Use finder to decide to show note to user

Given the note is about how to interpret ancestor clusters, use the
finder which actually knows if there are any ancestor clusters to find
out if the note should be shown, rather than passing the same info via a
view to a helper

Added note about Kaminari.paginate_array

Link to followup issue too

app/controllers/clusters/clusters_controller.rb

@@ -18,8 +18,20 @@ class Clusters::ClustersController < Clusters::BaseController
   STATUS_POLLING_INTERVAL = 10_000
 
   def index
-    clusters = ClusterAncestorsFinder.new(clusterable.subject, current_user).execute
+    finder = ClusterAncestorsFinder.new(clusterable.subject, current_user)
+    clusters = finder.execute
+
+    # Note: We are paginating through an array here but this should OK as:
+    #
+    # In CE, we can have a maximum group nesting depth of 21, so including
+    # project cluster, we can have max 22 clusters for a group hierachy.
+    # In EE (Premium) we can have any number, as multiple clusters are
+    # supported, but the number of clusters are fairly low currently.
+    #
+    # See https://gitlab.com/gitlab-org/gitlab-ce/issues/55260 also.
     @clusters = Kaminari.paginate_array(clusters).page(params[:page]).per(20)
+
+    @has_ancestor_clusters = finder.has_ancestor_clusters?
   end
 
   def new

app/finders/cluster_ancestors_finder.rb

 # frozen_string_literal: true
 
 class ClusterAncestorsFinder
+  include Gitlab::Utils::StrongMemoize
+
   def initialize(clusterable, current_user)
     @clusterable = clusterable
     @current_user = current_user
@@ -12,6 +14,10 @@ def execute
     clusterable.clusters + ancestor_clusters
   end
 
+  def has_ancestor_clusters?
+    ancestor_clusters.any?
+  end
+
   private
 
   attr_reader :clusterable, :current_user
@@ -20,7 +26,10 @@ def can_read_clusters?
     Ability.allowed?(current_user, :read_cluster, clusterable)
   end
 
+  # This unfortunately returns an Array, not a Relation!
   def ancestor_clusters
-    Clusters::Cluster.ancestor_clusters_for_clusterable(clusterable)
+    strong_memoize(:ancestor_clusters) do
+      Clusters::Cluster.ancestor_clusters_for_clusterable(clusterable)
+    end
   end
 end

app/helpers/clusters_helper.rb

@@ -14,8 +14,4 @@ def render_gcp_signup_offer
       render 'clusters/clusters/gcp_signup_offer_banner'
     end
   end
-
-  def render_cluster_help_content?(clusters, clusterable)
-    clusters.length > clusterable.clusters.length
-  end
 end

app/views/clusters/clusters/index.html.haml

@@ -12,7 +12,7 @@
         = s_("ClusterIntegration|Kubernetes clusters can be used to deploy applications and to provide Review Apps for this project")
       = render 'clusters/clusters/buttons'
 
-    - if render_cluster_help_content?(@clusters, clusterable)
+    - if @has_ancestor_clusters
       .bs-callout.bs-callout-info
         = s_("ClusterIntegration|Clusters are utilized by selecting the nearest ancestor with a matching environment scope. For example, project clusters will override group clusters.")
         %strong