Handle `TZInfo::AmbiguousTime` errors

Make `ActiveSupport::TimeWithZone` match Ruby's handling of ambiguous
times by choosing the later period, e.g.

Ruby:
```
ENV["TZ"] = "Europe/Moscow"
Time.local(2014, 10, 26, 1, 0, 0)   # => 2014-10-26 01:00:00 +0300
```

Before:
```
>> "2014-10-26 01:00:00".in_time_zone("Moscow")
TZInfo::AmbiguousTime: 26/10/2014 01:00 is an ambiguous local time.
```

After:
```
>> "2014-10-26 01:00:00".in_time_zone("Moscow")
=> Sun, 26 Oct 2014 01:00:00 MSK +03:00
```

Fixes #17395.

4 files changed, 66 insertions, 1 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 4762c7d21d..904dab0e05 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,30 @@
+*   Handle `TZInfo::AmbiguousTime` errors
+
+    Make `ActiveSupport::TimeWithZone` match Ruby's handling of ambiguous
+    times by choosing the later period, e.g.
+
+    Ruby:
+    ```
+    ENV["TZ"] = "Europe/Moscow"
+    Time.local(2014, 10, 26, 1, 0, 0)   # => 2014-10-26 01:00:00 +0300
+    ```
+
+    Before:
+    ```
+    >> "2014-10-26 01:00:00".in_time_zone("Moscow")
+    TZInfo::AmbiguousTime: 26/10/2014 01:00 is an ambiguous local time.
+    ```
+
+    After:
+    ```
+    >> "2014-10-26 01:00:00".in_time_zone("Moscow")
+    => Sun, 26 Oct 2014 01:00:00 MSK +03:00
+    ```
+
+    Fixes #17395.
+
+    *Andrew White*
+
 *   Redis cache store.
 
     ```
diff --git a/activesupport/lib/active_support/values/time_zone.rb b/activesupport/lib/active_support/values/time_zone.rb
index 4d1fbd4453..639a066e28 100644
--- a/activesupport/lib/active_support/values/time_zone.rb
+++ b/activesupport/lib/active_support/values/time_zone.rb
@@ -506,7 +506,7 @@ module ActiveSupport
     # Available so that TimeZone instances respond like TZInfo::Timezone
     # instances.
     def period_for_local(time, dst = true)
-      tzinfo.period_for_local(time, dst)
+      tzinfo.period_for_local(time, dst) { |periods| periods.last }
     end
 
     def periods_for_local(time) #:nodoc:
diff --git a/activesupport/test/core_ext/time_with_zone_test.rb b/activesupport/test/core_ext/time_with_zone_test.rb
index 0f80a24758..ab96568956 100644
--- a/activesupport/test/core_ext/time_with_zone_test.rb
+++ b/activesupport/test/core_ext/time_with_zone_test.rb
@@ -50,6 +50,12 @@ class TimeWithZoneTest < ActiveSupport::TestCase
     assert_raise(ArgumentError) { @twz.in_time_zone(Object.new) }
   end
 
+  def test_in_time_zone_with_ambiguous_time
+    with_env_tz "Europe/Moscow" do
+      assert_equal Time.utc(2014, 10, 25, 22, 0, 0), Time.local(2014, 10, 26, 1, 0, 0).in_time_zone("Moscow")
+    end
+  end
+
   def test_localtime
     assert_equal @twz.localtime, @twz.utc.getlocal
     assert_instance_of Time, @twz.localtime
@@ -1301,4 +1307,10 @@ class TimeWithZoneMethodsForString < ActiveSupport::TestCase
     assert_raise(ArgumentError) { @u.in_time_zone(Object.new) }
     assert_raise(ArgumentError) { @z.in_time_zone(Object.new) }
   end
+
+  def test_in_time_zone_with_ambiguous_time
+    with_tz_default "Moscow" do
+      assert_equal Time.utc(2014, 10, 25, 22, 0, 0), "2014-10-26 01:00:00".in_time_zone
+    end
+  end
 end
diff --git a/activesupport/test/time_zone_test.rb b/activesupport/test/time_zone_test.rb
index acb0ecd226..862e872494 100644
--- a/activesupport/test/time_zone_test.rb
+++ b/activesupport/test/time_zone_test.rb
@@ -32,6 +32,12 @@ class TimeZoneTest < ActiveSupport::TestCase
     end
   end
 
+  def test_period_for_local_with_ambigiuous_time
+    zone = ActiveSupport::TimeZone["Moscow"]
+    period = zone.period_for_local(Time.utc(2015, 1, 1))
+    assert_equal period, zone.period_for_local(Time.utc(2014, 10, 26, 1, 0, 0))
+  end
+
   def test_from_integer_to_map
     assert_instance_of ActiveSupport::TimeZone, ActiveSupport::TimeZone[-28800] # PST
   end
@@ -195,6 +201,11 @@ class TimeZoneTest < ActiveSupport::TestCase
     assert_equal "EDT", twz.zone
   end
 
+  def test_local_with_ambiguous_time
+    zone = ActiveSupport::TimeZone["Moscow"]
+    assert_equal Time.utc(2014, 10, 25, 22, 0, 0), zone.local(2014, 10, 26, 1, 0, 0)
+  end
+
   def test_at
     zone = ActiveSupport::TimeZone["Eastern Time (US & Canada)"]
     secs = 946684800.0
@@ -303,6 +314,11 @@ class TimeZoneTest < ActiveSupport::TestCase
     end
   end
 
+  def test_iso8601_with_ambiguous_time
+    zone = ActiveSupport::TimeZone["Moscow"]
+    assert_equal Time.utc(2014, 10, 25, 22, 0, 0), zone.parse("2014-10-26T01:00:00")
+  end
+
   def test_parse
     zone = ActiveSupport::TimeZone["Eastern Time (US & Canada)"]
     twz = zone.parse("1999-12-31 19:00:00")
@@ -412,6 +428,11 @@ class TimeZoneTest < ActiveSupport::TestCase
     assert_equal "argument out of range", exception.message
   end
 
+  def test_parse_with_ambiguous_time
+    zone = ActiveSupport::TimeZone["Moscow"]
+    assert_equal Time.utc(2014, 10, 25, 22, 0, 0), zone.parse("2014-10-26 01:00:00")
+  end
+
   def test_rfc3339
     zone = ActiveSupport::TimeZone["Eastern Time (US & Canada)"]
     twz = zone.rfc3339("1999-12-31T14:00:00-10:00")
@@ -604,6 +625,11 @@ class TimeZoneTest < ActiveSupport::TestCase
     end
   end
 
+  def test_strptime_with_ambiguous_time
+    zone = ActiveSupport::TimeZone["Moscow"]
+    assert_equal Time.utc(2014, 10, 25, 22, 0, 0), zone.strptime("2014-10-26 01:00:00", "%Y-%m-%d %H:%M:%S")
+  end
+
   def test_utc_offset_lazy_loaded_from_tzinfo_when_not_passed_in_to_initialize
     tzinfo = TZInfo::Timezone.get("America/New_York")
     zone = ActiveSupport::TimeZone.create(tzinfo.name, nil, tzinfo)