[Bug libstdc++/96161] New: istream::ignore sets eofbit too soon

[Bug libstdc++/96161] New: istream::ignore sets eofbit too soon

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96161

            Bug ID: 96161
           Summary: istream::ignore sets eofbit too soon
           Product: gcc
           Version: 10.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

#include <sstream>
#include <iostream>
int main()
{
  std::istringstream s("++");
  s.ignore(2, '-');
  std::cout << "EOF? " << s.eof() << std::endl;
}

This shows that we set eofbit when the end of the stream is reached, even
though we have already discarded as many characters as requested.

Libc++ and MSVC do not set eofbit here.

[Bug libstdc++/96161] istream::ignore sets eofbit too soon

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96161

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-07-10
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=94749
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org

[Bug libstdc++/96161] istream::ignore sets eofbit too soon

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96161

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:ba8fe4b4832e30277f2e4a73b5d35b2e55074d07

commit r11-2054-gba8fe4b4832e30277f2e4a73b5d35b2e55074d07
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jul 13 10:26:39 2020 +0100

    libstdc++: Fix istream::ignore exit conditions (PR 94749, PR 96161)

    My previous fix for PR 94749 did fix the reported case, so that the next
    character is not discarded if it happens to equal the delimiter when __n
    characters have already been read. But it introduced a new bug, which is
    that the delimiter character would *not* be discarded if the number of
    characters discarded is numeric_limits<streamsize>::max() or more before
    reaching the delimiter.

    The new bug happens because I changed the code to check _M_gcount < __n.
    But when __n == numeric_limits<streamsize>::max() that is false, and so
    we don't discard the delimiter. It's not sufficient to check for the
    delimiter when the __large_ignore condition is true, because there's an
    edge case where the delimiter is reached when _M_gcount == __n and so
    we break out of the loop without setting __large_ignore.

    PR 96161 is a similar bug to the original PR 94749 report, where eofbit
    is set after discarding __n characters if there happen to be no more
    characters in the stream.

    This patch fixes both cases (and the regression) by checking different
    conditions for the __n == max case and the __n < max case. For the
    former case, we know that we must have either reached the delimiter or
    EOF, and the value of _M_gcount doesn't matter (except to avoid integer
    overflow). For the latter case we need to check _M_gcount first and only
    set eofbit or discard the delimiter if it didn't reach __n. For the
    latter case overflow can't happen because _M_gcount <= __n < max.

    libstdc++-v3/ChangeLog:

            PR libstdc++/94749
            PR libstdc++/96161
            * include/bits/istream.tcc (basic_istream::ignore(streamsize))
            [n == max]: Check overflow conditions on _M_gcount. Rely on
            the fact that either EOF or the delimiter was reached.
            [n < max]: Check _M_gcount < n before checking for EOF or
            delimiter.
            (basic_istream::ignore(streamsize, char_type): Likewise.
            * src/c++98/compatibility.cc (istream::ignore(streamsize))
            (wistream::ignore(streamsize)): Likewise.
            * src/c++98/istream.cc (istream::ignore(streamsize, char_type))
            (wistream::ignore(streamsize, char_type)): Likewise.
            * testsuite/27_io/basic_istream/ignore/char/94749.cc: Check that
            delimiter is discarded if the number of characters ignored
            doesn't fit in streamsize.
            * testsuite/27_io/basic_istream/ignore/wchar_t/94749.cc:
            Likewise.
            * testsuite/27_io/basic_istream/ignore/char/96161.cc: New test.
            * testsuite/27_io/basic_istream/ignore/wchar_t/96161.cc: New test.

[Bug libstdc++/96161] istream::ignore sets eofbit too soon

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96161

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk so far.

[Bug libstdc++/96161] istream::ignore sets eofbit too soon

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96161

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
See also g:4e39f563c0cd25401f689c2093cb8c13692156ef

[Bug libstdc++/96161] istream::ignore sets eofbit too soon

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96161

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
           Assignee|redi at gcc dot gnu.org            |unassigned at gcc dot gnu.org