I am attaching another  patch (io2.diff). Please review. I am not sure
whether _adjust_chunk() should also adjust "readahead".

BTW, PEP 3116 says:

"If universal newlines without translation are requested on input (i.e.
newline=''), if a system read operation returns a buffer ending in '\r',
another system read operation is done to determine whether it is
followed by '\n' or not. In universal newlines mode with translation,
the second system read operation may be postponed until the next read
request, and if the following system read operation returns a buffer
starting with '\n', that character is simply discarded."

I suppose this issue is mainly talking about the latter (newline is
None). I don't understand what is meant by "enabling universal new line
mode without translation". Isn't the purpose of enabling universal new
line mode is to translate line endings? I may be missing something
basic, of course.