gh-76785: Improved Subinterpreters Compatibility with 3.12 (gh-115424)

For the most part, these changes make is substantially easier to backport subinterpreter-related code to 3.12, especially the related modules (e.g. _xxsubinterpreters). The main motivation is to support releasing a PyPI package with the 3.13 capabilities compiled for 3.12.

A lot of the changes here involve either hiding details behind macros/functions or splitting up some files.

Python/crossinterp_exceptions.h

@@ -0,0 +1,80 @@
+
+/* InterpreterError extends Exception */
+
+static PyTypeObject _PyExc_InterpreterError = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    .tp_name = "interpreters.InterpreterError",
+    .tp_doc = PyDoc_STR("A cross-interpreter operation failed"),
+    //.tp_base = (PyTypeObject *)PyExc_BaseException,
+};
+PyObject *PyExc_InterpreterError = (PyObject *)&_PyExc_InterpreterError;
+
+/* InterpreterNotFoundError extends InterpreterError */
+
+static PyTypeObject _PyExc_InterpreterNotFoundError = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    .tp_name = "interpreters.InterpreterNotFoundError",
+    .tp_doc = PyDoc_STR("An interpreter was not found"),
+    .tp_base = &_PyExc_InterpreterError,
+};
+PyObject *PyExc_InterpreterNotFoundError = (PyObject *)&_PyExc_InterpreterNotFoundError;
+
+/* NotShareableError extends ValueError */
+
+static int
+_init_not_shareable_error_type(PyInterpreterState *interp)
+{
+    const char *name = "interpreters.NotShareableError";
+    PyObject *base = PyExc_ValueError;
+    PyObject *ns = NULL;
+    PyObject *exctype = PyErr_NewException(name, base, ns);
+    if (exctype == NULL) {
+        return -1;
+    }
+
+    _PyInterpreterState_GetXIState(interp)->PyExc_NotShareableError = exctype;
+    return 0;
+}
+
+static void
+_fini_not_shareable_error_type(PyInterpreterState *interp)
+{
+    Py_CLEAR(_PyInterpreterState_GetXIState(interp)->PyExc_NotShareableError);
+}
+
+static PyObject *
+_get_not_shareable_error_type(PyInterpreterState *interp)
+{
+    assert(_PyInterpreterState_GetXIState(interp)->PyExc_NotShareableError != NULL);
+    return _PyInterpreterState_GetXIState(interp)->PyExc_NotShareableError;
+}
+
+
+/* lifecycle */
+
+static int
+init_exceptions(PyInterpreterState *interp)
+{
+    // builtin static types
+    _PyExc_InterpreterError.tp_base = (PyTypeObject *)PyExc_BaseException;
+    if (_PyStaticType_InitBuiltin(interp, &_PyExc_InterpreterError) < 0) {
+        return -1;
+    }
+    if (_PyStaticType_InitBuiltin(interp, &_PyExc_InterpreterNotFoundError) < 0) {
+        return -1;
+    }
+
+    // heap types
+    // We would  call _init_not_shareable_error_type() here too,
+    // but that leads to ref leaks
+
+    return 0;
+}
+
+static void
+fini_exceptions(PyInterpreterState *interp)
+{
+    // Likewise with _fini_not_shareable_error_type().
+    _PyStaticType_Dealloc(interp, &_PyExc_InterpreterNotFoundError);
+    _PyStaticType_Dealloc(interp, &_PyExc_InterpreterError);
+}