Generate a random hex string as the process id since Process.pid is not supported on all platforms (specifically, does not work on Heroku)

lib/sidekiq/util.rb

@@ -26,7 +26,7 @@ module Sidekiq
     end
 
     def process_id
-      Process.pid
+      @@process_id ||= SecureRandom.hex
     end
 
     def hostname

test/test_util.rb

@@ -0,0 +1,18 @@
+require 'helper'
+require 'sidekiq/util'
+
+class TestUtil < Minitest::Test
+  describe 'util' do
+    it 'generates the same process id when included in two or more classes' do
+      class One
+        include Sidekiq::Util
+      end
+
+      class Two
+        include Sidekiq::Util
+      end
+
+      assert_equal One.new.process_id, One.new.process_id
+    end
+  end
+end