[Bug libstdc++/93672] [11/12/13 Regression] std::basic_istream::ignore hangs if delim MSB is set

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93672

--- Comment #7 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:fcf60d0baafa1245f768ac375dc60a07e92e9673

commit r13-8675-gfcf60d0baafa1245f768ac375dc60a07e92e9673
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Apr 4 10:33:33 2024 +0100

    libstdc++: Fix infinite loop in std::istream::ignore(n, delim) [PR93672]

    A negative delim value passed to std::istream::ignore can never match
    any character in the stream, because the comparison is done using
    traits_type::eq_int_type(sb->sgetc(), delim) and sgetc() never returns
    negative values (except at EOF). The optimized version of ignore for the
    std::istream specialization uses traits_type::find to locate the delim
    character in the streambuf, which _can_ match a negative delim on
    platforms where char is signed, but then we do another comparison using
    eq_int_type which fails. The code then keeps looping forever, with
    traits_type::find locating the character and traits_type::eq_int_type
    saying it's not a match, so traits_type::find is used again and finds
    the same character again.

    A possible fix would be to check with eq_int_type after a successful
    find, to see whether we really have a match. However, that would be
    suboptimal since we know that a negative delimiter will never match
    using eq_int_type. So a better fix is to adjust the check at the top of
    the function that handles delim==eof(), so that we treat all negative
    delim values as equivalent to EOF. That way we don't bother using find
    to search for something that will never match with eq_int_type.

    The version of ignore in the primary template doesn't need a change,
    because it doesn't use traits_type::find, instead characters are
    extracted one-by-one and always matched using eq_int_type. That avoids
    the inconsistency between find and eq_int_type. The specialization for
    std::wistream does use traits_type::find, but traits_type::to_int_type
    is equivalent to an implicit conversion from wchar_t to wint_t, so
    passing a wchar_t directly to ignore without using to_int_type works.

    libstdc++-v3/ChangeLog:

            PR libstdc++/93672
            * src/c++98/istream.cc (istream::ignore(streamsize, int_type)):
            Treat all negative delimiter values as eof().
            * testsuite/27_io/basic_istream/ignore/char/93672.cc: New test.
            * testsuite/27_io/basic_istream/ignore/wchar_t/93672.cc: New
            test.

    (cherry picked from commit 2d694414ada8e3b58f504c1b175d31088529632e)