Create an immortal C locale

This adds a function fish_c_locale() which returns an immortal locale_t
corresponding to the C locale, and switches builtin_printf to use wcstod_l.

src/builtin_printf.cpp

@@ -271,19 +271,11 @@ template <>
 long double raw_string_to_scalar_type(const wchar_t *s, wchar_t **end) {
     double val = wcstod(s, end);
     if (**end == L'\0') return val;
-
     // The conversion using the user's locale failed. That may be due to the string not being a
     // valid floating point value. It could also be due to the locale using different separator
     // characters than the normal english convention. So try again by forcing the use of a locale
     // that employs the english convention for writing floating point numbers.
-    //
-    // TODO: switch to the wcstod_l() function to avoid changing the global locale.
-    char *saved_locale = strdup(setlocale(LC_NUMERIC, NULL));
-    setlocale(LC_NUMERIC, "C");
-    val = wcstod(s, end);
-    setlocale(LC_NUMERIC, saved_locale);
-    free(saved_locale);
-    return val;
+    return wcstod_l(s, end, fish_c_locale());
 }
 
 template <typename T>

src/wutil.cpp

@@ -517,6 +517,11 @@ int fish_wcswidth(const wchar_t *str) { return fish_wcswidth(str, wcslen(str));
 /// See fallback.h for the normal definitions.
 int fish_wcswidth(const wcstring &str) { return fish_wcswidth(str.c_str(), str.size()); }
 
+locale_t fish_c_locale() {
+    static locale_t loc = newlocale(LC_ALL_MASK, "C", NULL);
+    return loc;
+}
+
 /// Like fish_wcstol(), but fails on a value outside the range of an int.
 ///
 /// This is needed because BSD and GNU implementations differ in several ways that make it really

src/wutil.h

@@ -7,6 +7,7 @@
 #include <stdio.h>
 #include <sys/types.h>
 #include <time.h>
+#include <xlocale.h>
 #include <string>
 
 #include "common.h"
@@ -117,6 +118,9 @@ int fish_iswgraph(wint_t wc);
 int fish_wcswidth(const wchar_t *str);
 int fish_wcswidth(const wcstring &str);
 
+// returns an immortal locale_t corresponding to the C locale.
+locale_t fish_c_locale();
+
 int fish_wcstoi(const wchar_t *str, const wchar_t **endptr = NULL, int base = 10);
 long fish_wcstol(const wchar_t *str, const wchar_t **endptr = NULL, int base = 10);
 long long fish_wcstoll(const wchar_t *str, const wchar_t **endptr = NULL, int base = 10);