Re: [PATCH] Improve error reporting when serializing non-Unicode strings to JSON

[Philipp Stephani <p.stephani2@gmail.com>]

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Eli Zaretskii <eliz@gnu.org> schrieb am Sa., 23. Dez. 2017 um 15:52 Uhr:

> > From: Philipp Stephani <p.stephani2@gmail.com>
> > Date: Sat, 23 Dec 2017 14:29:56 +0000
> > Cc: emacs-devel@gnu.org, phst@google.com
> >
> >  OK, but why do we need external functions for doing that?  What is
> >  missing in our own code to detect such a situation?
> >
> > Not much I think, it's just easiest to use Gnulib functions because they
> are well-documented, have a clean
> > interface, and are probably bug-free.
> > coding.c has check_utf_8, which is quite similar, but has an
> incompatible interface (it takes struct
> > coding_system objects) and also checks for embedded newlines, which
> isn't necessary here.
>
> So let's use check_utf_8, as its downsides don't sound serious to me,
>

Well it needs to be rewritten significantly to take a char*, length
argument instead of the coding_system struct.


> and OTOH using unistring functions will bloat Emacs


u8-check.c is just 77 LoC (including all boilerplate, comments, and empty
lines), so I don't think it blows up Emacs in any significant way.


> for the benefit of
> a single use case, not to mention create two different methods for
> doing the same job, which IMO is even more confusing to any newcomer
> to the Emacs internals.
>

Agreed it's somewhat confusing, but I think not too much. The two functions
have quite different use cases: check_utf_8 is a specialized function that
requires a coding system with significant set-up and is only used once (in
decode_coding_gap), while u8_check is a general-purpose function.
Having not much experience with coding.c, I find the functions in that file
much more confusing and harder to understand than the ones from
libunistring. The libunistring functions tend to have a single, clear
purpose, while the coding.c functions often do many different things at
once.


>
> Btw, doesn't find_charsets_in_text do the same job cleaner and
> quicker?  AFAIU, all you need is make sure there are no characters
> from the 2 eight-bit-* charsets in the text, or did I miss something?
>

What I need to check is one of the following:
- Is the initial string either a well-formed UTF-8 unibyte string, or a
multibyte string that represents a Unicode scalar value sequence?
- Is the encoded string a well-formed UTF-8 unibyte string?
Given my understanding of the implementation of coding.c, these two
criteria should be equivalent. (Unfortunately that doesn't seem to be
documented.) So I choose to implement the second check, which is easier and
allows delaying the check until we know we have to signal an error.

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