add todo for corner case of transaction(&block)

src/db/begin_transaction.cr

@@ -7,6 +7,15 @@ module DB
       begin
         yield tx
       rescue e
+        # TODO check if `unless tx.closed?` should be added.
+        # If it is not added, when an exception occurs after the
+        # transaction is closed (explicit rollback/commit)
+        # the call to rollback will trigger an exception.
+        # Since the code will be in between `#transaction(&block)`
+        # seems reasonable to raise an error since it might trigger
+        # a design flaw. Transaction might been commited already.
+        # Maybe we should wrap e in another exception that clarifies
+        # this scenario.
         tx.rollback
         raise e unless e.is_a?(DB::Rollback)
       else