Re: [PATCH 0/2] Improve documentation on UTF-16

[Johannes Sixt <j6t@kdbg.org>]

Am 27.12.18 um 17:43 schrieb brian m. carlson:
> On Thu, Dec 27, 2018 at 11:06:17AM +0100, Johannes Sixt wrote:
>> It worries me that theoretical correctness is regarded higher than existing
>> practice. I do not care a lot what some RFC tells what programs should do if
>> the majority of the software does something different and that behavior has
>> been proven useful in practice.
> 
> The majority of OSes produce the behavior I document here, and they are
> the majority of systems on the Internet. Windows is the outlier here,
> although a significant one. It is a common user of UTF-16 and its
> variants, but so are Java and JavaScript, and they're present on a lot
> of devices. Swallowing the U+FEFF would break compatibility with those
> systems.
> 
> The issue that Windows users are seeing is that libiconv always produces
> big-endian data for UTF-16, and they always want little-endian. glibc
> produces native-endian data, which is what Windows users want. Git for
> Windows could patch libiconv to do that (and that is the simple,
> five-minute solution to this problem), but we'd still want to warn
> people that they're relying on unspecified behavior, hence this series.
> 
> I would even be willing to patch Git for Windows's libiconv if somebody
> could point me to the repo (although I obviously cannot test it, not
> being a Windows user). I feel strongly, though, that fixing this is
> outside of the scope of Git proper, and it's not a thing we should be
> handling here.

Please appologize that I leave the majority of what you said uncommented 
as I am not deep in the matter and don't have a firm understanding of 
all the issues. I'll just trust what you said is sound.

Just one thing: Please do the count by *users* (or existing files or 
number of charactes exchanged or something similar); do not just count 
OSs; I mean, Windows is *not* the outlier if it handles 90% of the 
UTF-16 data in the world. (I'm just making up numbers here, but I think 
you get the point.)

>> My understanding is that there is no such thing as a "byte order marker". It
>> just so happens that when the first character in some UTF-16 text file
>> begins with a ZWNBSP, then it is possible to derive the endianness of the
>> file automatically. Other then that, that very first code point U+FEFF *is
>> part of the data* and must not be removed when the data is reencoded. If Git
>> does something different, it is bogus, IMO.
> 
> You've got part of this. For UTF-16LE and UTF-16BE, a U+FEFF is part of
> the text, as would a second one be if we had two at the beginning of a
> UTF-16 or UTF-8 sequence. If someone produces UTF-16LE and places a
> U+FEFF at the beginning of it, when we encode to UTF-8, we emit only one
> U+FEFF, which has the wrong semantics.
> 
> To be correct here and accept a U+FEFF, we'd need to check for a U+FEFF
> at the beginning of a UTF-16LE or UTF-16BE sequence and ensure we encode
> an extra U+FEFF at the beginning of the UTF-8 data (one for BOM and one
> for the text) and then strip it off when we decode. That's kind of ugly,
> and since iconv doesn't do that itself, we'd have to.

But why do you add another U+FEFF on the way to UTF-8? There is one in 
the incoming UTF-16 data, and only *that* one must be converted. If 
there is no U+FEFF in the UTF-16 data, the should not be one in UTF-8, 
either. Puzzled...

-- Hannes