First feedback on new C locale problems

[Rich Felker <dalias@libc.org>]

So far I've gotten 2 reports of things breaking from the new C locale.
The first was in Alpine:

gpg-connect-agent: conversion from 'utf-8' to 'UTF-8-CODE-UNITS' not available

and turned out to be caused by --disable-nls omitting setlocale,
leading to nl_langinfo(CODESET) requesting the C locale's codeset
name. This could be fixed by making iconv support "UTF-8-CODE-UNITS"
and do something reasonable with it, but the second issue was uglier.
In Void Linux:

help2man:
Unknown encoding 'UTF-8-CODE-UNITS' at /usr/bin/help2man line 56.
(https://github.com/voidlinux/void-packages/issues/2425)

Oddly this one did not affect Alpine, for the same reason the first
one did: Alpine has gettext support turned off, and help2man omits the
offending code:

http://anonscm.debian.org/cgit/users/bod/help2man.git/tree/help2man.PL?id=9ce0caa4cf164261ddde3fe987a260f5ba0dd558#n117

(which is overriding the system locale with "C" by default) when
gettext support is disabled. Being that this is Perl code and it's
passing the charset name to Perl's conversion functions, we can't just
work around this by adding a new charset alias to iconv.

Note that fixing the broken programs to call setlocale properly and
honor the user's locale would make them work in the normal case, but
they would break again if the user explicitly invoked them with
LC_CTYPE=C.

Anyway, what I suspect is that we're going to find a fair number of
programs are calling nl_langinfo(CODESET) without actually having set
the locale properly. As long as they're not using multibyte functions
in libc to process text, failing to have called setlocale is not such
a bad thing; they can do character processing themselves if they know
the intended encoding, using iconv or native UTF-8 code or whatever.
And the situation we've got right now is that, despite best efforts
not to impact users who don't intentionally _try_ to get a byte-based
C locale, this functionality is causing actual regressions in musl's
promise of "always UTF-8".

What I'd like to do to fix it is just always return "UTF-8" for
nl_langinfo(CODESET) regardless of locale (rather than returning
"UTF-8-CODE-UNITS" when in C locale). POSIX places no requirements on
nl_langinfo that would preclude this, and it seems like it would
restore the desired properties and fix all the regressions.

I don't mind leaving it as-is for a little bit while we discuss this
though; hopefully we'll turn up some more interesting application bugs
to get fixed.

Rich