bug#48228: json-serialize should signal error when dll is not found [MS Windows]

[Dmitry Gutov]

On 04.05.2021 21:07, Eli Zaretskii wrote:
> > Cc: rpluim@gmail.com, p.stephani2@gmail.com, 48228@debbugs.gnu.org
> > From: Dmitry Gutov <dgutov@yandex.ru>
> > Date: Tue, 4 May 2021 20:47:26 +0300
> >
> > On 04.05.2021 20:42, Eli Zaretskii wrote:
> > > How is that different from similar code that relies on, say, librsvg
> > > to display SVG images?
> >
> > Does it have a Lisp entry point? If so, I suppose it should be fixed too.
>
> We have create-image, which currently explicitly checks for the
> relevant library to be available to Emacs.
>
> > My main experience with librsvg is creating image specs manually and
> > having them used via the 'display' text property. There is no obvious
> > place to signal an error in that scenario.
>
> There is: in create-image.

It does make sense to signal an error in that case, too (with a 
dedicated error symbol).

A bit less critical than the JSON case, because the latter can 
erroneously return nil (and print a message) in situations where nil is 
a valid return value. And one can create an image spec by hand without 
calling create-image, so the "real" error is going to happen somewhere 
else anyway (during redisplay, I imagine).

So I would probably split this change into 2 commits: the essential 
places where no valid code should proceed when there is no support, and 
cases like create-image, to be easily reverted if we see significant 
complaints.