1 files changed, 49 insertions, 0 deletions
diff --git a/app/graphql/resolvers/ci/runner_job_count_resolver.rb b/app/graphql/resolvers/ci/runner_job_count_resolver.rb
new file mode 100644
index 00000000000..a43d3f3a100
--- /dev/null
+++ b/app/graphql/resolvers/ci/runner_job_count_resolver.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+module Resolvers
+  module Ci
+    class RunnerJobCountResolver < BaseResolver
+      include Gitlab::Graphql::Authorize::AuthorizeResource
+
+      type GraphQL::Types::Int, null: true
+
+      authorize :read_runner
+      authorizes_object!
+
+      argument :statuses, [::Types::Ci::JobStatusEnum],
+        required: false,
+        description: 'Filter jobs by status.',
+        alpha: { milestone: '16.2' }
+
+      alias_method :runner, :object
+
+      def resolve(statuses: nil)
+        BatchLoader::GraphQL.for(runner.id).batch(key: [:job_count, statuses]) do |runner_ids, loader, _args|
+          counts_by_runner = calculate_job_count_per_runner(runner_ids, statuses)
+
+          runner_ids.each do |runner_id|
+            loader.call(runner_id, counts_by_runner[runner_id]&.count || 0)
+          end
+        end
+      end
+
+      private
+
+      def calculate_job_count_per_runner(runner_ids, statuses)
+        # rubocop: disable CodeReuse/ActiveRecord
+        builds_tbl = ::Ci::Build.arel_table
+        runners_tbl = ::Ci::Runner.arel_table
+        lateral_query = ::Ci::Build.select(1).where(builds_tbl['runner_id'].eq(runners_tbl['id']))
+        lateral_query = lateral_query.where(status: statuses) if statuses
+        # We limit to 1 above the JOB_COUNT_LIMIT to indicate that more items exist after JOB_COUNT_LIMIT
+        lateral_query = lateral_query.limit(::Types::Ci::RunnerType::JOB_COUNT_LIMIT + 1)
+        ::Ci::Runner.joins("JOIN LATERAL (#{lateral_query.to_sql}) builds_with_limit ON true")
+          .id_in(runner_ids)
+          .select(:id, Arel.star.count.as('count'))
+          .group(:id)
+          .index_by(&:id)
+        # rubocop: enable CodeReuse/ActiveRecord
+      end
+    end
+  end
+end