Restore the frozen state on rollback. Fixes #6417.

This is a 3-2-stable backport for #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.

1 files changed, 3 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/transactions.rb b/activerecord/lib/active_record/transactions.rb
index 2c70d31b94..8c93f50926 100644
--- a/activerecord/lib/active_record/transactions.rb
+++ b/activerecord/lib/active_record/transactions.rb
@@ -327,7 +327,8 @@ module ActiveRecord
         @_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 @@ module ActiveRecord
             @attributes.delete(self.class.primary_key)
             @attributes_cache.delete(self.class.primary_key)
           end
+          @attributes.freeze if was_frozen
         end
       end
     end