bpo-35081: Move Include/pyatomic.c to Include/internal/ (GH-10239)

Add pyatomic.h to the VS project (it wasn't referenced).

Include/Python.h

@@ -53,8 +53,6 @@
 #include "pyport.h"
 #include "pymacro.h"
 
-#include "pyatomic.h"
-
 /* Debug-mode build with pymalloc implies PYMALLOC_DEBUG.
  *  PYMALLOC_DEBUG is in error if pymalloc is not in use.
  */

Include/internal/ceval.h

@@ -4,7 +4,7 @@
 extern "C" {
 #endif
 
-#include "pyatomic.h"
+#include "internal/pyatomic.h"
 #include "pythread.h"
 
 struct _pending_calls {

Include/internal/gil.h

@@ -4,11 +4,11 @@
 extern "C" {
 #endif
 
-#include "pyatomic.h"
-
 #include "internal/condvar.h"
+#include "internal/pyatomic.h"
+
 #ifndef Py_HAVE_CONDVAR
-#error You need either a POSIX-compatible or a Windows system!
+#  error You need either a POSIX-compatible or a Windows system!
 #endif
 
 /* Enable if you want to force the switching of threads at least

Include/internal/pyatomic.h

@@ -1,6 +1,12 @@
 #ifndef Py_ATOMIC_H
 #define Py_ATOMIC_H
-#ifdef Py_BUILD_CORE
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "Py_BUILD_CORE must be defined to include this header"
+#endif
 
 #include "dynamic_annotations.h"
 
@@ -531,5 +537,8 @@ typedef struct _Py_atomic_int {
     _Py_atomic_store_explicit(ATOMIC_VAL, NEW_VAL, _Py_memory_order_relaxed)
 #define _Py_atomic_load_relaxed(ATOMIC_VAL) \
     _Py_atomic_load_explicit(ATOMIC_VAL, _Py_memory_order_relaxed)
-#endif  /* Py_BUILD_CORE */
+
+#ifdef __cplusplus
+}
+#endif
 #endif  /* Py_ATOMIC_H */

Include/internal/pystate.h

@@ -5,7 +5,6 @@ extern "C" {
 #endif
 
 #include "pystate.h"
-#include "pyatomic.h"
 #include "pythread.h"
 
 #include "internal/mem.h"