[Bug stdio/18241] failed fseek on memstream does not set errno and can fail when it shouldnt

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https://sourceware.org/bugzilla/show_bug.cgi?id=18241

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=645f97ced4d4b35deda3f8bde0927f898b163f5d

commit 645f97ced4d4b35deda3f8bde0927f898b163f5d
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Mon Jul 25 14:54:29 2016 -0300

    libio: Multiple fixes for open_{w}memstram (BZ#18241 and BZ#20181)

    This patches fixes multiples issues on open_{w}memstream reported on both
    BZ#18241 and BZ#20181:

      - failed fseek does not set errno.
      - negative offset in fseek fails even when resulting position is
        a valid one.
      - a flush after write if the current write position is not at the
        end of the stream currupt data.

    The main fix is on seek operation for memstream (_IO_{w}str_seekoff), where
    both _IO_read_ptr and _IO_read_end pointer are updated if a write operation
    has occured (similar to default file operations).  Also, to calculate the
    offset on both read and write pointers, a temporary value is instead of
    updating the argument supplied value.  Negative offset are valid if
resulting
    internal pointer is within the range of _IO_{read,write}_base and
    _IO_{read,write}_end.

    Also POSIX states that a null or wide null shall be appended to the current
    buffer iff a write moves the position to a value larger than the current
    lenght.  Current implementation appends a null or wide null regardless
    of this condition.  This patch fixes it by removing the 'else' condition
    on _IO_{w}mem_sync.

    Checked on x86_64.

        [BZ #18241]
        [BZ #20181]
        * libio/Makefile (test): Add tst-memstream3 and tst-wmemstream3.
        * libio/memstream.c (_IO_mem_sync): Only append a null byte if
        write position is at the end the buffer.
        * libio/wmemstream.c (_IO_wmem_sync): Likewise.
        * libio/strops.c (_IO_str_switch_to_get_mode): New function.
        (_IO_str_seekoff): Set correct offset from negative displacement and
        set EINVAL for invalid ones.
        * libio/wstrops.c (enlarge_userbuf): Use correct function to calculate
        buffer length.
        (_IO_wstr_switch_to_get_mode): New function.
        (_IO_wstr_seekoff): Set correct offset from negative displacement and
        set EINVAL for invalid ones.
        * libio/tst-memstream3.c: New file.
        * libio/tst-wmemstream3.c: Likewise.
        * manual/examples/memstrm.c: Remove warning when priting size_t.

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

Summary of changes:
 ChangeLog                                          |   20 +++
 libio/Makefile                                     |    4 +-
 libio/memstream.c                                  |    2 -
 libio/strops.c                                     |   81 +++++++---
 libio/tst-memstream3.c                             |  165 ++++++++++++++++++++
 .../tst-wcstod-round.c => libio/tst-wmemstream3.c  |   33 +++--
 libio/wmemstream.c                                 |    2 -
 libio/wstrops.c                                    |   89 +++++++----
 manual/examples/memstrm.c                          |    4 +-
 9 files changed, 327 insertions(+), 73 deletions(-)
 create mode 100644 libio/tst-memstream3.c
 copy wcsmbs/tst-wcstod-round.c => libio/tst-wmemstream3.c (55%)

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