Fix #microseconds conversion and #fast_string_to_time

* Use direct integer parsing in #fast_string_to_time to avoid convoluted
   conversions and errors due to truncation.
 * Use Float#round in #microseconds to avoid truncation errors.

Signed-off-by: Michael Koziarski <michael@koziarski.com>

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -13,6 +13,7 @@ class Column
       module Format
         ISO_DATE = /\A(\d{4})-(\d\d)-(\d\d)\z/
         ISO_DATETIME = /\A(\d{4})-(\d\d)-(\d\d) (\d\d):(\d\d):(\d\d)(\.\d+)?\z/
+        NEW_ISO_DATETIME = /\A(\d{4})-(\d\d)-(\d\d) (\d\d):(\d\d):(\d\d)(?:\.(\d+))?\z/
       end
 
       attr_reader :name, :default, :type, :limit, :null, :sql_type, :precision, :scale
@@ -167,10 +168,11 @@ def value_to_decimal(value)
         end
 
         protected
-          # '0.123456' -> 123456
-          # '1.123456' -> 123456
+          # Rational(123456, 1_000_000) -> 123456
+          # The sec_fraction component returned by Date._parse is a Rational fraction of a second or nil
+          # NB: This method is optimized for performance by immediately converting away from Rational.
           def microseconds(time)
-            ((time[:sec_fraction].to_f % 1) * 1_000_000).to_i
+            ((time[:sec_fraction].to_f % 1) * 1_000_000).round
           end
 
           def new_date(year, mon, mday)
@@ -194,9 +196,8 @@ def fast_string_to_date(string)
 
           # Doesn't handle time zones.
           def fast_string_to_time(string)
-            if string =~ Format::ISO_DATETIME
-              microsec = ($7.to_f * 1_000_000).to_i
-              new_time $1.to_i, $2.to_i, $3.to_i, $4.to_i, $5.to_i, $6.to_i, microsec
+            if md = Format::NEW_ISO_DATETIME.match(string)
+              new_time *md.to_a[1..7].map(&:to_i)
             end
           end
 

activerecord/test/cases/schema_definitions_test.rb

@@ -0,0 +1,43 @@
+require "cases/helper"
+
+class SchemaDefinitionsTest < ActiveRecord::TestCase
+
+  REGRESSION_SAMPLES = %w{000249 125014 003912 256051 524287}
+
+  test 'fast_string_to_time converts properly' do
+    converted = ActiveRecord::ConnectionAdapters::Column.send('fast_string_to_time', "2010-01-12 12:34:56.000249")
+    assert_equal Time.mktime(2010, 01, 12, 12, 34, 56, 249), converted
+  end
+
+  test 'fallback_string_to_time converts properly' do
+    converted = ActiveRecord::ConnectionAdapters::Column.send('fallback_string_to_time', "2010-01-12 12:34:56.000249")
+    assert_equal Time.mktime(2010, 01, 12, 12, 34, 56, 249), converted
+  end
+
+  test 'fallback_string_to_time converts properly with no microseconds' do
+    converted = ActiveRecord::ConnectionAdapters::Column.send('fallback_string_to_time', "2010-01-12 12:34:56")
+    assert_equal Time.mktime(2010, 01, 12, 12, 34, 56, 0), converted
+  end
+
+  test "fast_string_to_time can handle problematic microseconds" do
+    REGRESSION_SAMPLES.each do |u|
+      converted = ActiveRecord::ConnectionAdapters::Column.send('fast_string_to_time', "2010-01-12 12:34:56.#{u}")
+      assert_equal u.to_i, converted.usec
+    end
+  end
+
+  test "microseconds can handle problematic microseconds" do
+    REGRESSION_SAMPLES.each do |u|
+      i = u.to_i
+      converted = ActiveRecord::ConnectionAdapters::Column.send('microseconds', {:sec_fraction => Rational(i, 1_000_000)})
+      assert_equal i, converted
+
+      converted = ActiveRecord::ConnectionAdapters::Column.send('microseconds', {:sec_fraction => Rational(i, 1_000_000)})
+      assert_equal i, converted
+    end
+  end
+
+  test 'fast constant is equally restrictive' do
+    assert_match ActiveRecord::ConnectionAdapters::Column::Format::NEW_ISO_DATETIME, "2010-01-12 12:34:56.555493"
+  end
+end