Merge branch 'fix-mentioned-users-on-diff-notes' into 'master'

Fix mentioned users on diff notes

## Summary

`DiffNote`, and `LegacyDiffNote` returns empty array for `mentionable_attrs`, because `mentionable_attrs` is not inheritable by subclasses. The problem can be illustrated with this small sample:

```ruby
module Mentionable
  extend ActiveSupport::Concern

  module ClassMethods
    def attr_mentionable(attr)
      mentionable_attrs << [attr.to_s]
    end

    def mentionable_attrs
      @mentionable_attrs ||= []
    end
  end
end

class A
  include Mentionable

  attr_mentionable :foo
end

class B < A
end

A.mentionable_attrs
=> [["foo", {}]]

B.mentionable_attrs
=> []
```

Possible solution using `cattr_accessor`:

```ruby
module Mentionable
  extend ActiveSupport::Concern

  module ClassMethods
    def attr_mentionable(attr)
      mentionable_attrs << [attr.to_s]
    end
  end

  included do
    cattr_accessor :mentionable_attrs, instance_accessor: false do
      []
    end
  end
end

class A
  include Mentionable

  attr_mentionable :foo
end

class B < A
end

A.mentionable_attrs
=> [["foo"]]

B.mentionable_attrs
=> [["foo"]]

B.mentionable_attrs < [:bar]
=> [["foo"], ["bar"]]

A.mentionable_attrs
=> [["foo"], ["bar"]]
```

`mentionable_attrs` is inheritable by subclasses. If a subclass changes the value then that would also change the value for parent class. Similarly if parent class changes the value then that would change the value of subclasses too.

## What are the relevant issue numbers?

Fixes #19807 

Fixes #18022

/cc @stanhu  @DouweM @rspeicher 

See merge request !5243

CHANGELOG

@@ -57,6 +57,7 @@ v 8.10.0 (unreleased)
   - Throttle the update of `project.pushes_since_gc` to 1 minute.
   - Allow expanding and collapsing files in diff view (!4990)
   - Collapse large diffs by default (!4990)
+  - Fix mentioned users list on diff notes
   - Check for conflicts with existing Project's wiki path when creating a new project.
   - Show last push widget in upstream after push to fork
   - Cache todos pending/done dashboard query counts.

app/models/concerns/mentionable.rb

@@ -14,14 +14,14 @@ module Mentionable
       attr = attr.to_s
       mentionable_attrs << [attr, options]
     end
+  end
 
+  included do
     # Accessor for attributes marked mentionable.
-    def mentionable_attrs
-      @mentionable_attrs ||= []
+    cattr_accessor :mentionable_attrs, instance_accessor: false do
+      []
     end
-  end
 
-  included do
     if self < Participable
       participant -> (user, ext) { all_references(user, extractor: ext) }
     end

app/models/concerns/participable.rb

@@ -41,9 +41,12 @@ module Participable
     def participant(attr)
       participant_attrs << attr
     end
+  end
 
-    def participant_attrs
-      @participant_attrs ||= []
+  included do
+    # Accessor for participant attributes.
+    cattr_accessor :participant_attrs, instance_accessor: false do
+      []
     end
   end
 

spec/services/todo_service_spec.rb

@@ -435,6 +435,24 @@ describe TodoService, services: true do
         should_create_todo(user: author, target: mr_unassigned, action: Todo::MARKED)
       end
     end
+
+    describe '#new_note' do
+      let(:mention) { john_doe.to_reference }
+      let(:diff_note_on_merge_request) { create(:diff_note_on_merge_request, project: project, noteable: mr_unassigned, author: author, note: "Hey #{mention}") }
+      let(:legacy_diff_note_on_merge_request) { create(:legacy_diff_note_on_merge_request, project: project, noteable: mr_unassigned, author: author, note: "Hey #{mention}") }
+
+      it 'creates a todo for mentioned user on new diff note' do
+        service.new_note(diff_note_on_merge_request, author)
+
+        should_create_todo(user: john_doe, target: mr_unassigned, author: author, action: Todo::MENTIONED, note: diff_note_on_merge_request)
+      end
+
+      it 'creates a todo for mentioned user on legacy diff note' do
+        service.new_note(legacy_diff_note_on_merge_request, author)
+
+        should_create_todo(user: john_doe, target: mr_unassigned, author: author, action: Todo::MENTIONED, note: legacy_diff_note_on_merge_request)
+      end
+    end
   end
 
   it 'updates cached counts when a todo is created' do