[Bug stdio/20012] libio: fmemopen append mode failure

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https://sourceware.org/bugzilla/show_bug.cgi?id=20012

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=52affab7cc2787033713b73bbc2a9c412469012f

commit 52affab7cc2787033713b73bbc2a9c412469012f
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Wed Apr 27 11:51:01 2016 -0300

    libio: Fix fmemopen append mode failure (BZ# 20012)

    The fmemopen implementation does not account the file position correctly in
    append mode. The following example shows the failure:

    ===
    int main ()
    {
      char buf[10] = "test";
      FILE *fp = fmemopen (buf, 10, "a+");
      fseek (fp, 0, SEEK_SET);

      int gr;
      if ((gr = getc (fp)) != 't' ||
          (gr = getc (fp)) != 'e' ||
          (gr = getc (fp)) != 's' ||
          (gr = getc (fp)) != 't' ||
          (gr = getc (fp)) != EOF)
        {
          printf ("%s: getc failed returned %i\n", __FUNCTION__, gr);
          return 1;
        }

      return 0;
    }
    ===

    This is due both how read and write operation update the buffer position,
    taking in consideration buffer lenght instead of maximum position defined
    by the open mode.  This patch fixes it and also fixes fseek not returning
    EINVAL for invalid whence modes.

    Tested on x86_64 and i686.

    This is a backport of b65b205fbcabbb02463e31df17f5cabf7556f892.

        [BZ #20012]
        * libio/fmemopen.c (fmemopen_read): Use buffer maximum position, not
        length to calculate the buffer to read.
        (fmemopen_write): Set the buffer position based on bytes written.
        (fmemopen_seek): Return EINVAL for invalid whence modes.

    (cherry picked from commit 321e1cef26ccbece949b16622ef74c203bd8ecc6)

-----------------------------------------------------------------------

Summary of changes:
 libio/fmemopen.c             |   28 +++++-----
 stdio-common/tst-fmemopen3.c |  118 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 132 insertions(+), 14 deletions(-)

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