make the default locale (& a variant) failure-free cases for newlocale

commit aeeac9ca5490d7d90fe061ab72da446c01ddf746 introduced fail-safe
invariants that creating a locale_t object for the C locale or C.UTF-8
locale will always succeed. extend the guarantee to also cover the
following:

- newlocale(LC_ALL_MASK, "", 0)
- newlocale(LC_ALL_MASK-LC_CTYPE_MASK, "C", 0)

provided that the LANG/LC_* environment variables have not been
changed by the program. these usages are idiomatic for getting the
default locale, and for getting a locale that behaves as the C locale
except for honoring the default locale's character encoding.

src/locale/newlocale.c

@@ -1,10 +1,22 @@
 #include <stdlib.h>
 #include <string.h>
+#include <pthread.h>
 #include "locale_impl.h"
 
+static pthread_once_t default_locale_once;
+static struct __locale_struct default_locale, default_ctype_locale;
+
+static void default_locale_init(void)
+{
+	for (int i=0; i<LC_ALL; i++)
+		default_locale.cat[i] = __get_locale(i, "");
+	default_ctype_locale.cat[LC_CTYPE] = default_locale.cat[LC_CTYPE];
+}
+
 int __loc_is_allocated(locale_t loc)
 {
-	return loc && loc != C_LOCALE && loc != UTF8_LOCALE;
+	return loc && loc != C_LOCALE && loc != UTF8_LOCALE
+		&& loc != &default_locale && loc != &default_ctype_locale;
 }
 
 locale_t __newlocale(int mask, const char *name, locale_t loc)
@@ -30,6 +42,13 @@ locale_t __newlocale(int mask, const char *name, locale_t loc)
 	if (!memcmp(&tmp, C_LOCALE, sizeof tmp)) return C_LOCALE;
 	if (!memcmp(&tmp, UTF8_LOCALE, sizeof tmp)) return UTF8_LOCALE;
 
+	/* And provide builtins for the initial default locale, and a
+	 * variant of the C locale honoring the default locale's encoding. */
+	pthread_once(&default_locale_once, default_locale_init);
+	if (!memcmp(&tmp, &default_locale, sizeof tmp)) return &default_locale;
+	if (!memcmp(&tmp, &default_ctype_locale, sizeof tmp))
+		return &default_ctype_locale;
+
 	/* If no builtin locale matched, attempt to allocate and copy. */
 	if ((loc = malloc(sizeof *loc))) *loc = tmp;