Re: setlocale behavior with 'missing' locales

[Rich Felker <dalias@libc.org>]

On Thu, Mar 01, 2018 at 01:10:47PM -0600, William Pitcock wrote:
> >> One notable issue is that, right now, we rely on being able to set
> >> LC_MESSAGES to an arbitrary name even if there's no libc locale
> >> definition for it; this is because gettext() relies on the name of the
> >> current LC_MESSAGES locale to find (application-specific) translation
> >> files that might exist even without a libc translation. I'm not sure
> >> how we would best keep this working under changes similar to the
> >> above.
> >
> > Any further thoughts on this? I'd like to begin addressing these
> > issues in this release cycle.
> >
> > I think the above plan works (is conforming, doesn't break things)
> > except for the LC_MESSAGES issue mentioned at the end. I don't have
> > any good ideas still for dealing with that. Really since gettext can
> > be used with any category, not just LC_MESSAGES (although LC_MESSAGES
> > is the normal choice), it applies to all categories. Maybe we could
> > still use the ("nonexistant") requested locale name in this case, or
> > some derivative of it that clarifies that it's synthesized...?
> 
> +1 to using this approach.
> 
> We could use a locale name such as "en_US@virtual.UTF-8".
> 
> glibc uses this style of locale name for locales such as UK english
> with eurozone LC_CURRENCY: en_UK@euro.UTF-8.

I was actually just in the process of trying to work out something
very similar. Here's how I think it might work:

setlocale(cat, "") -- always succeeds, produces ll_TT@virtual (or
ll_TT@missing was my idea) if a locale file by the matching name is
not found.

setlocale(cat, "ll_TT@virtual") (or whatever name) - always succeeds.

setlocale(cat, "ll_TT[@other]") - succeeds only if a file matching the
name is found.

One thing I don't entirely like is repurposing the @ modifier for
this; it conflicts with (and perhaps fails to preserve) an existing
modifier if there is one, and affects how search for gettext
translation files would happen (searching extra @virtual paths).
Perhaps we should instead make it a separate component delimited in
some other way so it can always be dropped by gettext.

Rich