Add compatibility for Ruby 2.4 `to_time` changes

In Ruby 2.4 the `to_time` method for both `DateTime` and `Time` will
preserve the timezone of the receiver when converting to an instance
of `Time`. Since Rails 5.0 will support Ruby 2.2, 2.3 and later we
need to introduce a compatibility layer so that apps that upgrade do
not break. New apps will have a config initializer file that defaults
to match the new Ruby 2.4 behavior going forward.

For information about the changes to Ruby see:
https://bugs.ruby-lang.org/issues/12189
https://bugs.ruby-lang.org/issues/12271

Fixes #24617.

8 files changed, 51 insertions, 7 deletions

diff --git a/activesupport/lib/active_support.rb b/activesupport/lib/active_support.rb
index 72777baecd..11569add37 100644
--- a/activesupport/lib/active_support.rb
+++ b/activesupport/lib/active_support.rb
@@ -26,6 +26,7 @@ require "active_support/dependencies/autoload"
 require "active_support/version"
 require "active_support/logger"
 require "active_support/lazy_load_hooks"
+require "active_support/core_ext/date_and_time/compatibility"
 
 module ActiveSupport
   extend ActiveSupport::Autoload
@@ -85,6 +86,14 @@ module ActiveSupport
   def self.halt_callback_chains_on_return_false=(value)
     Callbacks.halt_and_display_warning_on_return_false = value
   end
+
+  def self.to_time_preserves_timezone
+    DateAndTime::Compatibility.preserve_timezone
+  end
+
+  def self.to_time_preserves_timezone=(value)
+    DateAndTime::Compatibility.preserve_timezone = value
+  end
 end
 
 autoload :I18n, "active_support/i18n"
diff --git a/activesupport/lib/active_support/core_ext/date_and_time/compatibility.rb b/activesupport/lib/active_support/core_ext/date_and_time/compatibility.rb
new file mode 100644
index 0000000000..b8eb587390
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/date_and_time/compatibility.rb
@@ -0,0 +1,16 @@
+module DateAndTime
+  module Compatibility
+    # If true, +to_time+ preserves the the timezone offset.
+    #
+    # NOTE: With Ruby 2.4+ the default for +to_time+ changed from
+    # converting to the local system time to preserving the offset
+    # of the receiver. For backwards compatibility we're overriding
+    # this behavior but new apps will have an initializer that sets
+    # this to true because the new behavior is preferred.
+    mattr_accessor(:preserve_timezone, instance_writer: false) { false }
+
+    def to_time
+      preserve_timezone ? getlocal(utc_offset) : getlocal
+    end
+  end
+end
diff --git a/activesupport/lib/active_support/core_ext/date_time.rb b/activesupport/lib/active_support/core_ext/date_time.rb
index 5450533935..86177488c0 100644
--- a/activesupport/lib/active_support/core_ext/date_time.rb
+++ b/activesupport/lib/active_support/core_ext/date_time.rb
@@ -1,4 +1,5 @@
 require 'active_support/core_ext/date_time/acts_like'
 require 'active_support/core_ext/date_time/blank'
 require 'active_support/core_ext/date_time/calculations'
+require 'active_support/core_ext/date_time/compatibility'
 require 'active_support/core_ext/date_time/conversions'
diff --git a/activesupport/lib/active_support/core_ext/date_time/compatibility.rb b/activesupport/lib/active_support/core_ext/date_time/compatibility.rb
new file mode 100644
index 0000000000..63ac4c2f3a
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/date_time/compatibility.rb
@@ -0,0 +1,16 @@
+require 'active_support/core_ext/date_and_time/compatibility'
+
+class DateTime
+  prepend DateAndTime::Compatibility
+
+  # Returns a <tt>Time.local()</tt> instance of the simultaneous time in your
+  # system's <tt>ENV['TZ']</tt> zone.
+  def getlocal(utc_offset = nil)
+    utc = getutc
+
+    Time.utc(
+      utc.year, utc.month, utc.day,
+      utc.hour, utc.min, utc.sec + utc.sec_fraction
+    ).getlocal(utc_offset)
+  end
+end
diff --git a/activesupport/lib/active_support/core_ext/string/conversions.rb b/activesupport/lib/active_support/core_ext/string/conversions.rb
index 71612e09fa..946976c5e9 100644
--- a/activesupport/lib/active_support/core_ext/string/conversions.rb
+++ b/activesupport/lib/active_support/core_ext/string/conversions.rb
@@ -32,7 +32,7 @@ class String
       parts.fetch(:offset, form == :utc ? 0 : nil)
     )
 
-    form == :utc ? time.utc : time.getlocal
+    form == :utc ? time.utc : time.to_time
   end
 
   # Converts a string to a Date value.
diff --git a/activesupport/lib/active_support/core_ext/time.rb b/activesupport/lib/active_support/core_ext/time.rb
index 72c3234630..0bce632222 100644
--- a/activesupport/lib/active_support/core_ext/time.rb
+++ b/activesupport/lib/active_support/core_ext/time.rb
@@ -1,4 +1,5 @@
 require 'active_support/core_ext/time/acts_like'
 require 'active_support/core_ext/time/calculations'
+require 'active_support/core_ext/time/compatibility'
 require 'active_support/core_ext/time/conversions'
 require 'active_support/core_ext/time/zones'
diff --git a/activesupport/lib/active_support/core_ext/time/compatibility.rb b/activesupport/lib/active_support/core_ext/time/compatibility.rb
new file mode 100644
index 0000000000..945319461b
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/time/compatibility.rb
@@ -0,0 +1,5 @@
+require 'active_support/core_ext/date_and_time/compatibility'
+
+class Time
+  prepend DateAndTime::Compatibility
+end
diff --git a/activesupport/lib/active_support/time_with_zone.rb b/activesupport/lib/active_support/time_with_zone.rb
index 79cc748cf5..a44041af82 100644
--- a/activesupport/lib/active_support/time_with_zone.rb
+++ b/activesupport/lib/active_support/time_with_zone.rb
@@ -1,6 +1,7 @@
 require 'active_support/duration'
 require 'active_support/values/time_zone'
 require 'active_support/core_ext/object/acts_like'
+require 'active_support/core_ext/date_and_time/compatibility'
 
 module ActiveSupport
   # A Time-like class that can represent a time in any time zone. Necessary
@@ -44,7 +45,7 @@ module ActiveSupport
     PRECISIONS = Hash.new { |h, n| h[n] = "%FT%T.%#{n}N".freeze }
     PRECISIONS[0] = '%FT%T'.freeze
 
-    include Comparable
+    include Comparable, DateAndTime::Compatibility
     attr_reader :time_zone
 
     def initialize(utc_time, time_zone, local_time = nil, period = nil)
@@ -401,11 +402,6 @@ module ActiveSupport
       utc.to_r
     end
 
-    # Returns an instance of Time in the system timezone.
-    def to_time
-      utc.to_time
-    end
-
     # Returns an instance of DateTime with the timezone's UTC offset
     #
     #   Time.zone.now.to_datetime                         # => Tue, 18 Aug 2015 02:32:20 +0000