Import Python 3.13.11

commit_hash:bbb53cefb159aa3e7afaa475fd19d5a03b66945f

1 files changed, 1922 insertions, 0 deletions

diff --git a/contrib/tools/python3/Python/crossinterp.c b/contrib/tools/python3/Python/crossinterp.c
new file mode 100644
index 00000000000..2f6324d300d
--- /dev/null
+++ b/contrib/tools/python3/Python/crossinterp.c
@@ -0,0 +1,1922 @@
+
+/* API for managing interactions between isolated interpreters */
+
+#include "Python.h"
+#include "pycore_ceval.h"         // _Py_simple_func
+#include "pycore_crossinterp.h"   // struct _xid
+#include "pycore_initconfig.h"    // _PyStatus_OK()
+#include "pycore_namespace.h"     //_PyNamespace_New()
+#include "pycore_pyerrors.h"      // _PyErr_Clear()
+#include "pycore_weakref.h"       // _PyWeakref_GET_REF()
+
+
+/**************/
+/* exceptions */
+/**************/
+
+static int init_exceptions(PyInterpreterState *);
+static void fini_exceptions(PyInterpreterState *);
+static int _init_not_shareable_error_type(PyInterpreterState *);
+static void _fini_not_shareable_error_type(PyInterpreterState *);
+static PyObject * _get_not_shareable_error_type(PyInterpreterState *);
+#include "crossinterp_exceptions.h"
+
+
+/***************************/
+/* cross-interpreter calls */
+/***************************/
+
+int
+_Py_CallInInterpreter(PyInterpreterState *interp,
+                      _Py_simple_func func, void *arg)
+{
+    if (interp == PyInterpreterState_Get()) {
+        return func(arg);
+    }
+    // XXX Emit a warning if this fails?
+    _PyEval_AddPendingCall(interp, (_Py_pending_call_func)func, arg, 0);
+    return 0;
+}
+
+int
+_Py_CallInInterpreterAndRawFree(PyInterpreterState *interp,
+                                _Py_simple_func func, void *arg)
+{
+    if (interp == PyInterpreterState_Get()) {
+        int res = func(arg);
+        PyMem_RawFree(arg);
+        return res;
+    }
+    // XXX Emit a warning if this fails?
+    _PyEval_AddPendingCall(interp, func, arg, _Py_PENDING_RAWFREE);
+    return 0;
+}
+
+
+/**************************/
+/* cross-interpreter data */
+/**************************/
+
+/* registry of {type -> crossinterpdatafunc} */
+
+/* For now we use a global registry of shareable classes.  An
+   alternative would be to add a tp_* slot for a class's
+   crossinterpdatafunc. It would be simpler and more efficient. */
+
+static void xid_lookup_init(PyInterpreterState *);
+static void xid_lookup_fini(PyInterpreterState *);
+static crossinterpdatafunc lookup_getdata(PyInterpreterState *, PyObject *);
+#include "crossinterp_data_lookup.h"
+
+
+/* lifecycle */
+
+_PyCrossInterpreterData *
+_PyCrossInterpreterData_New(void)
+{
+    _PyCrossInterpreterData *xid = PyMem_RawMalloc(
+                                            sizeof(_PyCrossInterpreterData));
+    if (xid == NULL) {
+        PyErr_NoMemory();
+    }
+    return xid;
+}
+
+void
+_PyCrossInterpreterData_Free(_PyCrossInterpreterData *xid)
+{
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    _PyCrossInterpreterData_Clear(interp, xid);
+    PyMem_RawFree(xid);
+}
+
+
+/* defining cross-interpreter data */
+
+static inline void
+_xidata_init(_PyCrossInterpreterData *data)
+{
+    // If the value is being reused
+    // then _xidata_clear() should have been called already.
+    assert(data->data == NULL);
+    assert(data->obj == NULL);
+    *data = (_PyCrossInterpreterData){0};
+    _PyCrossInterpreterData_INTERPID(data) = -1;
+}
+
+static inline void
+_xidata_clear(_PyCrossInterpreterData *data)
+{
+    // _PyCrossInterpreterData only has two members that need to be
+    // cleaned up, if set: "data" must be freed and "obj" must be decref'ed.
+    // In both cases the original (owning) interpreter must be used,
+    // which is the caller's responsibility to ensure.
+    if (data->data != NULL) {
+        if (data->free != NULL) {
+            data->free(data->data);
+        }
+        data->data = NULL;
+    }
+    Py_CLEAR(data->obj);
+}
+
+void
+_PyCrossInterpreterData_Init(_PyCrossInterpreterData *data,
+                             PyInterpreterState *interp,
+                             void *shared, PyObject *obj,
+                             xid_newobjectfunc new_object)
+{
+    assert(data != NULL);
+    assert(new_object != NULL);
+    _xidata_init(data);
+    data->data = shared;
+    if (obj != NULL) {
+        assert(interp != NULL);
+        // released in _PyCrossInterpreterData_Clear()
+        data->obj = Py_NewRef(obj);
+    }
+    // Ideally every object would know its owning interpreter.
+    // Until then, we have to rely on the caller to identify it
+    // (but we don't need it in all cases).
+    _PyCrossInterpreterData_INTERPID(data) = (interp != NULL)
+        ? PyInterpreterState_GetID(interp)
+        : -1;
+    data->new_object = new_object;
+}
+
+int
+_PyCrossInterpreterData_InitWithSize(_PyCrossInterpreterData *data,
+                                     PyInterpreterState *interp,
+                                     const size_t size, PyObject *obj,
+                                     xid_newobjectfunc new_object)
+{
+    assert(size > 0);
+    // For now we always free the shared data in the same interpreter
+    // where it was allocated, so the interpreter is required.
+    assert(interp != NULL);
+    _PyCrossInterpreterData_Init(data, interp, NULL, obj, new_object);
+    data->data = PyMem_RawMalloc(size);
+    if (data->data == NULL) {
+        return -1;
+    }
+    data->free = PyMem_RawFree;
+    return 0;
+}
+
+void
+_PyCrossInterpreterData_Clear(PyInterpreterState *interp,
+                              _PyCrossInterpreterData *data)
+{
+    assert(data != NULL);
+    // This must be called in the owning interpreter.
+    assert(interp == NULL
+           || _PyCrossInterpreterData_INTERPID(data) == -1
+           || _PyCrossInterpreterData_INTERPID(data) == PyInterpreterState_GetID(interp));
+    _xidata_clear(data);
+}
+
+
+/* using cross-interpreter data */
+
+static int
+_check_xidata(PyThreadState *tstate, _PyCrossInterpreterData *data)
+{
+    // data->data can be anything, including NULL, so we don't check it.
+
+    // data->obj may be NULL, so we don't check it.
+
+    if (_PyCrossInterpreterData_INTERPID(data) < 0) {
+        PyErr_SetString(PyExc_SystemError, "missing interp");
+        return -1;
+    }
+
+    if (data->new_object == NULL) {
+        PyErr_SetString(PyExc_SystemError, "missing new_object func");
+        return -1;
+    }
+
+    // data->free may be NULL, so we don't check it.
+
+    return 0;
+}
+
+static inline void
+_set_xid_lookup_failure(PyInterpreterState *interp,
+                        PyObject *obj, const char *msg)
+{
+    PyObject *exctype = _get_not_shareable_error_type(interp);
+    assert(exctype != NULL);
+    if (msg != NULL) {
+        assert(obj == NULL);
+        PyErr_SetString(exctype, msg);
+    }
+    else if (obj == NULL) {
+        PyErr_SetString(exctype,
+                        "object does not support cross-interpreter data");
+    }
+    else {
+        PyErr_Format(exctype,
+                     "%S does not support cross-interpreter data", obj);
+    }
+}
+
+int
+_PyObject_CheckCrossInterpreterData(PyObject *obj)
+{
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    crossinterpdatafunc getdata = lookup_getdata(interp, obj);
+    if (getdata == NULL) {
+        if (!PyErr_Occurred()) {
+            _set_xid_lookup_failure(interp, obj, NULL);
+        }
+        return -1;
+    }
+    return 0;
+}
+
+int
+_PyObject_GetCrossInterpreterData(PyObject *obj, _PyCrossInterpreterData *data)
+{
+    PyThreadState *tstate = PyThreadState_Get();
+    PyInterpreterState *interp = tstate->interp;
+
+    // Reset data before re-populating.
+    *data = (_PyCrossInterpreterData){0};
+    _PyCrossInterpreterData_INTERPID(data) = -1;
+
+    // Call the "getdata" func for the object.
+    Py_INCREF(obj);
+    crossinterpdatafunc getdata = lookup_getdata(interp, obj);
+    if (getdata == NULL) {
+        Py_DECREF(obj);
+        if (!PyErr_Occurred()) {
+            _set_xid_lookup_failure(interp, obj, NULL);
+        }
+        return -1;
+    }
+    int res = getdata(tstate, obj, data);
+    Py_DECREF(obj);
+    if (res != 0) {
+        return -1;
+    }
+
+    // Fill in the blanks and validate the result.
+    _PyCrossInterpreterData_INTERPID(data) = PyInterpreterState_GetID(interp);
+    if (_check_xidata(tstate, data) != 0) {
+        (void)_PyCrossInterpreterData_Release(data);
+        return -1;
+    }
+
+    return 0;
+}
+
+PyObject *
+_PyCrossInterpreterData_NewObject(_PyCrossInterpreterData *data)
+{
+    return data->new_object(data);
+}
+
+static int
+_call_clear_xidata(void *data)
+{
+    _xidata_clear((_PyCrossInterpreterData *)data);
+    return 0;
+}
+
+static int
+_xidata_release(_PyCrossInterpreterData *data, int rawfree)
+{
+    if ((data->data == NULL || data->free == NULL) && data->obj == NULL) {
+        // Nothing to release!
+        if (rawfree) {
+            PyMem_RawFree(data);
+        }
+        else {
+            data->data = NULL;
+        }
+        return 0;
+    }
+
+    // Switch to the original interpreter.
+    PyInterpreterState *interp = _PyInterpreterState_LookUpID(
+                                    _PyCrossInterpreterData_INTERPID(data));
+    if (interp == NULL) {
+        // The interpreter was already destroyed.
+        // This function shouldn't have been called.
+        // XXX Someone leaked some memory...
+        assert(PyErr_Occurred());
+        if (rawfree) {
+            PyMem_RawFree(data);
+        }
+        return -1;
+    }
+
+    // "Release" the data and/or the object.
+    if (rawfree) {
+        return _Py_CallInInterpreterAndRawFree(interp, _call_clear_xidata, data);
+    }
+    else {
+        return _Py_CallInInterpreter(interp, _call_clear_xidata, data);
+    }
+}
+
+int
+_PyCrossInterpreterData_Release(_PyCrossInterpreterData *data)
+{
+    return _xidata_release(data, 0);
+}
+
+int
+_PyCrossInterpreterData_ReleaseAndRawFree(_PyCrossInterpreterData *data)
+{
+    return _xidata_release(data, 1);
+}
+
+
+/*************************/
+/* convenience utilities */
+/*************************/
+
+static const char *
+_copy_string_obj_raw(PyObject *strobj, Py_ssize_t *p_size)
+{
+    Py_ssize_t size = -1;
+    const char *str = PyUnicode_AsUTF8AndSize(strobj, &size);
+    if (str == NULL) {
+        return NULL;
+    }
+
+    if (size != (Py_ssize_t)strlen(str)) {
+        PyErr_SetString(PyExc_ValueError, "found embedded NULL character");
+        return NULL;
+    }
+
+    char *copied = PyMem_RawMalloc(size+1);
+    if (copied == NULL) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+    strcpy(copied, str);
+    if (p_size != NULL) {
+        *p_size = size;
+    }
+    return copied;
+}
+
+
+static int
+_convert_exc_to_TracebackException(PyObject *exc, PyObject **p_tbexc)
+{
+    PyObject *args = NULL;
+    PyObject *kwargs = NULL;
+    PyObject *create = NULL;
+
+    // This is inspired by _PyErr_Display().
+    PyObject *tbmod = PyImport_ImportModule("traceback");
+    if (tbmod == NULL) {
+        return -1;
+    }
+    PyObject *tbexc_type = PyObject_GetAttrString(tbmod, "TracebackException");
+    Py_DECREF(tbmod);
+    if (tbexc_type == NULL) {
+        return -1;
+    }
+    create = PyObject_GetAttrString(tbexc_type, "from_exception");
+    Py_DECREF(tbexc_type);
+    if (create == NULL) {
+        return -1;
+    }
+
+    args = PyTuple_Pack(1, exc);
+    if (args == NULL) {
+        goto error;
+    }
+
+    kwargs = PyDict_New();
+    if (kwargs == NULL) {
+        goto error;
+    }
+    if (PyDict_SetItemString(kwargs, "save_exc_type", Py_False) < 0) {
+        goto error;
+    }
+    if (PyDict_SetItemString(kwargs, "lookup_lines", Py_False) < 0) {
+        goto error;
+    }
+
+    PyObject *tbexc = PyObject_Call(create, args, kwargs);
+    Py_DECREF(args);
+    Py_DECREF(kwargs);
+    Py_DECREF(create);
+    if (tbexc == NULL) {
+        goto error;
+    }
+
+    *p_tbexc = tbexc;
+    return 0;
+
+error:
+    Py_XDECREF(args);
+    Py_XDECREF(kwargs);
+    Py_XDECREF(create);
+    return -1;
+}
+
+// We accommodate backports here.
+#ifndef _Py_EMPTY_STR
+# define _Py_EMPTY_STR &_Py_STR(empty)
+#endif
+
+static const char *
+_format_TracebackException(PyObject *tbexc)
+{
+    PyObject *lines = PyObject_CallMethod(tbexc, "format", NULL);
+    if (lines == NULL) {
+        return NULL;
+    }
+    assert(_Py_EMPTY_STR != NULL);
+    PyObject *formatted_obj = PyUnicode_Join(_Py_EMPTY_STR, lines);
+    Py_DECREF(lines);
+    if (formatted_obj == NULL) {
+        return NULL;
+    }
+
+    Py_ssize_t size = -1;
+    const char *formatted = _copy_string_obj_raw(formatted_obj, &size);
+    Py_DECREF(formatted_obj);
+    // We remove trailing the newline added by TracebackException.format().
+    assert(formatted[size-1] == '\n');
+    ((char *)formatted)[size-1] = '\0';
+    return formatted;
+}
+
+
+static int
+_release_xid_data(_PyCrossInterpreterData *data, int rawfree)
+{
+    PyObject *exc = PyErr_GetRaisedException();
+    int res = rawfree
+        ? _PyCrossInterpreterData_ReleaseAndRawFree(data)
+        : _PyCrossInterpreterData_Release(data);
+    if (res < 0) {
+        /* The owning interpreter is already destroyed. */
+        _PyCrossInterpreterData_Clear(NULL, data);
+        // XXX Emit a warning?
+        PyErr_Clear();
+    }
+    PyErr_SetRaisedException(exc);
+    return res;
+}
+
+
+/***********************/
+/* exception snapshots */
+/***********************/
+
+static int
+_excinfo_init_type_from_exception(struct _excinfo_type *info, PyObject *exc)
+{
+    /* Note that this copies directly rather than into an intermediate
+       struct and does not clear on error.  If we need that then we
+       should have a separate function to wrap this one
+       and do all that there. */
+    PyObject *strobj = NULL;
+
+    PyTypeObject *type = Py_TYPE(exc);
+    if (type->tp_flags & _Py_TPFLAGS_STATIC_BUILTIN) {
+        assert(_Py_IsImmortal((PyObject *)type));
+        info->builtin = type;
+    }
+    else {
+        // Only builtin types are preserved.
+        info->builtin = NULL;
+    }
+
+    // __name__
+    strobj = PyType_GetName(type);
+    if (strobj == NULL) {
+        return -1;
+    }
+    info->name = _copy_string_obj_raw(strobj, NULL);
+    Py_DECREF(strobj);
+    if (info->name == NULL) {
+        return -1;
+    }
+
+    // __qualname__
+    strobj = PyType_GetQualName(type);
+    if (strobj == NULL) {
+        return -1;
+    }
+    info->qualname = _copy_string_obj_raw(strobj, NULL);
+    Py_DECREF(strobj);
+    if (info->qualname == NULL) {
+        return -1;
+    }
+
+    // __module__
+    strobj = PyType_GetModuleName(type);
+    if (strobj == NULL) {
+        return -1;
+    }
+    info->module = _copy_string_obj_raw(strobj, NULL);
+    Py_DECREF(strobj);
+    if (info->module == NULL) {
+        return -1;
+    }
+
+    return 0;
+}
+
+static int
+_excinfo_init_type_from_object(struct _excinfo_type *info, PyObject *exctype)
+{
+    PyObject *strobj = NULL;
+
+    // __name__
+    strobj = PyObject_GetAttrString(exctype, "__name__");
+    if (strobj == NULL) {
+        return -1;
+    }
+    info->name = _copy_string_obj_raw(strobj, NULL);
+    Py_DECREF(strobj);
+    if (info->name == NULL) {
+        return -1;
+    }
+
+    // __qualname__
+    strobj = PyObject_GetAttrString(exctype, "__qualname__");
+    if (strobj == NULL) {
+        return -1;
+    }
+    info->qualname = _copy_string_obj_raw(strobj, NULL);
+    Py_DECREF(strobj);
+    if (info->qualname == NULL) {
+        return -1;
+    }
+
+    // __module__
+    strobj = PyObject_GetAttrString(exctype, "__module__");
+    if (strobj == NULL) {
+        return -1;
+    }
+    info->module = _copy_string_obj_raw(strobj, NULL);
+    Py_DECREF(strobj);
+    if (info->module == NULL) {
+        return -1;
+    }
+
+    return 0;
+}
+
+static void
+_excinfo_clear_type(struct _excinfo_type *info)
+{
+    if (info->builtin != NULL) {
+        assert(info->builtin->tp_flags & _Py_TPFLAGS_STATIC_BUILTIN);
+        assert(_Py_IsImmortal((PyObject *)info->builtin));
+    }
+    if (info->name != NULL) {
+        PyMem_RawFree((void *)info->name);
+    }
+    if (info->qualname != NULL) {
+        PyMem_RawFree((void *)info->qualname);
+    }
+    if (info->module != NULL) {
+        PyMem_RawFree((void *)info->module);
+    }
+    *info = (struct _excinfo_type){NULL};
+}
+
+static void
+_excinfo_normalize_type(struct _excinfo_type *info,
+                        const char **p_module, const char **p_qualname)
+{
+    if (info->name == NULL) {
+        assert(info->builtin == NULL);
+        assert(info->qualname == NULL);
+        assert(info->module == NULL);
+        // This is inspired by TracebackException.format_exception_only().
+        *p_module = NULL;
+        *p_qualname = NULL;
+        return;
+    }
+
+    const char *module = info->module;
+    const char *qualname = info->qualname;
+    if (qualname == NULL) {
+        qualname = info->name;
+    }
+    assert(module != NULL);
+    if (strcmp(module, "builtins") == 0) {
+        module = NULL;
+    }
+    else if (strcmp(module, "__main__") == 0) {
+        module = NULL;
+    }
+    *p_qualname = qualname;
+    *p_module = module;
+}
+
+static void
+_PyXI_excinfo_Clear(_PyXI_excinfo *info)
+{
+    _excinfo_clear_type(&info->type);
+    if (info->msg != NULL) {
+        PyMem_RawFree((void *)info->msg);
+    }
+    if (info->errdisplay != NULL) {
+        PyMem_RawFree((void *)info->errdisplay);
+    }
+    *info = (_PyXI_excinfo){{NULL}};
+}
+
+PyObject *
+_PyXI_excinfo_format(_PyXI_excinfo *info)
+{
+    const char *module, *qualname;
+    _excinfo_normalize_type(&info->type, &module, &qualname);
+    if (qualname != NULL) {
+        if (module != NULL) {
+            if (info->msg != NULL) {
+                return PyUnicode_FromFormat("%s.%s: %s",
+                                            module, qualname, info->msg);
+            }
+            else {
+                return PyUnicode_FromFormat("%s.%s", module, qualname);
+            }
+        }
+        else {
+            if (info->msg != NULL) {
+                return PyUnicode_FromFormat("%s: %s", qualname, info->msg);
+            }
+            else {
+                return PyUnicode_FromString(qualname);
+            }
+        }
+    }
+    else if (info->msg != NULL) {
+        return PyUnicode_FromString(info->msg);
+    }
+    else {
+        Py_RETURN_NONE;
+    }
+}
+
+static const char *
+_PyXI_excinfo_InitFromException(_PyXI_excinfo *info, PyObject *exc)
+{
+    assert(exc != NULL);
+
+    if (PyErr_GivenExceptionMatches(exc, PyExc_MemoryError)) {
+        _PyXI_excinfo_Clear(info);
+        return NULL;
+    }
+    const char *failure = NULL;
+
+    if (_excinfo_init_type_from_exception(&info->type, exc) < 0) {
+        failure = "error while initializing exception type snapshot";
+        goto error;
+    }
+
+    // Extract the exception message.
+    PyObject *msgobj = PyObject_Str(exc);
+    if (msgobj == NULL) {
+        failure = "error while formatting exception";
+        goto error;
+    }
+    info->msg = _copy_string_obj_raw(msgobj, NULL);
+    Py_DECREF(msgobj);
+    if (info->msg == NULL) {
+        failure = "error while copying exception message";
+        goto error;
+    }
+
+    // Pickle a traceback.TracebackException.
+    PyObject *tbexc = NULL;
+    if (_convert_exc_to_TracebackException(exc, &tbexc) < 0) {
+#ifdef Py_DEBUG
+        PyErr_FormatUnraisable("Exception ignored while creating TracebackException");
+#endif
+        PyErr_Clear();
+    }
+    else {
+        info->errdisplay = _format_TracebackException(tbexc);
+        Py_DECREF(tbexc);
+        if (info->errdisplay == NULL) {
+#ifdef Py_DEBUG
+            PyErr_FormatUnraisable("Exception ignored while formatting TracebackException");
+#endif
+            PyErr_Clear();
+        }
+    }
+
+    return NULL;
+
+error:
+    assert(failure != NULL);
+    _PyXI_excinfo_Clear(info);
+    return failure;
+}
+
+static const char *
+_PyXI_excinfo_InitFromObject(_PyXI_excinfo *info, PyObject *obj)
+{
+    const char *failure = NULL;
+
+    PyObject *exctype = PyObject_GetAttrString(obj, "type");
+    if (exctype == NULL) {
+        failure = "exception snapshot missing 'type' attribute";
+        goto error;
+    }
+    int res = _excinfo_init_type_from_object(&info->type, exctype);
+    Py_DECREF(exctype);
+    if (res < 0) {
+        failure = "error while initializing exception type snapshot";
+        goto error;
+    }
+
+    // Extract the exception message.
+    PyObject *msgobj = PyObject_GetAttrString(obj, "msg");
+    if (msgobj == NULL) {
+        failure = "exception snapshot missing 'msg' attribute";
+        goto error;
+    }
+    info->msg = _copy_string_obj_raw(msgobj, NULL);
+    Py_DECREF(msgobj);
+    if (info->msg == NULL) {
+        failure = "error while copying exception message";
+        goto error;
+    }
+
+    // Pickle a traceback.TracebackException.
+    PyObject *errdisplay = PyObject_GetAttrString(obj, "errdisplay");
+    if (errdisplay == NULL) {
+        failure = "exception snapshot missing 'errdisplay' attribute";
+        goto error;
+    }
+    info->errdisplay = _copy_string_obj_raw(errdisplay, NULL);
+    Py_DECREF(errdisplay);
+    if (info->errdisplay == NULL) {
+        failure = "error while copying exception error display";
+        goto error;
+    }
+
+    return NULL;
+
+error:
+    assert(failure != NULL);
+    _PyXI_excinfo_Clear(info);
+    return failure;
+}
+
+static void
+_PyXI_excinfo_Apply(_PyXI_excinfo *info, PyObject *exctype)
+{
+    PyObject *tbexc = NULL;
+    if (info->errdisplay != NULL) {
+        tbexc = PyUnicode_FromString(info->errdisplay);
+        if (tbexc == NULL) {
+            PyErr_Clear();
+        }
+    }
+
+    PyObject *formatted = _PyXI_excinfo_format(info);
+    PyErr_SetObject(exctype, formatted);
+    Py_DECREF(formatted);
+
+    if (tbexc != NULL) {
+        PyObject *exc = PyErr_GetRaisedException();
+        if (PyObject_SetAttrString(exc, "_errdisplay", tbexc) < 0) {
+#ifdef Py_DEBUG
+            PyErr_FormatUnraisable("Exception ignored when setting _errdisplay");
+#endif
+            PyErr_Clear();
+        }
+        Py_DECREF(tbexc);
+        PyErr_SetRaisedException(exc);
+    }
+}
+
+static PyObject *
+_PyXI_excinfo_TypeAsObject(_PyXI_excinfo *info)
+{
+    PyObject *ns = _PyNamespace_New(NULL);
+    if (ns == NULL) {
+        return NULL;
+    }
+    int empty = 1;
+
+    if (info->type.name != NULL) {
+        PyObject *name = PyUnicode_FromString(info->type.name);
+        if (name == NULL) {
+            goto error;
+        }
+        int res = PyObject_SetAttrString(ns, "__name__", name);
+        Py_DECREF(name);
+        if (res < 0) {
+            goto error;
+        }
+        empty = 0;
+    }
+
+    if (info->type.qualname != NULL) {
+        PyObject *qualname = PyUnicode_FromString(info->type.qualname);
+        if (qualname == NULL) {
+            goto error;
+        }
+        int res = PyObject_SetAttrString(ns, "__qualname__", qualname);
+        Py_DECREF(qualname);
+        if (res < 0) {
+            goto error;
+        }
+        empty = 0;
+    }
+
+    if (info->type.module != NULL) {
+        PyObject *module = PyUnicode_FromString(info->type.module);
+        if (module == NULL) {
+            goto error;
+        }
+        int res = PyObject_SetAttrString(ns, "__module__", module);
+        Py_DECREF(module);
+        if (res < 0) {
+            goto error;
+        }
+        empty = 0;
+    }
+
+    if (empty) {
+        Py_CLEAR(ns);
+    }
+
+    return ns;
+
+error:
+    Py_DECREF(ns);
+    return NULL;
+}
+
+static PyObject *
+_PyXI_excinfo_AsObject(_PyXI_excinfo *info)
+{
+    PyObject *ns = _PyNamespace_New(NULL);
+    if (ns == NULL) {
+        return NULL;
+    }
+    int res;
+
+    PyObject *type = _PyXI_excinfo_TypeAsObject(info);
+    if (type == NULL) {
+        if (PyErr_Occurred()) {
+            goto error;
+        }
+        type = Py_NewRef(Py_None);
+    }
+    res = PyObject_SetAttrString(ns, "type", type);
+    Py_DECREF(type);
+    if (res < 0) {
+        goto error;
+    }
+
+    PyObject *msg = info->msg != NULL
+        ? PyUnicode_FromString(info->msg)
+        : Py_NewRef(Py_None);
+    if (msg == NULL) {
+        goto error;
+    }
+    res = PyObject_SetAttrString(ns, "msg", msg);
+    Py_DECREF(msg);
+    if (res < 0) {
+        goto error;
+    }
+
+    PyObject *formatted = _PyXI_excinfo_format(info);
+    if (formatted == NULL) {
+        goto error;
+    }
+    res = PyObject_SetAttrString(ns, "formatted", formatted);
+    Py_DECREF(formatted);
+    if (res < 0) {
+        goto error;
+    }
+
+    if (info->errdisplay != NULL) {
+        PyObject *tbexc = PyUnicode_FromString(info->errdisplay);
+        if (tbexc == NULL) {
+            PyErr_Clear();
+        }
+        else {
+            res = PyObject_SetAttrString(ns, "errdisplay", tbexc);
+            Py_DECREF(tbexc);
+            if (res < 0) {
+                goto error;
+            }
+        }
+    }
+
+    return ns;
+
+error:
+    Py_DECREF(ns);
+    return NULL;
+}
+
+
+int
+_PyXI_InitExcInfo(_PyXI_excinfo *info, PyObject *exc)
+{
+    assert(!PyErr_Occurred());
+    if (exc == NULL || exc == Py_None) {
+        PyErr_SetString(PyExc_ValueError, "missing exc");
+        return -1;
+    }
+    const char *failure;
+    if (PyExceptionInstance_Check(exc) || PyExceptionClass_Check(exc)) {
+        failure = _PyXI_excinfo_InitFromException(info, exc);
+    }
+    else {
+        failure = _PyXI_excinfo_InitFromObject(info, exc);
+    }
+    if (failure != NULL) {
+        PyErr_SetString(PyExc_Exception, failure);
+        return -1;
+    }
+    return 0;
+}
+
+PyObject *
+_PyXI_FormatExcInfo(_PyXI_excinfo *info)
+{
+    return _PyXI_excinfo_format(info);
+}
+
+PyObject *
+_PyXI_ExcInfoAsObject(_PyXI_excinfo *info)
+{
+    return _PyXI_excinfo_AsObject(info);
+}
+
+void
+_PyXI_ClearExcInfo(_PyXI_excinfo *info)
+{
+    _PyXI_excinfo_Clear(info);
+}
+
+
+/***************************/
+/* short-term data sharing */
+/***************************/
+
+/* error codes */
+
+static int
+_PyXI_ApplyErrorCode(_PyXI_errcode code, PyInterpreterState *interp)
+{
+    assert(!PyErr_Occurred());
+    switch (code) {
+    case _PyXI_ERR_NO_ERROR:  // fall through
+    case _PyXI_ERR_UNCAUGHT_EXCEPTION:
+        // There is nothing to apply.
+#ifdef Py_DEBUG
+        Py_UNREACHABLE();
+#endif
+        return 0;
+    case _PyXI_ERR_OTHER:
+        // XXX msg?
+        PyErr_SetNone(PyExc_InterpreterError);
+        break;
+    case _PyXI_ERR_NO_MEMORY:
+        PyErr_NoMemory();
+        break;
+    case _PyXI_ERR_ALREADY_RUNNING:
+        assert(interp != NULL);
+        // In 3.14+ we use _PyErr_SetInterpreterAlreadyRunning().
+        PyErr_SetString(PyExc_InterpreterError, "interpreter already running");
+        break;
+    case _PyXI_ERR_MAIN_NS_FAILURE:
+        PyErr_SetString(PyExc_InterpreterError,
+                        "failed to get __main__ namespace");
+        break;
+    case _PyXI_ERR_APPLY_NS_FAILURE:
+        PyErr_SetString(PyExc_InterpreterError,
+                        "failed to apply namespace to __main__");
+        break;
+    case _PyXI_ERR_NOT_SHAREABLE:
+        _set_xid_lookup_failure(interp, NULL, NULL);
+        break;
+    default:
+#ifdef Py_DEBUG
+        Py_UNREACHABLE();
+#else
+        PyErr_Format(PyExc_RuntimeError, "unsupported error code %d", code);
+#endif
+    }
+    assert(PyErr_Occurred());
+    return -1;
+}
+
+/* shared exceptions */
+
+static const char *
+_PyXI_InitError(_PyXI_error *error, PyObject *excobj, _PyXI_errcode code)
+{
+    if (error->interp == NULL) {
+        error->interp = PyInterpreterState_Get();
+    }
+
+    const char *failure = NULL;
+    if (code == _PyXI_ERR_UNCAUGHT_EXCEPTION) {
+        // There is an unhandled exception we need to propagate.
+        failure = _PyXI_excinfo_InitFromException(&error->uncaught, excobj);
+        if (failure != NULL) {
+            // We failed to initialize error->uncaught.
+            // XXX Print the excobj/traceback?  Emit a warning?
+            // XXX Print the current exception/traceback?
+            if (PyErr_ExceptionMatches(PyExc_MemoryError)) {
+                error->code = _PyXI_ERR_NO_MEMORY;
+            }
+            else {
+                error->code = _PyXI_ERR_OTHER;
+            }
+            PyErr_Clear();
+        }
+        else {
+            error->code = code;
+        }
+        assert(error->code != _PyXI_ERR_NO_ERROR);
+    }
+    else {
+        // There is an error code we need to propagate.
+        assert(excobj == NULL);
+        assert(code != _PyXI_ERR_NO_ERROR);
+        error->code = code;
+        _PyXI_excinfo_Clear(&error->uncaught);
+    }
+    return failure;
+}
+
+PyObject *
+_PyXI_ApplyError(_PyXI_error *error)
+{
+    if (error->code == _PyXI_ERR_UNCAUGHT_EXCEPTION) {
+        // Raise an exception that proxies the propagated exception.
+       return _PyXI_excinfo_AsObject(&error->uncaught);
+    }
+    else if (error->code == _PyXI_ERR_NOT_SHAREABLE) {
+        // Propagate the exception directly.
+        _set_xid_lookup_failure(error->interp, NULL, error->uncaught.msg);
+    }
+    else {
+        // Raise an exception corresponding to the code.
+        assert(error->code != _PyXI_ERR_NO_ERROR);
+        (void)_PyXI_ApplyErrorCode(error->code, error->interp);
+        if (error->uncaught.type.name != NULL || error->uncaught.msg != NULL) {
+            // __context__ will be set to a proxy of the propagated exception.
+            PyObject *exc = PyErr_GetRaisedException();
+            _PyXI_excinfo_Apply(&error->uncaught, PyExc_InterpreterError);
+            PyObject *exc2 = PyErr_GetRaisedException();
+            PyException_SetContext(exc, exc2);
+            PyErr_SetRaisedException(exc);
+        }
+    }
+    assert(PyErr_Occurred());
+    return NULL;
+}
+
+/* shared namespaces */
+
+/* Shared namespaces are expected to have relatively short lifetimes.
+   This means dealloc of a shared namespace will normally happen "soon".
+   Namespace items hold cross-interpreter data, which must get released.
+   If the namespace/items are cleared in a different interpreter than
+   where the items' cross-interpreter data was set then that will cause
+   pending calls to be used to release the cross-interpreter data.
+   The tricky bit is that the pending calls can happen sufficiently
+   later that the namespace/items might already be deallocated.  This is
+   a problem if the cross-interpreter data is allocated as part of a
+   namespace item.  If that's the case then we must ensure the shared
+   namespace is only cleared/freed *after* that data has been released. */
+
+typedef struct _sharednsitem {
+    const char *name;
+    _PyCrossInterpreterData *data;
+    // We could have a "PyCrossInterpreterData _data" field, so it would
+    // be allocated as part of the item and avoid an extra allocation.
+    // However, doing so adds a bunch of complexity because we must
+    // ensure the item isn't freed before a pending call might happen
+    // in a different interpreter to release the XI data.
+} _PyXI_namespace_item;
+
+static int
+_sharednsitem_is_initialized(_PyXI_namespace_item *item)
+{
+    if (item->name != NULL) {
+        return 1;
+    }
+    return 0;
+}
+
+static int
+_sharednsitem_init(_PyXI_namespace_item *item, PyObject *key)
+{
+    item->name = _copy_string_obj_raw(key, NULL);
+    if (item->name == NULL) {
+        assert(!_sharednsitem_is_initialized(item));
+        return -1;
+    }
+    item->data = NULL;
+    assert(_sharednsitem_is_initialized(item));
+    return 0;
+}
+
+static int
+_sharednsitem_has_value(_PyXI_namespace_item *item, int64_t *p_interpid)
+{
+    if (item->data == NULL) {
+        return 0;
+    }
+    if (p_interpid != NULL) {
+        *p_interpid = _PyCrossInterpreterData_INTERPID(item->data);
+    }
+    return 1;
+}
+
+static int
+_sharednsitem_set_value(_PyXI_namespace_item *item, PyObject *value)
+{
+    assert(_sharednsitem_is_initialized(item));
+    assert(item->data == NULL);
+    item->data = PyMem_RawMalloc(sizeof(_PyCrossInterpreterData));
+    if (item->data == NULL) {
+        PyErr_NoMemory();
+        return -1;
+    }
+    if (_PyObject_GetCrossInterpreterData(value, item->data) != 0) {
+        PyMem_RawFree(item->data);
+        item->data = NULL;
+        // The caller may want to propagate PyExc_NotShareableError
+        // if currently switched between interpreters.
+        return -1;
+    }
+    return 0;
+}
+
+static void
+_sharednsitem_clear_value(_PyXI_namespace_item *item)
+{
+    _PyCrossInterpreterData *data = item->data;
+    if (data != NULL) {
+        item->data = NULL;
+        int rawfree = 1;
+        (void)_release_xid_data(data, rawfree);
+    }
+}
+
+static void
+_sharednsitem_clear(_PyXI_namespace_item *item)
+{
+    if (item->name != NULL) {
+        PyMem_RawFree((void *)item->name);
+        item->name = NULL;
+    }
+    _sharednsitem_clear_value(item);
+}
+
+static int
+_sharednsitem_copy_from_ns(struct _sharednsitem *item, PyObject *ns)
+{
+    assert(item->name != NULL);
+    assert(item->data == NULL);
+    PyObject *value = PyDict_GetItemString(ns, item->name);  // borrowed
+    if (value == NULL) {
+        if (PyErr_Occurred()) {
+            return -1;
+        }
+        // When applied, this item will be set to the default (or fail).
+        return 0;
+    }
+    if (_sharednsitem_set_value(item, value) < 0) {
+        return -1;
+    }
+    return 0;
+}
+
+static int
+_sharednsitem_apply(_PyXI_namespace_item *item, PyObject *ns, PyObject *dflt)
+{
+    PyObject *name = PyUnicode_FromString(item->name);
+    if (name == NULL) {
+        return -1;
+    }
+    PyObject *value;
+    if (item->data != NULL) {
+        value = _PyCrossInterpreterData_NewObject(item->data);
+        if (value == NULL) {
+            Py_DECREF(name);
+            return -1;
+        }
+    }
+    else {
+        value = Py_NewRef(dflt);
+    }
+    int res = PyDict_SetItem(ns, name, value);
+    Py_DECREF(name);
+    Py_DECREF(value);
+    return res;
+}
+
+struct _sharedns {
+    Py_ssize_t len;
+    _PyXI_namespace_item *items;
+};
+
+static _PyXI_namespace *
+_sharedns_new(void)
+{
+    _PyXI_namespace *ns = PyMem_RawCalloc(sizeof(_PyXI_namespace), 1);
+    if (ns == NULL) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+    *ns = (_PyXI_namespace){ 0 };
+    return ns;
+}
+
+static int
+_sharedns_is_initialized(_PyXI_namespace *ns)
+{
+    if (ns->len == 0) {
+        assert(ns->items == NULL);
+        return 0;
+    }
+
+    assert(ns->len > 0);
+    assert(ns->items != NULL);
+    assert(_sharednsitem_is_initialized(&ns->items[0]));
+    assert(ns->len == 1
+           || _sharednsitem_is_initialized(&ns->items[ns->len - 1]));
+    return 1;
+}
+
+#define HAS_COMPLETE_DATA 1
+#define HAS_PARTIAL_DATA 2
+
+static int
+_sharedns_has_xidata(_PyXI_namespace *ns, int64_t *p_interpid)
+{
+    // We expect _PyXI_namespace to always be initialized.
+    assert(_sharedns_is_initialized(ns));
+    int res = 0;
+    _PyXI_namespace_item *item0 = &ns->items[0];
+    if (!_sharednsitem_is_initialized(item0)) {
+        return 0;
+    }
+    int64_t interpid0 = -1;
+    if (!_sharednsitem_has_value(item0, &interpid0)) {
+        return 0;
+    }
+    if (ns->len > 1) {
+        // At this point we know it is has at least partial data.
+        _PyXI_namespace_item *itemN = &ns->items[ns->len-1];
+        if (!_sharednsitem_is_initialized(itemN)) {
+            res = HAS_PARTIAL_DATA;
+            goto finally;
+        }
+        int64_t interpidN = -1;
+        if (!_sharednsitem_has_value(itemN, &interpidN)) {
+            res = HAS_PARTIAL_DATA;
+            goto finally;
+        }
+        assert(interpidN == interpid0);
+    }
+    res = HAS_COMPLETE_DATA;
+    *p_interpid = interpid0;
+
+finally:
+    return res;
+}
+
+static void
+_sharedns_clear(_PyXI_namespace *ns)
+{
+    if (!_sharedns_is_initialized(ns)) {
+        return;
+    }
+
+    // If the cross-interpreter data were allocated as part of
+    // _PyXI_namespace_item (instead of dynamically), this is where
+    // we would need verify that we are clearing the items in the
+    // correct interpreter, to avoid a race with releasing the XI data
+    // via a pending call.  See _sharedns_has_xidata().
+    for (Py_ssize_t i=0; i < ns->len; i++) {
+        _sharednsitem_clear(&ns->items[i]);
+    }
+    PyMem_RawFree(ns->items);
+    ns->items = NULL;
+    ns->len = 0;
+}
+
+static void
+_sharedns_free(_PyXI_namespace *ns)
+{
+    _sharedns_clear(ns);
+    PyMem_RawFree(ns);
+}
+
+static int
+_sharedns_init(_PyXI_namespace *ns, PyObject *names)
+{
+    assert(!_sharedns_is_initialized(ns));
+    assert(names != NULL);
+    Py_ssize_t len = PyDict_CheckExact(names)
+        ? PyDict_Size(names)
+        : PySequence_Size(names);
+    if (len < 0) {
+        return -1;
+    }
+    if (len == 0) {
+        PyErr_SetString(PyExc_ValueError, "empty namespaces not allowed");
+        return -1;
+    }
+    assert(len > 0);
+
+    // Allocate the items.
+    _PyXI_namespace_item *items =
+            PyMem_RawCalloc(sizeof(struct _sharednsitem), len);
+    if (items == NULL) {
+        PyErr_NoMemory();
+        return -1;
+    }
+
+    // Fill in the names.
+    Py_ssize_t i = -1;
+    if (PyDict_CheckExact(names)) {
+        Py_ssize_t pos = 0;
+        for (i=0; i < len; i++) {
+            PyObject *key;
+            if (!PyDict_Next(names, &pos, &key, NULL)) {
+                // This should not be possible.
+                assert(0);
+                goto error;
+            }
+            if (_sharednsitem_init(&items[i], key) < 0) {
+                goto error;
+            }
+        }
+    }
+    else if (PySequence_Check(names)) {
+        for (i=0; i < len; i++) {
+            PyObject *key = PySequence_GetItem(names, i);
+            if (key == NULL) {
+                goto error;
+            }
+            int res = _sharednsitem_init(&items[i], key);
+            Py_DECREF(key);
+            if (res < 0) {
+                goto error;
+            }
+        }
+    }
+    else {
+        PyErr_SetString(PyExc_NotImplementedError,
+                        "non-sequence namespace not supported");
+        goto error;
+    }
+
+    ns->items = items;
+    ns->len = len;
+    assert(_sharedns_is_initialized(ns));
+    return 0;
+
+error:
+    for (Py_ssize_t j=0; j < i; j++) {
+        _sharednsitem_clear(&items[j]);
+    }
+    PyMem_RawFree(items);
+    assert(!_sharedns_is_initialized(ns));
+    return -1;
+}
+
+void
+_PyXI_FreeNamespace(_PyXI_namespace *ns)
+{
+    if (!_sharedns_is_initialized(ns)) {
+        return;
+    }
+
+    int64_t interpid = -1;
+    if (!_sharedns_has_xidata(ns, &interpid)) {
+        _sharedns_free(ns);
+        return;
+    }
+
+    if (interpid == PyInterpreterState_GetID(PyInterpreterState_Get())) {
+        _sharedns_free(ns);
+    }
+    else {
+        // If we weren't always dynamically allocating the cross-interpreter
+        // data in each item then we would need to using a pending call
+        // to call _sharedns_free(), to avoid the race between freeing
+        // the shared namespace and releasing the XI data.
+        _sharedns_free(ns);
+    }
+}
+
+_PyXI_namespace *
+_PyXI_NamespaceFromNames(PyObject *names)
+{
+    if (names == NULL || names == Py_None) {
+        return NULL;
+    }
+
+    _PyXI_namespace *ns = _sharedns_new();
+    if (ns == NULL) {
+        return NULL;
+    }
+
+    if (_sharedns_init(ns, names) < 0) {
+        PyMem_RawFree(ns);
+        if (PySequence_Size(names) == 0) {
+            PyErr_Clear();
+        }
+        return NULL;
+    }
+
+    return ns;
+}
+
+#ifndef NDEBUG
+static int _session_is_active(_PyXI_session *);
+#endif
+static void _propagate_not_shareable_error(_PyXI_session *);
+
+int
+_PyXI_FillNamespaceFromDict(_PyXI_namespace *ns, PyObject *nsobj,
+                            _PyXI_session *session)
+{
+    // session must be entered already, if provided.
+    assert(session == NULL || _session_is_active(session));
+    assert(_sharedns_is_initialized(ns));
+    for (Py_ssize_t i=0; i < ns->len; i++) {
+        _PyXI_namespace_item *item = &ns->items[i];
+        if (_sharednsitem_copy_from_ns(item, nsobj) < 0) {
+            _propagate_not_shareable_error(session);
+            // Clear out the ones we set so far.
+            for (Py_ssize_t j=0; j < i; j++) {
+                _sharednsitem_clear_value(&ns->items[j]);
+            }
+            return -1;
+        }
+    }
+    return 0;
+}
+
+// All items are expected to be shareable.
+static _PyXI_namespace *
+_PyXI_NamespaceFromDict(PyObject *nsobj, _PyXI_session *session)
+{
+    // session must be entered already, if provided.
+    assert(session == NULL || _session_is_active(session));
+    if (nsobj == NULL || nsobj == Py_None) {
+        return NULL;
+    }
+    if (!PyDict_CheckExact(nsobj)) {
+        PyErr_SetString(PyExc_TypeError, "expected a dict");
+        return NULL;
+    }
+
+    _PyXI_namespace *ns = _sharedns_new();
+    if (ns == NULL) {
+        return NULL;
+    }
+
+    if (_sharedns_init(ns, nsobj) < 0) {
+        if (PyDict_Size(nsobj) == 0) {
+            PyMem_RawFree(ns);
+            PyErr_Clear();
+            return NULL;
+        }
+        goto error;
+    }
+
+    if (_PyXI_FillNamespaceFromDict(ns, nsobj, session) < 0) {
+        goto error;
+    }
+
+    return ns;
+
+error:
+    assert(PyErr_Occurred()
+           || (session != NULL && session->error_override != NULL));
+    _sharedns_free(ns);
+    return NULL;
+}
+
+int
+_PyXI_ApplyNamespace(_PyXI_namespace *ns, PyObject *nsobj, PyObject *dflt)
+{
+    for (Py_ssize_t i=0; i < ns->len; i++) {
+        if (_sharednsitem_apply(&ns->items[i], nsobj, dflt) != 0) {
+            return -1;
+        }
+    }
+    return 0;
+}
+
+
+/**********************/
+/* high-level helpers */
+/**********************/
+
+/* enter/exit a cross-interpreter session */
+
+static void
+_enter_session(_PyXI_session *session, PyInterpreterState *interp)
+{
+    // Set here and cleared in _exit_session().
+    assert(!session->own_init_tstate);
+    assert(session->init_tstate == NULL);
+    assert(session->prev_tstate == NULL);
+    // Set elsewhere and cleared in _exit_session().
+    assert(!session->running);
+    assert(session->main_ns == NULL);
+    // Set elsewhere and cleared in _capture_current_exception().
+    assert(session->error_override == NULL);
+    // Set elsewhere and cleared in _PyXI_ApplyCapturedException().
+    assert(session->error == NULL);
+
+    // Switch to interpreter.
+    PyThreadState *tstate = PyThreadState_Get();
+    PyThreadState *prev = tstate;
+    if (interp != tstate->interp) {
+        tstate = _PyThreadState_NewBound(interp, _PyThreadState_WHENCE_EXEC);
+        // XXX Possible GILState issues?
+        session->prev_tstate = PyThreadState_Swap(tstate);
+        assert(session->prev_tstate == prev);
+        session->own_init_tstate = 1;
+    }
+    session->init_tstate = tstate;
+    session->prev_tstate = prev;
+}
+
+static void
+_exit_session(_PyXI_session *session)
+{
+    PyThreadState *tstate = session->init_tstate;
+    assert(tstate != NULL);
+    assert(PyThreadState_Get() == tstate);
+
+    // Release any of the entered interpreters resources.
+    if (session->main_ns != NULL) {
+        Py_CLEAR(session->main_ns);
+    }
+
+    // Ensure this thread no longer owns __main__.
+    if (session->running) {
+        _PyInterpreterState_SetNotRunningMain(tstate->interp);
+        assert(!PyErr_Occurred());
+        session->running = 0;
+    }
+
+    // Switch back.
+    assert(session->prev_tstate != NULL);
+    if (session->prev_tstate != session->init_tstate) {
+        assert(session->own_init_tstate);
+        session->own_init_tstate = 0;
+        PyThreadState_Clear(tstate);
+        PyThreadState_Swap(session->prev_tstate);
+        PyThreadState_Delete(tstate);
+    }
+    else {
+        assert(!session->own_init_tstate);
+    }
+    session->prev_tstate = NULL;
+    session->init_tstate = NULL;
+}
+
+#ifndef NDEBUG
+static int
+_session_is_active(_PyXI_session *session)
+{
+    return (session->init_tstate != NULL);
+}
+#endif
+
+static void
+_propagate_not_shareable_error(_PyXI_session *session)
+{
+    if (session == NULL) {
+        return;
+    }
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    if (PyErr_ExceptionMatches(_get_not_shareable_error_type(interp))) {
+        // We want to propagate the exception directly.
+        session->_error_override = _PyXI_ERR_NOT_SHAREABLE;
+        session->error_override = &session->_error_override;
+    }
+}
+
+static void
+_capture_current_exception(_PyXI_session *session)
+{
+    assert(session->error == NULL);
+    if (!PyErr_Occurred()) {
+        assert(session->error_override == NULL);
+        return;
+    }
+
+    // Handle the exception override.
+    _PyXI_errcode *override = session->error_override;
+    session->error_override = NULL;
+    _PyXI_errcode errcode = override != NULL
+        ? *override
+        : _PyXI_ERR_UNCAUGHT_EXCEPTION;
+
+    // Pop the exception object.
+    PyObject *excval = NULL;
+    if (errcode == _PyXI_ERR_UNCAUGHT_EXCEPTION) {
+        // We want to actually capture the current exception.
+        excval = PyErr_GetRaisedException();
+    }
+    else if (errcode == _PyXI_ERR_ALREADY_RUNNING) {
+        // We don't need the exception info.
+        PyErr_Clear();
+    }
+    else {
+        // We could do a variety of things here, depending on errcode.
+        // However, for now we simply capture the exception and save
+        // the errcode.
+        excval = PyErr_GetRaisedException();
+    }
+
+    // Capture the exception.
+    _PyXI_error *err = &session->_error;
+    *err = (_PyXI_error){
+        .interp = session->init_tstate->interp,
+    };
+    const char *failure;
+    if (excval == NULL) {
+        failure = _PyXI_InitError(err, NULL, errcode);
+    }
+    else {
+        failure = _PyXI_InitError(err, excval, _PyXI_ERR_UNCAUGHT_EXCEPTION);
+        Py_DECREF(excval);
+        if (failure == NULL && override != NULL) {
+            err->code = errcode;
+        }
+    }
+
+    // Handle capture failure.
+    if (failure != NULL) {
+        // XXX Make this error message more generic.
+        fprintf(stderr,
+                "RunFailedError: script raised an uncaught exception (%s)",
+                failure);
+        err = NULL;
+    }
+
+    // Finished!
+    assert(!PyErr_Occurred());
+    session->error  = err;
+}
+
+PyObject *
+_PyXI_ApplyCapturedException(_PyXI_session *session)
+{
+    assert(!PyErr_Occurred());
+    assert(session->error != NULL);
+    PyObject *res = _PyXI_ApplyError(session->error);
+    assert((res == NULL) != (PyErr_Occurred() == NULL));
+    session->error = NULL;
+    return res;
+}
+
+int
+_PyXI_HasCapturedException(_PyXI_session *session)
+{
+    return session->error != NULL;
+}
+
+int
+_PyXI_Enter(_PyXI_session *session,
+            PyInterpreterState *interp, PyObject *nsupdates)
+{
+    // Convert the attrs for cross-interpreter use.
+    _PyXI_namespace *sharedns = NULL;
+    if (nsupdates != NULL) {
+        assert(PyDict_Check(nsupdates));
+        sharedns = _PyXI_NamespaceFromDict(nsupdates, NULL);
+        if (sharedns == NULL && PyErr_Occurred()) {
+            assert(session->error == NULL);
+            return -1;
+        }
+    }
+
+    // Switch to the requested interpreter (if necessary).
+    _enter_session(session, interp);
+    _PyXI_errcode errcode = _PyXI_ERR_UNCAUGHT_EXCEPTION;
+
+    // Ensure this thread owns __main__.
+    if (_PyInterpreterState_SetRunningMain(interp) < 0) {
+        // In the case where we didn't switch interpreters, it would
+        // be more efficient to leave the exception in place and return
+        // immediately.  However, life is simpler if we don't.
+        errcode = _PyXI_ERR_ALREADY_RUNNING;
+        goto error;
+    }
+    session->running = 1;
+
+    // Cache __main__.__dict__.
+    PyObject *main_mod = PyUnstable_InterpreterState_GetMainModule(interp);
+    if (main_mod == NULL) {
+        errcode = _PyXI_ERR_MAIN_NS_FAILURE;
+        goto error;
+    }
+    PyObject *ns = PyModule_GetDict(main_mod);  // borrowed
+    Py_DECREF(main_mod);
+    if (ns == NULL) {
+        errcode = _PyXI_ERR_MAIN_NS_FAILURE;
+        goto error;
+    }
+    session->main_ns = Py_NewRef(ns);
+
+    // Apply the cross-interpreter data.
+    if (sharedns != NULL) {
+        if (_PyXI_ApplyNamespace(sharedns, ns, NULL) < 0) {
+            errcode = _PyXI_ERR_APPLY_NS_FAILURE;
+            goto error;
+        }
+        _PyXI_FreeNamespace(sharedns);
+    }
+
+    errcode = _PyXI_ERR_NO_ERROR;
+    assert(!PyErr_Occurred());
+    return 0;
+
+error:
+    assert(PyErr_Occurred());
+    // We want to propagate all exceptions here directly (best effort).
+    assert(errcode != _PyXI_ERR_UNCAUGHT_EXCEPTION);
+    session->error_override = &errcode;
+    _capture_current_exception(session);
+    _exit_session(session);
+    if (sharedns != NULL) {
+        _PyXI_FreeNamespace(sharedns);
+    }
+    return -1;
+}
+
+void
+_PyXI_Exit(_PyXI_session *session)
+{
+    _capture_current_exception(session);
+    _exit_session(session);
+}
+
+
+/*********************/
+/* runtime lifecycle */
+/*********************/
+
+PyStatus
+_PyXI_Init(PyInterpreterState *interp)
+{
+    // Initialize the XID lookup state (e.g. registry).
+    xid_lookup_init(interp);
+
+    // Initialize exceptions (heap types).
+    if (_init_not_shareable_error_type(interp) < 0) {
+        return _PyStatus_ERR("failed to initialize NotShareableError");
+    }
+
+    return _PyStatus_OK();
+}
+
+// _PyXI_Fini() must be called before the interpreter is cleared,
+// since we must clear some heap objects.
+
+void
+_PyXI_Fini(PyInterpreterState *interp)
+{
+    // Finalize exceptions (heap types).
+    _fini_not_shareable_error_type(interp);
+
+    // Finalize the XID lookup state (e.g. registry).
+    xid_lookup_fini(interp);
+}
+
+PyStatus
+_PyXI_InitTypes(PyInterpreterState *interp)
+{
+    if (init_exceptions(interp) < 0) {
+        PyErr_PrintEx(0);
+        return _PyStatus_ERR("failed to initialize an exception type");
+    }
+    return _PyStatus_OK();
+}
+
+void
+_PyXI_FiniTypes(PyInterpreterState *interp)
+{
+    fini_exceptions(interp);
+}
+
+
+/*************/
+/* other API */
+/*************/
+
+PyInterpreterState *
+_PyXI_NewInterpreter(PyInterpreterConfig *config, long *maybe_whence,
+                     PyThreadState **p_tstate, PyThreadState **p_save_tstate)
+{
+    PyThreadState *save_tstate = PyThreadState_Swap(NULL);
+    assert(save_tstate != NULL);
+
+    PyThreadState *tstate;
+    PyStatus status = Py_NewInterpreterFromConfig(&tstate, config);
+    if (PyStatus_Exception(status)) {
+        // Since no new thread state was created, there is no exception
+        // to propagate; raise a fresh one after swapping back in the
+        // old thread state.
+        PyThreadState_Swap(save_tstate);
+        _PyErr_SetFromPyStatus(status);
+        PyObject *exc = PyErr_GetRaisedException();
+        PyErr_SetString(PyExc_InterpreterError,
+                        "sub-interpreter creation failed");
+        _PyErr_ChainExceptions1(exc);
+        return NULL;
+    }
+    assert(tstate != NULL);
+    PyInterpreterState *interp = PyThreadState_GetInterpreter(tstate);
+
+    long whence = _PyInterpreterState_WHENCE_XI;
+    if (maybe_whence != NULL) {
+        whence = *maybe_whence;
+    }
+    _PyInterpreterState_SetWhence(interp, whence);
+
+    if (p_tstate != NULL) {
+        // We leave the new thread state as the current one.
+        *p_tstate = tstate;
+    }
+    else {
+        // Throw away the initial tstate.
+        PyThreadState_Clear(tstate);
+        PyThreadState_Swap(save_tstate);
+        PyThreadState_Delete(tstate);
+        save_tstate = NULL;
+    }
+    if (p_save_tstate != NULL) {
+        *p_save_tstate = save_tstate;
+    }
+    return interp;
+}
+
+void
+_PyXI_EndInterpreter(PyInterpreterState *interp,
+                     PyThreadState *tstate, PyThreadState **p_save_tstate)
+{
+#ifndef NDEBUG
+    long whence = _PyInterpreterState_GetWhence(interp);
+#endif
+    assert(whence != _PyInterpreterState_WHENCE_RUNTIME);
+
+    if (!_PyInterpreterState_IsReady(interp)) {
+        assert(whence == _PyInterpreterState_WHENCE_UNKNOWN);
+        // PyInterpreterState_Clear() requires the GIL,
+        // which a not-ready does not have, so we don't clear it.
+        // That means there may be leaks here until clearing the
+        // interpreter is fixed.
+        PyInterpreterState_Delete(interp);
+        return;
+    }
+    assert(whence != _PyInterpreterState_WHENCE_UNKNOWN);
+
+    PyThreadState *save_tstate = NULL;
+    PyThreadState *cur_tstate = PyThreadState_GET();
+    if (tstate == NULL) {
+        if (PyThreadState_GetInterpreter(cur_tstate) == interp) {
+            tstate = cur_tstate;
+        }
+        else {
+            tstate = _PyThreadState_NewBound(interp, _PyThreadState_WHENCE_FINI);
+            assert(tstate != NULL);
+            save_tstate = PyThreadState_Swap(tstate);
+        }
+    }
+    else {
+        assert(PyThreadState_GetInterpreter(tstate) == interp);
+        if (tstate != cur_tstate) {
+            assert(PyThreadState_GetInterpreter(cur_tstate) != interp);
+            save_tstate = PyThreadState_Swap(tstate);
+        }
+    }
+
+    Py_EndInterpreter(tstate);
+
+    if (p_save_tstate != NULL) {
+        save_tstate = *p_save_tstate;
+    }
+    PyThreadState_Swap(save_tstate);
+}