Do not cache ActiveSupport::TimeZone#utc_offset

This can be an issue when TZInfo::TimeZone#current_period is refreshed
due to timezone period transition, but it's not reflected in
ActiveSupport::TimeZone object.

For example, on Sun, 26 Oct 2014 22:00 UTC, Moscow changed its TZ from
MSK +04:00 to MSK +03:00 (-1 hour). If ActiveSupport::TimeZone['Moscow']
happens to be initialized just before the timezone transition, it will
cache its stale utc_offset even after the timezone transition.

This commit removes cache and fixes this issue.

Signed-off-by: Jeremy Daer <jeremydaer@gmail.com>

activesupport/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Time zones: Ensure that the UTC offset reflects DST changes that occurred
+    since the app started. Removes UTC offset caching, reducing performance,
+    but this is still relatively quick and isn't in any hot paths.
+
+    *Alexey Shein*
+
 *   Make `getlocal` and `getutc` always return instances of `Time` for
     `ActiveSupport::TimeWithZone` and `DateTime`. This eliminates a possible
     stack level too deep error in `to_time` where `ActiveSupport::TimeWithZone`

activesupport/lib/active_support/values/time_zone.rb

@@ -278,16 +278,14 @@ def initialize(name, utc_offset = nil, tzinfo = nil)
       @name = name
       @utc_offset = utc_offset
       @tzinfo = tzinfo || TimeZone.find_tzinfo(name)
-      @current_period = nil
     end
 
     # Returns the offset of this time zone from UTC in seconds.
     def utc_offset
       if @utc_offset
         @utc_offset
       else
-        @current_period ||= tzinfo.current_period if tzinfo
-        @current_period.utc_offset if @current_period
+        tzinfo.current_period.utc_offset if tzinfo && tzinfo.current_period
       end
     end
 

activesupport/test/time_zone_test.rb

@@ -395,6 +395,17 @@ def test_utc_offset_lazy_loaded_from_tzinfo_when_not_passed_in_to_initialize
     assert_equal(-18_000, zone.utc_offset)
   end
 
+  def test_utc_offset_is_not_cached_when_current_period_gets_stale
+    tz = ActiveSupport::TimeZone.create('Moscow')
+    travel_to(Time.utc(2014, 10, 25, 21)) do # 1 hour before TZ change
+      assert_equal 14400, tz.utc_offset, 'utc_offset should be initialized according to current_period'
+    end
+
+    travel_to(Time.utc(2014, 10, 25, 22)) do # after TZ change
+      assert_equal 10800, tz.utc_offset, 'utc_offset should not be cached when current_period gets stale'
+    end
+  end
+
   def test_seconds_to_utc_offset_with_colon
     assert_equal "-06:00", ActiveSupport::TimeZone.seconds_to_utc_offset(-21_600)
     assert_equal "+00:00", ActiveSupport::TimeZone.seconds_to_utc_offset(0)