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

(cherry picked from commit 83998c0a33c70c786f48eef5ce723eb6dd76761f)

53dc9e83 Cache feature check for 5 minutes for MethodCall instrumentation toggle
b503e6ff Implement simple in memory cache that expires after 5 minutes
6af84964 Set cache expire only once the cache is filled,
5904b033 Implemente measurement enabled cache using AtomicReference
408208bc Use AtomicFixNum to implement CAS isolated cache update.
ca176a9b move call_measurement_enabled? method to the bottom of the file
fd0a5168 use class variables instead of CONSTANTs
da19ce62 Expire feature flag cache after 1minute
51668d3e Use class variable and add rubocop exception
db9e5bf7 fix test case description

2 files changed, 48 insertions, 1 deletions

diff --git a/lib/gitlab/metrics/method_call.rb b/lib/gitlab/metrics/method_call.rb
index 65d55576ac2..9112164f22e 100644
--- a/lib/gitlab/metrics/method_call.rb
+++ b/lib/gitlab/metrics/method_call.rb
@@ -1,7 +1,11 @@
+# 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
diff --git a/spec/lib/gitlab/metrics/method_call_spec.rb b/spec/lib/gitlab/metrics/method_call_spec.rb
index 5341addf911..78767d06462 100644
--- a/spec/lib/gitlab/metrics/method_call_spec.rb
+++ b/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