Allow subsecond resolution in `travel_to` helper

The `travel_to` helper didn’t support subsecond time traveling in order
to prevent problems with external services, such as MySQL. This,
however, makes it impossible to test features that use durations less
than a second ("subsecond").

This therefore allows `travel_to` to stub `Time.now` with the accurate
time passed to it as `date_or_time` if the `with_usec` optional argument
is set to true:

    Time.now.inspect # => 2022-01-05 22:36:54.613371959 +0000
    travel_to(0.1.seconds.from_now, with_usec: true)
    Time.now.inspect # => 2022-01-05 22:36:54.713371959 +0000

activesupport/lib/active_support/testing/time_helpers.rb

@@ -115,7 +115,8 @@ module ActiveSupport
       #
       # Note that the usec for the time passed will be set to 0 to prevent rounding
       # errors with external services, like MySQL (which will round instead of floor,
-      # leading to off-by-one-second errors).
+      # leading to off-by-one-second errors), unless the <tt>with_usec</tt> argument
+      # is set to <tt>true</tt>.
       #
       # This method also accepts a block, which will return the current time back to its original
       # state at the end of the block:
@@ -125,7 +126,7 @@ module ActiveSupport
       #     Time.current # => Wed, 24 Nov 2004 01:04:44 EST -05:00
       #   end
       #   Time.current # => Sat, 09 Nov 2013 15:34:49 EST -05:00
-      def travel_to(date_or_time)
+      def travel_to(date_or_time, with_usec: false)
         if block_given? && in_block
           travel_to_nested_block_call = <<~MSG
 
@@ -158,12 +159,14 @@ module ActiveSupport
           now = date_or_time.midnight.to_time
         elsif date_or_time.is_a?(String)
           now = Time.zone.parse(date_or_time)
+        elsif with_usec
+          now = date_or_time.to_time
         else
           now = date_or_time.to_time.change(usec: 0)
         end
 
         stubbed_time = Time.now if simple_stubs.stubbing(Time, :now)
-        simple_stubs.stub_object(Time, :now) { at(now.to_i) }
+        simple_stubs.stub_object(Time, :now) { at(now.to_f) }
         simple_stubs.stub_object(Date, :today) { jd(now.to_date.jd) }
         simple_stubs.stub_object(DateTime, :now) { jd(now.to_date.jd, now.hour, now.min, now.sec, Rational(now.utc_offset, 86400)) }
 

activesupport/test/time_travel_test.rb

@@ -186,6 +186,41 @@ class TimeTravelTest < ActiveSupport::TestCase
     end
   end
 
+  def test_time_helper_travel_to_with_usec
+    Time.stub(:now, Time.now) do
+      duration_usec = 0.1.seconds
+      traveled_time = Time.new(2004, 11, 24, 1, 4, 44) + duration_usec
+      expected_time = Time.new(2004, 11, 24, 1, 4, 44)
+
+      assert_nothing_raised do
+        travel_to traveled_time
+
+        assert_equal expected_time, Time.now
+
+        travel_back
+      end
+    ensure
+      travel_back
+    end
+  end
+
+  def test_time_helper_travel_to_with_usec_true
+    Time.stub(:now, Time.now) do
+      duration_usec = 0.1.seconds
+      expected_time = Time.new(2004, 11, 24, 1, 4, 44) + duration_usec
+
+      assert_nothing_raised do
+        travel_to expected_time, with_usec: true
+
+        assert_equal expected_time.to_f, Time.now.to_f
+
+        travel_back
+      end
+    ensure
+      travel_back
+    end
+  end
+
   def test_time_helper_travel_with_subsequent_block
     Time.stub(:now, Time.now) do
       outer_expected_time = Time.new(2004, 11, 24, 1, 4, 44)