Cygwin: _sys_mbstowcs: fix handling invalid 4-byte UTF-8 sequences

When a 4 byte utf-8 sequence has an invalid 4th byte, it's actually
an invalid 3 byte sequence.  In this case we already generated the
high surrogate and only realize the problem when byte 4 doesn't
match.

At this point _sys_mbstowcs transposes the invalid 4th byte into the
private use area.

This is wrong.  The invalid byte sequence here is the 3 byte sequence
already converted to a high surrogate, not the trailing 4th byte.

Fix this by backtracking to the start of the broken sequence and
overwrite the already written high surrogate with a sequence of
the original three bytes transposed to the private use area. Reset
the mbstate and restart normal conversion at the non-matching
4th byte, which might start a new multibyte sequence.

The resulting wide-char string can be converted back to multibyte
and back again to wide-char, and the result will be identical, even
if the multibyte sequence differs from the original sequence.

Fixes: e44b9069cd227 ("* strfuncs.cc (sys_cp_mbstowcs): Treat src as unsigned char *.  Convert failure of f_mbtowc into a single malformed utf-16 value.")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

0 files changed, 0 insertions, 0 deletions