Introduce CursorPager service

Introduce CursorPager service

lib/gitlab/pagination/keyset/cursor_based_request_context.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Pagination
+    module Keyset
+      class CursorBasedRequestContext
+        attr_reader :request
+        delegate :params, :header, to: :request
+
+        def initialize(request)
+          @request = request
+        end
+
+        def per_page
+          params[:per_page]
+        end
+
+        def cursor
+          params[:cursor]
+        end
+
+        def apply_headers(cursor_for_next_page)
+          Gitlab::Pagination::Keyset::HeaderBuilder
+            .new(self)
+            .add_next_page_header({ cursor: cursor_for_next_page })
+        end
+      end
+    end
+  end
+end

lib/gitlab/pagination/keyset/cursor_pager.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Pagination
+    module Keyset
+      class CursorPager < Gitlab::Pagination::Base
+        attr_reader :cursor_based_request_context, :paginator
+
+        def initialize(cursor_based_request_context)
+          @cursor_based_request_context = cursor_based_request_context
+        end
+
+        def paginate(relation)
+          @paginator ||= relation.keyset_paginate(
+            per_page: cursor_based_request_context.per_page,
+            cursor: cursor_based_request_context.cursor
+          )
+
+          paginator.records
+        end
+
+        def finalize(_records = [])
+          # can be called only after executing `paginate(relation)`
+          apply_headers
+        end
+
+        private
+
+        def apply_headers
+          return unless paginator.has_next_page?
+
+          cursor_based_request_context
+            .apply_headers(paginator.cursor_for_next_page)
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/pagination/keyset/cursor_based_request_context_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Pagination::Keyset::CursorBasedRequestContext do
+  let(:params) { { per_page: 2, cursor: 'eyJuYW1lIjoiR2l0TGFiIEluc3RhbmNlIiwiaWQiOiI1MiIsIl9rZCI6Im4ifQ==' } }
+  let(:request) { double('request', params: params) }
+
+  describe '#per_page' do
+    subject(:per_page) { described_class.new(request).per_page }
+
+    it { is_expected.to eq 2 }
+  end
+
+  describe '#cursor' do
+    subject(:cursor) { described_class.new(request).cursor }
+
+    it { is_expected.to eq 'eyJuYW1lIjoiR2l0TGFiIEluc3RhbmNlIiwiaWQiOiI1MiIsIl9rZCI6Im4ifQ==' }
+  end
+
+  describe '#apply_headers' do
+    let(:request) { double('request', url: "http://#{Gitlab.config.gitlab.host}/api/v4/projects?per_page=3", params: params) }
+    let(:params) { { per_page: 3 } }
+    let(:cursor_for_next_page) { 'eyJuYW1lIjoiSDVicCIsImlkIjoiMjgiLCJfa2QiOiJuIn0=' }
+
+    subject(:apply_headers) { described_class.new(request).apply_headers(cursor_for_next_page) }
+
+    it 'sets Link header with same host/path as the original request' do
+      orig_uri = URI.parse(request.url)
+
+      expect(request).to receive(:header).once do |name, header|
+        first_link, _ = /<([^>]+)>; rel="next"/.match(header).captures
+
+        uri = URI.parse(first_link)
+
+        expect(name).to eq('Link')
+        expect(uri.host).to eq(orig_uri.host)
+        expect(uri.path).to eq(orig_uri.path)
+      end
+
+      apply_headers
+    end
+
+    it 'sets Link header with a cursor to the next page' do
+      orig_uri = URI.parse(request.url)
+
+      expect(request).to receive(:header).once do |name, header|
+        first_link, _ = /<([^>]+)>; rel="next"/.match(header).captures
+
+        query = CGI.parse(URI.parse(first_link).query)
+
+        expect(name).to eq('Link')
+        expect(query.except('cursor')).to eq(CGI.parse(orig_uri.query).except('cursor'))
+        expect(query['cursor']).to eq([cursor_for_next_page])
+      end
+
+      apply_headers
+    end
+  end
+end

spec/lib/gitlab/pagination/keyset/cursor_pager_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Pagination::Keyset::CursorPager do
+  let(:relation) { Group.all.order(:name, :id) }
+  let(:per_page) { 3 }
+  let(:params) { { cursor: nil, per_page: per_page } }
+  let(:request) { double('request', params: params) }
+  let(:cursor_based_request_context) { Gitlab::Pagination::Keyset::CursorBasedRequestContext.new(request) }
+
+  before_all do
+    create_list(:group, 7)
+  end
+
+  describe '#paginate' do
+    subject(:paginated_result) { described_class.new(cursor_based_request_context).paginate(relation) }
+
+    it 'returns the limited relation' do
+      expect(paginated_result).to eq(relation.limit(per_page))
+    end
+  end
+
+  describe '#finalize' do
+    subject(:finalize) do
+      service = described_class.new(cursor_based_request_context)
+      # we need to do this because `finalize` can only be called
+      # after `paginate` is called. Otherwise the `paginator` object won't be set.
+      service.paginate(relation)
+      service.finalize
+    end
+
+    it 'passes information about next page to request' do
+      cursor_for_next_page = relation.keyset_paginate(**params).cursor_for_next_page
+
+      expect_next_instance_of(Gitlab::Pagination::Keyset::HeaderBuilder, cursor_based_request_context) do |builder|
+        expect(builder).to receive(:add_next_page_header).with({ cursor: cursor_for_next_page })
+      end
+
+      finalize
+    end
+
+    context 'when retrieving the last page' do
+      let(:relation) { Group.where('id > ?', Group.maximum(:id) - per_page).order(:name, :id) }
+
+      it 'does not build information about the next page' do
+        expect(Gitlab::Pagination::Keyset::HeaderBuilder).not_to receive(:new)
+
+        finalize
+      end
+    end
+
+    context 'when retrieving an empty page' do
+      let(:relation) { Group.where('id > ?', Group.maximum(:id) + 1).order(:name, :id) }
+
+      it 'does not build information about the next page' do
+        expect(Gitlab::Pagination::Keyset::HeaderBuilder).not_to receive(:new)
+
+        finalize
+      end
+    end
+  end
+end