1 files changed, 89 insertions, 0 deletions
diff --git a/lib/gitlab/pagination/keyset/request_context.rb b/lib/gitlab/pagination/keyset/request_context.rb
new file mode 100644
index 00000000000..aeaed7587b3
--- /dev/null
+++ b/lib/gitlab/pagination/keyset/request_context.rb
@@ -0,0 +1,89 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Pagination
+    module Keyset
+      class RequestContext
+        attr_reader :request
+
+        DEFAULT_SORT_DIRECTION = :desc
+        PRIMARY_KEY = :id
+
+        # A tie breaker is added as an additional order-by column
+        # to establish a well-defined order. We use the primary key
+        # column here.
+        TIE_BREAKER = { PRIMARY_KEY => DEFAULT_SORT_DIRECTION }.freeze
+
+        def initialize(request)
+          @request = request
+        end
+
+        # extracts Paging information from request parameters
+        def page
+          @page ||= Page.new(order_by: order_by, per_page: params[:per_page])
+        end
+
+        def apply_headers(next_page)
+          request.header('Links', pagination_links(next_page))
+        end
+
+        private
+
+        def order_by
+          return TIE_BREAKER.dup unless params[:order_by]
+
+          order_by = { params[:order_by].to_sym => params[:sort]&.to_sym || DEFAULT_SORT_DIRECTION }
+
+          # Order by an additional unique key, we use the primary key here
+          order_by = order_by.merge(TIE_BREAKER) unless order_by[PRIMARY_KEY]
+
+          order_by
+        end
+
+        def params
+          @params ||= request.params
+        end
+
+        def lower_bounds_params(page)
+          page.lower_bounds.each_with_object({}) do |(column, value), params|
+            filter = filter_with_comparator(page, column)
+            params[filter] = value
+          end
+        end
+
+        def filter_with_comparator(page, column)
+          direction = page.order_by[column]
+
+          if direction&.to_sym == :desc
+            "#{column}_before"
+          else
+            "#{column}_after"
+          end
+        end
+
+        def page_href(page)
+          base_request_uri.tap do |uri|
+            uri.query = query_params_for(page).to_query
+          end.to_s
+        end
+
+        def pagination_links(next_page)
+          return if next_page.end_reached?
+
+          %(<#{page_href(next_page)}>; rel="next")
+        end
+
+        def base_request_uri
+          @base_request_uri ||= URI.parse(request.request.url).tap do |uri|
+            uri.host = Gitlab.config.gitlab.host
+            uri.port = Gitlab.config.gitlab.port
+          end
+        end
+
+        def query_params_for(page)
+          request.params.merge(lower_bounds_params(page))
+        end
+      end
+    end
+  end
+end