Check availability of keyset pagination

We specifically only enable keyset pagination for reviewed endpoints and
order-by combinations (in this case Project and only ordering by id).

lib/api/helpers/pagination.rb

@@ -4,8 +4,13 @@ module API
   module Helpers
     module Pagination
       def paginate(relation)
-        if params[:pagination] == "keyset"
+        if params[:pagination] == "keyset" && Feature.enabled?(:api_keyset_pagination)
           request_context = Gitlab::Pagination::Keyset::RequestContext.new(self)
+
+          unless Gitlab::Pagination::Keyset.available?(request_context, relation)
+            return error!('Keyset pagination is not yet available for this type of request', 501)
+          end
+
           Gitlab::Pagination::Keyset.paginate(request_context, relation)
         else
           Gitlab::Pagination::OffsetPagination.new(self).paginate(relation)

lib/gitlab/pagination/keyset.rb

@@ -6,6 +6,16 @@ module Gitlab
       def self.paginate(request_context, relation)
         Gitlab::Pagination::Keyset::Pager.new(request_context).paginate(relation)
       end
+
+      def self.available?(request_context, relation)
+        order_by = request_context.page.order_by
+
+        # This is only available for Project and order-by id (asc/desc)
+        return false unless relation.klass == Project
+        return false unless order_by[:id]
+
+        true
+      end
     end
   end
 end

spec/lib/api/helpers/pagination_spec.rb

@@ -31,14 +31,26 @@ describe API::Helpers::Pagination do
       let(:params) { { pagination: 'keyset' } }
       let(:request_context) { double('request context') }
 
+      before do
+        allow(Gitlab::Pagination::Keyset::RequestContext).to receive(:new).with(subject).and_return(request_context)
+        allow(Gitlab::Pagination::Keyset).to receive(:available?).and_return(true)
+      end
+
       it 'delegates to KeysetPagination' do
-        expect(Gitlab::Pagination::Keyset::RequestContext).to receive(:new).with(subject).and_return(request_context)
         expect(Gitlab::Pagination::Keyset).to receive(:paginate).with(request_context, relation).and_return(expected_result)
 
         result = subject.paginate(relation)
 
         expect(result).to eq(expected_result)
       end
+
+      it 'renders a 501 error if keyset pagination isnt available yet' do
+        expect(Gitlab::Pagination::Keyset).to receive(:available?).with(request_context, relation).and_return(false)
+        expect(Gitlab::Pagination::Keyset).not_to receive(:paginate)
+        expect(subject).to receive(:error!).with(/not yet available/, 501)
+
+        subject.paginate(relation)
+      end
     end
   end
 end

spec/lib/gitlab/pagination/keyset_spec.rb

@@ -18,4 +18,44 @@ describe Gitlab::Pagination::Keyset do
       expect(subject).to eq(result)
     end
   end
+
+  describe '.available?' do
+    subject { described_class }
+
+    let(:request_context) { double("request context", page: page)}
+    let(:page) { double("page", order_by: order_by) }
+
+    shared_examples_for 'keyset pagination is available' do
+      it 'returns true for Project' do
+        expect(subject.available?(request_context, Project.all)).to be_truthy
+      end
+
+      it 'return false for other types of relations' do
+        expect(subject.available?(request_context, User.all)).to be_falsey
+      end
+    end
+
+    context 'with order-by id asc' do
+      let(:order_by) { { id: :asc } }
+
+      it_behaves_like 'keyset pagination is available'
+    end
+
+    context 'with order-by id desc' do
+      let(:order_by) { { id: :desc } }
+
+      it_behaves_like 'keyset pagination is available'
+    end
+
+    context 'with other order-by columns' do
+      let(:order_by) { { created_at: :desc } }
+      it 'returns false for Project' do
+        expect(subject.available?(request_context, Project.all)).to be_falsey
+      end
+
+      it 'return false for other types of relations' do
+        expect(subject.available?(request_context, User.all)).to be_falsey
+      end
+    end
+  end
 end