Re: setlocale behavior with 'missing' locales

[Rich Felker <dalias@libc.org>]

On Thu, Mar 01, 2018 at 02:25:45PM -0500, Rich Felker wrote:
> 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.

Implementation notes if we do this:

__get_locale is the internal backend that loads locale maps, and looks
like the point at which this all should be implemented.

Presently __get_locale has no means to return an error; a null return
value indicates the C locale, which is represented everywhere by the
lack of any locale map.

It seems __get_locale has all the information it needs to decide how
to act:

- If the argument is "", missing/virtual locale synthesis should
  happen. If allocation failures etc. prevent synthesis, it should
  behave as if the argument had been "C.UTF-8".

- If the argument is one of the builtin locales (C/C.UTF-8/POSIX) it
  can return one of the builtin maps. Right now it oddly replaces
  "C.UTF-8" with just plain "C" (null return value) in all categories
  except LC_CTYPE. This behavior might should be revisited but
  newlocale.c and perhaps other places encode assumptions that it's
  done this way.

- If the argument is another name that can't be found, an error should
  be returned to the caller somehow. We could perhaps use MAP_FAILED.
  The alternative seems to be reworking the contract so that null
  doesn't mean C and either using a real locale_map object for the C
  locale or translating to null in the caller, but these choices seem
  to impose worse costs/effects elsewhere.

None of the above covers anything about _how_ the synthesis of names
for missing locales should happen, just where/when it should happen.

Rich