Graphql - fix pagination bug due to ms precision

When we use keyset pagination inside GraphQL our cursors are missing
millisecond data for timestamps, which causes pagination to not work
properly when objects have the same timestamp up to the second, as the
database sees for example '23:14:56' as '23:14:56.0000' which is
different from any other timestamps with millisecond values that are not
'0000'

changelogs/unreleased/221174-graphql-pagination-bug.yml

+---
+title: GraphQL - properly handle pagination of millisecond-precision timestamps
+merge_request: 34352
+author:
+type: fixed

lib/gitlab/graphql/pagination/keyset/connection.rb

@@ -194,7 +194,12 @@ module Gitlab
 
             order_list.each do |field|
               field_name = field.attribute_name
-              ordering[field_name] = node[field_name].to_s
+              field_value = node[field_name]
+              ordering[field_name] = if field_value.is_a?(Time)
+                                       field_value.strftime('%Y-%m-%d %H:%M:%S.%N %Z')
+                                     else
+                                       field_value.to_s
+                                     end
             end
 
             encode(ordering.to_json)

spec/lib/gitlab/graphql/pagination/keyset/connection_spec.rb

@@ -33,7 +33,7 @@ describe Gitlab::Graphql::Pagination::Keyset::Connection do
       let(:nodes) { Project.order(:updated_at) }
 
       it 'returns the encoded value of the order' do
-        expect(decoded_cursor(cursor)).to include('updated_at' => project.updated_at.to_s)
+        expect(decoded_cursor(cursor)).to include('updated_at' => project.updated_at.strftime('%Y-%m-%d %H:%M:%S.%N %Z'))
       end
 
       it 'includes the :id even when not specified in the order' do
@@ -45,7 +45,7 @@ describe Gitlab::Graphql::Pagination::Keyset::Connection do
       let(:nodes) { Project.order(:updated_at).order(:created_at) }
 
       it 'returns the encoded value of the order' do
-        expect(decoded_cursor(cursor)).to include('updated_at' => project.updated_at.to_s)
+        expect(decoded_cursor(cursor)).to include('updated_at' => project.updated_at.strftime('%Y-%m-%d %H:%M:%S.%N %Z'))
       end
     end
 
@@ -53,7 +53,7 @@ describe Gitlab::Graphql::Pagination::Keyset::Connection do
       let(:nodes) { Project.order(Arel.sql('projects.updated_at IS NULL')).order(:updated_at).order(:id) }
 
       it 'returns the encoded value of the order' do
-        expect(decoded_cursor(cursor)).to include('updated_at' => project.updated_at.to_s)
+        expect(decoded_cursor(cursor)).to include('updated_at' => project.updated_at.strftime('%Y-%m-%d %H:%M:%S.%N %Z'))
       end
     end
   end

spec/requests/api/graphql_spec.rb

@@ -187,4 +187,62 @@ describe 'GraphQL' do
       end
     end
   end
+
+  describe 'keyset pagination' do
+    let_it_be(:project) { create(:project, :public) }
+    let_it_be(:issues) { create_list(:issue, 10, project: project, created_at: Time.now.change(usec: 200)) }
+
+    let(:page_size) { 6 }
+    let(:issues_edges) { %w(data project issues edges) }
+    let(:end_cursor) { %w(data project issues pageInfo endCursor) }
+    let(:query) do
+      <<~GRAPHQL
+        query project($fullPath: ID!, $first: Int, $after: String) {
+            project(fullPath: $fullPath) {
+              issues(first: $first, after: $after) {
+                edges { node { iid } }
+                pageInfo { endCursor }
+              }
+            }
+        }
+      GRAPHQL
+    end
+
+    # TODO: Switch this to use `post_graphql`
+    # This is not performing an actual GraphQL request because the
+    # variables end up being strings when passed through the `post_graphql`
+    # helper.
+    #
+    # https://gitlab.com/gitlab-org/gitlab/-/issues/222432
+    def execute_query(after: nil)
+      GitlabSchema.execute(
+        query,
+        context: { current_user: nil },
+        variables: {
+          fullPath: project.full_path,
+          first: page_size,
+          after: after
+        }
+      )
+    end
+
+    it 'paginates datetimes correctly when they have millisecond data' do
+      # let's make sure we're actually querying a timestamp, just in case
+      expect(Gitlab::Graphql::Pagination::Keyset::QueryBuilder)
+        .to receive(:new).with(anything, anything, hash_including('created_at'), anything).and_call_original
+
+      first_page = execute_query
+      edges = first_page.dig(*issues_edges)
+      cursor = first_page.dig(*end_cursor)
+
+      expect(edges.count).to eq(6)
+      expect(edges.last['node']['iid']).to eq(issues[4].iid.to_s)
+
+      second_page = execute_query(after: cursor)
+      edges = second_page.dig(*issues_edges)
+
+      expect(edges.count).to eq(4)
+      expect(edges.last['node']['iid']).to eq(issues[0].iid.to_s)
+    end
+  end
 end