crossinterp.c

/* API for managing interactions between isolated interpreters */

#include "Python.h"
#include "marshal.h"              // PyMarshal_WriteObjectToString()
#include "osdefs.h"               // MAXPATHLEN
#include "pycore_ceval.h"         // _Py_simple_func
#include "pycore_crossinterp.h"   // _PyXIData_t
#include "pycore_initconfig.h"    // _PyStatus_OK()
#include "pycore_namespace.h"     // _PyNamespace_New()
#include "pycore_typeobject.h"    // _PyStaticType_InitBuiltin()


static Py_ssize_t
_Py_GetMainfile(char *buffer, size_t maxlen)
{
    // We don't expect subinterpreters to have the __main__ module's
    // __name__ set, but proceed just in case.
    PyThreadState *tstate = _PyThreadState_GET();
    PyObject *module = _Py_GetMainModule(tstate);
    if (_Py_CheckMainModule(module) < 0) {
        return -1;
    }
    Py_ssize_t size = _PyModule_GetFilenameUTF8(module, buffer, maxlen);
    Py_DECREF(module);
    return size;
}


static PyObject *
import_get_module(PyThreadState *tstate, const char *modname)
{
    PyObject *module = NULL;
    if (strcmp(modname, "__main__") == 0) {
        module = _Py_GetMainModule(tstate);
        if (_Py_CheckMainModule(module) < 0) {
            assert(_PyErr_Occurred(tstate));
            return NULL;
        }
    }
    else {
        module = PyImport_ImportModule(modname);
        if (module == NULL) {
            return NULL;
        }
    }
    return module;
}


static PyObject *
runpy_run_path(const char *filename, const char *modname)
{
    PyObject *run_path = PyImport_ImportModuleAttrString("runpy", "run_path");
    if (run_path == NULL) {
        return NULL;
    }
    PyObject *args = Py_BuildValue("(sOs)", filename, Py_None, modname);
    if (args == NULL) {
        Py_DECREF(run_path);
        return NULL;
    }
    PyObject *ns = PyObject_Call(run_path, args, NULL);
    Py_DECREF(run_path);
    Py_DECREF(args);
    return ns;
}


static PyObject *
pyerr_get_message(PyObject *exc)
{
    assert(!PyErr_Occurred());
    PyObject *args = PyException_GetArgs(exc);
    if (args == NULL || args == Py_None || PyObject_Size(args) < 1) {
        return NULL;
    }
    if (PyUnicode_Check(args)) {
        return args;
    }
    PyObject *msg = PySequence_GetItem(args, 0);
    Py_DECREF(args);
    if (msg == NULL) {
        PyErr_Clear();
        return NULL;
    }
    if (!PyUnicode_Check(msg)) {
        Py_DECREF(msg);
        return NULL;
    }
    return msg;
}

#define MAX_MODNAME (255)
#define MAX_ATTRNAME (255)

struct attributeerror_info {
    char modname[MAX_MODNAME+1];
    char attrname[MAX_ATTRNAME+1];
};

static int
_parse_attributeerror(PyObject *exc, struct attributeerror_info *info)
{
    assert(exc != NULL);
    assert(PyErr_GivenExceptionMatches(exc, PyExc_AttributeError));
    int res = -1;

    PyObject *msgobj = pyerr_get_message(exc);
    if (msgobj == NULL) {
        return -1;
    }
    const char *err = PyUnicode_AsUTF8(msgobj);

    if (strncmp(err, "module '", 8) != 0) {
        goto finally;
    }
    err += 8;

    const char *matched = strchr(err, '\'');
    if (matched == NULL) {
        goto finally;
    }
    Py_ssize_t len = matched - err;
    if (len > MAX_MODNAME) {
        goto finally;
    }
    (void)strncpy(info->modname, err, len);
    info->modname[len] = '\0';
    err = matched;

    if (strncmp(err, "' has no attribute '", 20) != 0) {
        goto finally;
    }
    err += 20;

    matched = strchr(err, '\'');
    if (matched == NULL) {
        goto finally;
    }
    len = matched - err;
    if (len > MAX_ATTRNAME) {
        goto finally;
    }
    (void)strncpy(info->attrname, err, len);
    info->attrname[len] = '\0';
    err = matched + 1;

    if (strlen(err) > 0) {
        goto finally;
    }
    res = 0;

finally:
    Py_DECREF(msgobj);
    return res;
}

#undef MAX_MODNAME
#undef MAX_ATTRNAME


/**************/
/* exceptions */
/**************/

typedef struct xi_exceptions exceptions_t;
static int init_static_exctypes(exceptions_t *, PyInterpreterState *);
static void fini_static_exctypes(exceptions_t *, PyInterpreterState *);
static int init_heap_exctypes(exceptions_t *);
static void fini_heap_exctypes(exceptions_t *);
#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 -> xidatafunc} */

/* For now we use a global registry of shareable classes.  An
   alternative would be to add a tp_* slot for a class's
   xidatafunc. It would be simpler and more efficient. */

static void xid_lookup_init(_PyXIData_lookup_t *);
static void xid_lookup_fini(_PyXIData_lookup_t *);
struct _dlcontext;
static xidatafunc lookup_getdata(struct _dlcontext *, PyObject *);
#include "crossinterp_data_lookup.h"


/* lifecycle */

_PyXIData_t *
_PyXIData_New(void)
{
    _PyXIData_t *xid = PyMem_RawCalloc(1, sizeof(_PyXIData_t));
    if (xid == NULL) {
        PyErr_NoMemory();
    }
    return xid;
}

void
_PyXIData_Free(_PyXIData_t *xid)
{
    PyInterpreterState *interp = PyInterpreterState_Get();
    _PyXIData_Clear(interp, xid);
    PyMem_RawFree(xid);
}


/* defining cross-interpreter data */

static inline void
_xidata_init(_PyXIData_t *xidata)
{
    // If the value is being reused
    // then _xidata_clear() should have been called already.
    assert(xidata->data == NULL);
    assert(xidata->obj == NULL);
    *xidata = (_PyXIData_t){0};
    _PyXIData_INTERPID(xidata) = -1;
}

static inline void
_xidata_clear(_PyXIData_t *xidata)
{
    // _PyXIData_t only has two members that need to be
    // cleaned up, if set: "xidata" 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 (xidata->data != NULL) {
        if (xidata->free != NULL) {
            xidata->free(xidata->data);
        }
        xidata->data = NULL;
    }
    Py_CLEAR(xidata->obj);
}

void
_PyXIData_Init(_PyXIData_t *xidata,
               PyInterpreterState *interp,
               void *shared, PyObject *obj,
               xid_newobjfunc new_object)
{
    assert(xidata != NULL);
    assert(new_object != NULL);
    _xidata_init(xidata);
    xidata->data = shared;
    if (obj != NULL) {
        assert(interp != NULL);
        // released in _PyXIData_Clear()
        xidata->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).
    _PyXIData_INTERPID(xidata) = (interp != NULL)
        ? PyInterpreterState_GetID(interp)
        : -1;
    xidata->new_object = new_object;
}

int
_PyXIData_InitWithSize(_PyXIData_t *xidata,
                       PyInterpreterState *interp,
                       const size_t size, PyObject *obj,
                       xid_newobjfunc 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);
    _PyXIData_Init(xidata, interp, NULL, obj, new_object);
    xidata->data = PyMem_RawCalloc(1, size);
    if (xidata->data == NULL) {
        return -1;
    }
    xidata->free = PyMem_RawFree;
    return 0;
}

void
_PyXIData_Clear(PyInterpreterState *interp, _PyXIData_t *xidata)
{
    assert(xidata != NULL);
    // This must be called in the owning interpreter.
    assert(interp == NULL
           || _PyXIData_INTERPID(xidata) == -1
           || _PyXIData_INTERPID(xidata) == PyInterpreterState_GetID(interp));
    _xidata_clear(xidata);
}


/* getting cross-interpreter data */

static inline void
_set_xid_lookup_failure(PyThreadState *tstate, PyObject *obj, const char *msg,
                        PyObject *cause)
{
    if (msg != NULL) {
        assert(obj == NULL);
        set_notshareableerror(tstate, cause, 0, msg);
    }
    else if (obj == NULL) {
        msg = "object does not support cross-interpreter data";
        set_notshareableerror(tstate, cause, 0, msg);
    }
    else {
        msg = "%S does not support cross-interpreter data";
        format_notshareableerror(tstate, cause, 0, msg, obj);
    }
}


int
_PyObject_CheckXIData(PyThreadState *tstate, PyObject *obj)
{
    dlcontext_t ctx;
    if (get_lookup_context(tstate, &ctx) < 0) {
        return -1;
    }
    xidatafunc getdata = lookup_getdata(&ctx, obj);
    if (getdata == NULL) {
        if (!_PyErr_Occurred(tstate)) {
            _set_xid_lookup_failure(tstate, obj, NULL, NULL);
        }
        return -1;
    }
    return 0;
}

static int
_check_xidata(PyThreadState *tstate, _PyXIData_t *xidata)
{
    // xidata->data can be anything, including NULL, so we don't check it.

    // xidata->obj may be NULL, so we don't check it.

    if (_PyXIData_INTERPID(xidata) < 0) {
        PyErr_SetString(PyExc_SystemError, "missing interp");
        return -1;
    }

    if (xidata->new_object == NULL) {
        PyErr_SetString(PyExc_SystemError, "missing new_object func");
        return -1;
    }

    // xidata->free may be NULL, so we don't check it.

    return 0;
}

int
_PyObject_GetXIData(PyThreadState *tstate,
                    PyObject *obj, _PyXIData_t *xidata)
{
    PyInterpreterState *interp = tstate->interp;

    assert(xidata->data == NULL);
    assert(xidata->obj == NULL);
    if (xidata->data != NULL || xidata->obj != NULL) {
        _PyErr_SetString(tstate, PyExc_ValueError, "xidata not cleared");
    }

    // Call the "getdata" func for the object.
    dlcontext_t ctx;
    if (get_lookup_context(tstate, &ctx) < 0) {
        return -1;
    }
    Py_INCREF(obj);
    xidatafunc getdata = lookup_getdata(&ctx, obj);
    if (getdata == NULL) {
        if (PyErr_Occurred()) {
            Py_DECREF(obj);
            return -1;
        }
        // Fall back to obj
        Py_DECREF(obj);
        if (!_PyErr_Occurred(tstate)) {
            _set_xid_lookup_failure(tstate, obj, NULL, NULL);
        }
        return -1;
    }
    int res = getdata(tstate, obj, xidata);
    Py_DECREF(obj);
    if (res != 0) {
        PyObject *cause = _PyErr_GetRaisedException(tstate);
        assert(cause != NULL);
        _set_xid_lookup_failure(tstate, obj, NULL, cause);
        Py_XDECREF(cause);
        return -1;
    }

    // Fill in the blanks and validate the result.
    _PyXIData_INTERPID(xidata) = PyInterpreterState_GetID(interp);
    if (_check_xidata(tstate, xidata) != 0) {
        (void)_PyXIData_Release(xidata);
        return -1;
    }

    return 0;
}


/* pickle C-API */

struct _pickle_context {
    PyThreadState *tstate;
};

static PyObject *
_PyPickle_Dumps(struct _pickle_context *ctx, PyObject *obj)
{
    PyObject *dumps = PyImport_ImportModuleAttrString("pickle", "dumps");
    if (dumps == NULL) {
        return NULL;
    }
    PyObject *bytes = PyObject_CallOneArg(dumps, obj);
    Py_DECREF(dumps);
    return bytes;
}


struct sync_module_result {
    PyObject *module;
    PyObject *loaded;
    PyObject *failed;
};

struct sync_module {
    const char *filename;
    char _filename[MAXPATHLEN+1];
    struct sync_module_result cached;
};

static void
sync_module_clear(struct sync_module *data)
{
    data->filename = NULL;
    Py_CLEAR(data->cached.module);
    Py_CLEAR(data->cached.loaded);
    Py_CLEAR(data->cached.failed);
}


struct _unpickle_context {
    PyThreadState *tstate;
    // We only special-case the __main__ module,
    // since other modules behave consistently.
    struct sync_module main;
};

static void
_unpickle_context_clear(struct _unpickle_context *ctx)
{
    sync_module_clear(&ctx->main);
}

static struct sync_module_result
_unpickle_context_get_module(struct _unpickle_context *ctx,
                             const char *modname)
{
    if (strcmp(modname, "__main__") == 0) {
        return ctx->main.cached;
    }
    else {
        return (struct sync_module_result){
            .failed = PyExc_NotImplementedError,
        };
    }
}

static struct sync_module_result
_unpickle_context_set_module(struct _unpickle_context *ctx,
                             const char *modname)
{
    struct sync_module_result res = {0};
    struct sync_module_result *cached = NULL;
    const char *filename = NULL;
    if (strcmp(modname, "__main__") == 0) {
        cached = &ctx->main.cached;
        filename = ctx->main.filename;
    }
    else {
        res.failed = PyExc_NotImplementedError;
        goto finally;
    }

    res.module = import_get_module(ctx->tstate, modname);
    if (res.module == NULL) {
        res.failed = _PyErr_GetRaisedException(ctx->tstate);
        assert(res.failed != NULL);
        goto finally;
    }

    if (filename == NULL) {
        Py_CLEAR(res.module);
        res.failed = PyExc_NotImplementedError;
        goto finally;
    }
    res.loaded = runpy_run_path(filename, modname);
    if (res.loaded == NULL) {
        Py_CLEAR(res.module);
        res.failed = _PyErr_GetRaisedException(ctx->tstate);
        assert(res.failed != NULL);
        goto finally;
    }

finally:
    if (cached != NULL) {
        assert(cached->module == NULL);
        assert(cached->loaded == NULL);
        assert(cached->failed == NULL);
        *cached = res;
    }
    return res;
}


static int
_handle_unpickle_missing_attr(struct _unpickle_context *ctx, PyObject *exc)
{
    // The caller must check if an exception is set or not when -1 is returned.
    assert(!_PyErr_Occurred(ctx->tstate));
    assert(PyErr_GivenExceptionMatches(exc, PyExc_AttributeError));
    struct attributeerror_info info;
    if (_parse_attributeerror(exc, &info) < 0) {
        return -1;
    }

    // Get the module.
    struct sync_module_result mod = _unpickle_context_get_module(ctx, info.modname);
    if (mod.failed != NULL) {
        // It must have failed previously.
        return -1;
    }
    if (mod.module == NULL) {
        mod = _unpickle_context_set_module(ctx, info.modname);
        if (mod.failed != NULL) {
            return -1;
        }
        assert(mod.module != NULL);
    }

    // Bail out if it is unexpectedly set already.
    if (PyObject_HasAttrString(mod.module, info.attrname)) {
        return -1;
    }

    // Try setting the attribute.
    PyObject *value = NULL;
    if (PyDict_GetItemStringRef(mod.loaded, info.attrname, &value) <= 0) {
        return -1;
    }
    assert(value != NULL);
    int res = PyObject_SetAttrString(mod.module, info.attrname, value);
    Py_DECREF(value);
    if (res < 0) {
        return -1;
    }

    return 0;
}

static PyObject *
_PyPickle_Loads(struct _unpickle_context *ctx, PyObject *pickled)
{
    PyObject *loads = PyImport_ImportModuleAttrString("pickle", "loads");
    if (loads == NULL) {
        return NULL;
    }
    PyObject *obj = PyObject_CallOneArg(loads, pickled);
    if (ctx != NULL) {
        while (obj == NULL) {
            assert(_PyErr_Occurred(ctx->tstate));
            if (!PyErr_ExceptionMatches(PyExc_AttributeError)) {
                // We leave other failures unhandled.
                break;
            }
            // Try setting the attr if not set.
            PyObject *exc = _PyErr_GetRaisedException(ctx->tstate);
            if (_handle_unpickle_missing_attr(ctx, exc) < 0) {
                // Any resulting exceptions are ignored
                // in favor of the original.
                _PyErr_SetRaisedException(ctx->tstate, exc);
                break;
            }
            Py_CLEAR(exc);
            // Retry with the attribute set.
            obj = PyObject_CallOneArg(loads, pickled);
        }
    }
    Py_DECREF(loads);
    return obj;
}


/* pickle wrapper */

struct _pickle_xid_context {
    // __main__.__file__
    struct {
        const char *utf8;
        size_t len;
        char _utf8[MAXPATHLEN+1];
    } mainfile;
};

static int
_set_pickle_xid_context(PyThreadState *tstate, struct _pickle_xid_context *ctx)
{
    // Set mainfile if possible.
    Py_ssize_t len = _Py_GetMainfile(ctx->mainfile._utf8, MAXPATHLEN);
    if (len < 0) {
        // For now we ignore any exceptions.
        PyErr_Clear();
    }
    else if (len > 0) {
        ctx->mainfile.utf8 = ctx->mainfile._utf8;
        ctx->mainfile.len = (size_t)len;
    }

    return 0;
}


struct _shared_pickle_data {
    _PyBytes_data_t pickled;  // Must be first if we use _PyBytes_FromXIData().
    struct _pickle_xid_context ctx;
};

PyObject *
_PyPickle_LoadFromXIData(_PyXIData_t *xidata)
{
    PyThreadState *tstate = _PyThreadState_GET();
    struct _shared_pickle_data *shared =
                            (struct _shared_pickle_data *)xidata->data;
    // We avoid copying the pickled data by wrapping it in a memoryview.
    // The alternative is to get a bytes object using _PyBytes_FromXIData().
    PyObject *pickled = PyMemoryView_FromMemory(
            (char *)shared->pickled.bytes, shared->pickled.len, PyBUF_READ);
    if (pickled == NULL) {
        return NULL;
    }

    // Unpickle the object.
    struct _unpickle_context ctx = {
        .tstate = tstate,
        .main = {
            .filename = shared->ctx.mainfile.utf8,
        },
    };
    PyObject *obj = _PyPickle_Loads(&ctx, pickled);
    Py_DECREF(pickled);
    _unpickle_context_clear(&ctx);
    if (obj == NULL) {
        PyObject *cause = _PyErr_GetRaisedException(tstate);
        assert(cause != NULL);
        _set_xid_lookup_failure(
                    tstate, NULL, "object could not be unpickled", cause);
        Py_DECREF(cause);
    }
    return obj;
}


int
_PyPickle_GetXIData(PyThreadState *tstate, PyObject *obj, _PyXIData_t *xidata)
{
    // Pickle the object.
    struct _pickle_context ctx = {
        .tstate = tstate,
    };
    PyObject *bytes = _PyPickle_Dumps(&ctx, obj);
    if (bytes == NULL) {
        PyObject *cause = _PyErr_GetRaisedException(tstate);
        assert(cause != NULL);
        _set_xid_lookup_failure(
                    tstate, NULL, "object could not be pickled", cause);
        Py_DECREF(cause);
        return -1;
    }

    // If we had an "unwrapper" mechnanism, we could call
    // _PyObject_GetXIData() on the bytes object directly and add
    // a simple unwrapper to call pickle.loads() on the bytes.
    size_t size = sizeof(struct _shared_pickle_data);
    struct _shared_pickle_data *shared =
            (struct _shared_pickle_data *)_PyBytes_GetXIDataWrapped(
                    tstate, bytes, size, _PyPickle_LoadFromXIData, xidata);
    Py_DECREF(bytes);
    if (shared == NULL) {
        return -1;
    }

    // If it mattered, we could skip getting __main__.__file__
    // when "__main__" doesn't show up in the pickle bytes.
    if (_set_pickle_xid_context(tstate, &shared->ctx) < 0) {
        _xidata_clear(xidata);
        return -1;
    }

    return 0;
}


/* marshal wrapper */

PyObject *
_PyMarshal_ReadObjectFromXIData(_PyXIData_t *xidata)
{
    PyThreadState *tstate = _PyThreadState_GET();
    _PyBytes_data_t *shared = (_PyBytes_data_t *)xidata->data;
    PyObject *obj = PyMarshal_ReadObjectFromString(shared->bytes, shared->len);
    if (obj == NULL) {
        PyObject *cause = _PyErr_GetRaisedException(tstate);
        assert(cause != NULL);
        _set_xid_lookup_failure(
                    tstate, NULL, "object could not be unmarshalled", cause);
        Py_DECREF(cause);
        return NULL;
    }
    return obj;
}

int
_PyMarshal_GetXIData(PyThreadState *tstate, PyObject *obj, _PyXIData_t *xidata)
{
    PyObject *bytes = PyMarshal_WriteObjectToString(obj, Py_MARSHAL_VERSION);
    if (bytes == NULL) {
        PyObject *cause = _PyErr_GetRaisedException(tstate);
        assert(cause != NULL);
        _set_xid_lookup_failure(
                    tstate, NULL, "object could not be marshalled", cause);
        Py_DECREF(cause);
        return -1;
    }
    size_t size = sizeof(_PyBytes_data_t);
    _PyBytes_data_t *shared = _PyBytes_GetXIDataWrapped(
            tstate, bytes, size, _PyMarshal_ReadObjectFromXIData, xidata);
    Py_DECREF(bytes);
    if (shared == NULL) {
        return -1;
    }
    return 0;
}


/* using cross-interpreter data */

PyObject *
_PyXIData_NewObject(_PyXIData_t *xidata)
{
    return xidata->new_object(xidata);
}

static int
_call_clear_xidata(void *data)
{
    _xidata_clear((_PyXIData_t *)data);
    return 0;
}

static int
_xidata_release(_PyXIData_t *xidata, int rawfree)
{
    if ((xidata->data == NULL || xidata->free == NULL) && xidata->obj == NULL) {
        // Nothing to release!
        if (rawfree) {
            PyMem_RawFree(xidata);
        }
        else {
            xidata->data = NULL;
        }
        return 0;
    }

    // Switch to the original interpreter.
    PyInterpreterState *interp = _PyInterpreterState_LookUpID(
                                        _PyXIData_INTERPID(xidata));
    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(xidata);
        }
        return -1;
    }

    // "Release" the data and/or the object.
    if (rawfree) {
        return _Py_CallInInterpreterAndRawFree(interp, _call_clear_xidata, xidata);
    }
    else {
        return _Py_CallInInterpreter(interp, _call_clear_xidata, xidata);
    }
}

int
_PyXIData_Release(_PyXIData_t *xidata)
{
    return _xidata_release(xidata, 0);
}

int
_PyXIData_ReleaseAndRawFree(_PyXIData_t *xidata)
{
    return _xidata_release(xidata, 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 *tbexc_type = PyImport_ImportModuleAttrString(
        "traceback",
        "TracebackException");
    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(_PyXIData_t *xidata, int rawfree)
{
    PyObject *exc = PyErr_GetRaisedException();
    int res = rawfree
        ? _PyXIData_Release(xidata)
        : _PyXIData_ReleaseAndRawFree(xidata);
    if (res < 0) {
        /* The owning interpreter is already destroyed. */
        _PyXIData_Clear(NULL, xidata);
        // 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.