[Bug stdio/1190] fgetc()/fread() behaviour is not POSIX compliant

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https://sourceware.org/bugzilla/show_bug.cgi?id=1190

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The branch, zack/sticky-eof has been created
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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=b8efec55de001d0f2517a8a7037673ef931336e9

commit b8efec55de001d0f2517a8a7037673ef931336e9
Author: Zack Weinberg <zackw@panix.com>
Date:   Wed Feb 21 19:12:51 2018 -0500

    [BZ 1190] Make EOF sticky in stdio.

    C99 specifies that the EOF condition on a file is "sticky": once EOF
    has been encountered, all subsequent reads should continue to return
    EOF until the file is closed or something clears the "end-of-file
    indicator" (e.g. fseek, clearerr).  This is arguably a change from
    C89, where the wording was ambiguous; the BSDs always had sticky EOF,
    but the System V lineage would attempt to read from the underlying fd
    again.  GNU libc has followed System V for as long as we've been
    using libio---the relevant chunk of code is

        int
        _IO_new_file_underflow (_IO_FILE *fp)
        {
          _IO_ssize_t count;
        #if 0
          /* SysV does not make this test; take it out for compatibility */
          if (fp->_flags & _IO_EOF_SEEN)
            return (EOF);
        #endif

    That's been unchanged since before 1995.  This only matters if the
    underlying file has changed in the meantime, of course; perhaps that's
    why nobody got around to filing a bug report until 2005, six years
    after C99 was published.  And nobody took that bug seriously until
    2012, at which time there was a long but inconclusive discussion on
    libc-alpha regarding whether it would break applications to change
    anything, see <https://sourceware.org/ml/libc-alpha/2012-09/msg00343.html>.

    It is my considered opinion that we should just go ahead and fix the
    bug, and that a backward compatibility mode is not required, because
    the BSDs have always had sticky EOF, so portable code has always had
    to be prepared to deal with that behavior.  Nowadays, the lineages we
    should be worrying most about compatibility with are all BSD-derived,
    anyway.  Thus, this patch.

    You might wonder if changing the _underflow impls is sufficient to
    apply the C99 semantics to all of the many stdio functions that
    perform input.  It should be enough to cover all paths to _IO_SYSREAD,
    and the only other functions that call _IO_SYSREAD are the _seekoff
    impls, which is OK because seeking clears EOF, and the _xsgetn impls,
    which, as far as I can tell, are unused within glibc.

    There is some question as to whether the test case in bug #19476
    (one of the several duplicate bug reports) is valid, so instead I have
    written a test case that uses a pseudoterminal to set up the necessary
    conditions -- actually two test cases, one for narrow and one for wide
    streams.  To facilitate this I added a new test-support function that
    sets up a pair of pty file descriptors for you; it's almost the same
    as BSD openpty, the only differences are that it allocates the
    optionally-returned tty pathname with malloc, and that it crashes if
    anything goes wrong.

    zw

        [BZ 1190]
        * libio/fileops.c (_IO_new_file_underflow): Return EOF immediately
        if the _IO_EOF_SEEN bit is already set; update commentary.
        * libio/oldfileops.c (_IO_old_file_underflow): Likewise.
        * libio/wfileops.c (_IO_wfile_underflow): Likewise.

        * support/support_openpty.c, support/tty.h: New files.
        * support/Makefile (libsupport-routines): Add support_openpty.

        * libio/tst-fgetc-after-eof.c, wcsmbs/test-fgetwc-after-eof.c:
        New test cases.
        * libio/Makefile (tests): Add tst-fgetc-after-eof.
        * wcsmbs/Makefile (tests): Add tst-fgetwc-after-eof.

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