Set the usage ping cron based on UUID

Review comments

config/settings.rb

@@ -184,14 +184,15 @@ class Settings < Settingslogic
       URI.parse(url_without_path).host
     end
 
-    # Runs at a random time of day on a consistent day of the week based on
+    # Runs at a consistent random time of day on a day of the week based on
     # the instance UUID. This is to balance the load on the service receiving
     # these pings. The sidekiq job handles temporary http failures.
     def cron_for_usage_ping
-      hour = rand(24)
-      minute = rand(60)
       # Set a default UUID for the case when the UUID hasn't been initialized.
       uuid = Gitlab::CurrentSettings.uuid || 'uuid-not-set'
+
+      minute = Digest::MD5.hexdigest(uuid + 'minute').to_i(16) % 60
+      hour = Digest::MD5.hexdigest(uuid + 'hour').to_i(16) % 24
       day_of_week = Digest::MD5.hexdigest(uuid).to_i(16) % 7
 
       "#{minute} #{hour} * * #{day_of_week}"

spec/config/settings_spec.rb

@@ -112,4 +112,26 @@ RSpec.describe Settings do
       end
     end
   end
+
+  describe '.cron_for_usage_ping' do
+    it 'returns correct crontab for some manually calculated example' do
+      allow(Gitlab::CurrentSettings)
+        .to receive(:uuid) { 'd9e2f4e8-db1f-4e51-b03d-f427e1965c4a'}
+
+      expect(described_class.send(:cron_for_usage_ping)).to eq('21 18 * * 4')
+    end
+
+    it 'returns min, hour, day in the valid range' do
+      allow(Gitlab::CurrentSettings)
+        .to receive(:uuid) { SecureRandom.uuid }
+
+      10.times do
+        cron = described_class.send(:cron_for_usage_ping).split(/\s/)
+
+        expect(cron[0].to_i).to be_between(0, 59)
+        expect(cron[1].to_i).to be_between(0, 23)
+        expect(cron[4].to_i).to be_between(0, 6)
+      end
+    end
+  end
 end