Hi,

I think there is a bug in `io.open` in --safer mode:

    \directlua{
      io.open(tex.jobname .. '.tex'):close()
    }
    \bye

If this document is run with `luatex --safer` it leads to `bad argument #1 to 'gsub' (string expected, got nil)`.

The problem is a missing `not` in luatex-core.lua, function luatex_io_open_readonly:
The default argument 'r' is used if an explicit argument is given and when no "how" argument
is given it tries to interpret the non existing argument.