Force a period when local times are ambiguous

This commit includes the essence of
rails#17409.

activesupport/lib/active_support/time_with_zone.rb

@@ -338,7 +338,7 @@ def get_period_and_ensure_valid_local_time
         # so transfer time values to a utc constructor if necessary
         @time = transfer_time_values_to_utc_constructor(@time) unless @time.utc?
         begin
-          @time_zone.period_for_local(@time)
+          @time_zone.period_for_local(@time) { |periods| periods.first }
         rescue ::TZInfo::PeriodNotFound
           # time is in the "spring forward" hour gap, so we're moving the time forward one hour and trying again
           @time += 1.hour

activesupport/lib/active_support/values/time_zone.rb

@@ -309,8 +309,8 @@ def utc_to_local(time)
     end
 
     # Adjust the given time to the simultaneous time in UTC. Returns a Time.utc() instance.
-    def local_to_utc(time, dst=true)
-      tzinfo.local_to_utc(time, dst)
+    def local_to_utc(time, dst=true, &block)
+      tzinfo.local_to_utc(time, dst, &block)
     end
 
     # Available so that TimeZone instances respond like TZInfo::Timezone instances
@@ -319,8 +319,8 @@ def period_for_utc(time)
     end
 
     # Available so that TimeZone instances respond like TZInfo::Timezone instances
-    def period_for_local(time, dst=true)
-      tzinfo.period_for_local(time, dst)
+    def period_for_local(time, dst=true, &block)
+      tzinfo.period_for_local(time, dst, &block)
     end
 
     def self.find_tzinfo(name)

activesupport/test/time_zone_test.rb

@@ -136,6 +136,17 @@ def test_local_enforces_fall_dst_rules
     assert_equal 'EDT', twz.zone
   end
 
+  def test_local_handles_non_dst_clock_set_back
+    # If the clocks are set back during a transition from one non-DST observance
+    # to another, the time of the transition will be ambiguous. The earlier
+    # observance will be selected to mirror behaviour of DST to non-DST transitions.
+    # Such a transition occurred at 02:00 local time in Moscow on 26 October 2014.
+    zone = ActiveSupport::TimeZone['Moscow']
+    twz = zone.local(2014,10,26,1)
+    assert_equal Time.utc(2014,10,26,1), twz.time
+    assert_equal Time.utc(2014,10,25,21), twz.utc
+  end
+
   def test_at
     zone = ActiveSupport::TimeZone['Eastern Time (US & Canada)']
     secs = 946684800.0