Add a cop for `FinderMethods`bvl-finder-methods-cop

This notifies developers when calling `find(_by!)` chained on
`execute`. And suggests using the methods from `FinderMethods`. These
will perform the correct authorization checks on the resource when it
is found.

2 files changed, 53 insertions, 0 deletions

diff --git a/rubocop/cop/gitlab/finder_with_find_by.rb b/rubocop/cop/gitlab/finder_with_find_by.rb
new file mode 100644
index 00000000000..f45a37ddc06
--- /dev/null
+++ b/rubocop/cop/gitlab/finder_with_find_by.rb
@@ -0,0 +1,52 @@
+module RuboCop
+  module Cop
+    module Gitlab
+      class FinderWithFindBy < RuboCop::Cop::Cop
+        FIND_PATTERN = /\Afind(_by\!?)?\z/
+        ALLOWED_MODULES = ['FinderMethods'].freeze
+
+        def message(used_method)
+          <<~MSG
+          Don't chain finders `#execute` method with `##{used_method}`.
+          Instead include `FinderMethods` in the Finder and call `##{used_method}`
+          directly on the finder instance.
+
+          This will make sure all authorization checks are performed on the resource.
+          MSG
+        end
+
+        def on_send(node)
+          if find_on_execute?(node) && !allowed_module?(node)
+            add_offense(node, location: :selector, message: message(node.method_name))
+          end
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            upto_including_execute = node.descendants.first.source_range
+            before_execute = node.descendants[1].source_range
+            range_to_remove = node.source_range
+                                .with(begin_pos: before_execute.end_pos,
+                                      end_pos: upto_including_execute.end_pos)
+
+            corrector.remove(range_to_remove)
+          end
+        end
+
+        def find_on_execute?(node)
+          chained_on_node = node.descendants.first
+          node.method_name.to_s =~ FIND_PATTERN &&
+            chained_on_node&.method_name == :execute
+        end
+
+        def allowed_module?(node)
+          ALLOWED_MODULES.include?(node.parent_module_name)
+        end
+
+        def method_name_for_node(node)
+          children[1].to_s
+        end
+      end
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index f05990232ab..aa7ae601f75 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -2,6 +2,7 @@
 require_relative 'cop/gitlab/module_with_instance_variables'
 require_relative 'cop/gitlab/predicate_memoization'
 require_relative 'cop/gitlab/httparty'
+require_relative 'cop/gitlab/finder_with_find_by'
 require_relative 'cop/include_sidekiq_worker'
 require_relative 'cop/avoid_return_from_blocks'
 require_relative 'cop/avoid_break_from_strong_memoize'