mbrtowc, wcwidth: Fix MT-safety bug (regression from 2018-06-23).

* lib/mbrtowc.c (enc_t): New enum type.
(locale_enc, locale_enc_cached): New functions.
(mbrtowc): Eliminate static variables. Use locale_enc_cached instead.
* lib/wcwidth.c (is_locale_utf8, is_locale_utf8_cached): New functions.
(wcwidth): Eliminate static variables. Use is_locale_utf8_cached
instead.
* m4/mbrtowc.m4 (gl_PREREQ_MBRTOWC): Require AC_C_INLINE.
* m4/wcwidth.m4 (gl_PREREQ_WCWIDTH): New macro.
* modules/wcwidth (configure.ac): Invoke it.

1 files changed, 25 insertions, 13 deletions

diff --git a/lib/wcwidth.c b/lib/wcwidth.c
index d03a50f3eb..d33b6a9a55 100644
--- a/lib/wcwidth.c
+++ b/lib/wcwidth.c
@@ -26,28 +26,40 @@
 #include "streq.h"
 #include "uniwidth.h"
 
-int
-wcwidth (wchar_t wc)
-#undef wcwidth
+/* Returns 1 if the current locale is an UTF-8 locale, 0 otherwise.  */
+static inline int
+is_locale_utf8 (void)
 {
-  static int utf8_charset = -1;
-  static const char *encoding;
+  const char *encoding = locale_charset ();
+  return STREQ_OPT (encoding, "UTF-8", 'U', 'T', 'F', '-', '8', 0, 0, 0, 0);
+}
 
 #if GNULIB_WCHAR_SINGLE
-  if (utf8_charset == -1)
+/* When we know that the locale does not change, provide a speedup by
+   caching the value of is_locale_utf8.  */
+static int cached_is_locale_utf8 = -1;
+static inline int
+is_locale_utf8_cached (void)
+{
+  if (cached_is_locale_utf8 < 0)
+    cached_is_locale_utf8 = is_locale_utf8 ();
+  return cached_is_locale_utf8;
+}
+#else
+/* By default, don't make assumptions, hence no caching.  */
+# define is_locale_utf8_cached is_locale_utf8
 #endif
-    {
-      encoding = locale_charset ();
-      utf8_charset = STREQ_OPT (encoding,
-                                "UTF-8", 'U', 'T', 'F', '-', '8', 0, 0, 0 ,0);
-    }
 
+int
+wcwidth (wchar_t wc)
+#undef wcwidth
+{
   /* In UTF-8 locales, use a Unicode aware width function.  */
-  if (utf8_charset)
+  if (is_locale_utf8_cached ())
     {
       /* We assume that in a UTF-8 locale, a wide character is the same as a
          Unicode character.  */
-      return uc_width (wc, encoding);
+      return uc_width (wc, "UTF-8");
     }
   else
     {