[RFC] Strategy for 9533

[Paolo Carlini]

Pétur Runólfsson wrote:

>Excellent, except I think III should be called from
>filebuf::showmanyc, not filebuf::open so ifstream::readsome doesn't
>stop after BUFSIZ characters, as in the original fix for 6746.
>
Specifically, when the testcase stops, in.rdbuf()->in_avail() is zero, 
therefore
there is nothing wrong with both the old and the new behaviour.

Otherwise, explain in better detail which alternate solution you are 
suggesting.

Paolo.