Support resetting of Prometheus metrics between test runs

Adding the :prometheus tag to an rspec test will clear out
memory-mapped files and reset the registry.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/39968

2 files changed, 25 insertions, 0 deletions

diff --git a/spec/lib/gitlab/metrics/prometheus_spec.rb b/spec/lib/gitlab/metrics/prometheus_spec.rb
new file mode 100644
index 00000000000..3d4dd5fdf01
--- /dev/null
+++ b/spec/lib/gitlab/metrics/prometheus_spec.rb
@@ -0,0 +1,18 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::Prometheus, :prometheus do
+  let(:all_metrics) { Gitlab::Metrics }
+  let(:registry) { all_metrics.registry }
+
+  describe '#reset_registry!' do
+    it 'clears existing metrics' do
+      registry.counter(:test, 'test metric')
+
+      expect(registry.metrics.count).to eq(1)
+
+      all_metrics.reset_registry!
+
+      expect(all_metrics.registry.metrics.count).to eq(0)
+    end
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index b4fc596a751..d3de2331244 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -137,6 +137,13 @@ RSpec.configure do |config|
     reset_delivered_emails!
   end
 
+  config.before(:example, :prometheus) do
+    matching_files = File.join(::Prometheus::Client.configuration.multiprocess_files_dir, "*.db")
+    Dir[matching_files].map { |filename| File.delete(filename) if File.file?(filename) }
+
+    Gitlab::Metrics.reset_registry!
+  end
+
   config.around(:each, :use_clean_rails_memory_store_caching) do |example|
     caching_store = Rails.cache
     Rails.cache = ActiveSupport::Cache::MemoryStore.new