From 408208bc2b75d28235092e9bb3821242fdad08fb Mon Sep 17 00:00:00 2001
From: Pawel Chojnacki <pawel@chojnacki.ws>
Date: Mon, 11 Dec 2017 23:39:40 +0100
Subject: Use AtomicFixNum to implement CAS isolated cache update.

i.e.
Using compare and swap we update the expires_at value.
The thread that actually is able to update this value will also set
the cache holding method_call enabled state
---
 lib/gitlab/metrics/method_call.rb           | 17 +++++++----------
 spec/lib/gitlab/metrics/method_call_spec.rb | 13 +++++++++----
 2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/lib/gitlab/metrics/method_call.rb b/lib/gitlab/metrics/method_call.rb
index dc56a10957d..2c1cb789314 100644
--- a/lib/gitlab/metrics/method_call.rb
+++ b/lib/gitlab/metrics/method_call.rb
@@ -2,7 +2,8 @@ module Gitlab
   module Metrics
     # Class for tracking timing information about method calls
     class MethodCall
-      MEASUREMENT_ENABLED_CACHE = Concurrent::AtomicReference.new({ enabled: false, expires_at: Time.now })
+      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
@@ -20,18 +21,14 @@ module Gitlab
       end
 
       def call_measurement_enabled?
-        res = MEASUREMENT_ENABLED_CACHE.update do |cache|
-          if cache[:expires_at] < Time.now
-            {
-              enabled: Feature.get(:prometheus_metrics_method_instrumentation).enabled?,
-              expires_at: Time.now + 5.minutes
-            }
-          else
-            cache
+        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, (Time.now + 30.seconds).to_i)
+            MEASUREMENT_ENABLED_CACHE.value = Feature.get(:prometheus_metrics_method_instrumentation).enabled?
           end
         end
 
-        res[:enabled]
+        MEASUREMENT_ENABLED_CACHE.value
       end
 
       # name - The full name of the method (including namespace) such as
diff --git a/spec/lib/gitlab/metrics/method_call_spec.rb b/spec/lib/gitlab/metrics/method_call_spec.rb
index eca6a2cb54a..5ce62504245 100644
--- a/spec/lib/gitlab/metrics/method_call_spec.rb
+++ b/spec/lib/gitlab/metrics/method_call_spec.rb
@@ -21,7 +21,7 @@ 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.set({enabled: false, expires_at: Time.now - 1.second})
+          described_class::MEASUREMENT_ENABLED_CACHE_EXPIRES_AT.value = Time.now.to_i - 1
           Feature.get(:prometheus_metrics_method_instrumentation).enable
         end
 
@@ -39,12 +39,16 @@ describe Gitlab::Metrics::MethodCall do
           expect(Feature.get(:prometheus_metrics_method_instrumentation)).to have_received(:enabled?).once
         end
 
-        it 'expires feature check cache after 5 minutes' do
+        it 'expires feature check cache after 30 seconds' do
           10.times do
             method_call.measure { 'foo' }
           end
 
-          Timecop.travel(Time.now + 5.minutes) do
+          Timecop.travel(Time.now + 30.seconds) do
+            method_call.measure { 'foo' }
+          end
+
+          Timecop.travel(Time.now + 31.seconds) do
             method_call.measure { 'foo' }
           end
 
@@ -62,7 +66,8 @@ describe Gitlab::Metrics::MethodCall do
 
       context 'prometheus instrumentation is disabled' do
         before do
-          described_class::MEASUREMENT_ENABLED_CACHE.set({enabled: false, expires_at: Time.now})
+          described_class::MEASUREMENT_ENABLED_CACHE_EXPIRES_AT.value = Time.now.to_i - 1
+
           Feature.get(:prometheus_metrics_method_instrumentation).disable
         end
 
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