Ensure duration parsing is consistent across DST changes

Previously `ActiveSupport::Duration.parse` used `Time.current` and
`Time#advance` to calculate the number of seconds in the duration
from an arbitrary collection of parts. However as `advance` tries to
be consistent across DST boundaries this meant that either the
duration was shorter or longer depending on the time of year.

This was fixed by using an absolute reference point in UTC which
isn't subject to DST transitions. An arbitrary date of Jan 1st, 2000
was chosen for no other reason that it seemed appropriate.

Additionally, duration parsing should now be marginally faster as we
are no longer creating instances of `ActiveSupport::TimeWithZone`
every time we parse a duration string.

Fixes #26941.

activesupport/CHANGELOG.md

@@ -1,3 +1,23 @@
*   Ensure duration parsing is consistent across DST changes

    Previously `ActiveSupport::Duration.parse` used `Time.current` and
    `Time#advance` to calculate the number of seconds in the duration
    from an arbitrary collection of parts. However as `advance` tries to
    be consistent across DST boundaries this meant that either the
    duration was shorter or longer depending on the time of year.

    This was fixed by using an absolute reference point in UTC which
    isn't subject to DST transitions. An arbitrary date of Jan 1st, 2000
    was chosen for no other reason that it seemed appropriate.

    Additionally, duration parsing should now be marginally faster as we
    are no longer creating instances of `ActiveSupport::TimeWithZone`
    every time we parse a duration string.

    Fixes #26941.

    *Andrew White*

*   Use `Hash#compact` and `Hash#compact!` from Ruby 2.4. Old Ruby versions
    will continue to get these methods from Active Support as before.

activesupport/lib/active_support/duration.rb

@@ -7,6 +7,8 @@ module ActiveSupport
  #
  #   1.month.ago       # equivalent to Time.now.advance(months: -1)
  class Duration
    EPOCH = ::Time.utc(2000)

    attr_accessor :value, :parts

    autoload :ISO8601Parser,     "active_support/duration/iso8601_parser"
@@ -140,8 +142,7 @@ def respond_to_missing?(method, include_private = false) #:nodoc:
    # If invalid string is provided, it will raise +ActiveSupport::Duration::ISO8601Parser::ParsingError+.
    def self.parse(iso8601duration)
      parts = ISO8601Parser.new(iso8601duration).parse!
      time  = ::Time.current
      new(time.advance(parts) - time, parts)
      new(EPOCH.advance(parts) - EPOCH, parts)
    end

    # Build ISO 8601 Duration string for this duration.

activesupport/test/core_ext/duration_test.rb

@@ -322,4 +322,35 @@ def test_iso8601_output_and_reparsing
      assert_equal time + duration, time + ActiveSupport::Duration.parse(duration.iso8601), pattern.inspect
    end
  end

  def test_iso8601_parsing_across_spring_dst_boundary
    with_env_tz eastern_time_zone do
      with_tz_default "Eastern Time (US & Canada)" do
        travel_to Time.utc(2016, 3, 11) do
          assert_equal 604800, ActiveSupport::Duration.parse("P7D").to_i
          assert_equal 604800, ActiveSupport::Duration.parse("P1W").to_i
        end
      end
    end
  end

  def test_iso8601_parsing_across_autumn_dst_boundary
    with_env_tz eastern_time_zone do
      with_tz_default "Eastern Time (US & Canada)" do
        travel_to Time.utc(2016, 11, 4) do
          assert_equal 604800, ActiveSupport::Duration.parse("P7D").to_i
          assert_equal 604800, ActiveSupport::Duration.parse("P1W").to_i
        end
      end
    end
  end

  private
    def eastern_time_zone
      if Gem.win_platform?
        "EST5EDT"
      else
        "America/New_York"
      end
    end
end