.
+
+              #{details}
+            TEXT
+        end
+
+        private
+
+        def generate_error(method_name, target_class, delegator_class)
+          target_location = extract_location(target_class, method_name)
+          delegator_location = extract_location(delegator_class, method_name)
+          Error.new(method_name, target_class, target_location, delegator_class, delegator_location)
+        end
+
+        def extract_location(klass, method_name)
+          klass.instance_method(method_name).source_location&.join(':') || 'unknown'
+        end
+
+        def allowlist
+          [].tap do |allowed|
+            allowed.concat(allowed_method_names)
+            allowed.concat(Object.instance_methods)
+            allowed.concat(::Delegator.instance_methods)
+          end
+        end
+      end
+    end
+  end
+end