Restore the frozen state on rollback. Fixes rails#6417.

This is a 3-2-stable backport for rails#6420 which was merged into master.

Currently, when saving a frozen record, an exception would be thrown
which causes a rollback. However, there is a bug in active record that
"defrost" the record as a side effect:

    >> t = Topic.new
    => #<Topic id: nil, ...>
    >> t.freeze
    => #<Topic id: nil, ...>
    >> t.save
    RuntimeError: can't modify a frozen Hash
    >> t.frozen?
    => false
    >> t.save
    => true

This patch fixes the bug by explictly restoring the frozen state on the
attributes Hash after every rollback.

activerecord/lib/active_record/transactions.rb

@@ -327,7 +327,8 @@ def restore_transaction_record_state(force = false) #:nodoc:
         @_start_transaction_state[:level] = (@_start_transaction_state[:level] || 0) - 1
         if @_start_transaction_state[:level] < 1
           restore_state = remove_instance_variable(:@_start_transaction_state)
-          @attributes = @attributes.dup if @attributes.frozen?
+          was_frozen = @attributes.frozen?
+          @attributes = @attributes.dup if was_frozen
           @new_record = restore_state[:new_record]
           @destroyed  = restore_state[:destroyed]
           if restore_state.has_key?(:id)
@@ -336,6 +337,7 @@ def restore_transaction_record_state(force = false) #:nodoc:
             @attributes.delete(self.class.primary_key)
             @attributes_cache.delete(self.class.primary_key)
           end
+          @attributes.freeze if was_frozen
         end
       end
     end

activerecord/test/cases/transactions_test.rb

@@ -362,6 +362,16 @@ def test_rollback_when_commit_raises
     end
   end
 
+  def test_rollback_when_saving_a_frozen_record
+    topic = Topic.new(:title => 'test')
+    topic.freeze
+    e = assert_raise(RuntimeError) { topic.save }
+    assert_equal "can't modify frozen Hash", e.message
+    assert !topic.persisted?, 'not persisted'
+    assert_nil topic.id
+    assert topic.frozen?, 'not frozen'
+  end
+
   def test_restore_active_record_state_for_all_records_in_a_transaction
     topic_1 = Topic.new(:title => 'test_1')
     topic_2 = Topic.new(:title => 'test_2')