locale.c: Handle input parameter

calculate_LC_ALL_string() takes an input parameter specifying where the
caller wants the return value to be placed.  It wasn't handling one of
the possible values for that parameter.  This was spotted by Tony Cook.

I stashed the code that does the handling during a 'rebase -i', and
forgot to unstash it.

This fixes #20810

Author: khwilliamson

locale.c

@@ -2759,6 +2759,12 @@ S_calculate_LC_ALL_string(pTHX_ const char ** category_locales_list,
      *                              doesn't have to worry about it being
      *                              per-thread, nor needs to arrange for its
      *                              clean-up.
+     *      WANT_PL_setlocale_buf  the function stores the calculated string
+     *                              into the per-thread buffer PL_setlocale_buf
+     *                              and returns a pointer to that.  The buffer
+     *                              is cleaned up automatically in process
+     *                              destruction.  This return method avoids
+     *                              extra copies in some circumstances.
      *      WANT_VOID           NULL is returned.  This is used when the
      *                              function is being called only for its side
      *                              effect of updating
@@ -2903,6 +2909,14 @@ S_calculate_LC_ALL_string(pTHX_ const char ** category_locales_list,
 
     /* If all categories have the same locale, we already know the answer */
     if (! disparate) {
+        if (returning == WANT_PL_setlocale_buf) {
+            save_to_buffer(locales_list[0],
+                           &PL_setlocale_buf,
+                           &PL_setlocale_bufsize);
+            retval = PL_setlocale_buf;
+        }
+        else {
+
             retval = locales_list[0];
 
             /* If a temporary is wanted for the return, and we had to create
@@ -2917,11 +2931,22 @@ S_calculate_LC_ALL_string(pTHX_ const char ** category_locales_list,
             /* In all cases here, there's nothing we create that needs to be
              * freed, so leave 'free_if_void_return' set to the default
              * 'false'. */
+        }
     }
     else {  /* Here, not all categories have the same locale */
 
         char * constructed;
 
+        /* If returning to PL_setlocale_buf, set up to write directly to it,
+         * being sure it is resized to be large enough */
+        if (returning == WANT_PL_setlocale_buf) {
+            set_save_buffer_min_size(total_len,
+                                     &PL_setlocale_buf,
+                                     &PL_setlocale_bufsize);
+            constructed = PL_setlocale_buf;
+        }
+        else {  /* Otherwise we need new memory to hold the calculated value. */
+
             Newx(constructed, total_len, char);
 
             /* If returning the new memory, it must be set up to be freed
@@ -2932,6 +2957,7 @@ S_calculate_LC_ALL_string(pTHX_ const char ** category_locales_list,
             else {
                 free_if_void_return = true;
             }
+        }
 
         constructed[0] = '\0';