Only allow multiple todos per action per object

Changes the behavior of the multiple_todos feature flag so that only one
todo per action type is allowed per object.

An exception is made for mentions so that users can still have multiple
mention todos since these link to different notes.

This reduces noise for the other todo actions where they link to the
same thing.

app/finders/pending_todos_finder.rb

@@ -27,7 +27,8 @@ class PendingTodosFinder
     todos = by_target_id(todos)
     todos = by_target_type(todos)
     todos = by_discussion(todos)
-    by_commit_id(todos)
+    todos = by_commit_id(todos)
+    by_action(todos)
   end
 
   def by_project(todos)
@@ -69,4 +70,10 @@ class PendingTodosFinder
       todos
     end
   end
+
+  def by_action(todos)
+    return todos if params[:action].blank?
+
+    todos.for_action(params[:action])
+  end
 end

app/models/todo.rb

@@ -34,6 +34,8 @@ class Todo < ApplicationRecord
     ATTENTION_REQUESTED => :attention_requested
   }.freeze
 
+  ACTIONS_MULTIPLE_ALLOWED = [Todo::MENTIONED, Todo::DIRECTLY_ADDRESSED].freeze
+
   belongs_to :author, class_name: "User"
   belongs_to :note
   belongs_to :project

app/services/todo_service.rb

@@ -9,6 +9,7 @@
 #
 class TodoService
   include Gitlab::Utils::UsageData
+
   # When create an issue we should:
   #
   #  * create a todo for assignee if issue is assigned
@@ -229,8 +230,24 @@ class TodoService
 
     return if users.empty?
 
-    users_with_pending_todos = pending_todos(users, attributes).distinct_user_ids
-    users.reject! { |user| users_with_pending_todos.include?(user.id) && Feature.disabled?(:multiple_todos, user) }
+    users_single_todos, users_multiple_todos = users.partition { |u| Feature.disabled?(:multiple_todos, u) }
+    excluded_user_ids = []
+
+    if users_single_todos.present?
+      excluded_user_ids += pending_todos(
+        users_single_todos,
+        attributes.slice(:project_id, :target_id, :target_type, :commit_id, :discussion)
+      ).distinct_user_ids
+    end
+
+    if users_multiple_todos.present? && !Todo::ACTIONS_MULTIPLE_ALLOWED.include?(attributes.fetch(:action))
+      excluded_user_ids += pending_todos(
+        users_multiple_todos,
+        attributes.slice(:project_id, :target_id, :target_type, :commit_id, :discussion, :action)
+      ).distinct_user_ids
+    end
+
+    users.reject! { |user| excluded_user_ids.include?(user.id) }
 
     todos = users.map do |user|
       issue_type = attributes.delete(:issue_type)

doc/user/todos.md

@@ -62,8 +62,8 @@ ask an administrator to [enable the feature flag](../administration/feature_flag
 On GitLab.com, this feature is not available.
 The feature is not ready for production use.
 
-When you enable this feature, new actions for the same user on the same object
-create new to-do items.
+When you enable this feature, mentions for the same user on the same object
+create new to-do items. Other actions are limited to one to-do item per action type.
 
 ## Create a to-do item
 

spec/finders/pending_todos_finder_spec.rb

@@ -75,5 +75,15 @@ RSpec.describe PendingTodosFinder do
 
       expect(todos).to contain_exactly(todo1, todo2)
     end
+
+    it 'supports retrieving of todos for a specific action' do
+      todo = create(:todo, :pending, user: user, target: issue, action: Todo::MENTIONED)
+
+      create(:todo, :pending, user: user, target: issue, action: Todo::ASSIGNED)
+
+      todos = described_class.new(users, action: Todo::MENTIONED).execute
+
+      expect(todos).to contain_exactly(todo)
+    end
   end
 end

spec/services/todo_service_spec.rb

@@ -628,12 +628,32 @@ RSpec.describe TodoService do
         stub_feature_flags(multiple_todos: true)
       end
 
-      it 'creates a todo even if user already has a pending todo' do
+      it 'creates a MENTIONED todo even if user already has a pending MENTIONED todo' do
         create(:todo, :mentioned, user: member, project: project, target: issue, author: author)
 
         expect { service.update_issue(issue, author) }.to change(member.todos, :count)
       end
 
+      it 'creates a DIRECTLY_ADDRESSED todo even if user already has a pending DIRECTLY_ADDRESSED todo' do
+        create(:todo, :directly_addressed, user: member, project: project, target: issue, author: author)
+
+        issue.update!(description: "#{member.to_reference}, what do you think?")
+
+        expect { service.update_issue(issue, author) }.to change(member.todos, :count)
+      end
+
+      it 'creates an ASSIGNED todo even if user already has a pending MARKED todo' do
+        create(:todo, :marked, user: john_doe, project: project, target: assigned_issue, author: author)
+
+        expect { service.reassigned_assignable(assigned_issue, author) }.to change(john_doe.todos, :count)
+      end
+
+      it 'does not create an ASSIGNED todo if user already has an ASSIGNED todo' do
+        create(:todo, :assigned, user: john_doe, project: project, target: assigned_issue, author: author)
+
+        expect { service.reassigned_assignable(assigned_issue, author) }.not_to change(john_doe.todos, :count)
+      end
+
       it 'creates multiple todos if a user is assigned and mentioned in a new issue' do
         assigned_issue.description = mentions
         service.new_issue(assigned_issue, author)