Gain control of macro namespace visibility

This commit adds the capability to undefine macros that are visible to
XS code but shouldn't be.  This can be used to stop macro namespace
pollution by perl.

It works by changing embed.h to have two modes, controlled by a #ifdef
that is set by perl.h.  perl.h now #includes embed.h twice.  The first
time works as it always has.  The second sets the #ifdef, and causes
embed.h to #undef the macros that shouldn't be visible.  This call is
just before perl.h returns to its includer, so that these macros have
come and gone before the file that #included perl.h is affected by them.
It comes after the inline headers get included, so they have access to
all the symbols that are defined.

The list of macros is determined by the visibility given by the apidoc
lines documenting them, plus several exception lists that allow a symbol
to be visible even though it is not documented as such.

In this commit, the main exception list contains everything that is
currently visible outside the Perl core, so this should not break any
code.  But it means that the visibility control is established for
future changes to our code base.  New macros will not be visible except
when documented as needing to be such.  We can no longer inadvertently
add new names to pollute the user's.

I expect that over time, the exception list will become smaller, as we
go through it and remove the items that really shouldn't be visible.  We
can then see via smoking if someone is actually using them, and either
decide that these should be visible, or work with the module author for
another way to accomplish their needs.  (I would hope this would lead to
proper documentation of the ones that need to be visible.)

There are currently four lists of symbols.

One list is for symbols that are used by libc functions, and that Perl
may redefine (usually so that code doesn't have to know if it is running
on a platform that is lacking the given feature.)  The algorithm added
here catches most of these and keeps them visible, but there are a few
items that currently must be manually listed.

A second list is of symbols that the re extension to Perl requires, but
no one else needs to.  This list is currently empty, as everything
initially is in the main exception list.

A third list is for items that other Perl extensions require, but no one
else needs to.  This list is currently empty, as everything initially is
in the main exception list.

The final list is for items that currently are visible to the whole
world.  It contains thousands of items.  This list should be examined
for:

    1) Names that shouldn't be so visible; and
    2) Names that need to remain visible but should be changed so they
       are less likely to clash with anything the user might come up
       with.

I have wanted this ability to happen for a long time; and now things
have come together to enable it.

This allows us to have a clear-cut boundary with CPAN.

It means you can add macros that have internal-only use without having
to worry about making them likely not to clash with user names.

It shows precisely in one place what our names are that are visible to
CPAN.

Author: khwilliamson

embed.h

@@ -21,77 +21,11 @@
 
 /* (Doing namespace management portably in C is really gross.) */
 
-/* By defining PERL_NO_SHORT_NAMES (not done by default) the short forms
- * (like warn instead of Perl_warn) for the API are not defined.
- * Not defining the short forms is a good thing for cleaner embedding.
- * BEWARE that a bunch of macros don't have long names, so either must be
- * added or don't use them if you define this symbol */
+/* When this symbol is defined, we undef various symbols we have defined
+ * earlier when this file was #included with this symbol undefined */
+#if !defined(PERL_DO_UNDEFS)
 
-#if !defined(MULTIPLICITY)
-/* undefined symbols, point them back at the usual ones */
-# define Perl_deb_nocontext                     Perl_deb
-# define Perl_form_nocontext                    Perl_form
-# define Perl_load_module_nocontext             Perl_load_module
-# define Perl_mess_nocontext                    Perl_mess
-# define Perl_newSVpvf_nocontext                Perl_newSVpvf
-# define Perl_sv_catpvf_nocontext               Perl_sv_catpvf
-# define Perl_sv_catpvf_mg_nocontext            Perl_sv_catpvf_mg
-# define Perl_sv_setpvf_nocontext               Perl_sv_setpvf
-# define Perl_sv_setpvf_mg_nocontext            Perl_sv_setpvf_mg
-# define Perl_warn_nocontext                    Perl_warn
-# define Perl_warner_nocontext                  Perl_warner
-#endif /* !defined(MULTIPLICITY) */
-#if !defined(PERL_CORE)
-/* Compatibility stubs.  Compile extensions with -DPERL_NOCOMPAT to
- * disable them.
- */
-# define sv_setptrobj(rv,ptr,name)              sv_setref_iv(rv,name,PTR2IV(ptr))
-# define sv_setptrref(rv,ptr)                   sv_setref_iv(rv,NULL,PTR2IV(ptr))
-# if !defined(PERL_NOCOMPAT)
-
-/* Compatibility for this renamed function. */
-#   define perl_atexit(a,b)                     Perl_call_atexit(aTHX_ a,b)
-
-/* Compatibility for these functions that had a 'perl_' prefix before
- * 'Perl_' became the standard */
-#   define perl_call_argv(a,b,c)                Perl_call_argv(aTHX_ a,b,c)
-#   define perl_call_method(a,b)                Perl_call_method(aTHX_ a,b)
-#   define perl_call_pv(a,b)                    Perl_call_pv(aTHX_ a,b)
-#   define perl_call_sv(a,b)                    Perl_call_sv(aTHX_ a,b)
-#   define perl_eval_pv(a,b)                    Perl_eval_pv(aTHX_ a,b)
-#   define perl_eval_sv(a,b)                    Perl_eval_sv(aTHX_ a,b)
-#   define perl_get_av(a,b)                     Perl_get_av(aTHX_ a,b)
-#   define perl_get_cv(a,b)                     Perl_get_cv(aTHX_ a,b)
-#   define perl_get_hv(a,b)                     Perl_get_hv(aTHX_ a,b)
-#   define perl_get_sv(a,b)                     Perl_get_sv(aTHX_ a,b)
-#   define perl_init_i18nl10n(a)                Perl_init_i18nl10n(aTHX_ a)
-#   define perl_require_pv(a)                   Perl_require_pv(aTHX_ a)
-
-/* Before C99, macros could not wrap varargs functions. This
-   provides a set of compatibility functions that don't take an
-   extra argument but grab the context pointer using the macro dTHX.
- */
-
-#   if  defined(MULTIPLICITY) && !defined(PERL_NO_SHORT_NAMES) && \
-       !defined(PERL_WANT_VARARGS)
-#     define deb                                Perl_deb_nocontext
-#     define form                               Perl_form_nocontext
-#     define load_module                        Perl_load_module_nocontext
-#     define mess                               Perl_mess_nocontext
-#     define newSVpvf                           Perl_newSVpvf_nocontext
-#     define sv_catpvf                          Perl_sv_catpvf_nocontext
-#     define sv_catpvf_mg                       Perl_sv_catpvf_mg_nocontext
-#     define sv_setpvf                          Perl_sv_setpvf_nocontext
-#     define sv_setpvf_mg                       Perl_sv_setpvf_mg_nocontext
-#     define warn                               Perl_warn_nocontext
-#     define warner                             Perl_warner_nocontext
-#   endif /*  defined(MULTIPLICITY) && !defined(PERL_NO_SHORT_NAMES) &&
-             !defined(PERL_WANT_VARARGS) */
-# endif /* !defined(PERL_NOCOMPAT) */
-#endif /* !defined(PERL_CORE) */
-#if !defined(PERL_NO_SHORT_NAMES)
-
-/* Hide global symbols */
+/* Create short name macros that hide any need for thread context */
 
 # define AvFILL_(a)                             Perl_AvFILL_(aTHX_ a)
 # define Gv_AMupdate(a,b)                       Perl_Gv_AMupdate(aTHX_ a,b)
@@ -908,7 +842,20 @@
 #   define sv_setpvf_nocontext                  Perl_sv_setpvf_nocontext
 #   define warn_nocontext                       Perl_warn_nocontext
 #   define warner_nocontext                     Perl_warner_nocontext
-# endif /* defined(MULTIPLICITY) */
+# else /* if !defined(MULTIPLICITY) */
+/* undefined symbols, point them back at the usual ones */
+#   define Perl_deb_nocontext                   Perl_deb
+#   define Perl_form_nocontext                  Perl_form
+#   define Perl_load_module_nocontext           Perl_load_module
+#   define Perl_mess_nocontext                  Perl_mess
+#   define Perl_newSVpvf_nocontext              Perl_newSVpvf
+#   define Perl_sv_catpvf_nocontext             Perl_sv_catpvf
+#   define Perl_sv_catpvf_mg_nocontext          Perl_sv_catpvf_mg
+#   define Perl_sv_setpvf_nocontext             Perl_sv_setpvf
+#   define Perl_sv_setpvf_mg_nocontext          Perl_sv_setpvf_mg
+#   define Perl_warn_nocontext                  Perl_warn
+#   define Perl_warner_nocontext                Perl_warner
+# endif /* !defined(MULTIPLICITY) */
 # if !defined(MULTIPLICITY) || defined(PERL_CORE) || \
       defined(PERL_WANT_VARARGS)
 #   define deb(...)                             Perl_deb(aTHX_ __VA_ARGS__)
@@ -1834,7 +1781,52 @@
 #   else
 #     define do_exec3(a,b,c)                    Perl_do_exec3(aTHX_ a,b,c)
 #   endif
-# endif /* defined(PERL_CORE) */
+# else /* if !defined(PERL_CORE) */
+/* Compatibility stubs.  Compile extensions with -DPERL_NOCOMPAT to
+ * disable them.
+ */
+#   define sv_setptrobj(rv,ptr,name)            sv_setref_iv(rv,name,PTR2IV(ptr))
+#   define sv_setptrref(rv,ptr)                 sv_setref_iv(rv,NULL,PTR2IV(ptr))
+#   if !defined(PERL_NOCOMPAT)
+
+/* Compatibility for this renamed function. */
+#     define perl_atexit(a,b)                   Perl_call_atexit(aTHX_ a,b)
+
+/* Compatibility for these functions that had a 'perl_' prefix before
+ * 'Perl_' became the standard */
+#     define perl_call_argv(a,b,c)              Perl_call_argv(aTHX_ a,b,c)
+#     define perl_call_method(a,b)              Perl_call_method(aTHX_ a,b)
+#     define perl_call_pv(a,b)                  Perl_call_pv(aTHX_ a,b)
+#     define perl_call_sv(a,b)                  Perl_call_sv(aTHX_ a,b)
+#     define perl_eval_pv(a,b)                  Perl_eval_pv(aTHX_ a,b)
+#     define perl_eval_sv(a,b)                  Perl_eval_sv(aTHX_ a,b)
+#     define perl_get_av(a,b)                   Perl_get_av(aTHX_ a,b)
+#     define perl_get_cv(a,b)                   Perl_get_cv(aTHX_ a,b)
+#     define perl_get_hv(a,b)                   Perl_get_hv(aTHX_ a,b)
+#     define perl_get_sv(a,b)                   Perl_get_sv(aTHX_ a,b)
+#     define perl_init_i18nl10n(a)              Perl_init_i18nl10n(aTHX_ a)
+#     define perl_require_pv(a)                 Perl_require_pv(aTHX_ a)
+
+/* Before C99, macros could not wrap varargs functions. This
+   provides a set of compatibility functions that don't take an
+   extra argument but grab the context pointer using the macro dTHX.
+ */
+
+#     if defined(MULTIPLICITY) && !defined(PERL_WANT_VARARGS)
+#       define deb                              Perl_deb_nocontext
+#       define form                             Perl_form_nocontext
+#       define load_module                      Perl_load_module_nocontext
+#       define mess                             Perl_mess_nocontext
+#       define newSVpvf                         Perl_newSVpvf_nocontext
+#       define sv_catpvf                        Perl_sv_catpvf_nocontext
+#       define sv_catpvf_mg                     Perl_sv_catpvf_mg_nocontext
+#       define sv_setpvf                        Perl_sv_setpvf_nocontext
+#       define sv_setpvf_mg                     Perl_sv_setpvf_mg_nocontext
+#       define warn                             Perl_warn_nocontext
+#       define warner                           Perl_warner_nocontext
+#     endif /* defined(MULTIPLICITY) && !defined(PERL_WANT_VARARGS) */
+#   endif /* !defined(PERL_NOCOMPAT) */
+# endif /* !defined(PERL_CORE) */
 # if defined(PERL_CORE) || defined(PERL_EXT)
 #   define append_utf8_from_native_byte         Perl_append_utf8_from_native_byte
 #   define av_reify(a)                          Perl_av_reify(aTHX_ a)
@@ -2474,6 +2466,6 @@
 # else
 #   define get_context                          Perl_get_context
 # endif
-#endif /* !defined(PERL_NO_SHORT_NAMES) */
+#endif /* !defined(PERL_DO_UNDEFS) */
 
 /* ex: set ro ft=c: */

perl.h

@@ -9354,6 +9354,12 @@ END_EXTERN_C
 
 */
 
+/* #including a second time causes it to #undef any unwanted symbols to avoid
+ * polluting the user name space */
+#  define PERL_DO_UNDEFS
+#  include "embed.h"
+#  undef PERL_DO_UNDEFS
+
 #endif /* Include guard */
 
 /*