bpo-35081: Internal headers require Py_BUILD_CORE (GH-10363)

* All internal header files now require Py_BUILD_CORE or
  Py_BUILD_CORE_BUILTIN to be defined.
* _json.c is now compiled with Py_BUILD_CORE_BUILTIN to access
  pycore_accu.h header.
* Add an example to Modules/Setup to show how to build _json
  as a built-in module; it requires non trivial compiler options.

Author: vstinner

Include/internal/pycore_accu.h

@@ -9,6 +9,10 @@ extern "C" {
  *** Its definition may be changed or removed at any moment.
  ***/
 
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
+#endif
+
 /*
  * A two-level accumulator of unicode objects that avoids both the overhead
  * of keeping a huge number of small separate objects, and the quadratic

Include/internal/pycore_atomic.h

@@ -4,8 +4,8 @@
 extern "C" {
 #endif
 
-#ifndef Py_BUILD_CORE
-#  error "Py_BUILD_CORE must be defined to include this header"
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
 #endif
 
 #include "dynamic_annotations.h"

Include/internal/pycore_ceval.h

@@ -4,6 +4,10 @@
 extern "C" {
 #endif
 
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
+#endif
+
 #include "pycore_atomic.h"
 #include "pythread.h"
 

Include/internal/pycore_condvar.h

@@ -1,6 +1,10 @@
 #ifndef Py_INTERNAL_CONDVAR_H
 #define Py_INTERNAL_CONDVAR_H
 
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
+#endif
+
 #ifndef _POSIX_THREADS
 /* This means pthreads are not implemented in libc headers, hence the macro
    not present in unistd.h. But they still can be implemented as an external

Include/internal/pycore_context.h

@@ -1,10 +1,12 @@
 #ifndef Py_INTERNAL_CONTEXT_H
 #define Py_INTERNAL_CONTEXT_H
 
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
+#endif
 
 #include "pycore_hamt.h"
 
-
 struct _pycontextobject {
     PyObject_HEAD
     PyContext *ctx_prev;
@@ -37,5 +39,4 @@ struct _pycontexttokenobject {
 int _PyContext_Init(void);
 void _PyContext_Fini(void);
 
-
 #endif /* !Py_INTERNAL_CONTEXT_H */

Include/internal/pycore_getopt.h

@@ -1,6 +1,10 @@
 #ifndef Py_INTERNAL_PYGETOPT_H
 #define Py_INTERNAL_PYGETOPT_H
 
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
+#endif
+
 extern int _PyOS_opterr;
 extern int _PyOS_optind;
 extern wchar_t *_PyOS_optarg;

Include/internal/pycore_gil.h

@@ -4,6 +4,10 @@
 extern "C" {
 #endif
 
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
+#endif
+
 #include "pycore_condvar.h"
 #include "pycore_atomic.h"
 

Include/internal/pycore_hamt.h

@@ -1,6 +1,9 @@
 #ifndef Py_INTERNAL_HAMT_H
 #define Py_INTERNAL_HAMT_H
 
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
+#endif
 
 #define _Py_HAMT_MAX_TREE_DEPTH 7
 

Include/internal/pycore_hash.h

@@ -1,6 +1,10 @@
 #ifndef Py_INTERNAL_HASH_H
 #define Py_INTERNAL_HASH_H
 
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
+#endif
+
 uint64_t _Py_KeyedHash(uint64_t, const char *, Py_ssize_t);
 
 #endif

Include/internal/pycore_lifecycle.h

@@ -4,8 +4,8 @@
 extern "C" {
 #endif
 
-#ifndef Py_BUILD_CORE
-#  error "Py_BUILD_CORE must be defined to include this header"
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
 #endif
 
 PyAPI_FUNC(int) _Py_UnixMain(int argc, char **argv);