[3.14] gh-132775: Support Fallbacks in _PyObject_GetXIData() (gh-134418)

It now supports a "full" fallback to _PyFunction_GetXIData() and then `_PyPickle_GetXIData()`.
There's also room for other fallback modes if that later makes sense.

(cherry picked from commit 88f8102, AKA gh-133482)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

Author: miss-islington

Doc/data/python3.14.abi

@@ -131,7 +131,23 @@ PyAPI_FUNC(void) _PyXIData_Clear(PyInterpreterState *, _PyXIData_t *);
 
 /* getting cross-interpreter data */
 
-typedef int (*xidatafunc)(PyThreadState *tstate, PyObject *, _PyXIData_t *);
+typedef int xidata_fallback_t;
+#define _PyXIDATA_XIDATA_ONLY (0)
+#define _PyXIDATA_FULL_FALLBACK (1)
+
+// Technically, we don't need two different function types;
+// we could go with just the fallback one.  However, only container
+// types like tuple need it, so always having the extra arg would be
+// a bit unfortunate.  It's also nice to be able to clearly distinguish
+// between types that might call _PyObject_GetXIData() and those that won't.
+//
+typedef int (*xidatafunc)(PyThreadState *, PyObject *, _PyXIData_t *);
+typedef int (*xidatafbfunc)(
+        PyThreadState *, PyObject *, xidata_fallback_t, _PyXIData_t *);
+typedef struct {
+    xidatafunc basic;
+    xidatafbfunc fallback;
+} _PyXIData_getdata_t;
 
 PyAPI_FUNC(PyObject *) _PyXIData_GetNotShareableErrorType(PyThreadState *);
 PyAPI_FUNC(void) _PyXIData_SetNotShareableError(PyThreadState *, const char *);
@@ -140,16 +156,21 @@ PyAPI_FUNC(void) _PyXIData_FormatNotShareableError(
         const char *,
         ...);
 
-PyAPI_FUNC(xidatafunc) _PyXIData_Lookup(
+PyAPI_FUNC(_PyXIData_getdata_t) _PyXIData_Lookup(
         PyThreadState *,
         PyObject *);
 PyAPI_FUNC(int) _PyObject_CheckXIData(
         PyThreadState *,
         PyObject *);
 
+PyAPI_FUNC(int) _PyObject_GetXIDataNoFallback(
+        PyThreadState *,
+        PyObject *,
+        _PyXIData_t *);
 PyAPI_FUNC(int) _PyObject_GetXIData(
         PyThreadState *,
         PyObject *,
+        xidata_fallback_t,
         _PyXIData_t *);
 
 // _PyObject_GetXIData() for bytes

Include/internal/pycore_crossinterp.h

@@ -17,7 +17,7 @@ typedef struct _xid_regitem {
     /* This is NULL for builtin types. */
     PyObject *weakref;
     size_t refcount;
-    xidatafunc getdata;
+    _PyXIData_getdata_t getdata;
 } _PyXIData_regitem_t;
 
 typedef struct {
@@ -30,7 +30,7 @@ typedef struct {
 PyAPI_FUNC(int) _PyXIData_RegisterClass(
     PyThreadState *,
     PyTypeObject *,
-    xidatafunc);
+    _PyXIData_getdata_t);
 PyAPI_FUNC(int) _PyXIData_UnregisterClass(
     PyThreadState *,
     PyTypeObject *);

Include/internal/pycore_crossinterp_data_registry.h

@@ -438,5 +438,5 @@ def ensure_module_imported(name, *, clearnone=True):
     if sys.modules.get(name) is not None:
         mod = sys.modules[name]
     else:
-        mod, _, _ = _force_import(name, False, True, clearnone)
+        mod, _, _ = _ensure_module(name, False, True, clearnone)
     return mod