Why fixing 9533 doesn't fix 7774: revealed!

[Paolo Carlini]

Hi,

seriously, after a little more gdb along the lines suggested
by Nathan, I know why and I'd like to ask your opinion about
what should we actually do now that we know what's going on.
This is 7744 testcase:

void f2()
{
 cout
 << "\n:: f2() ::\n"
 << "Type in the characters 'abc' and press <ENTER>: ";
 cin.peek();
 cout
 << "The number of unread characters should be 4 (a, b, c, \\n): "
 << cin.rdbuf()->in_avail()
 << '\n';
}

As Nathan explained, in_avail shouldn't call showmanyc but in
fact it does. Why?

Because after the in.peek(), egptr == gptr! Indeed, in.peek()
calls sgetc(), which calls underflow(), which does:

           __elen = _M_file.xsgetn(reinterpret_cast<char*>(_M_in_beg),
                                   _M_buf_size);
           __ilen = __elen;

           if (0 < __ilen)
             {
                _M_set_determinate(__ilen);
                if (__testout)
                  _M_out_cur = _M_in_cur;
                __ret = traits_type::to_int_type(*_M_in_cur);
                if (__bump)
                  _M_in_cur_move(1);
                else if (_M_buf_size == 1)
                  {
                    // If we are synced with stdio, we have to unget the
                    // character we just read so that the file pointer
                    // doesn't move.
                    
_M_file.sys_ungetc(traits_type::to_int_type(*_M_in_cur));
                    _M_set_indeterminate();
                  }
             }         

There are no chances that 4 chars are put in the buffer (stdio is synced
and _M_buf_size == 1) but we expect at least one!

That single char _is_ in fact put into the buffer, but, since 
_M_buf_size == 1
the char is unget and _M_set_indeterminate called, which basically reset
all the get area pointers to _M_buf and the information on the char present
in the buffer ends up _not_ being propagated (as egptr == gptr + 1) to 
in_avail.

So, a few question:
1- Is that _M_set_indeterminate() call really necessary for other reasons?
2- Is it ok to have _M_buf_size == 1 when synced with stdio?

Thanks,
Paolo.