mem_collxfrm(): Return early if locale collation not sane

This changes a subsidiary function's return value from void to bool,
returning false if it finds the locale doesn't have sane collation.

The calling function is changed to check this, and give up immediately
if the locale isn't sane.

Author: khwilliamson

locale.c

@@ -9451,11 +9451,32 @@ Perl_init_i18nl10n(pTHX_ int printwarn)
 #undef GET_DESCRIPTION
 #ifdef USE_LOCALE_COLLATE
 
-STATIC void
+STATIC bool
 S_compute_collxfrm_coefficients(pTHX)
 {
-
-    /* A locale collation definition includes primary, secondary, tertiary,
+    /* This is called from mem_collxfrm() the first time the latter is called
+     * on the current locale to do initialization for it.
+     *
+     * This returns true and initializes the coefficients for a linear equation
+     * that, given a string of some length, predicts how much memory it will
+     * take to hold the result of calling mem_collxfrm() on that string.  The
+     * equation is of the form:
+     *      m * length + b
+     * where m = PL_collxfrm_mult and b = PL_collxfrm_base
+     *
+     * It returns false if the locale does not appear to be sane.
+     *
+     * The prediction is just an educated guess to save time and,
+     * mem_collxrfm() may adjust it based on experience with strings it
+     * encounters.
+     *
+     * This function also:
+     *      sets 'PL_in_utf8_COLLATE_locale' to indicate if the locale is a
+     *          UTF-8 one
+     *      initializes 'PL_strxfrm_NUL_replacement' to NUL
+     *      initializes 'PL_strxfrm_max_cp' = 0;
+     *
+     * A locale collation definition includes primary, secondary, tertiary,
      * etc. weights for each character.  To sort, the primary weights are used,
      * and only if they compare equal, then the secondary weights are used, and
      * only if they compare equal, then the tertiary, etc.
@@ -9564,8 +9585,9 @@ S_compute_collxfrm_coefficients(pTHX)
                 "Disabling locale collation for LC_COLLATE='%s';"
                 " length for shorter sample=%zu; longer=%zu\n",
                 PL_collation_name, x_len_shorter, x_len_longer));
+        return false;
     }
-    else {
+
         SSize_t base;       /* Temporary */
 
         /* We have both: m * strlen(longer)  + b = x_len_longer
@@ -9597,7 +9619,6 @@ S_compute_collxfrm_coefficients(pTHX)
 
         /* Add 1 for the trailing NUL */
         PL_collxfrm_base = base + 1;
-    }
 
     DEBUG_L(PerlIO_printf(Perl_debug_log,
                           "?UTF-8 locale=%d; x_len_shorter=%zu, "
@@ -9606,6 +9627,7 @@ S_compute_collxfrm_coefficients(pTHX)
                           PL_in_utf8_COLLATE_locale,
                           x_len_shorter, x_len_longer,
                           PL_collxfrm_mult, PL_collxfrm_base));
+    return true;
 }
 
 char *
@@ -9668,7 +9690,9 @@ Perl_mem_collxfrm_(pTHX_ const char *input_string,
 
         /* (mult, base) == (0,0) means we need to calculate mult and base
          * before proceeding */
-        S_compute_collxfrm_coefficients(aTHX);
+        if (! S_compute_collxfrm_coefficients(aTHX)) {
+            return NULL;    /* locale collation not sane */
+        }
     }
 
     /* Replace any embedded NULs with the control that sorts before any others.