Re: iconv UTF-8 <--> CP1255 roundtrip possible bug?

[Will Dietz <w@wdtz.org>]

Nothing yet, I've not been able to spend more time on this lately sorry :).

I'll let you know if I do find anything,
and at least I'll be trying the latest changes you've pushed.

(yay all issues I've run into appear fixed :D)

Thanks!

~Will

On Sat, Jun 2, 2018 at 9:26 PM, Rich Felker <dalias@libc.org> wrote:
> On Wed, May 16, 2018 at 08:48:08PM -0500, Will Dietz wrote:
>> On Wed, May 16, 2018 at 6:04 PM, Rich Felker <dalias@libc.org> wrote:
>> > On Wed, May 16, 2018 at 12:22:36PM -0500, Will Dietz wrote:
>> >> I admit to being a bit unsure, but the behavior shown below doesn't
>> >> seem obviously right --LMK if I'm missing something :).
>> >>
>> >> Input file attached for inspection without relying on it getting
>> >> through byte-identical to what I have--
>> >> indeed I'm not sure copy+paste into this is working correctly (the
>> >> characters look different in my terminal :)).  Anyway:
>> >>
>> >> $ cat cp1255-snippet.xxd
>> >> 00000000: efac b3d6 b8d7 9d0a                      ........
>> >> $ xxd -r cp1255-snippet.xxd
>> >> דָּם
>> >>
>> >> Attempt to round-trip this from UTF-8 to CP1255 and back,
>> >> first with glibc's iconv (2.26):
>> >>
>> >> $ xxd -r cp1255-snippet.xxd|iconv -f UTF-8 -t CP1255|iconv -f CP1255
>> >> -t UTF-8 | xxd
>> >> 00000000: efac b3d6 b8d7 9d0a
>> >>
>> >> Looks good, same as what was sent in.
>> >>
>> >> Using musl-based iconv utility (1.1.19):
>> >> $ xxd -r cp1255-snippet.xxd|$ICONV -f UTF-8 -t CP1255|$ICONV -f CP1255
>> >> -t UTF-8 | xxd
>> >> 00000000: 2ad6 b8d7 9d0a                           *.....
>> >>
>> >> Indeed, the result looks different than what was started with:
>> >>
>> >> *ָם
>> >>
>> >> (again apologies if that doesn't survive mailing and such)
>> >>
>> >> This input was taken from gnu libiconv's test suite, in particular the
>> >> first line of tests/CP1255-snippet.UTF-8.  Since it's 2 characters,
>> >> and test data, I hope there's no problem re:licensing O:).
>> >>
>> >> I've reproduced the same behavior using iconv() directly, I can share
>> >> that if that would be preferable. It's the same code from earlier
>> >> iconv threads on the ML.
>> >
>> > No need; it's easy to reproduce, and I'm leaning towards saying the
>> > test is invalid. U+FB33 is a precomposed ligature form (from the
>> > Alphabetic Presentation Forms block), roughly equivalent in status to
>> > stuff like "fi" (U+FB01). An iconv implementation could perform an
>> > approximate conversion for such characters, returning a positive value
>> > indicating the number of such substitutions made, but silently
>> > converting it in a lossy way is not conforming, and of there's
>> > apparently no lossless way to convert it since CP1255 has no dedicated
>> > character slot for it (at least based on the definition of the
>> > codepage I'm using).
>>
>> Thanks for looking into this and for the great information!
>> I'll investigate more tomorrow, but wanted to respond to your inquiry
>> since it's easy to produce and might help explain things :).
>
> Any further findings?
>
>> > Do you know how/why they expect it to round-trip? What does glibc do
>> > when converting it -- can you show the intermediate (CP1255) form as a
>> > hexdump?
>>
>> Sure!
>>
>> Here's the intermediates for libiconv first, then w/musl:
>>
>> $ cat libiconv-cp1255.xxd
>> 00000000: e3cc c8ed 0a                             .....
>> $ cat musl-iconv-cp1255.xxd
>> 00000000: 2ac8 ed0a                                *...
>
> This is a plausible/reasonable conversion GNU iconv is doing...
>
>> Here's what happens when each of these are feed through both:
>>
>> ---- using libiconv's intermediate:
>> $ xxd -r ./libiconv-cp1255.xxd|iconv -f CP1255 -t UTF-8
>> דָּם
>> $ xxd -r ./libiconv-cp1255.xxd|iconv -f CP1255 -t UTF-8|xxd
>> 00000000: efac b3d6 b8d7 9d0a                      ........
>> $ xxd -r ./libiconv-cp1255.xxd|$ICONV -f CP1255 -t UTF-8
>> דָּם
>> $ xxd -r ./libiconv-cp1255.xxd|$ICONV -f CP1255 -t UTF-8|xxd
>> 00000000: d793 d6bc d6b8 d79d 0a                   .........
>
> ...but the GNU iconv behavior here is completely unreasonable/wrong.
> The first character it outputs is a presentation form for a ligature.
> There is no reason iconv should be doing this kind of renormalization
> when the original representation as two separate characters is
> available in the dest charset.
>
> Rich