Currently travel time helpers don't restore to proper previous state …

…if we have called them in a row, or in a nested fashion.

We simply unstub all previous occurrences. Consider following scenario:

 travel_to outer_expected_time do
     travel_to inner_expected_time do
         #perform some operations from some state here
       end
   #perform some operations from some outer state here
 end

This will fail for outer scenario in that, we simply just unstub all stubbed objects.
Here onwards we also take into account the object into account to create alias, to be more safe in returning back to previous states.

Fixes #24689

activesupport/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   `travel/travel_to` travel time helpers, now support nested calls,  
+     and restore properly to time state for respective blocks.  
+
+        travel_to outer_expected_time do
+          travel_to inner_expected_time do
+            #will work with inner_expected_time as time values
+            end
+          #will work with outer_expected_time as time values
+        end
+
+    *Vipul A M*
+
 *   Add `ActiveSupport.to_time_preserves_timezone` config option to control
     how `to_time` handles timezones. In Ruby 2.4+ the behavior will change
     from converting to the local system timezone to preserving the timezone

activesupport/lib/active_support/testing/time_helpers.rb

@@ -1,32 +1,48 @@
 module ActiveSupport
   module Testing
     class SimpleStubs # :nodoc:
-      Stub = Struct.new(:object, :method_name, :original_method)
+      Stub = Struct.new(:object, :method_name, :original_method, :return_value)
 
       def initialize
-        @stubs = {}
+        @stubs = Hash.new { |h,k| h[k] = [] }
       end
 
       def stub_object(object, method_name, return_value)
         key = [object.object_id, method_name]
 
-        if stub = @stubs[key]
-          unstub_object(stub)
+        if @stubs[key].any?
+          stub = @stubs[key].last
+          unstub_object(stub) if stub
         end
 
         new_name = "__simple_stub__#{method_name}"
 
-        @stubs[key] = Stub.new(object, method_name, new_name)
+        @stubs[key].push(Stub.new(object, method_name, new_name, return_value))
 
         object.singleton_class.send :alias_method, new_name, method_name
         object.define_singleton_method(method_name) { return_value }
       end
 
       def unstub_all!
-        @stubs.each_value do |stub|
-          unstub_object(stub)
+        @stubs.each_value do |stubs|
+          stub = stubs.first
+          unstub_object(stub) if stub
+        end
+        @stubs.clear
+      end
+
+      def unstub_to_previous_stub_state(object, method_name, return_value)
+        key = [object.object_id, method_name]
+
+        if @stubs[key].any?
+          current_stub = @stubs[key].pop
+
+          if stub = @stubs[key].last
+            stub_object(stub.object, stub.method_name, stub.return_value)
+          else
+            unstub_object(current_stub)
+          end
         end
-        @stubs = {}
       end
 
       private
@@ -98,16 +114,16 @@ def travel_to(date_or_time)
         else
           now = date_or_time.to_time.change(usec: 0)
         end
+        to_date_value = now.to_date
+        to_datetime_value = now.to_datetime
 
-        simple_stubs.stub_object(Time, :now, now)
-        simple_stubs.stub_object(Date, :today, now.to_date)
-        simple_stubs.stub_object(DateTime, :now, now.to_datetime)
+        stub_unstub_date_time_objects(:stub_object, now, to_date_value, to_datetime_value)
 
         if block_given?
           begin
             yield
           ensure
-            travel_back
+            stub_unstub_date_time_objects(:unstub_to_previous_stub_state, now, to_date_value, to_datetime_value)
           end
         end
       end
@@ -129,6 +145,12 @@ def travel_back
         def simple_stubs
           @simple_stubs ||= SimpleStubs.new
         end
+
+        def stub_unstub_date_time_objects(method, now, to_date_value, to_datetime_value)
+          simple_stubs.public_send(method, Time, :now, now)
+          simple_stubs.public_send(method, Date, :today, to_date_value)
+          simple_stubs.public_send(method, DateTime, :now, to_datetime_value)
+        end
     end
   end
 end

activesupport/test/time_travel_test.rb

@@ -45,6 +45,31 @@ def test_time_helper_travel_to
     end
   end
 
+  def test_time_helper_travel_to_with_nested_calls
+    Time.stub(:now, Time.now) do
+      outer_expected_time = Time.new(2004, 11, 24, 01, 04, 44)
+      inner_expected_time = Time.new(2004, 10, 24, 01, 04, 44)
+      travel_to outer_expected_time do
+        travel_to inner_expected_time do
+          assert_equal inner_expected_time, Time.now
+        end
+        assert_equal outer_expected_time, Time.now
+      end
+    end
+  end
+
+  def test_time_helper_travel_to_with_nested_calls_and_travel_back
+    current_time = Time.now
+    Time.stub(:now, Time.now) do
+      expected_time = Time.new(2004, 11, 24, 01, 04, 44)
+      travel_to expected_time do
+        assert_equal expected_time, Time.now
+        travel_back
+      end
+      assert_equal current_time.to_i, Time.now.to_i
+    end
+  end
+
   def test_time_helper_travel_to_with_block
     Time.stub(:now, Time.now) do
       expected_time = Time.new(2004, 11, 24, 01, 04, 44)