fix inexact number printing when LC_NUMERIC locale is not "C"

Relevant to PR 11802

src/racket/src/numstr.c

@@ -1569,7 +1569,9 @@ static char *double_to_string (double d, int alloc, int was_single)
     /* Initial count for significant digits is 14. That's big enough to
        get most right, small enough to avoid nonsense digits. But we'll
        loop in case it's not precise enough to get read-write invariance: */
+    GC_CAN_IGNORE char *loc;
     digits = 14;
+    loc = scheme_push_c_numeric_locale();
     while (digits < 30) {
       double check;
       GC_CAN_IGNORE char *ptr;
@@ -1583,6 +1585,7 @@ static char *double_to_string (double d, int alloc, int was_single)
 
       digits++;
     }
+    scheme_pop_c_numeric_locale(loc);
     
     l = strlen(buffer);
     for (i = 0; i < l; i++) {

src/racket/src/schpriv.h

@@ -1693,6 +1693,9 @@ void scheme_clear_bignum_cache(void);
 
 intptr_t scheme_integer_length(Scheme_Object *n);
 
+char *scheme_push_c_numeric_locale();
+void scheme_pop_c_numeric_locale(char *prev);
+
 /****** Rational numbers *******/
 
 typedef struct {

src/racket/src/string.c

@@ -3508,6 +3508,26 @@ static void reset_locale(void)
   }
 }
 
+char *scheme_push_c_numeric_locale()
+{
+#ifndef DONT_USE_LOCALE
+  GC_CAN_IGNORE char *prev;
+  prev = setlocale(LC_NUMERIC, NULL);
+  if (!strcmp(prev, "C"))
+    return NULL;
+  else
+    return setlocale(LC_NUMERIC, "C");
+#endif  
+}
+
+void scheme_pop_c_numeric_locale(char *prev)
+{
+#ifndef DONT_USE_LOCALE
+  if (prev)
+    setlocale(LC_NUMERIC, prev);
+#endif  
+}
+
 static int find_special_casing(int ch)
 {
   /* Binary search */