Merge branch 'sh-reset-prometheus-metrics-tests' into 'master'

Support resetting of Prometheus metrics between test runs

Closes #39968

See merge request gitlab-org/gitlab-ce!18836

6 files changed, 41 insertions, 3 deletions

diff --git a/Gemfile b/Gemfile
index 2df14c032ed..ccab7685b2e 100644
--- a/Gemfile
+++ b/Gemfile
@@ -294,7 +294,7 @@ group :metrics do
   gem 'influxdb', '~> 0.2', require: false
 
   # Prometheus
-  gem 'prometheus-client-mmap', '~> 0.9.1'
+  gem 'prometheus-client-mmap', '~> 0.9.2'
   gem 'raindrops', '~> 0.18'
 end
 
diff --git a/Gemfile.lock b/Gemfile.lock
index ee28465decd..201dba5fc5a 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -632,7 +632,7 @@ GEM
       parser
       unparser
     procto (0.0.3)
-    prometheus-client-mmap (0.9.1)
+    prometheus-client-mmap (0.9.2)
     pry (0.10.4)
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)
@@ -1130,7 +1130,7 @@ DEPENDENCIES
   peek-sidekiq (~> 1.0.3)
   pg (~> 0.18.2)
   premailer-rails (~> 1.9.7)
-  prometheus-client-mmap (~> 0.9.1)
+  prometheus-client-mmap (~> 0.9.2)
   pry-byebug (~> 3.4.1)
   pry-rails (~> 0.3.4)
   rack-attack (~> 4.4.1)
diff --git a/doc/development/testing_guide/best_practices.md b/doc/development/testing_guide/best_practices.md
index 7b32e0a47e1..61fa5459b91 100644
--- a/doc/development/testing_guide/best_practices.md
+++ b/doc/development/testing_guide/best_practices.md
@@ -230,6 +230,11 @@ describe "#==" do
 end
 ```
 
+### Prometheus tests
+
+Prometheus metrics may be preserved from one test run to another. To ensure that metrics are
+reset before each example, add the `:prometheus` tag to the Rspec test.
+
 ### Matchers
 
 Custom matchers should be created to clarify the intent and/or hide the
diff --git a/lib/gitlab/metrics/prometheus.rb b/lib/gitlab/metrics/prometheus.rb
index d12ba0ec176..d41a855bff1 100644
--- a/lib/gitlab/metrics/prometheus.rb
+++ b/lib/gitlab/metrics/prometheus.rb
@@ -25,6 +25,14 @@ module Gitlab
           end
         end
 
+        def reset_registry!
+          clear_memoization(:registry)
+
+          REGISTRY_MUTEX.synchronize do
+            ::Prometheus::Client.reset!
+          end
+        end
+
         def registry
           strong_memoize(:registry) do
             REGISTRY_MUTEX.synchronize do
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