OpenHome/venv/Lib/site-packages/greenlet/greenlet.c

/* -*- indent-tabs-mode: nil; tab-width: 4; -*- */
/* Format with:
 *  clang-format -i --style=file src/greenlet/greenlet.c
 *
 *
 * Fix missing braces with:
 *   clang-tidy src/greenlet/greenlet.c -fix -checks="readability-braces-around-statements"
*/
#define GREENLET_MODULE

#include "greenlet.h"

#include "structmember.h"

#ifdef __clang__
#    pragma clang diagnostic push
#    pragma clang diagnostic ignored "-Wunused-parameter"
#    pragma clang diagnostic ignored "-Wmissing-field-initializers"
#endif

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

A PyGreenlet is a range of C stack addresses that must be
saved and restored in such a way that the full range of the
stack contains valid data when we switch to it.

Stack layout for a greenlet:

               |     ^^^       |
               |  older data   |
               |               |
  stack_stop . |_______________|
        .      |               |
        .      | greenlet data |
        .      |   in stack    |
        .    * |_______________| . .  _____________  stack_copy + stack_saved
        .      |               |     |             |
        .      |     data      |     |greenlet data|
        .      |   unrelated   |     |    saved    |
        .      |      to       |     |   in heap   |
 stack_start . |     this      | . . |_____________| stack_copy
               |   greenlet    |
               |               |
               |  newer data   |
               |     vvv       |


Note that a greenlet's stack data is typically partly at its correct
place in the stack, and partly saved away in the heap, but always in
the above configuration: two blocks, the more recent one in the heap
and the older one still in the stack (either block may be empty).

Greenlets are chained: each points to the previous greenlet, which is
the one that owns the data currently in the C stack above my
stack_stop.  The currently running greenlet is the first element of
this chain.  The main (initial) greenlet is the last one.  Greenlets
whose stack is entirely in the heap can be skipped from the chain.

The chain is not related to execution order, but only to the order
in which bits of C stack happen to belong to greenlets at a particular
point in time.

The main greenlet doesn't have a stack_stop: it is responsible for the
complete rest of the C stack, and we don't know where it begins.  We
use (char*) -1, the largest possible address.

States:
  stack_stop == NULL && stack_start == NULL:  did not start yet
  stack_stop != NULL && stack_start == NULL:  already finished
  stack_stop != NULL && stack_start != NULL:  active

The running greenlet's stack_start is undefined but not NULL.

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

/*** global state ***/

/* In the presence of multithreading, this is a bit tricky:

   - ts_current always store a reference to a greenlet, but it is
     not really the current greenlet after a thread switch occurred.

   - each *running* greenlet uses its run_info field to know which
     thread it is attached to.  A greenlet can only run in the thread
     where it was created.  This run_info is a ref to tstate->dict.

   - the thread state dict is used to save and restore ts_current,
     using the dictionary key 'ts_curkey'.
*/

extern PyTypeObject PyGreenlet_Type;

#if PY_VERSION_HEX >= 0x030700A3
#    define GREENLET_PY37 1
#else
#    define GREENLET_PY37 0
#endif

#if PY_VERSION_HEX >= 0x30A00B1
/*
Python 3.10 beta 1 changed tstate->use_tracing to a nested cframe member.
See https://github.com/python/cpython/pull/25276
We have to save and restore this as well.
*/
#define TSTATE_USE_TRACING(tstate) (tstate->cframe->use_tracing)
#define GREENLET_USE_CFRAME 1
#else
#define TSTATE_USE_TRACING(tstate) (tstate->use_tracing)
#define GREENLET_USE_CFRAME 0
#endif

#ifndef Py_SET_REFCNT
/* Py_REFCNT and Py_SIZE macros are converted to functions
https://bugs.python.org/issue39573 */
#    define Py_SET_REFCNT(obj, refcnt) Py_REFCNT(obj) = (refcnt)
#endif

#ifndef _Py_DEC_REFTOTAL
/* _Py_DEC_REFTOTAL macro has been removed from Python 3.9 by:
  https://github.com/python/cpython/commit/49932fec62c616ec88da52642339d83ae719e924
*/
#    ifdef Py_REF_DEBUG
#        define _Py_DEC_REFTOTAL _Py_RefTotal--
#    else
#        define _Py_DEC_REFTOTAL
#    endif
#endif

/* Weak reference to the switching-to greenlet during the slp switch */
static PyGreenlet* volatile ts_target = NULL;
/* Strong reference to the switching from greenlet after the switch */
static PyGreenlet* volatile ts_origin = NULL;
/* Strong reference to the current greenlet in this thread state */
static PyGreenlet* volatile ts_current = NULL;
/* NULL if error, otherwise args tuple to pass around during slp switch */
static PyObject* volatile ts_passaround_args = NULL;
static PyObject* volatile ts_passaround_kwargs = NULL;

/***********************************************************/
/* Thread-aware routines, switching global variables when needed */

#define STATE_OK                                          \
    (ts_current->run_info == PyThreadState_GET()->dict || \
     !green_updatecurrent())

static PyObject* ts_curkey;
static PyObject* ts_delkey;
static PyObject* ts_tracekey;
static PyObject* ts_event_switch;
static PyObject* ts_event_throw;
static PyObject* PyExc_GreenletError;
static PyObject* PyExc_GreenletExit;
static PyObject* ts_empty_tuple;
static PyObject* ts_empty_dict;

#define GREENLET_GC_FLAGS Py_TPFLAGS_HAVE_GC
#define GREENLET_tp_alloc PyType_GenericAlloc
#define GREENLET_tp_free PyObject_GC_Del
#define GREENLET_tp_traverse green_traverse
#define GREENLET_tp_clear green_clear
#define GREENLET_tp_is_gc green_is_gc

static void
green_clear_exc(PyGreenlet* g)
{
#if GREENLET_PY37
    g->exc_info = NULL;
    g->exc_state.exc_type = NULL;
    g->exc_state.exc_value = NULL;
    g->exc_state.exc_traceback = NULL;
    g->exc_state.previous_item = NULL;
#else
    g->exc_type = NULL;
    g->exc_value = NULL;
    g->exc_traceback = NULL;
#endif
}

static PyGreenlet*
green_create_main(void)
{
    PyGreenlet* gmain;
    PyObject* dict = PyThreadState_GetDict();
    if (dict == NULL) {
        if (!PyErr_Occurred()) {
            PyErr_NoMemory();
        }
        return NULL;
    }

    /* create the main greenlet for this thread */
    gmain = (PyGreenlet*)PyType_GenericAlloc(&PyGreenlet_Type, 0);
    if (gmain == NULL) {
        return NULL;
    }
    gmain->stack_start = (char*)1;
    gmain->stack_stop = (char*)-1;
    gmain->run_info = dict;
    Py_INCREF(dict);
    return gmain;
}

static int
green_updatecurrent(void)
{
    PyObject *exc, *val, *tb;
    PyThreadState* tstate;
    PyGreenlet* current;
    PyGreenlet* previous;
    PyObject* deleteme;

green_updatecurrent_restart:
    /* save current exception */
    PyErr_Fetch(&exc, &val, &tb);

    /* get ts_current from the active tstate */
    tstate = PyThreadState_GET();
    if (tstate->dict &&
        (current = (PyGreenlet*)PyDict_GetItem(tstate->dict, ts_curkey))) {
        /* found -- remove it, to avoid keeping a ref */
        Py_INCREF(current);
        PyDict_DelItem(tstate->dict, ts_curkey);
    }
    else {
        /* first time we see this tstate */
        current = green_create_main();
        if (current == NULL) {
            Py_XDECREF(exc);
            Py_XDECREF(val);
            Py_XDECREF(tb);
            return -1;
        }
    }
    assert(current->run_info == tstate->dict);

green_updatecurrent_retry:
    /* update ts_current as soon as possible, in case of nested switches */
    Py_INCREF(current);
    previous = ts_current;
    ts_current = current;

    /* save ts_current as the current greenlet of its own thread */
    if (PyDict_SetItem(previous->run_info, ts_curkey, (PyObject*)previous)) {
        Py_DECREF(previous);
        Py_DECREF(current);
        Py_XDECREF(exc);
        Py_XDECREF(val);
        Py_XDECREF(tb);
        return -1;
    }
    Py_DECREF(previous);

    /* green_dealloc() cannot delete greenlets from other threads, so
       it stores them in the thread dict; delete them now. */
    deleteme = PyDict_GetItem(tstate->dict, ts_delkey);
    if (deleteme != NULL) {
        PyList_SetSlice(deleteme, 0, INT_MAX, NULL);
    }

    if (ts_current != current) {
        /* some Python code executed above and there was a thread switch,
         * so ts_current points to some other thread again. We need to
         * delete ts_curkey (it's likely there) and retry. */
        PyDict_DelItem(tstate->dict, ts_curkey);
        goto green_updatecurrent_retry;
    }

    /* release an extra reference */
    Py_DECREF(current);

    /* restore current exception */
    PyErr_Restore(exc, val, tb);

    /* thread switch could happen during PyErr_Restore, in that
       case there's nothing to do except restart from scratch. */
    if (ts_current->run_info != tstate->dict) {
        goto green_updatecurrent_restart;
    }

    return 0;
}

static PyObject*
green_statedict(PyGreenlet* g)
{
    while (!PyGreenlet_STARTED(g)) {
        g = g->parent;
        if (g == NULL) {
            /* garbage collected greenlet in chain */
            return NULL;
        }
    }
    return g->run_info;
}

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

/* Some functions must not be inlined:
   * slp_restore_state, when inlined into slp_switch might cause
     it to restore stack over its own local variables
   * slp_save_state, when inlined would add its own local
     variables to the saved stack, wasting space
   * slp_switch, cannot be inlined for obvious reasons
   * g_initialstub, when inlined would receive a pointer into its
     own stack frame, leading to incomplete stack save/restore
*/

#if defined(__GNUC__) && \
    (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4))
#    define GREENLET_NOINLINE_SUPPORTED
#    define GREENLET_NOINLINE(name) __attribute__((noinline)) name
#elif defined(_MSC_VER) && (_MSC_VER >= 1300)
#    define GREENLET_NOINLINE_SUPPORTED
#    define GREENLET_NOINLINE(name) __declspec(noinline) name
#endif

#ifdef GREENLET_NOINLINE_SUPPORTED
/* add forward declarations */
static void GREENLET_NOINLINE(slp_restore_state)(void);
static int GREENLET_NOINLINE(slp_save_state)(char*);
#    if !(defined(MS_WIN64) && defined(_M_X64))
static int GREENLET_NOINLINE(slp_switch)(void);
#    endif
static int GREENLET_NOINLINE(g_initialstub)(void*);
#    define GREENLET_NOINLINE_INIT() \
        do {                         \
        } while (0)
#else
/* force compiler to call functions via pointers */
static void (*slp_restore_state)(void);
static int (*slp_save_state)(char*);
static int (*slp_switch)(void);
static int (*g_initialstub)(void*);
#    define GREENLET_NOINLINE(name) cannot_inline_##name
#    define GREENLET_NOINLINE_INIT()                                  \
        do {                                                          \
            slp_restore_state = GREENLET_NOINLINE(slp_restore_state); \
            slp_save_state = GREENLET_NOINLINE(slp_save_state);       \
            slp_switch = GREENLET_NOINLINE(slp_switch);               \
            g_initialstub = GREENLET_NOINLINE(g_initialstub);         \
        } while (0)
#endif

/*
 * the following macros are spliced into the OS/compiler
 * specific code, in order to simplify maintenance.
 */

#define SLP_SAVE_STATE(stackref, stsizediff) \
    stackref += STACK_MAGIC;                 \
    if (slp_save_state((char*)stackref))     \
        return -1;                           \
    if (!PyGreenlet_ACTIVE(ts_target))       \
        return 1;                            \
    stsizediff = ts_target->stack_start - (char*)stackref

#define SLP_RESTORE_STATE() slp_restore_state()

#define SLP_EVAL
#define slp_switch GREENLET_NOINLINE(slp_switch)
#include "slp_platformselect.h"
#undef slp_switch

#ifndef STACK_MAGIC
#    error \
        "greenlet needs to be ported to this platform, or taught how to detect your compiler properly."
#endif /* !STACK_MAGIC */

#ifdef EXTERNAL_ASM
/* CCP addition: Make these functions, to be called from assembler.
 * The token include file for the given platform should enable the
 * EXTERNAL_ASM define so that this is included.
 */

intptr_t
slp_save_state_asm(intptr_t* ref)
{
    intptr_t diff;
    SLP_SAVE_STATE(ref, diff);
    return diff;
}

void
slp_restore_state_asm(void)
{
    SLP_RESTORE_STATE();
}

extern int
slp_switch(void);

#endif

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

static int
g_save(PyGreenlet* g, char* stop)
{
    /* Save more of g's stack into the heap -- at least up to 'stop'

       g->stack_stop |________|
                     |        |
                     |    __ stop       . . . . .
                     |        |    ==>  .       .
                     |________|          _______
                     |        |         |       |
                     |        |         |       |
      g->stack_start |        |         |_______| g->stack_copy

     */
    intptr_t sz1 = g->stack_saved;
    intptr_t sz2 = stop - g->stack_start;
    assert(g->stack_start != NULL);
    if (sz2 > sz1) {
        char* c = (char*)PyMem_Realloc(g->stack_copy, sz2);
        if (!c) {
            PyErr_NoMemory();
            return -1;
        }
        memcpy(c + sz1, g->stack_start + sz1, sz2 - sz1);
        g->stack_copy = c;
        g->stack_saved = sz2;
    }
    return 0;
}

static void GREENLET_NOINLINE(slp_restore_state)(void)
{
    PyGreenlet* g = ts_target;
    PyGreenlet* owner = ts_current;

#ifdef SLP_BEFORE_RESTORE_STATE
    SLP_BEFORE_RESTORE_STATE();
#endif

    /* Restore the heap copy back into the C stack */
    if (g->stack_saved != 0) {
        memcpy(g->stack_start, g->stack_copy, g->stack_saved);
        PyMem_Free(g->stack_copy);
        g->stack_copy = NULL;
        g->stack_saved = 0;
    }
    if (owner->stack_start == NULL) {
        owner = owner->stack_prev; /* greenlet is dying, skip it */
    }
    while (owner && owner->stack_stop <= g->stack_stop) {
        owner = owner->stack_prev; /* find greenlet with more stack */
    }
    g->stack_prev = owner;
}

static int GREENLET_NOINLINE(slp_save_state)(char* stackref)
{
    /* must free all the C stack up to target_stop */
    char* target_stop = ts_target->stack_stop;
    PyGreenlet* owner = ts_current;
    assert(owner->stack_saved == 0);
    if (owner->stack_start == NULL) {
        owner = owner->stack_prev; /* not saved if dying */
    }
    else {
        owner->stack_start = stackref;
    }

#ifdef SLP_BEFORE_SAVE_STATE
    SLP_BEFORE_SAVE_STATE();
#endif

    while (owner->stack_stop < target_stop) {
        /* ts_current is entierely within the area to free */
        if (g_save(owner, owner->stack_stop)) {
            return -1; /* XXX */
        }
        owner = owner->stack_prev;
    }
    if (owner != ts_target) {
        if (g_save(owner, target_stop)) {
            return -1; /* XXX */
        }
    }
    return 0;
}

static int
g_switchstack(void)
{
    /* Perform a stack switch according to some global variables
       that must be set before:
       - ts_current: current greenlet (holds a reference)
       - ts_target: greenlet to switch to (weak reference)
       - ts_passaround_args: NULL if PyErr_Occurred(),
           else a tuple of args sent to ts_target (holds a reference)
       - ts_passaround_kwargs: switch kwargs (holds a reference)
       On return results are passed via global variables as well:
       - ts_origin: originating greenlet (holds a reference)
       - ts_current: current greenlet (holds a reference)
       - ts_passaround_args: NULL if PyErr_Occurred(),
           else a tuple of args sent to ts_current (holds a reference)
       - ts_passaround_kwargs: switch kwargs (holds a reference)
       It is very important that stack switch is 'atomic', i.e. no
       calls into other Python code allowed (except very few that
       are safe), because global variables are very fragile.
    */
    int err;
    { /* save state */
        PyGreenlet* current = ts_current;
        PyThreadState* tstate = PyThreadState_GET();
        current->recursion_depth = tstate->recursion_depth;
        current->top_frame = tstate->frame;
#if GREENLET_PY37
        current->context = tstate->context;
#endif
#if GREENLET_PY37
        current->exc_info = tstate->exc_info;
        current->exc_state = tstate->exc_state;
#else
        current->exc_type = tstate->exc_type;
        current->exc_value = tstate->exc_value;
        current->exc_traceback = tstate->exc_traceback;
#endif
#if GREENLET_USE_CFRAME
        current->cframe = tstate->cframe;
#endif
    }
    err = slp_switch();
    if (err < 0) { /* error */
        PyGreenlet* current = ts_current;
        current->top_frame = NULL;
#if GREENLET_PY37
        green_clear_exc(current);
#else
        current->exc_type = NULL;
        current->exc_value = NULL;
        current->exc_traceback = NULL;
#endif

        assert(ts_origin == NULL);
        ts_target = NULL;
    }
    else {
        PyGreenlet* target = ts_target;
        PyGreenlet* origin = ts_current;
        PyThreadState* tstate = PyThreadState_GET();
        tstate->recursion_depth = target->recursion_depth;
        tstate->frame = target->top_frame;
        target->top_frame = NULL;

#if GREENLET_PY37
        tstate->context = target->context;
        target->context = NULL;
        /* Incrementing this value invalidates the contextvars cache,
           which would otherwise remain valid across switches */
        tstate->context_ver++;
#endif

#if GREENLET_PY37
        tstate->exc_state = target->exc_state;
        tstate->exc_info =
            target->exc_info ? target->exc_info : &tstate->exc_state;
#else
        tstate->exc_type = target->exc_type;
        tstate->exc_value = target->exc_value;
        tstate->exc_traceback = target->exc_traceback;
#endif
        green_clear_exc(target);

#if GREENLET_USE_CFRAME
        tstate->cframe = target->cframe;
#endif

        assert(ts_origin == NULL);
        Py_INCREF(target);
        ts_current = target;
        ts_origin = origin;
        ts_target = NULL;
    }
    return err;
}

static int
g_calltrace(PyObject* tracefunc, PyObject* event, PyGreenlet* origin,
            PyGreenlet* target)
{
    PyObject* retval;
    PyObject *exc_type, *exc_val, *exc_tb;
    PyThreadState* tstate;
    PyErr_Fetch(&exc_type, &exc_val, &exc_tb);
    tstate = PyThreadState_GET();
    tstate->tracing++;
    TSTATE_USE_TRACING(tstate) = 0;
    retval = PyObject_CallFunction(tracefunc, "O(OO)", event, origin, target);
    tstate->tracing--;
    TSTATE_USE_TRACING(tstate) =
        (tstate->tracing <= 0 &&
         ((tstate->c_tracefunc != NULL) || (tstate->c_profilefunc != NULL)));
    if (retval == NULL) {
        /* In case of exceptions trace function is removed */
        if (PyDict_GetItem(tstate->dict, ts_tracekey)) {
            PyDict_DelItem(tstate->dict, ts_tracekey);
        }
        Py_XDECREF(exc_type);
        Py_XDECREF(exc_val);
        Py_XDECREF(exc_tb);
        return -1;
    }
    else {
        Py_DECREF(retval);
    }
    PyErr_Restore(exc_type, exc_val, exc_tb);
    return 0;
}

static PyObject*
g_switch(PyGreenlet* target, PyObject* args, PyObject* kwargs)
{
    /* _consumes_ a reference to the args tuple and kwargs dict,
       and return a new tuple reference */
    int err = 0;
    PyObject* run_info;

    /* check ts_current */
    if (!STATE_OK) {
        Py_XDECREF(args);
        Py_XDECREF(kwargs);
        return NULL;
    }
    run_info = green_statedict(target);
    if (run_info == NULL || run_info != ts_current->run_info) {
        Py_XDECREF(args);
        Py_XDECREF(kwargs);
        PyErr_SetString(PyExc_GreenletError,
                        run_info ?
                            "cannot switch to a different thread" :
                            "cannot switch to a garbage collected greenlet");
        return NULL;
    }

    ts_passaround_args = args;
    ts_passaround_kwargs = kwargs;

    /* find the real target by ignoring dead greenlets,
       and if necessary starting a greenlet. */
    while (target) {
        if (PyGreenlet_ACTIVE(target)) {
            ts_target = target;
            err = g_switchstack();
            break;
        }
        if (!PyGreenlet_STARTED(target)) {
            void* dummymarker;
            ts_target = target;
            err = g_initialstub(&dummymarker);
            if (err == 1) {
                continue; /* retry the switch */
            }
            break;
        }
        target = target->parent;
    }

    /* For a very short time, immediately after the 'atomic'
       g_switchstack() call, global variables are in a known state.
       We need to save everything we need, before it is destroyed
       by calls into arbitrary Python code. */
    args = ts_passaround_args;
    ts_passaround_args = NULL;
    kwargs = ts_passaround_kwargs;
    ts_passaround_kwargs = NULL;
    if (err < 0) {
        /* Turn switch errors into switch throws */
        assert(ts_origin == NULL);
        Py_CLEAR(kwargs);
        Py_CLEAR(args);
    }
    else {
        PyGreenlet* origin;
        PyGreenlet* current;
        PyObject* tracefunc;
        origin = ts_origin;
        ts_origin = NULL;

        current = ts_current;
        if ((tracefunc = PyDict_GetItem(current->run_info, ts_tracekey)) !=
            NULL) {
            Py_INCREF(tracefunc);
            if (g_calltrace(tracefunc,
                            args ? ts_event_switch : ts_event_throw,
                            origin,
                            current) < 0) {
                /* Turn trace errors into switch throws */
                Py_CLEAR(kwargs);
                Py_CLEAR(args);
            }
            Py_DECREF(tracefunc);
        }

        Py_DECREF(origin);
    }

    /* We need to figure out what values to pass to the target greenlet
       based on the arguments that have been passed to greenlet.switch(). If
       switch() was just passed an arg tuple, then we'll just return that.
       If only keyword arguments were passed, then we'll pass the keyword
       argument dict. Otherwise, we'll create a tuple of (args, kwargs) and
       return both. */
    if (kwargs == NULL) {
        return args;
    }
    else if (PyDict_Size(kwargs) == 0) {
        Py_DECREF(kwargs);
        return args;
    }
    else if (PySequence_Length(args) == 0) {
        Py_DECREF(args);
        return kwargs;
    }
    else {
        PyObject* tuple = PyTuple_New(2);
        if (tuple == NULL) {
            Py_DECREF(args);
            Py_DECREF(kwargs);
            return NULL;
        }
        PyTuple_SET_ITEM(tuple, 0, args);
        PyTuple_SET_ITEM(tuple, 1, kwargs);
        return tuple;
    }
}

static PyObject*
g_handle_exit(PyObject* result)
{
    if (result == NULL && PyErr_ExceptionMatches(PyExc_GreenletExit)) {
        /* catch and ignore GreenletExit */
        PyObject *exc, *val, *tb;
        PyErr_Fetch(&exc, &val, &tb);
        if (val == NULL) {
            Py_INCREF(Py_None);
            val = Py_None;
        }
        result = val;
        Py_DECREF(exc);
        Py_XDECREF(tb);
    }
    if (result != NULL) {
        /* package the result into a 1-tuple */
        PyObject* r = result;
        result = PyTuple_New(1);
        if (result) {
            PyTuple_SET_ITEM(result, 0, r);
        }
        else {
            Py_DECREF(r);
        }
    }
    return result;
}

static int GREENLET_NOINLINE(g_initialstub)(void* mark)
{
    int err;
    PyObject *o, *run;
    PyObject *exc, *val, *tb;
    PyObject* run_info;
    PyGreenlet* self = ts_target;
    PyObject* args = ts_passaround_args;
    PyObject* kwargs = ts_passaround_kwargs;

    /* save exception in case getattr clears it */
    PyErr_Fetch(&exc, &val, &tb);
    /* self.run is the object to call in the new greenlet */
    run = PyObject_GetAttrString((PyObject*)self, "run");
    if (run == NULL) {
        Py_XDECREF(exc);
        Py_XDECREF(val);
        Py_XDECREF(tb);
        return -1;
    }
    /* restore saved exception */
    PyErr_Restore(exc, val, tb);

    /* recheck the state in case getattr caused thread switches */
    if (!STATE_OK) {
        Py_DECREF(run);
        return -1;
    }

    /* recheck run_info in case greenlet reparented anywhere above */
    run_info = green_statedict(self);
    if (run_info == NULL || run_info != ts_current->run_info) {
        Py_DECREF(run);
        PyErr_SetString(PyExc_GreenletError,
                        run_info ?
                            "cannot switch to a different thread" :
                            "cannot switch to a garbage collected greenlet");
        return -1;
    }

    /* by the time we got here another start could happen elsewhere,
     * that means it should now be a regular switch
     */
    if (PyGreenlet_STARTED(self)) {
        Py_DECREF(run);
        ts_passaround_args = args;
        ts_passaround_kwargs = kwargs;
        return 1;
    }

    /* start the greenlet */
    self->stack_start = NULL;
    self->stack_stop = (char*)mark;
    if (ts_current->stack_start == NULL) {
        /* ts_current is dying */
        self->stack_prev = ts_current->stack_prev;
    }
    else {
        self->stack_prev = ts_current;
    }
    self->top_frame = NULL;
    green_clear_exc(self);
    self->recursion_depth = PyThreadState_GET()->recursion_depth;

    /* restore arguments in case they are clobbered */
    ts_target = self;
    ts_passaround_args = args;
    ts_passaround_kwargs = kwargs;

    /* perform the initial switch */
    err = g_switchstack();

    /* returns twice!
       The 1st time with err=1: we are in the new greenlet
       The 2nd time with err=0: back in the caller's greenlet
    */
    if (err == 1) {
        /* in the new greenlet */
        PyGreenlet* origin;
        PyObject* tracefunc;
        PyObject* result;
        PyGreenlet* parent;
        self->stack_start = (char*)1; /* running */

        /* grab origin while we still can */
        origin = ts_origin;
        ts_origin = NULL;

        /* now use run_info to store the statedict */
        o = self->run_info;
        self->run_info = green_statedict(self->parent);
        Py_INCREF(self->run_info);
        Py_XDECREF(o);

        if ((tracefunc = PyDict_GetItem(self->run_info, ts_tracekey)) !=
            NULL) {
            Py_INCREF(tracefunc);
            if (g_calltrace(tracefunc,
                            args ? ts_event_switch : ts_event_throw,
                            origin,
                            self) < 0) {
                /* Turn trace errors into switch throws */
                Py_CLEAR(kwargs);
                Py_CLEAR(args);
            }
            Py_DECREF(tracefunc);
        }

        Py_DECREF(origin);

        if (args == NULL) {
            /* pending exception */
            result = NULL;
        }
        else {
            /* call g.run(*args, **kwargs) */
            result = PyObject_Call(run, args, kwargs);
            Py_DECREF(args);
            Py_XDECREF(kwargs);
        }
        Py_DECREF(run);
        result = g_handle_exit(result);

        /* jump back to parent */
        self->stack_start = NULL; /* dead */
        for (parent = self->parent; parent != NULL; parent = parent->parent) {
            result = g_switch(parent, result, NULL);
            /* Return here means switch to parent failed,
             * in which case we throw *current* exception
             * to the next parent in chain.
             */
            assert(result == NULL);
        }
        /* We ran out of parents, cannot continue */
        PyErr_WriteUnraisable((PyObject*)self);
        Py_FatalError("greenlets cannot continue");
    }
    /* back in the parent */
    if (err < 0) {
        /* start failed badly, restore greenlet state */
        self->stack_start = NULL;
        self->stack_stop = NULL;
        self->stack_prev = NULL;
    }
    return err;
}

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

static PyObject*
green_new(PyTypeObject* type, PyObject* args, PyObject* kwds)
{
    PyObject* o =
        PyBaseObject_Type.tp_new(type, ts_empty_tuple, ts_empty_dict);
    if (o != NULL) {
        if (!STATE_OK) {
            Py_DECREF(o);
            return NULL;
        }
        Py_INCREF(ts_current);
        ((PyGreenlet*)o)->parent = ts_current;
#if GREENLET_USE_CFRAME
        ((PyGreenlet*)o)->cframe = &PyThreadState_GET()->root_cframe;
#endif
    }
    return o;
}

static int
green_setrun(PyGreenlet* self, PyObject* nrun, void* c);
static int
green_setparent(PyGreenlet* self, PyObject* nparent, void* c);

static int
green_init(PyGreenlet* self, PyObject* args, PyObject* kwargs)
{
    PyObject* run = NULL;
    PyObject* nparent = NULL;
    static char* kwlist[] = {"run", "parent", 0};
    if (!PyArg_ParseTupleAndKeywords(
            args, kwargs, "|OO:green", kwlist, &run, &nparent)) {
        return -1;
    }

    if (run != NULL) {
        if (green_setrun(self, run, NULL)) {
            return -1;
        }
    }
    if (nparent != NULL && nparent != Py_None) {
        return green_setparent(self, nparent, NULL);
    }
    return 0;
}

static int
kill_greenlet(PyGreenlet* self)
{
    /* Cannot raise an exception to kill the greenlet if
       it is not running in the same thread! */
    if (self->run_info == PyThreadState_GET()->dict) {
        /* The dying greenlet cannot be a parent of ts_current
           because the 'parent' field chain would hold a
           reference */
        PyObject* result;
        PyGreenlet* oldparent;
        PyGreenlet* tmp;
        if (!STATE_OK) {
            return -1;
        }
        oldparent = self->parent;
        self->parent = ts_current;
        Py_INCREF(self->parent);
        /* Send the greenlet a GreenletExit exception. */
        PyErr_SetNone(PyExc_GreenletExit);
        result = g_switch(self, NULL, NULL);
        tmp = self->parent;
        self->parent = oldparent;
        Py_XDECREF(tmp);
        if (result == NULL) {
            return -1;
        }
        Py_DECREF(result);
        return 0;
    }
    else {
        /* Not the same thread! Temporarily save the greenlet
           into its thread's ts_delkey list. */
        PyObject* lst;
        lst = PyDict_GetItem(self->run_info, ts_delkey);
        if (lst == NULL) {
            lst = PyList_New(0);
            if (lst == NULL ||
                PyDict_SetItem(self->run_info, ts_delkey, lst) < 0) {
                return -1;
            }
        }
        if (PyList_Append(lst, (PyObject*)self) < 0) {
            return -1;
        }
        if (!STATE_OK) { /* to force ts_delkey to be reconsidered */
            return -1;
        }
        return 0;
    }
}

static int
green_traverse(PyGreenlet* self, visitproc visit, void* arg)
{
    /* We must only visit referenced objects, i.e. only objects
       Py_INCREF'ed by this greenlet (directly or indirectly):
       - stack_prev is not visited: holds previous stack pointer, but it's not
       referenced
       - frames are not visited: alive greenlets are not garbage collected
       anyway */
    Py_VISIT((PyObject*)self->parent);
    Py_VISIT(self->run_info);
#if GREENLET_PY37
    Py_VISIT(self->context);
#endif
#if GREENLET_PY37
    Py_VISIT(self->exc_state.exc_type);
    Py_VISIT(self->exc_state.exc_value);
    Py_VISIT(self->exc_state.exc_traceback);
#else
    Py_VISIT(self->exc_type);
    Py_VISIT(self->exc_value);
    Py_VISIT(self->exc_traceback);
#endif
    Py_VISIT(self->dict);
    return 0;
}

static int
green_is_gc(PyGreenlet* self)
{
    /* Main greenlet can be garbage collected since it can only
       become unreachable if the underlying thread exited.
       Active greenlet cannot be garbage collected, however. */
    if (PyGreenlet_MAIN(self) || !PyGreenlet_ACTIVE(self)) {
        return 1;
    }
    return 0;
}

static int
green_clear(PyGreenlet* self)
{
    /* Greenlet is only cleared if it is about to be collected.
       Since active greenlets are not garbage collectable, we can
       be sure that, even if they are deallocated during clear,
       nothing they reference is in unreachable or finalizers,
       so even if it switches we are relatively safe. */
    Py_CLEAR(self->parent);
    Py_CLEAR(self->run_info);
#if GREENLET_PY37
    Py_CLEAR(self->context);
#endif
#if GREENLET_PY37
    Py_CLEAR(self->exc_state.exc_type);
    Py_CLEAR(self->exc_state.exc_value);
    Py_CLEAR(self->exc_state.exc_traceback);
#else
    Py_CLEAR(self->exc_type);
    Py_CLEAR(self->exc_value);
    Py_CLEAR(self->exc_traceback);
#endif
    Py_CLEAR(self->dict);
    return 0;
}

static void
green_dealloc(PyGreenlet* self)
{
    PyObject *error_type, *error_value, *error_traceback;
    Py_ssize_t refcnt;

    PyObject_GC_UnTrack(self);

    if (PyGreenlet_ACTIVE(self) && self->run_info != NULL &&
        !PyGreenlet_MAIN(self)) {
        /* Hacks hacks hacks copied from instance_dealloc() */
        /* Temporarily resurrect the greenlet. */
        assert(Py_REFCNT(self) == 0);
        Py_SET_REFCNT(self, 1);
        /* Save the current exception, if any. */
        PyErr_Fetch(&error_type, &error_value, &error_traceback);
        if (kill_greenlet(self) < 0) {
            PyErr_WriteUnraisable((PyObject*)self);
            /* XXX what else should we do? */
        }
        /* Check for no resurrection must be done while we keep
         * our internal reference, otherwise PyFile_WriteObject
         * causes recursion if using Py_INCREF/Py_DECREF
         */
        if (Py_REFCNT(self) == 1 && PyGreenlet_ACTIVE(self)) {
            /* Not resurrected, but still not dead!
               XXX what else should we do? we complain. */
            PyObject* f = PySys_GetObject("stderr");
            Py_INCREF(self); /* leak! */
            if (f != NULL) {
                PyFile_WriteString("GreenletExit did not kill ", f);
                PyFile_WriteObject((PyObject*)self, f, 0);
                PyFile_WriteString("\n", f);
            }
        }
        /* Restore the saved exception. */
        PyErr_Restore(error_type, error_value, error_traceback);
        /* Undo the temporary resurrection; can't use DECREF here,
         * it would cause a recursive call.
         */
        assert(Py_REFCNT(self) > 0);

        refcnt = Py_REFCNT(self) - 1;
        Py_SET_REFCNT(self, refcnt);
        if (refcnt != 0) {
            /* Resurrected! */
            _Py_NewReference((PyObject*)self);
            Py_SET_REFCNT(self, refcnt);

            PyObject_GC_Track((PyObject*)self);

            _Py_DEC_REFTOTAL;
#ifdef COUNT_ALLOCS
            --Py_TYPE(self)->tp_frees;
            --Py_TYPE(self)->tp_allocs;
#endif /* COUNT_ALLOCS */
            return;
        }
    }
    if (self->weakreflist != NULL) {
        PyObject_ClearWeakRefs((PyObject*)self);
    }
    Py_CLEAR(self->parent);
    Py_CLEAR(self->run_info);
#if GREENLET_PY37
    Py_CLEAR(self->context);
#endif
#if GREENLET_PY37
    Py_CLEAR(self->exc_state.exc_type);
    Py_CLEAR(self->exc_state.exc_value);
    Py_CLEAR(self->exc_state.exc_traceback);
#else
    Py_CLEAR(self->exc_type);
    Py_CLEAR(self->exc_value);
    Py_CLEAR(self->exc_traceback);
#endif
    Py_CLEAR(self->dict);
    Py_TYPE(self)->tp_free((PyObject*)self);
}

static PyObject*
single_result(PyObject* results)
{
    if (results != NULL && PyTuple_Check(results) &&
        PyTuple_GET_SIZE(results) == 1) {
        PyObject* result = PyTuple_GET_ITEM(results, 0);
        Py_INCREF(result);
        Py_DECREF(results);
        return result;
    }
    else {
        return results;
    }
}

static PyObject*
throw_greenlet(PyGreenlet* self, PyObject* typ, PyObject* val, PyObject* tb)
{
    /* Note: _consumes_ a reference to typ, val, tb */
    PyObject* result = NULL;
    PyErr_Restore(typ, val, tb);
    if (PyGreenlet_STARTED(self) && !PyGreenlet_ACTIVE(self)) {
        /* dead greenlet: turn GreenletExit into a regular return */
        result = g_handle_exit(result);
    }
    return single_result(g_switch(self, result, NULL));
}

PyDoc_STRVAR(
    green_switch_doc,
    "switch(*args, **kwargs)\n"
    "\n"
    "Switch execution to this greenlet.\n"
    "\n"
    "If this greenlet has never been run, then this greenlet\n"
    "will be switched to using the body of ``self.run(*args, **kwargs)``.\n"
    "\n"
    "If the greenlet is active (has been run, but was switch()'ed\n"
    "out before leaving its run function), then this greenlet will\n"
    "be resumed and the return value to its switch call will be\n"
    "None if no arguments are given, the given argument if one\n"
    "argument is given, or the args tuple and keyword args dict if\n"
    "multiple arguments are given.\n"
    "\n"
    "If the greenlet is dead, or is the current greenlet then this\n"
    "function will simply return the arguments using the same rules as\n"
    "above.\n");

static PyObject*
green_switch(PyGreenlet* self, PyObject* args, PyObject* kwargs)
{
    Py_INCREF(args);
    Py_XINCREF(kwargs);
    return single_result(g_switch(self, args, kwargs));
}

PyDoc_STRVAR(
    green_throw_doc,
    "Switches execution to this greenlet, but immediately raises the\n"
    "given exception in this greenlet.  If no argument is provided, the "
    "exception\n"
    "defaults to `greenlet.GreenletExit`.  The normal exception\n"
    "propagation rules apply, as described for `switch`.  Note that calling "
    "this\n"
    "method is almost equivalent to the following::\n"
    "\n"
    "    def raiser():\n"
    "        raise typ, val, tb\n"
    "    g_raiser = greenlet(raiser, parent=g)\n"
    "    g_raiser.switch()\n"
    "\n"
    "except that this trick does not work for the\n"
    "`greenlet.GreenletExit` exception, which would not propagate\n"
    "from ``g_raiser`` to ``g``.\n");

static PyObject*
green_throw(PyGreenlet* self, PyObject* args)
{
    PyObject* typ = PyExc_GreenletExit;
    PyObject* val = NULL;
    PyObject* tb = NULL;

    if (!PyArg_ParseTuple(args, "|OOO:throw", &typ, &val, &tb)) {
        return NULL;
    }

    /* First, check the traceback argument, replacing None, with NULL */
    if (tb == Py_None) {
        tb = NULL;
    }
    else if (tb != NULL && !PyTraceBack_Check(tb)) {
        PyErr_SetString(PyExc_TypeError,
                        "throw() third argument must be a traceback object");
        return NULL;
    }

    Py_INCREF(typ);
    Py_XINCREF(val);
    Py_XINCREF(tb);

    if (PyExceptionClass_Check(typ)) {
        PyErr_NormalizeException(&typ, &val, &tb);
    }
    else if (PyExceptionInstance_Check(typ)) {
        /* Raising an instance. The value should be a dummy. */
        if (val && val != Py_None) {
            PyErr_SetString(
                PyExc_TypeError,
                "instance exception may not have a separate value");
            goto failed_throw;
        }
        else {
            /* Normalize to raise <class>, <instance> */
            Py_XDECREF(val);
            val = typ;
            typ = PyExceptionInstance_Class(typ);
            Py_INCREF(typ);
        }
    }
    else {
        /* Not something you can raise. throw() fails. */
        PyErr_Format(PyExc_TypeError,
                     "exceptions must be classes, or instances, not %s",
                     Py_TYPE(typ)->tp_name);
        goto failed_throw;
    }

    return throw_greenlet(self, typ, val, tb);

failed_throw:
    /* Didn't use our arguments, so restore their original refcounts */
    Py_DECREF(typ);
    Py_XDECREF(val);
    Py_XDECREF(tb);
    return NULL;
}

static int
green_bool(PyGreenlet* self)
{
    return PyGreenlet_ACTIVE(self);
}

static PyObject*
green_getdict(PyGreenlet* self, void* c)
{
    if (self->dict == NULL) {
        self->dict = PyDict_New();
        if (self->dict == NULL) {
            return NULL;
        }
    }
    Py_INCREF(self->dict);
    return self->dict;
}

static int
green_setdict(PyGreenlet* self, PyObject* val, void* c)
{
    PyObject* tmp;

    if (val == NULL) {
        PyErr_SetString(PyExc_TypeError, "__dict__ may not be deleted");
        return -1;
    }
    if (!PyDict_Check(val)) {
        PyErr_SetString(PyExc_TypeError, "__dict__ must be a dictionary");
        return -1;
    }
    tmp = self->dict;
    Py_INCREF(val);
    self->dict = val;
    Py_XDECREF(tmp);
    return 0;
}

static int
_green_not_dead(PyGreenlet* self)
{
    return PyGreenlet_ACTIVE(self) || !PyGreenlet_STARTED(self);
}


static PyObject*
green_getdead(PyGreenlet* self, void* c)
{
    if (_green_not_dead(self)) {
        Py_RETURN_FALSE;
    }
    else {
        Py_RETURN_TRUE;
    }
}

static PyObject*
green_get_stack_saved(PyGreenlet* self, void* c)
{
    return PyLong_FromSsize_t(self->stack_saved);
}

static PyObject*
green_getrun(PyGreenlet* self, void* c)
{
    if (PyGreenlet_STARTED(self) || self->run_info == NULL) {
        PyErr_SetString(PyExc_AttributeError, "run");
        return NULL;
    }
    Py_INCREF(self->run_info);
    return self->run_info;
}

static int
green_setrun(PyGreenlet* self, PyObject* nrun, void* c)
{
    PyObject* o;
    if (PyGreenlet_STARTED(self)) {
        PyErr_SetString(PyExc_AttributeError,
                        "run cannot be set "
                        "after the start of the greenlet");
        return -1;
    }
    o = self->run_info;
    self->run_info = nrun;
    Py_XINCREF(nrun);
    Py_XDECREF(o);
    return 0;
}

static PyObject*
green_getparent(PyGreenlet* self, void* c)
{
    PyObject* result = self->parent ? (PyObject*)self->parent : Py_None;
    Py_INCREF(result);
    return result;
}

static int
green_setparent(PyGreenlet* self, PyObject* nparent, void* c)
{
    PyGreenlet* p;
    PyObject* run_info = NULL;
    if (nparent == NULL) {
        PyErr_SetString(PyExc_AttributeError, "can't delete attribute");
        return -1;
    }
    if (!PyGreenlet_Check(nparent)) {
        PyErr_SetString(PyExc_TypeError, "parent must be a greenlet");
        return -1;
    }
    for (p = (PyGreenlet*)nparent; p; p = p->parent) {
        if (p == self) {
            PyErr_SetString(PyExc_ValueError, "cyclic parent chain");
            return -1;
        }
        run_info = PyGreenlet_ACTIVE(p) ? p->run_info : NULL;
    }
    if (run_info == NULL) {
        PyErr_SetString(PyExc_ValueError,
                        "parent must not be garbage collected");
        return -1;
    }
    if (PyGreenlet_STARTED(self) && self->run_info != run_info) {
        PyErr_SetString(PyExc_ValueError,
                        "parent cannot be on a different thread");
        return -1;
    }
    p = self->parent;
    self->parent = (PyGreenlet*)nparent;
    Py_INCREF(nparent);
    Py_XDECREF(p);
    return 0;
}

#ifdef Py_CONTEXT_H
#    define GREENLET_NO_CONTEXTVARS_REASON "This build of greenlet"
#else
#    define GREENLET_NO_CONTEXTVARS_REASON "This Python interpreter"
#endif

static PyObject*
green_getcontext(PyGreenlet* self, void* c)
{
#if GREENLET_PY37
    PyThreadState* tstate = PyThreadState_GET();
    PyObject* result;

    if (!STATE_OK) {
        return NULL;
    }
    if (PyGreenlet_ACTIVE(self) && self->top_frame == NULL) {
        /* Currently running greenlet: context is stored in the thread state,
           not the greenlet object. */
        if (self == ts_current) {
            result = tstate->context;
        }
        else {
            PyErr_SetString(PyExc_ValueError,
                            "cannot get context of a "
                            "greenlet that is running in a different thread");
            return NULL;
        }
    }
    else {
        /* Greenlet is not running: just return context. */
        result = self->context;
    }
    if (result == NULL) {
        result = Py_None;
    }
    Py_INCREF(result);
    return result;
#else
    PyErr_SetString(PyExc_AttributeError,
                    GREENLET_NO_CONTEXTVARS_REASON
                    " does not support context variables");
    return NULL;
#endif
}

static int
green_setcontext(PyGreenlet* self, PyObject* nctx, void* c)
{
#if GREENLET_PY37
    PyThreadState* tstate;
    PyObject* octx = NULL;
    if (!STATE_OK) {
        return -1;
    }
    if (nctx == NULL) {
        PyErr_SetString(PyExc_AttributeError, "can't delete attribute");
        return -1;
    }
    if (nctx == Py_None) {
        /* "Empty context" is stored as NULL, not None. */
        nctx = NULL;
    }
    else if (!PyContext_CheckExact(nctx)) {
        PyErr_SetString(PyExc_TypeError,
                        "greenlet context must be a "
                        "contextvars.Context or None");
        return -1;
    }
    tstate = PyThreadState_GET();
    if (PyGreenlet_ACTIVE(self) && self->top_frame == NULL) {
        /* Currently running greenlet: context is stored in the thread state,
           not the greenlet object. */
        if (self == ts_current) {
            octx = tstate->context;
            tstate->context = nctx;
            tstate->context_ver++;
            Py_XINCREF(nctx);
        }
        else {
            PyErr_SetString(PyExc_ValueError,
                            "cannot set context of a "
                            "greenlet that is running in a different thread");
            return -1;
        }
    }
    else {
        /* Greenlet is not running: just set context. */
        octx = self->context;
        self->context = nctx;
        Py_XINCREF(nctx);
    }
    Py_XDECREF(octx);
    return 0;
#else
    PyErr_SetString(PyExc_AttributeError,
                    GREENLET_NO_CONTEXTVARS_REASON
                    " does not support context variables");
    return -1;
#endif
}

#undef GREENLET_NO_CONTEXTVARS_REASON

static PyObject*
green_getframe(PyGreenlet* self, void* c)
{
    PyObject* result = self->top_frame ? (PyObject*)self->top_frame : Py_None;
    Py_INCREF(result);
    return result;
}

static PyObject*
green_getstate(PyGreenlet* self)
{
    PyErr_Format(PyExc_TypeError,
                 "cannot serialize '%s' object",
                 Py_TYPE(self)->tp_name);
    return NULL;
}

static PyObject*
green_repr(PyGreenlet* self)
{
    /*
      Return a string like
      <greenlet.greenlet at 0xdeadbeef [current][active started]|dead main>

      The handling of greenlets across threads is not super good.
      We mostly use the internal definitions of these terms, but they
      generally should make sense to users as well.
     */
    PyObject* result;
    int never_started = !PyGreenlet_STARTED(self) && !PyGreenlet_ACTIVE(self);

    if (!STATE_OK) {
        return NULL;
    }

#if PY_MAJOR_VERSION >= 3
# define GNative_FromFormat PyUnicode_FromFormat
#else
# define GNative_FromFormat PyString_FromFormat
#endif

    if (_green_not_dead(self)) {
        result = GNative_FromFormat(
            "<%s object at %p (otid=%p)%s%s%s%s>",
            Py_TYPE(self)->tp_name,
            self,
            self->run_info,
            ts_current == self
                ? " current"
                : (PyGreenlet_STARTED(self) ? " suspended" : ""),
            PyGreenlet_ACTIVE(self) ? " active" : "",
            never_started ? " pending" : " started",
            PyGreenlet_MAIN(self) ? " main" : ""
        );
    }
    else {
        /* main greenlets never really appear dead. */
        result = GNative_FromFormat(
            "<%s object at %p (otid=%p) dead>",
            Py_TYPE(self)->tp_name,
            self,
            self->run_info
            );
    }
#undef GNative_FromFormat

    return result;
}

/*****************************************************************************
 * C interface
 *
 * These are exported using the CObject API
 */

static PyGreenlet*
PyGreenlet_GetCurrent(void)
{
    if (!STATE_OK) {
        return NULL;
    }
    Py_INCREF(ts_current);
    return ts_current;
}

static int
PyGreenlet_SetParent(PyGreenlet* g, PyGreenlet* nparent)
{
    if (!PyGreenlet_Check(g)) {
        PyErr_SetString(PyExc_TypeError, "parent must be a greenlet");
        return -1;
    }

    return green_setparent((PyGreenlet*)g, (PyObject*)nparent, NULL);
}

static PyGreenlet*
PyGreenlet_New(PyObject* run, PyGreenlet* parent)
{
    /* XXX: Why doesn't this call green_new()? There's some duplicate
     code. */
    PyGreenlet* g = NULL;
    g = (PyGreenlet*)PyType_GenericAlloc(&PyGreenlet_Type, 0);
    if (g == NULL) {
        return NULL;
    }

    if (run != NULL) {
        Py_INCREF(run);
        g->run_info = run;
    }

    if (parent != NULL) {
        if (PyGreenlet_SetParent(g, parent)) {
            Py_DECREF(g);
            return NULL;
        }
    }
    else {
        if ((g->parent = PyGreenlet_GetCurrent()) == NULL) {
            Py_DECREF(g);
            return NULL;
        }
    }
#if GREENLET_USE_CFRAME
    g->cframe = &PyThreadState_GET()->root_cframe;
#endif
    return g;
}

static PyObject*
PyGreenlet_Switch(PyGreenlet* g, PyObject* args, PyObject* kwargs)
{
    PyGreenlet* self = (PyGreenlet*)g;

    if (!PyGreenlet_Check(self)) {
        PyErr_BadArgument();
        return NULL;
    }

    if (args == NULL) {
        args = Py_BuildValue("()");
    }
    else {
        Py_INCREF(args);
    }

    if (kwargs != NULL && PyDict_Check(kwargs)) {
        Py_INCREF(kwargs);
    }
    else {
        kwargs = NULL;
    }

    return single_result(g_switch(self, args, kwargs));
}

static PyObject*
PyGreenlet_Throw(PyGreenlet* self, PyObject* typ, PyObject* val, PyObject* tb)
{
    if (!PyGreenlet_Check(self)) {
        PyErr_BadArgument();
        return NULL;
    }
    Py_INCREF(typ);
    Py_XINCREF(val);
    Py_XINCREF(tb);
    return throw_greenlet(self, typ, val, tb);
}

/** End C API ****************************************************************/

static PyMethodDef green_methods[] = {
    {"switch",
     (PyCFunction)green_switch,
     METH_VARARGS | METH_KEYWORDS,
     green_switch_doc},
    {"throw", (PyCFunction)green_throw, METH_VARARGS, green_throw_doc},
    {"__getstate__", (PyCFunction)green_getstate, METH_NOARGS, NULL},
    {NULL, NULL} /* sentinel */
};

static PyGetSetDef green_getsets[] = {
    {"__dict__", (getter)green_getdict, (setter)green_setdict, /*XXX*/ NULL},
    {"run", (getter)green_getrun, (setter)green_setrun, /*XXX*/ NULL},
    {"parent", (getter)green_getparent, (setter)green_setparent, /*XXX*/ NULL},
    {"gr_frame", (getter)green_getframe, NULL, /*XXX*/ NULL},
    {"gr_context",
     (getter)green_getcontext,
     (setter)green_setcontext,
     /*XXX*/ NULL},
    {"dead", (getter)green_getdead, NULL, /*XXX*/ NULL},
    {"_stack_saved", (getter)green_get_stack_saved, NULL, /*XXX*/ NULL},
    {NULL}};

static PyNumberMethods green_as_number = {
    NULL, /* nb_add */
    NULL, /* nb_subtract */
    NULL, /* nb_multiply */
#if PY_MAJOR_VERSION < 3
    NULL, /* nb_divide */
#endif
    NULL,                /* nb_remainder */
    NULL,                /* nb_divmod */
    NULL,                /* nb_power */
    NULL,                /* nb_negative */
    NULL,                /* nb_positive */
    NULL,                /* nb_absolute */
    (inquiry)green_bool, /* nb_bool */
};

PyTypeObject PyGreenlet_Type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "greenlet.greenlet", /* tp_name */
    sizeof(PyGreenlet),  /* tp_basicsize */
    0,                   /* tp_itemsize */
    /* methods */
    (destructor)green_dealloc, /* tp_dealloc */
    0,                         /* tp_print */
    0,                         /* tp_getattr */
    0,                         /* tp_setattr */
    0,                         /* tp_compare */
    (reprfunc)green_repr,      /* tp_repr */
    &green_as_number,          /* tp_as _number*/
    0,                         /* tp_as _sequence*/
    0,                         /* tp_as _mapping*/
    0,                         /* tp_hash */
    0,                         /* tp_call */
    0,                         /* tp_str */
    0,                         /* tp_getattro */
    0,                         /* tp_setattro */
    0,                         /* tp_as_buffer*/
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE |
        GREENLET_GC_FLAGS, /* tp_flags */
    "greenlet(run=None, parent=None) -> greenlet\n\n"
    "Creates a new greenlet object (without running it).\n\n"
    " - *run* -- The callable to invoke.\n"
    " - *parent* -- The parent greenlet. The default is the current "
    "greenlet.",                        /* tp_doc */
    (traverseproc)GREENLET_tp_traverse, /* tp_traverse */
    (inquiry)GREENLET_tp_clear,         /* tp_clear */
    0,                                  /* tp_richcompare */
    offsetof(PyGreenlet, weakreflist),  /* tp_weaklistoffset */
    0,                                  /* tp_iter */
    0,                                  /* tp_iternext */
    green_methods,                      /* tp_methods */
    0,                                  /* tp_members */
    green_getsets,                      /* tp_getset */
    0,                                  /* tp_base */
    0,                                  /* tp_dict */
    0,                                  /* tp_descr_get */
    0,                                  /* tp_descr_set */
    offsetof(PyGreenlet, dict),         /* tp_dictoffset */
    (initproc)green_init,               /* tp_init */
    GREENLET_tp_alloc,                  /* tp_alloc */
    green_new,                          /* tp_new */
    GREENLET_tp_free,                   /* tp_free */
    (inquiry)GREENLET_tp_is_gc,         /* tp_is_gc */
};

PyDoc_STRVAR(mod_getcurrent_doc,
             "getcurrent() -> greenlet\n"
             "\n"
             "Returns the current greenlet (i.e. the one which called this "
             "function).\n");

static PyObject*
mod_getcurrent(PyObject* self)
{
    if (!STATE_OK) {
        return NULL;
    }
    Py_INCREF(ts_current);
    return (PyObject*)ts_current;
}

PyDoc_STRVAR(mod_settrace_doc,
             "settrace(callback) -> object\n"
             "\n"
             "Sets a new tracing function and returns the previous one.\n");
static PyObject*
mod_settrace(PyObject* self, PyObject* args)
{
    int err;
    PyObject* previous;
    PyObject* tracefunc;
    PyGreenlet* current;
    if (!PyArg_ParseTuple(args, "O", &tracefunc)) {
        return NULL;
    }
    if (!STATE_OK) {
        return NULL;
    }
    current = ts_current;
    previous = PyDict_GetItem(current->run_info, ts_tracekey);
    if (previous == NULL) {
        previous = Py_None;
    }
    Py_INCREF(previous);
    if (tracefunc == Py_None) {
        err = previous != Py_None ?
                  PyDict_DelItem(current->run_info, ts_tracekey) :
                  0;
    }
    else {
        err = PyDict_SetItem(current->run_info, ts_tracekey, tracefunc);
    }
    if (err < 0) {
        Py_CLEAR(previous);
    }
    return previous;
}

PyDoc_STRVAR(mod_gettrace_doc,
             "gettrace() -> object\n"
             "\n"
             "Returns the currently set tracing function, or None.\n");

static PyObject*
mod_gettrace(PyObject* self)
{
    PyObject* tracefunc;
    if (!STATE_OK) {
        return NULL;
    }
    tracefunc = PyDict_GetItem(ts_current->run_info, ts_tracekey);
    if (tracefunc == NULL) {
        tracefunc = Py_None;
    }
    Py_INCREF(tracefunc);
    return tracefunc;
}

static PyMethodDef GreenMethods[] = {
    {"getcurrent",
     (PyCFunction)mod_getcurrent,
     METH_NOARGS,
     mod_getcurrent_doc},
    {"settrace", (PyCFunction)mod_settrace, METH_VARARGS, mod_settrace_doc},
    {"gettrace", (PyCFunction)mod_gettrace, METH_NOARGS, mod_gettrace_doc},
    {NULL, NULL} /* Sentinel */
};

static char* copy_on_greentype[] = {
    "getcurrent", "error", "GreenletExit", "settrace", "gettrace", NULL};

#if PY_MAJOR_VERSION >= 3
#    define INITERROR return NULL

static struct PyModuleDef greenlet_module_def = {
    PyModuleDef_HEAD_INIT,
    "greenlet._greenlet",
    NULL,
    -1,
    GreenMethods,
};

PyMODINIT_FUNC
PyInit__greenlet(void)
#else
#    define INITERROR return

PyMODINIT_FUNC
init_greenlet(void)
#endif
{
    PyObject* m = NULL;
    char** p = NULL;
    PyObject* c_api_object;
    static void* _PyGreenlet_API[PyGreenlet_API_pointers];

    GREENLET_NOINLINE_INIT();

#if PY_MAJOR_VERSION >= 3
    m = PyModule_Create(&greenlet_module_def);
#else
    m = Py_InitModule("greenlet._greenlet", GreenMethods);
#endif
    if (m == NULL) {
        INITERROR;
    }

#if PY_MAJOR_VERSION >= 3
#    define Greenlet_Intern PyUnicode_InternFromString
#else
#    define Greenlet_Intern PyString_InternFromString
#endif
    ts_curkey = Greenlet_Intern("__greenlet_ts_curkey");
    ts_delkey = Greenlet_Intern("__greenlet_ts_delkey");
    ts_tracekey = Greenlet_Intern("__greenlet_ts_tracekey");
    ts_event_switch = Greenlet_Intern("switch");
    ts_event_throw = Greenlet_Intern("throw");
#undef Greenlet_Intern

    if (ts_curkey == NULL || ts_delkey == NULL) {
        INITERROR;
    }
    if (PyType_Ready(&PyGreenlet_Type) < 0) {
        INITERROR;
    }
    PyExc_GreenletError = PyErr_NewException("greenlet.error", NULL, NULL);
    if (PyExc_GreenletError == NULL) {
        INITERROR;
    }
    PyExc_GreenletExit =
        PyErr_NewException("greenlet.GreenletExit", PyExc_BaseException, NULL);
    if (PyExc_GreenletExit == NULL) {
        INITERROR;
    }

    ts_empty_tuple = PyTuple_New(0);
    if (ts_empty_tuple == NULL) {
        INITERROR;
    }

    ts_empty_dict = PyDict_New();
    if (ts_empty_dict == NULL) {
        INITERROR;
    }

    ts_current = green_create_main();
    if (ts_current == NULL) {
        INITERROR;
    }

    Py_INCREF(&PyGreenlet_Type);
    PyModule_AddObject(m, "greenlet", (PyObject*)&PyGreenlet_Type);
    Py_INCREF(PyExc_GreenletError);
    PyModule_AddObject(m, "error", PyExc_GreenletError);
    Py_INCREF(PyExc_GreenletExit);
    PyModule_AddObject(m, "GreenletExit", PyExc_GreenletExit);

    PyModule_AddObject(m, "GREENLET_USE_GC", PyBool_FromLong(1));
    PyModule_AddObject(m, "GREENLET_USE_TRACING", PyBool_FromLong(1));
    PyModule_AddObject(
        m, "GREENLET_USE_CONTEXT_VARS", PyBool_FromLong(GREENLET_PY37));

    /* also publish module-level data as attributes of the greentype. */
    /* XXX: Why? */
    for (p = copy_on_greentype; *p; p++) {
        PyObject* o = PyObject_GetAttrString(m, *p);
        if (!o) {
            continue;
        }
        PyDict_SetItemString(PyGreenlet_Type.tp_dict, *p, o);
        Py_DECREF(o);
    }

    /*
     * Expose C API
     */

    /* types */
    _PyGreenlet_API[PyGreenlet_Type_NUM] = (void*)&PyGreenlet_Type;

    /* exceptions */
    _PyGreenlet_API[PyExc_GreenletError_NUM] = (void*)PyExc_GreenletError;
    _PyGreenlet_API[PyExc_GreenletExit_NUM] = (void*)PyExc_GreenletExit;

    /* methods */
    _PyGreenlet_API[PyGreenlet_New_NUM] = (void*)PyGreenlet_New;
    _PyGreenlet_API[PyGreenlet_GetCurrent_NUM] = (void*)PyGreenlet_GetCurrent;
    _PyGreenlet_API[PyGreenlet_Throw_NUM] = (void*)PyGreenlet_Throw;
    _PyGreenlet_API[PyGreenlet_Switch_NUM] = (void*)PyGreenlet_Switch;
    _PyGreenlet_API[PyGreenlet_SetParent_NUM] = (void*)PyGreenlet_SetParent;

    /* XXX: Note that our module name is ``greenlet._greenlet``, but for
       backwards compatibility with existing C code, we need the _C_API to
       be directly in greenlet.
    */
    c_api_object =
        PyCapsule_New((void*)_PyGreenlet_API, "greenlet._C_API", NULL);
    if (c_api_object != NULL) {
        PyModule_AddObject(m, "_C_API", c_api_object);
    }

#if PY_MAJOR_VERSION >= 3
    return m;
#endif
}

#ifdef __clang__
#    pragma clang diagnostic pop
#endif