Optional clock gettime 2 (#427)

* Disable Process#clock_gettime mocking by default

* prepare release 0.9.9

* update readme

---------

Co-authored-by: Alex Watt <alex@alexcwatt.com>

README.markdown

@@ -129,6 +129,15 @@ Timecop.freeze
 # => Timecop::SafeModeException: Safe mode is enabled, only calls passing a block are allowed.
 ```
 
+### Configuring Mocking Process.clock_gettime
+
+By default Timecop does not mock Process.clock_gettime. You must enable it like this:
+
+``` ruby
+# turn on
+Timecop.mock_process_clock = true
+```
+
 ### Rails v Ruby Date/Time libraries
 
 Sometimes [Rails Date/Time methods don't play nicely with Ruby Date/Time methods.](https://rails.lighthouseapp.com/projects/8994/tickets/6410-dateyesterday-datetoday)

lib/timecop/time_extensions.rb

@@ -181,7 +181,7 @@ def clock_gettime_mock_time(clock_id, unit = :float_second)
                       mock_time_realtime
                     end
 
-        return clock_gettime_without_mock(clock_id, unit) unless mock_time
+        return clock_gettime_without_mock(clock_id, unit) unless Timecop.mock_process_clock? && mock_time
 
         divisor = case unit
                   when :float_second

lib/timecop/timecop.rb

@@ -142,6 +142,14 @@ def scaled?
       !instance.stack.empty? && instance.stack.last.mock_type == :scale
     end
 
+    def mock_process_clock=(mock)
+      @mock_process_clock = mock
+    end
+
+    def mock_process_clock?
+      @mock_process_clock ||= false
+    end
+
     private
     def send_travel(mock_type, *args, &block)
       val = instance.travel(mock_type, *args, &block)

test/timecop_with_process_clock_test.rb

@@ -4,11 +4,24 @@
 class TestTimecopWithProcessClock < Minitest::Test
   TIME_EPSILON = 0.001 # seconds - represents enough time for Process.clock_gettime to have advanced if not frozen
 
+  def setup
+    Timecop.mock_process_clock = true
+  end
+
   def teardown
     Timecop.return
+    Timecop.mock_process_clock = false
   end
 
   if RUBY_VERSION >= '2.1.0'
+    def test_process_clock_mock_disabled
+      Timecop.mock_process_clock = false
+
+      Timecop.freeze do
+        refute_same(*consecutive_monotonic, "CLOCK_MONOTONIC is frozen")
+      end
+    end
+
     def test_process_clock_gettime_monotonic
       Timecop.freeze do
         assert_same(*consecutive_monotonic, "CLOCK_MONOTONIC is not frozen")