Add service to cache group issues count

- Replace count in group template

app/models/group.rb

@@ -620,6 +620,18 @@ class Group < Namespace
     Gitlab::ServiceDesk.supported? && all_projects.service_desk_enabled.exists?
   end
 
+  # rubocop: disable CodeReuse/ServiceClass
+  def open_issues_count(current_user = nil)
+    Groups::OpenIssuesCountService.new(self, current_user).count
+  end
+  # rubocop: enable CodeReuse/ServiceClass
+
+  # rubocop: disable CodeReuse/ServiceClass
+  def update_group_issues_counter_cache
+    Groups::OpenIssuesCountService.new(self).refresh_cache
+  end
+  # rubocop: enable CodeReuse/ServiceClass
+
   private
 
   def update_two_factor_requirement

app/services/groups/open_issues_count_service.rb

+# frozen_string_literal: true
+
+module Groups
+  # Service class for counting and caching the number of open issues of a group.
+  class OpenIssuesCountService < OpenIssuesCountBaseService
+    include Gitlab::Utils::StrongMemoize
+
+    PUBLIC_COUNT_KEY = 'group_public_open_issues_count'
+    TOTAL_COUNT_KEY = 'group_total_open_issues_count'
+
+    def cache_options
+      super.merge({ expires_in: 24.hours })
+    end
+
+    def self.query(group, user: nil, public_only: true)
+      IssuesFinder.new(user, group_id: group.id, state: 'opened', non_archived: true, include_subgroups: true, public_only: public_only).execute
+    end
+  end
+end

app/services/issuables_count_service.rb

+# frozen_string_literal: true
+
+class IssuablesCountService < BaseCountService
+  # The version of the cache format. This should be bumped whenever the
+  # underlying logic changes. This removes the need for explicitly flushing
+  # all caches.
+  VERSION = 1
+
+  def initialize(parent)
+    @parent = parent
+  end
+
+  def relation_for_count
+    self.class.query(@parent.id)
+  end
+
+  def cache_key_name
+    raise(
+      NotImplementedError,
+      '"cache_key_name" must be implemented and return a String'
+    )
+  end
+
+  def cache_key(key = nil)
+    cache_key = key || cache_key_name
+
+    ['parents', 'count_service', VERSION, @parent.id, cache_key]
+  end
+
+  def self.query(parent_ids)
+    raise(
+      NotImplementedError,
+      '"query" must be implemented and return an ActiveRecord::Relation'
+    )
+  end
+end

app/services/open_issues_count_base_service.rb

+# frozen_string_literal: true
+
+# Service class for counting and caching the number of open issues of a group or a project.
+class OpenIssuesCountBaseService < IssuablesCountService
+  include Gitlab::Utils::StrongMemoize
+
+  # Cache keys used to store issues count
+  PUBLIC_COUNT_KEY = ''
+  TOTAL_COUNT_KEY = ''
+
+  def initialize(group, user = nil)
+    @user = user
+
+    super(group)
+  end
+
+  def cache_key_name
+    public_only? ? PUBLIC_COUNT_KEY : TOTAL_COUNT_KEY
+  end
+
+  def public_only?
+    !user_is_at_least_reporter?
+  end
+
+  def relation_for_count
+    self.class.query(@parent, user: @user, public_only: public_only?)
+  end
+
+  def user_is_at_least_reporter?
+    strong_memoize(:user_is_at_least_reporter) do
+      @user && @parent.member?(@user, Gitlab::Access::REPORTER)
+    end
+  end
+
+  def public_count_cache_key
+    cache_key(PUBLIC_COUNT_KEY)
+  end
+
+  def total_count_cache_key
+    cache_key(TOTAL_COUNT_KEY)
+  end
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def refresh_cache(&block)
+    if block_given?
+      super(&block)
+    else
+      count_grouped_by_confidential = self.class.query(@parent, user: @user, public_only: false).group(:confidential).count
+      public_count = count_grouped_by_confidential[false] || 0
+      total_count = public_count + (count_grouped_by_confidential[true] || 0)
+
+      update_cache_for_key(group_public_count_cache_key) do
+        public_count
+      end
+
+      update_cache_for_key(group_total_count_cache_key) do
+        total_count
+      end
+    end
+  end
+
+  def self.query(parent, user: nil, public_only: true)
+    raise(
+      NotImplementedError,
+      '"query" must be implemented and return an ActiveRecord::Relation'
+    )
+  end
+end

app/views/layouts/nav/sidebar/_group.html.haml

-- issues_count = group_issues_count(state: 'opened')
 - merge_requests_count = group_merge_requests_count(state: 'opened')
 
 .nav-sidebar{ class: ("sidebar-collapsed-desktop" if collapsed_sidebar?), **tracking_attrs('groups_side_navigation', 'render', 'groups_side_navigation') }
@@ -54,14 +53,16 @@
               = sprite_icon('issues')
             %span.nav-item-name
               = _('Issues')
-            %span.badge.badge-pill.count= number_with_delimiter(issues_count)
+            %span.badge.badge-pill.count
+              = number_with_delimiter(@group.open_issues_count(current_user))
 
           %ul.sidebar-sub-level-items{ data: { qa_selector: 'group_issues_sidebar_submenu'} }
             = nav_link(path: ['groups#issues', 'labels#index', 'milestones#index', 'iterations#index'], html_options: { class: "fly-out-top-item" } ) do
               = link_to issues_group_path(@group) do
                 %strong.fly-out-top-item-name
                   = _('Issues')
-                %span.badge.badge-pill.count.issue_counter.fly-out-badge= number_with_delimiter(issues_count)
+                %span.badge.badge-pill.count.issue_counter.fly-out-badge
+                  = number_with_delimiter(@group.open_issues_count(current_user))
 
             %li.divider.fly-out-top-item
             = nav_link(path: 'groups#issues', html_options: { class: 'home' }) do