Add a `WithPagination` concern to reuse across serializers

app/serializers/concerns/with_pagination.rb

+module WithPagination
+  def with_pagination(request, response)
+    tap { @paginator = Gitlab::Serializer::Pagination.new(request, response) }
+  end
+
+  def paginated?
+    @paginator.present?
+  end
+
+  # super is `BaseSerializer#represent` here.
+  #
+  # we shouldn't try to paginate single resources
+  def represent(resource, opts = {})
+    if paginated? && resource.respond_to?(:page)
+      super(@paginator.paginate(resource), opts)
+    else
+      super(resource, opts)
+    end
+  end
+end

app/serializers/environment_serializer.rb

 class EnvironmentSerializer < BaseSerializer
+  include WithPagination
+
   Item = Struct.new(:name, :size, :latest)
   entity EnvironmentEntity
@@ -7,18 +9,10 @@ class EnvironmentSerializer < BaseSerializer
     tap { @itemize = true }
   end
-  def with_pagination(request, response)
-    tap { @paginator = Gitlab::Serializer::Pagination.new(request, response) }
-  end
-
   def itemized?
     @itemize
   end
-  def paginated?
-    @paginator.present?
-  end
-
   def represent(resource, opts = {})
     if itemized?
       itemize(resource).map do |item|
@@ -27,8 +21,6 @@ class EnvironmentSerializer < BaseSerializer
           latest: super(item.latest, opts) }
       end
     else
-      resource = @paginator.paginate(resource) if paginated?
-
       super(resource, opts)
     end
   end

app/serializers/group_serializer.rb

 class GroupSerializer < BaseSerializer
-  entity GroupEntity
-
-  def with_pagination(request, response)
-    tap { @paginator = Gitlab::Serializer::Pagination.new(request, response) }
-  end
-  def paginated?
-    @paginator.present?
-  end
-
-  def represent(resource, opts = {})
-    if paginated?
-      super(@paginator.paginate(resource), opts)
-    else
-      super(resource, opts)
-    end
-  end
+  include WithPagination
+  entity GroupEntity
 end

app/serializers/pipeline_serializer.rb

 class PipelineSerializer < BaseSerializer
+  include WithPagination
+
   InvalidResourceError = Class.new(StandardError)
   entity PipelineDetailsEntity
-  def with_pagination(request, response)
-    tap { @paginator = Gitlab::Serializer::Pagination.new(request, response) }
-  end
-
-  def paginated?
-    @paginator.present?
-  end
-
   def represent(resource, opts = {})
     if resource.is_a?(ActiveRecord::Relation)

config/application.rb

@@ -29,6 +29,7 @@ module Gitlab
                                      #{config.root}/app/models/project_services
                                      #{config.root}/app/workers/concerns
                                      #{config.root}/app/services/concerns
+                                     #{config.root}/app/serializers/concerns
                                      #{config.root}/app/finders/concerns])
     config.generators.templates.push("#{config.root}/generator_templates")