Clean up perl.h/makedef.pl common logic

This has gotten two twisty little mazy over time.  Clean it up, add
comments, and make sure the logic is the same on both.

Author: khwilliamson

makedef.pl

@@ -107,6 +107,7 @@ BEGIN
 }
 close(CFG);
 
+#==========================================================================
 # perl.h logic duplication begins
 
 if ($define{USE_ITHREADS}) {
@@ -143,33 +144,45 @@ BEGIN
 if ($define{USE_ITHREADS} && ! $define{NO_LOCALE_THREADS}) {
     $define{USE_LOCALE_THREADS} = 1;
 }
-if (! $define{HAS_SETLOCALE} && $define{HAS_POSIX_2008_LOCALE}) {
+
+if (   $define{HAS_POSIX_2008_LOCALE}
+    && (  ! $define{HAS_SETLOCALE} || (     $define{USE_LOCALE_THREADS}
+                                       && ! $define{NO_POSIX_2008_LOCALE})))
+{
     $define{USE_POSIX_2008_LOCALE} = 1;
-    $define{USE_THREAD_SAFE_LOCALE} = 1;
 }
-elsif (   ($define{USE_LOCALE_THREADS} || $define{USE_THREAD_SAFE_LOCALE})
-       && (    $define{HAS_POSIX_2008_LOCALE}
-           || ($ARGS{PLATFORM} eq 'win32' && (   $cctype !~ /\D/
-                                              && $cctype >= 80)))
-       && ! $define{NO_THREAD_SAFE_LOCALE})
+
+if ($define{USE_LOCALE_THREADS} && ! $define{NO_THREAD_SAFE_LOCALE})
+{
+    if (    $define{USE_POSIX_2008_LOCALE}
+        || ($ARGS{PLATFORM} eq 'win32' && (   $cctype !~ /\D/
+                                           && $cctype >= 80)))
+    {
+        $define{USE_THREAD_SAFE_LOCALE} = 1;
+    }
+}
+
+if ($define{USE_POSIX_2008_LOCALE} && $define{HAS_QUERYLOCALE})
 {
-    $define{USE_THREAD_SAFE_LOCALE} = 1 unless $define{USE_THREAD_SAFE_LOCALE};
-    $define{USE_POSIX_2008_LOCALE} = 1 if $define{HAS_POSIX_2008_LOCALE};
+    $define{USE_QUERYLOCALE} = 1;
+
+    # Don't need glibc only code from perl.h
 }
 
-if (   ($define{USE_POSIX_2008_LOCALE} && ! $define{HAS_QUERYLOCALE}))
+if ($define{USE_POSIX_2008_LOCALE} && ! $define{USE_QUERYLOCALE})
 {
     $define{USE_PL_CURLOCALES} = 1;
 }
 
-if (   $ARGS{PLATFORM} eq 'win32'
-    && $define{USE_THREAD_SAFE_LOCALE}
-    && $cctype < 140)
+if ($ARGS{PLATFORM} eq 'win32' && $define{USE_THREAD_SAFE_LOCALE})
 {
-    $define{TS_W32_BROKEN_LOCALECONV} = 1;
+    if ($cctype < 140) {
+        $define{TS_W32_BROKEN_LOCALECONV} = 1;
+    }
 }
 
 # perl.h logic duplication ends
+#==========================================================================
 
 print STDERR "Defines: (" . join(' ', sort keys %define) . ")\n"
      unless $ARGS{PLATFORM} eq 'test';

perl.h

@@ -1064,6 +1064,10 @@ violations are fatal.
 
 #include "perl_langinfo.h"    /* Needed for _NL_LOCALE_NAME */
 
+/* =========================================================================
+ * The defines from here to the following ===== line are unfortunately
+ * duplicated in makedef.pl, and changes here MUST also be made there */
+
 /* If not forbidden, we enable locale handling if either 1) the POSIX 2008
  * functions are available, or 2) just the setlocale() function.  This logic is
  * repeated in t/loc_tools.pl and makedef.pl;  The three should be kept in
@@ -1215,35 +1219,32 @@ violations are fatal.
 #    define LC_ALL_INDEX_               PERL_DUMMY_TOD_ + 1
 #  endif
 
-/* XXX The next few defines are unfortunately duplicated in makedef.pl, and
- * changes here MUST also be made there */
 
 #  if defined(USE_ITHREADS) && ! defined(NO_LOCALE_THREADS)
 #    define USE_LOCALE_THREADS
 #  endif
-#  if ! defined(HAS_SETLOCALE) && defined(HAS_POSIX_2008_LOCALE)
-#      define USE_POSIX_2008_LOCALE
-#      ifdef USE_LOCALE_THREADS
-#        define USE_THREAD_SAFE_LOCALE
-#      endif
-                                   /* If compiled with
-                                    * -DUSE_THREAD_SAFE_LOCALE, will do so even
-                                    * on unthreaded builds */
-#  elif   (defined(USE_LOCALE_THREADS) || defined(USE_THREAD_SAFE_LOCALE))   \
-       && (    defined(HAS_POSIX_2008_LOCALE)                                \
-           || (defined(WIN32) && defined(_MSC_VER)))                         \
-       && ! defined(NO_THREAD_SAFE_LOCALE)
-#    ifndef USE_THREAD_SAFE_LOCALE
+
+   /* Use POSIX 2008 locales if available, and no alternative exists
+    * ('setlocale()' is the alternative); or is threaded and not forbidden to
+    * use them */
+#  if defined(HAS_POSIX_2008_LOCALE) && (  ! defined(HAS_SETLOCALE)            \
+                                         || (     defined(USE_LOCALE_THREADS)  \
+                                             && ! defined(NO_POSIX_2008_LOCALE)))
+#    define USE_POSIX_2008_LOCALE
+#  endif
+
+   /* On threaded builds, use thread-safe locales if they are available and not
+    * forbidden.  Availability is when we are using POSIX 2008 locales, or
+    * Windows for quite a few releases now. */
+#  if defined(USE_LOCALE_THREADS) && ! defined(NO_THREAD_SAFE_LOCALE)
+#    if defined(USE_POSIX_2008_LOCALE) || (defined(WIN32) && defined(_MSC_VER))
 #      define USE_THREAD_SAFE_LOCALE
 #    endif
-#    ifdef HAS_POSIX_2008_LOCALE
-#      define USE_POSIX_2008_LOCALE
-#    endif
 #  endif
 
 #  include "perl_langinfo.h"    /* Needed for _NL_LOCALE_NAME */
 
-/* Allow use of glib's undocumented querylocale() equivalent if asked for, and
+/* Allow use of glibc's undocumented querylocale() equivalent if asked for, and
  * appropriate */
 #  ifdef USE_POSIX_2008_LOCALE
 #    if  defined(HAS_QUERYLOCALE)                                           \
@@ -1262,21 +1263,28 @@ violations are fatal.
 #    endif
 #  endif
 
+   /* POSIX 2008 has no means of finding out the current locale without a
+    * querylocale; so must keep track of it ourselves */
 #  if (defined(USE_POSIX_2008_LOCALE) && ! defined(USE_QUERYLOCALE))
 #    define USE_PL_CURLOCALES
 #  endif
 
-/*  Microsoft documentation reads in the change log for VS 2015:
- *     "The localeconv function declared in locale.h now works correctly when
- *     per-thread locale is enabled. In previous versions of the library, this
- *     function would return the lconv data for the global locale, not the
- *     thread's locale."
- */
-#  if defined(WIN32) && defined(USE_THREAD_SAFE_LOCALE) && _MSC_VER < 1900
-#  define TS_W32_BROKEN_LOCALECONV
+#  if defined(WIN32) && defined(USE_THREAD_SAFE_LOCALE)
+
+   /* Microsoft documentation reads in the change log for VS 2015: "The
+    * localeconv function declared in locale.h now works correctly when
+    * per-thread locale is enabled. In previous versions of the library, this
+    * function would return the lconv data for the global locale, not the
+    * thread's locale." */
+#    if _MSC_VER < 1900
+#      define TS_W32_BROKEN_LOCALECONV
+#    endif
 #  endif
 #endif
 
+/* end of makedef.pl logic duplication
+ * ========================================================================= */
+
 #ifdef PERL_CORE
 
 /* Both typedefs are used in locale.c only, but defined here so that embed.fnc