Merge branch 'pawel/cache_feature_check_for_5_minutes_for_method_call' into 'master'

Cache feature check for 1 minute for MethodCall instrumentation toggle

See merge request gitlab-org/gitlab-ce!15800

lib/gitlab/metrics/method_call.rb

+# rubocop:disable Style/ClassVars
+
 module Gitlab
   module Metrics
     # Class for tracking timing information about method calls
     class MethodCall
+      @@measurement_enabled_cache = Concurrent::AtomicBoolean.new(false)
+      @@measurement_enabled_cache_expires_at = Concurrent::AtomicFixnum.new(Time.now.to_i)
       MUTEX = Mutex.new
       BASE_LABELS = { module: nil, method: nil }.freeze
       attr_reader :real_time, :cpu_time, :call_count, :labels
@@ -18,6 +22,10 @@ module Gitlab
         end
       end
 
+      def self.measurement_enabled_cache_expires_at
+        @@measurement_enabled_cache_expires_at
+      end
+
       # name - The full name of the method (including namespace) such as
       #        `User#sign_in`.
       #
@@ -72,7 +80,14 @@ module Gitlab
       end
 
       def call_measurement_enabled?
-        Feature.get(:prometheus_metrics_method_instrumentation).enabled?
+        expires_at = @@measurement_enabled_cache_expires_at.value
+        if expires_at < Time.now.to_i
+          if @@measurement_enabled_cache_expires_at.compare_and_set(expires_at, 1.minute.from_now.to_i)
+            @@measurement_enabled_cache.value = Feature.get(:prometheus_metrics_method_instrumentation).enabled?
+          end
+        end
+
+        @@measurement_enabled_cache.value
       end
     end
   end

spec/lib/gitlab/metrics/method_call_spec.rb

@@ -20,9 +20,39 @@ describe Gitlab::Metrics::MethodCall do
 
       context 'prometheus instrumentation is enabled' do
         before do
+          allow(Feature.get(:prometheus_metrics_method_instrumentation)).to receive(:enabled?).and_call_original
+          described_class.measurement_enabled_cache_expires_at.value = Time.now.to_i - 1
           Feature.get(:prometheus_metrics_method_instrumentation).enable
         end
 
+        around do |example|
+          Timecop.freeze do
+            example.run
+          end
+        end
+
+        it 'caches subsequent invocations of feature check' do
+          10.times do
+            method_call.measure { 'foo' }
+          end
+
+          expect(Feature.get(:prometheus_metrics_method_instrumentation)).to have_received(:enabled?).once
+        end
+
+        it 'expires feature check cache after 1 minute' do
+          method_call.measure { 'foo' }
+
+          Timecop.travel(1.minute.from_now) do
+            method_call.measure { 'foo' }
+          end
+
+          Timecop.travel(1.minute.from_now + 1.second) do
+            method_call.measure { 'foo' }
+          end
+
+          expect(Feature.get(:prometheus_metrics_method_instrumentation)).to have_received(:enabled?).twice
+        end
+
         it 'observes the performance of the supplied block' do
           expect(described_class.call_duration_histogram)
             .to receive(:observe)
@@ -34,6 +64,8 @@ describe Gitlab::Metrics::MethodCall do
 
       context 'prometheus instrumentation is disabled' do
         before do
+          described_class.measurement_enabled_cache_expires_at.value = Time.now.to_i - 1
+
           Feature.get(:prometheus_metrics_method_instrumentation).disable
         end