gh-112538: Add internal-only _PyThreadStateImpl "wrapper" for PyThreadState (gh-112560)

Every PyThreadState instance is now actually a _PyThreadStateImpl.
It is safe to cast from `PyThreadState*` to `_PyThreadStateImpl*` and back.
The _PyThreadStateImpl will contain fields that we do not want to expose
in the public C API.

Include/internal/pycore_interp.h

@@ -29,6 +29,7 @@ extern "C" {
 #include "pycore_list.h"          // struct _Py_list_state
 #include "pycore_object_state.h"  // struct _py_object_state
 #include "pycore_obmalloc.h"      // struct _obmalloc_state
+#include "pycore_tstate.h"        // _PyThreadStateImpl
 #include "pycore_tuple.h"         // struct _Py_tuple_state
 #include "pycore_typeobject.h"    // struct types_state
 #include "pycore_unicodeobject.h" // struct _Py_unicode_state
@@ -210,8 +211,8 @@ struct _is {
     struct _Py_interp_cached_objects cached_objects;
     struct _Py_interp_static_objects static_objects;
 
-   /* the initial PyInterpreterState.threads.head */
-    PyThreadState _initial_thread;
+    /* the initial PyInterpreterState.threads.head */
+    _PyThreadStateImpl _initial_thread;
     Py_ssize_t _interactive_src_count;
 };
 

Include/internal/pycore_runtime_init.h

@@ -186,7 +186,12 @@ extern PyTypeObject _PyExc_MemoryError;
                 }, \
             }, \
         }, \
-        ._initial_thread = _PyThreadState_INIT, \
+        ._initial_thread = _PyThreadStateImpl_INIT, \
+    }
+
+#define _PyThreadStateImpl_INIT \
+    { \
+        .base = _PyThreadState_INIT, \
     }
 
 #define _PyThreadState_INIT \

Include/internal/pycore_tstate.h

@@ -0,0 +1,26 @@
+#ifndef Py_INTERNAL_TSTATE_H
+#define Py_INTERNAL_TSTATE_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+
+// Every PyThreadState is actually allocated as a _PyThreadStateImpl. The
+// PyThreadState fields are exposed as part of the C API, although most fields
+// are intended to be private. The _PyThreadStateImpl fields not exposed.
+typedef struct _PyThreadStateImpl {
+    // semi-public fields are in PyThreadState.
+    PyThreadState base;
+
+    // TODO: add private fields here
+} _PyThreadStateImpl;
+
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_TSTATE_H */