_warnings.c

#include "Python.h"
#include "pycore_critical_section.h"  // Py_BEGIN_CRITICAL_SECTION_MUT()
#include "pycore_interp.h"        // PyInterpreterState.warnings
#include "pycore_long.h"          // _PyLong_GetZero()
#include "pycore_pyerrors.h"      // _PyErr_Occurred()
#include "pycore_pylifecycle.h"   // _Py_IsInterpreterFinalizing()
#include "pycore_pystate.h"       // _PyThreadState_GET()
#include "pycore_sysmodule.h"     // _PySys_GetOptionalAttr()
#include "pycore_traceback.h"     // _Py_DisplaySourceLine()

#include <stdbool.h>

#include "clinic/_warnings.c.h"

#define MODULE_NAME "_warnings"

PyDoc_STRVAR(warnings__doc__,
MODULE_NAME " provides basic warning filtering support.\n"
"It is a helper module to speed up interpreter start-up.");


/*************************************************************************/

typedef struct _warnings_runtime_state WarningsState;

static inline int
check_interp(PyInterpreterState *interp)
{
    if (interp == NULL) {
        PyErr_SetString(PyExc_RuntimeError,
                        "warnings_get_state: could not identify "
                        "current interpreter");
        return 0;
    }
    return 1;
}

static inline PyInterpreterState *
get_current_interp(void)
{
    PyInterpreterState *interp = _PyInterpreterState_GET();
    return check_interp(interp) ? interp : NULL;
}

static inline PyThreadState *
get_current_tstate(void)
{
    PyThreadState *tstate = _PyThreadState_GET();
    if (tstate == NULL) {
        (void)check_interp(NULL);
        return NULL;
    }
    return check_interp(tstate->interp) ? tstate : NULL;
}

/* Given a module object, get its per-module state. */
static WarningsState *
warnings_get_state(PyInterpreterState *interp)
{
    return &interp->warnings;
}

/* Clear the given warnings module state. */
static void
warnings_clear_state(WarningsState *st)
{
    Py_CLEAR(st->filters);
    Py_CLEAR(st->once_registry);
    Py_CLEAR(st->default_action);
}

#ifndef Py_DEBUG
static PyObject *
create_filter(PyObject *category, PyObject *action_str, const char *modname)
{
    PyObject *modname_obj = NULL;

    /* Default to "no module name" for initial filter set */
    if (modname != NULL) {
        modname_obj = PyUnicode_InternFromString(modname);
        if (modname_obj == NULL) {
            return NULL;
        }
    } else {
        modname_obj = Py_NewRef(Py_None);
    }

    /* This assumes the line number is zero for now. */
    PyObject *filter = PyTuple_Pack(5, action_str, Py_None,
                                    category, modname_obj, _PyLong_GetZero());
    Py_DECREF(modname_obj);
    return filter;
}
#endif

static PyObject *
init_filters(PyInterpreterState *interp)
{
#ifdef Py_DEBUG
    /* Py_DEBUG builds show all warnings by default */
    return PyList_New(0);
#else
    /* Other builds ignore a number of warning categories by default */
    PyObject *filters = PyList_New(5);
    if (filters == NULL) {
        return NULL;
    }

    size_t pos = 0;  /* Post-incremented in each use. */
#define ADD(TYPE, ACTION, MODNAME) \
    PyList_SET_ITEM(filters, pos++, \
                    create_filter(TYPE, &_Py_ID(ACTION), MODNAME));
    ADD(PyExc_DeprecationWarning, default, "__main__");
    ADD(PyExc_DeprecationWarning, ignore, NULL);
    ADD(PyExc_PendingDeprecationWarning, ignore, NULL);
    ADD(PyExc_ImportWarning, ignore, NULL);
    ADD(PyExc_ResourceWarning, ignore, NULL);
#undef ADD

    for (size_t x = 0; x < pos; x++) {
        if (PyList_GET_ITEM(filters, x) == NULL) {
            Py_DECREF(filters);
            return NULL;
        }
    }
    return filters;
#endif
}

/* Initialize the given warnings module state. */
int
_PyWarnings_InitState(PyInterpreterState *interp)
{
    WarningsState *st = &interp->warnings;

    if (st->filters == NULL) {
        st->filters = init_filters(interp);
        if (st->filters == NULL) {
            return -1;
        }
    }

    if (st->once_registry == NULL) {
        st->once_registry = PyDict_New();
        if (st->once_registry == NULL) {
            return -1;
        }
    }

    if (st->default_action == NULL) {
        st->default_action = PyUnicode_FromString("default");
        if (st->default_action == NULL) {
            return -1;
        }
    }

    st->filters_version = 0;
    return 0;
}


/*************************************************************************/

static int
check_matched(PyInterpreterState *interp, PyObject *obj, PyObject *arg)
{
    PyObject *result;
    int rc;

    /* A 'None' filter always matches */
    if (obj == Py_None)
        return 1;

    /* An internal plain text default filter must match exactly */
    if (PyUnicode_CheckExact(obj)) {
        int cmp_result = PyUnicode_Compare(obj, arg);
        if (cmp_result == -1 && PyErr_Occurred()) {
            return -1;
        }
        return !cmp_result;
    }

    /* Otherwise assume a regex filter and call its match() method */
    result = PyObject_CallMethodOneArg(obj, &_Py_ID(match), arg);
    if (result == NULL)
        return -1;

    rc = PyObject_IsTrue(result);
    Py_DECREF(result);
    return rc;
}

#define GET_WARNINGS_ATTR(interp, ATTR, try_import) \
    get_warnings_attr(interp, &_Py_ID(ATTR), try_import)

/*
   Returns a new reference.
   A NULL return value can mean false or an error.
*/
static PyObject *
get_warnings_attr(PyInterpreterState *interp, PyObject *attr, int try_import)
{
    PyObject *warnings_module, *obj;

    /* don't try to import after the start of the Python finallization */
    if (try_import && !_Py_IsInterpreterFinalizing(interp)) {
        warnings_module = PyImport_Import(&_Py_ID(warnings));
        if (warnings_module == NULL) {
            /* Fallback to the C implementation if we cannot get
               the Python implementation */
            if (PyErr_ExceptionMatches(PyExc_ImportError)) {
                PyErr_Clear();
            }
            return NULL;
        }
    }
    else {
        /* if we're so late into Python finalization that the module dict is
           gone, then we can't even use PyImport_GetModule without triggering
           an interpreter abort.
        */
        if (!_PyImport_GetModules(interp)) {
            return NULL;
        }
        warnings_module = PyImport_GetModule(&_Py_ID(warnings));
        if (warnings_module == NULL)
            return NULL;
    }

    (void)PyObject_GetOptionalAttr(warnings_module, attr, &obj);
    Py_DECREF(warnings_module);
    return obj;
}


static PyObject *
get_once_registry(PyInterpreterState *interp)
{
    WarningsState *st = warnings_get_state(interp);
    assert(st != NULL);

    _Py_CRITICAL_SECTION_ASSERT_MUTEX_LOCKED(&st->mutex);

    PyObject *registry = GET_WARNINGS_ATTR(interp, onceregistry, 0);
    if (registry == NULL) {
        if (PyErr_Occurred())
            return NULL;
        assert(st->once_registry);
        return st->once_registry;
    }
    if (!PyDict_Check(registry)) {
        PyErr_Format(PyExc_TypeError,
                     MODULE_NAME ".onceregistry must be a dict, "
                     "not '%.200s'",
                     Py_TYPE(registry)->tp_name);
        Py_DECREF(registry);
        return NULL;
    }
    Py_SETREF(st->once_registry, registry);
    return registry;
}


static PyObject *
get_default_action(PyInterpreterState *interp)
{
    WarningsState *st = warnings_get_state(interp);
    assert(st != NULL);

    _Py_CRITICAL_SECTION_ASSERT_MUTEX_LOCKED(&st->mutex);

    PyObject *default_action = GET_WARNINGS_ATTR(interp, defaultaction, 0);
    if (default_action == NULL) {
        if (PyErr_Occurred()) {
            return NULL;
        }
        assert(st->default_action);
        return st->default_action;
    }
    if (!PyUnicode_Check(default_action)) {
        PyErr_Format(PyExc_TypeError,
                     MODULE_NAME ".defaultaction must be a string, "
                     "not '%.200s'",
                     Py_TYPE(default_action)->tp_name);
        Py_DECREF(default_action);
        return NULL;
    }
    Py_SETREF(st->default_action, default_action);
    return default_action;
}


/* The item is a new reference. */
static PyObject*
get_filter(PyInterpreterState *interp, PyObject *category,
           PyObject *text, Py_ssize_t lineno,
           PyObject *module, PyObject **item)
{
    WarningsState *st = warnings_get_state(interp);
    assert(st != NULL);

    _Py_CRITICAL_SECTION_ASSERT_MUTEX_LOCKED(&st->mutex);

    PyObject *warnings_filters = GET_WARNINGS_ATTR(interp, filters, 0);
    if (warnings_filters == NULL) {
        if (PyErr_Occurred())
            return NULL;
    }
    else {
        Py_SETREF(st->filters, warnings_filters);
    }

    PyObject *filters = st->filters;
    if (filters == NULL || !PyList_Check(filters)) {
        PyErr_SetString(PyExc_ValueError,
                        MODULE_NAME ".filters must be a list");
        return NULL;
    }

    /* WarningsState.filters could change while we are iterating over it. */
    for (Py_ssize_t i = 0; i < PyList_GET_SIZE(filters); i++) {
        PyObject *tmp_item, *action, *msg, *cat, *mod, *ln_obj;
        Py_ssize_t ln;
        int is_subclass, good_msg, good_mod;

        tmp_item = PyList_GET_ITEM(filters, i);
        if (!PyTuple_Check(tmp_item) || PyTuple_GET_SIZE(tmp_item) != 5) {
            PyErr_Format(PyExc_ValueError,
                         MODULE_NAME ".filters item %zd isn't a 5-tuple", i);
            return NULL;
        }

        /* Python code: action, msg, cat, mod, ln = item */
        Py_INCREF(tmp_item);
        action = PyTuple_GET_ITEM(tmp_item, 0);
        msg = PyTuple_GET_ITEM(tmp_item, 1);
        cat = PyTuple_GET_ITEM(tmp_item, 2);
        mod = PyTuple_GET_ITEM(tmp_item, 3);
        ln_obj = PyTuple_GET_ITEM(tmp_item, 4);

        if (!PyUnicode_Check(action)) {
            PyErr_Format(PyExc_TypeError,
                         "action must be a string, not '%.200s'",
                         Py_TYPE(action)->tp_name);
            Py_DECREF(tmp_item);
            return NULL;
        }

        good_msg = check_matched(interp, msg, text);
        if (good_msg == -1) {
            Py_DECREF(tmp_item);
            return NULL;
        }

        good_mod = check_matched(interp, mod, module);
        if (good_mod == -1) {
            Py_DECREF(tmp_item);
            return NULL;
        }

        is_subclass = PyObject_IsSubclass(category, cat);
        if (is_subclass == -1) {
            Py_DECREF(tmp_item);
            return NULL;
        }

        ln = PyLong_AsSsize_t(ln_obj);
        if (ln == -1 && PyErr_Occurred()) {
            Py_DECREF(tmp_item);
            return NULL;
        }

        if (good_msg && is_subclass && good_mod && (ln == 0 || lineno == ln)) {
            *item = tmp_item;
            return action;
        }

        Py_DECREF(tmp_item);
    }

    PyObject *action = get_default_action(interp);
    if (action != NULL) {
        *item = Py_NewRef(Py_None);
        return action;
    }

    return NULL;
}


static int
already_warned(PyInterpreterState *interp, PyObject *registry, PyObject *key,
               int should_set)
{
    PyObject *already_warned;

    if (key == NULL)
        return -1;

    WarningsState *st = warnings_get_state(interp);
    assert(st != NULL);
    _Py_CRITICAL_SECTION_ASSERT_MUTEX_LOCKED(&st->mutex);

    PyObject *version_obj;
    if (PyDict_GetItemRef(registry, &_Py_ID(version), &version_obj) < 0) {
        return -1;
    }
    bool should_update_version = (
        version_obj == NULL
        || !PyLong_CheckExact(version_obj)
        || PyLong_AsLong(version_obj) != st->filters_version
    );
    Py_XDECREF(version_obj);
    if (should_update_version) {
        PyDict_Clear(registry);
        version_obj = PyLong_FromLong(st->filters_version);
        if (version_obj == NULL)
            return -1;
        if (PyDict_SetItem(registry, &_Py_ID(version), version_obj) < 0) {
            Py_DECREF(version_obj);
            return -1;
        }
        Py_DECREF(version_obj);
    }
    else {
        if (PyDict_GetItemRef(registry, key, &already_warned) < 0) {
            return -1;
        }
        if (already_warned != NULL) {
            int rc = PyObject_IsTrue(already_warned);
            Py_DECREF(already_warned);
            if (rc != 0)
                return rc;
        }
    }

    /* This warning wasn't found in the registry, set it. */
    if (should_set)
        return PyDict_SetItem(registry, key, Py_True);
    return 0;
}

/* New reference. */
static PyObject *
normalize_module(PyObject *filename)
{
    PyObject *module;
    int kind;
    const void *data;
    Py_ssize_t len;

    len = PyUnicode_GetLength(filename);
    if (len < 0)
        return NULL;

    if (len == 0)
        return PyUnicode_FromString("<unknown>");

    kind = PyUnicode_KIND(filename);
    data = PyUnicode_DATA(filename);

    /* if filename.endswith(".py"): */
    if (len >= 3 &&
        PyUnicode_READ(kind, data, len-3) == '.' &&
        PyUnicode_READ(kind, data, len-2) == 'p' &&
        PyUnicode_READ(kind, data, len-1) == 'y')
    {
        module = PyUnicode_Substring(filename, 0, len-3);
    }
    else {
        module = Py_NewRef(filename);
    }
    return module;
}

static int
update_registry(PyInterpreterState *interp, PyObject *registry, PyObject *text,
                PyObject *category, int add_zero)
{
    PyObject *altkey;
    int rc;

    if (add_zero)
        altkey = PyTuple_Pack(3, text, category, _PyLong_GetZero());
    else
        altkey = PyTuple_Pack(2, text, category);

    rc = already_warned(interp, registry, altkey, 1);
    Py_XDECREF(altkey);
    return rc;
}

static void
show_warning(PyThreadState *tstate, PyObject *filename, int lineno,
             PyObject *text, PyObject *category, PyObject *sourceline)
{
    PyObject *f_stderr = NULL;
    PyObject *name;
    char lineno_str[128];

    PyOS_snprintf(lineno_str, sizeof(lineno_str), ":%d: ", lineno);

    name = PyObject_GetAttr(category, &_Py_ID(__name__));
    if (name == NULL) {
        goto error;
    }

    if (_PySys_GetOptionalAttr(&_Py_ID(stderr), &f_stderr) <= 0) {
        fprintf(stderr, "lost sys.stderr\n");
        goto error;
    }

    /* Print "filename:lineno: category: text\n" */
    if (PyFile_WriteObject(filename, f_stderr, Py_PRINT_RAW) < 0)
        goto error;
    if (PyFile_WriteString(lineno_str, f_stderr) < 0)
        goto error;
    if (PyFile_WriteObject(name, f_stderr, Py_PRINT_RAW) < 0)
        goto error;
    if (PyFile_WriteString(": ", f_stderr) < 0)
        goto error;
    if (PyFile_WriteObject(text, f_stderr, Py_PRINT_RAW) < 0)
        goto error;
    if (PyFile_WriteString("\n", f_stderr) < 0)
        goto error;
    Py_CLEAR(name);

    /* Print "  source_line\n" */
    if (sourceline) {
        int kind;
        const void *data;
        Py_ssize_t i, len;
        Py_UCS4 ch;
        PyObject *truncated;

        kind = PyUnicode_KIND(sourceline);
        data = PyUnicode_DATA(sourceline);
        len = PyUnicode_GET_LENGTH(sourceline);
        for (i=0; i<len; i++) {
            ch = PyUnicode_READ(kind, data, i);
            if (ch != ' ' && ch != '\t' && ch != '\014')
                break;
        }

        truncated = PyUnicode_Substring(sourceline, i, len);
        if (truncated == NULL)
            goto error;

        PyFile_WriteObject(sourceline, f_stderr, Py_PRINT_RAW);
        Py_DECREF(truncated);
        PyFile_WriteString("\n", f_stderr);
    }
    else {
        _Py_DisplaySourceLine(f_stderr, filename, lineno, 2, NULL, NULL);
    }

error:
    Py_XDECREF(f_stderr);
    Py_XDECREF(name);
    PyErr_Clear();
}

static int
call_show_warning(PyThreadState *tstate, PyObject *category,
                  PyObject *text, PyObject *message,
                  PyObject *filename, int lineno, PyObject *lineno_obj,
                  PyObject *sourceline, PyObject *source)
{
    PyObject *show_fn, *msg, *res, *warnmsg_cls = NULL;
    PyInterpreterState *interp = tstate->interp;

    /* The Python implementation is able to log the traceback where the source
       was allocated, whereas the C implementation doesn't. */
    show_fn = GET_WARNINGS_ATTR(interp, _showwarnmsg, 1);
    if (show_fn == NULL) {
        if (PyErr_Occurred())
            return -1;
        show_warning(tstate, filename, lineno, text, category, sourceline);
        return 0;
    }

    if (!PyCallable_Check(show_fn)) {
        PyErr_SetString(PyExc_TypeError,
                "warnings._showwarnmsg() must be set to a callable");
        goto error;
    }

    warnmsg_cls = GET_WARNINGS_ATTR(interp, WarningMessage, 0);
    if (warnmsg_cls == NULL) {
        if (!PyErr_Occurred()) {
            PyErr_SetString(PyExc_RuntimeError,
                    "unable to get warnings.WarningMessage");
        }
        goto error;
    }

    msg = PyObject_CallFunctionObjArgs(warnmsg_cls, message, category,
            filename, lineno_obj, Py_None, Py_None, source,
            NULL);
    Py_DECREF(warnmsg_cls);
    if (msg == NULL)
        goto error;

    res = PyObject_CallOneArg(show_fn, msg);
    Py_DECREF(show_fn);
    Py_DECREF(msg);

    if (res == NULL)
        return -1;

    Py_DECREF(res);
    return 0;

error:
    Py_XDECREF(show_fn);
    return -1;
}

static PyObject *
warn_explicit(PyThreadState *tstate, PyObject *category, PyObject *message,
              PyObject *filename, int lineno,
              PyObject *module, PyObject *registry, PyObject *sourceline,
              PyObject *source)
{
    PyObject *key = NULL, *text = NULL, *result = NULL, *lineno_obj = NULL;
    PyObject *item = NULL;
    PyObject *action;
    int rc;
    PyInterpreterState *interp = tstate->interp;

    /* module can be None if a warning is emitted late during Python shutdown.
       In this case, the Python warnings module was probably unloaded, filters
       are no more available to choose as action. It is safer to ignore the
       warning and do nothing. */
    if (module == Py_None)
        Py_RETURN_NONE;

    if (registry && !PyDict_Check(registry) && (registry != Py_None)) {
        PyErr_SetString(PyExc_TypeError, "'registry' must be a dict or None");
        return NULL;
    }

    /* Normalize module. */
    if (module == NULL) {
        module = normalize_module(filename);
        if (module == NULL)
            return NULL;
    }
    else
        Py_INCREF(module);

    /* Normalize message. */
    Py_INCREF(message);  /* DECREF'ed in cleanup. */
    rc = PyObject_IsInstance(message, PyExc_Warning);
    if (rc == -1) {
        goto cleanup;
    }
    if (rc == 1) {
        text = PyObject_Str(message);
        if (text == NULL)
            goto cleanup;
        category = (PyObject*)Py_TYPE(message);
    }
    else {
        text = message;
        message = PyObject_CallOneArg(category, message);
        if (message == NULL)
            goto cleanup;
    }

    lineno_obj = PyLong_FromLong(lineno);
    if (lineno_obj == NULL)
        goto cleanup;

    if (source == Py_None) {
        source = NULL;
    }

    /* Create key. */
    key = PyTuple_Pack(3, text, category, lineno_obj);
    if (key == NULL)
        goto cleanup;

    if ((registry != NULL) && (registry != Py_None)) {
        rc = already_warned(interp, registry, key, 0);
        if (rc == -1)
            goto cleanup;
        else if (rc == 1)
            goto return_none;
        /* Else this warning hasn't been generated before. */
    }

    action = get_filter(interp, category, text, lineno, module, &item);
    if (action == NULL)
        goto cleanup;

    if (_PyUnicode_EqualToASCIIString(action, "error")) {
        PyErr_SetObject(category, message);
        goto cleanup;
    }

    if (_PyUnicode_EqualToASCIIString(action, "ignore")) {
        goto return_none;
    }

    /* Store in the registry that we've been here, *except* when the action
       is "always". */
    rc = 0;
    if (!_PyUnicode_EqualToASCIIString(action, "always")) {
        if (registry != NULL && registry != Py_None &&
            PyDict_SetItem(registry, key, Py_True) < 0)
        {
            goto cleanup;
        }

        if (_PyUnicode_EqualToASCIIString(action, "once")) {
            if (registry == NULL || registry == Py_None) {
                registry = get_once_registry(interp);
                if (registry == NULL)
                    goto cleanup;
            }
            /* WarningsState.once_registry[(text, category)] = 1 */
            rc = update_registry(interp, registry, text, category, 0);
        }
        else if (_PyUnicode_EqualToASCIIString(action, "module")) {
            /* registry[(text, category, 0)] = 1 */
            if (registry != NULL && registry != Py_None)
                rc = update_registry(interp, registry, text, category, 0);
        }
        else if (!_PyUnicode_EqualToASCIIString(action, "default")) {
            PyErr_Format(PyExc_RuntimeError,
                        "Unrecognized action (%R) in warnings.filters:\n %R",
                        action, item);
            goto cleanup;
        }
    }

    if (rc == 1)  /* Already warned for this module. */
        goto return_none;
    if (rc == 0) {
        if (call_show_warning(tstate, category, text, message, filename,
                              lineno, lineno_obj, sourceline, source) < 0)
            goto cleanup;
    }
    else /* if (rc == -1) */
        goto cleanup;

 return_none:
    result = Py_NewRef(Py_None);

 cleanup:
    Py_XDECREF(item);
    Py_XDECREF(key);
    Py_XDECREF(text);
    Py_XDECREF(lineno_obj);
    Py_DECREF(module);
    Py_XDECREF(message);
    return result;  /* Py_None or NULL. */
}

static PyObject *
get_frame_filename(PyFrameObject *frame)
{
    PyCodeObject *code = PyFrame_GetCode(frame);
    PyObject *filename = code->co_filename;
    Py_DECREF(code);
    return filename;
}

static bool
is_internal_filename(PyObject *filename)
{
    if (!PyUnicode_Check(filename)) {
        return false;
    }

    int contains = PyUnicode_Contains(filename, &_Py_ID(importlib));
    if (contains < 0) {
        return false;
    }
    else if (contains > 0) {
        contains = PyUnicode_Contains(filename, &_Py_ID(_bootstrap));
        if (contains < 0) {
            return false;
        }
        else if (contains > 0) {
            return true;
        }
    }

    return false;
}

static bool
is_filename_to_skip(PyObject *filename, PyTupleObject *skip_file_prefixes)
{
    if (skip_file_prefixes) {
        if (!PyUnicode_Check(filename)) {
            return false;
        }

        Py_ssize_t prefixes = PyTuple_GET_SIZE(skip_file_prefixes);
        for (Py_ssize_t idx = 0; idx < prefixes; ++idx)
        {
            PyObject *prefix = PyTuple_GET_ITEM(skip_file_prefixes, idx);
            Py_ssize_t found = PyUnicode_Tailmatch(filename, prefix, 0, -1, -1);
            if (found == 1) {
                return true;
            }
            if (found < 0) {
                return false;
            }
        }
    }
    return false;
}

static bool
is_internal_frame(PyFrameObject *frame)
{
    if (frame == NULL) {
        return false;
    }

    PyObject *filename = get_frame_filename(frame);
    if (filename == NULL) {
        return false;
    }

    return is_internal_filename(filename);
}

static PyFrameObject *
next_external_frame(PyFrameObject *frame, PyTupleObject *skip_file_prefixes)
{
    PyObject *frame_filename;
    do {
        PyFrameObject *back = PyFrame_GetBack(frame);
        Py_SETREF(frame, back);
    } while (frame != NULL && (frame_filename = get_frame_filename(frame)) &&
             (is_internal_filename(frame_filename) ||
              is_filename_to_skip(frame_filename, skip_file_prefixes)));

    return frame;
}

/* filename, module, and registry are new refs, globals is borrowed */
/* skip_file_prefixes is either NULL or a tuple of strs. */
/* Returns 0 on error (no new refs), 1 on success */
static int
setup_context(Py_ssize_t stack_level,
              PyTupleObject *skip_file_prefixes,
              PyObject **filename, int *lineno,
              PyObject **module, PyObject **registry)
{
    PyObject *globals;

    /* Setup globals, filename and lineno. */
    PyThreadState *tstate = get_current_tstate();
    if (tstate == NULL) {
        return 0;
    }
    if (skip_file_prefixes) {
        /* Type check our data structure up front. Later code that uses it
         * isn't structured to report errors. */
        Py_ssize_t prefixes = PyTuple_GET_SIZE(skip_file_prefixes);
        for (Py_ssize_t idx = 0; idx < prefixes; ++idx)
        {
            PyObject *prefix = PyTuple_GET_ITEM(skip_file_prefixes, idx);
            if (!PyUnicode_Check(prefix)) {
                PyErr_Format(PyExc_TypeError,
                             "Found non-str '%s' in skip_file_prefixes.",
                             Py_TYPE(prefix)->tp_name);
                return 0;
            }
        }
    }
    PyInterpreterState *interp = tstate->interp;
    PyFrameObject *f = PyThreadState_GetFrame(tstate);
    // Stack level comparisons to Python code is off by one as there is no
    // warnings-related stack level to avoid.
    if (stack_level <= 0 || is_internal_frame(f)) {
        while (--stack_level > 0 && f != NULL) {
            PyFrameObject *back = PyFrame_GetBack(f);
            Py_SETREF(f, back);
        }
    }
    else {
        while (--stack_level > 0 && f != NULL) {
            f = next_external_frame(f, skip_file_prefixes);
        }
    }

    if (f == NULL) {
        globals = interp->sysdict;
        *filename = PyUnicode_FromString("<sys>");
        *lineno = 0;
    }
    else {
        globals = f->f_frame->f_globals;
        *filename = Py_NewRef(_PyFrame_GetCode(f->f_frame)->co_filename);
        *lineno = PyFrame_GetLineNumber(f);
        Py_DECREF(f);
    }

    *module = NULL;

    /* Setup registry. */
    assert(globals != NULL);
    assert(PyDict_Check(globals));
    int rc = PyDict_GetItemRef(globals, &_Py_ID(__warningregistry__),
                               registry);
    if (rc < 0) {
        goto handle_error;
    }
    if (*registry == NULL) {
        *registry = PyDict_New();
        if (*registry == NULL)
            goto handle_error;

         rc = PyDict_SetItem(globals, &_Py_ID(__warningregistry__), *registry);
         if (rc < 0)
            goto handle_error;
    }

    /* Setup module. */
    rc = PyDict_GetItemRef(globals, &_Py_ID(__name__), module);
    if (rc < 0) {
        goto handle_error;
    }
    if (rc > 0) {
        if (Py_IsNone(*module) || PyUnicode_Check(*module)) {
            return 1;
        }
        Py_DECREF(*module);
    }
    *module = PyUnicode_FromString("<string>");
    if (*module == NULL) {
        goto handle_error;
    }

    return 1;

 handle_error:
    Py_XDECREF(*registry);
    Py_XDECREF(*module);
    Py_DECREF(*filename);
    return 0;
}

static PyObject *
get_category(PyObject *message, PyObject *category)
{
    int rc;

    /* Get category. */
    rc = PyObject_IsInstance(message, PyExc_Warning);
    if (rc == -1)
        return NULL;

    if (rc == 1)
        category = (PyObject*)Py_TYPE(message);
    else if (category == NULL || category == Py_None)
        category = PyExc_UserWarning;

    /* Validate category. */
    rc = PyObject_IsSubclass(category, PyExc_Warning);
    /* category is not a subclass of PyExc_Warning or
       PyObject_IsSubclass raised an error */
    if (rc == -1 || rc == 0) {
        PyErr_Format(PyExc_TypeError,
                     "category must be a Warning subclass, not '%s'",
                     Py_TYPE(category)->tp_name);
        return NULL;
    }

    return category;
}

static PyObject *
do_warn(PyObject *message, PyObject *category, Py_ssize_t stack_level,
        PyObject *source, PyTupleObject *skip_file_prefixes)
{
    PyObject *filename, *module, *registry, *res;
    int lineno;

    PyThreadState *tstate = get_current_tstate();
    if (tstate == NULL) {
        return NULL;
    }

    if (!setup_context(stack_level, skip_file_prefixes,
                       &filename, &lineno, &module, &registry))
        return NULL;

#ifdef Py_GIL_DISABLED
    WarningsState *st = warnings_get_state(tstate->interp);
    assert(st != NULL);
#endif