Modules/faulthandler.c

#include "Python.h"
#include "pythread.h"
#include <signal.h>
#include <object.h>
#include <frameobject.h>
#include <signal.h>
#if defined(HAVE_PTHREAD_SIGMASK) && !defined(HAVE_BROKEN_PTHREAD_SIGMASK)
#include <pthread.h>
#endif

/* Allocate at maximum 100 MB of the stack to raise the stack overflow */
#define STACK_OVERFLOW_MAX_SIZE (100*1024*1024)

#ifdef WITH_THREAD
#  define FAULTHANDLER_LATER
#  define FAULTHANDLER_WATCHDOG
#endif

#ifndef MS_WINDOWS
   /* register() is useless on Windows, because only SIGSEGV, SIGABRT and
      SIGILL can be handled by the process, and these signals can only be used
      with enable(), not using register() */
#  define FAULTHANDLER_USER
#endif

#define PUTS(fd, str) write(fd, str, strlen(str))

#ifdef HAVE_SIGACTION
typedef struct sigaction _Py_sighandler_t;
#else
typedef PyOS_sighandler_t _Py_sighandler_t;
#endif

typedef struct {
    int signum;
    int enabled;
    const char* name;
    _Py_sighandler_t previous;
    int all_threads;
} fault_handler_t;

static struct {
    int enabled;
    PyObject *file;
    int fd;
    int all_threads;
    PyInterpreterState *interp;
} fatal_error = {0, NULL, -1, 0};

#ifdef FAULTHANDLER_LATER
static struct {
    PyObject *file;
    int fd;
    PY_TIMEOUT_T timeout_us;   /* timeout in microseconds */
    int repeat;
    PyInterpreterState *interp;
    int exit;
    char *header;
    size_t header_len;
    /* The main thread always holds this lock. It is only released when
       faulthandler_thread() is interrupted before this thread exits, or at
       Python exit. */
    PyThread_type_lock cancel_event;
    /* released by child thread when joined */
    PyThread_type_lock running;
} thread;
#endif

#ifdef FAULTHANDLER_WATCHDOG
static struct {
    int rfd;
    int wfd;
    PY_TIMEOUT_T period_us;   /* period in microseconds */
    /* The main thread always holds this lock. It is only released when
       faulthandler_watchdog() is interrupted before this thread exits, or at
       Python exit. */
    PyThread_type_lock cancel_event;
    /* released by child thread when joined */
    PyThread_type_lock running;
} watchdog;
#endif

#ifdef FAULTHANDLER_USER
typedef struct {
    int enabled;
    PyObject *file;
    int fd;
    int all_threads;
    int chain;
    _Py_sighandler_t previous;
    PyInterpreterState *interp;
} user_signal_t;

static user_signal_t *user_signals;

/* the following macros come from Python: Modules/signalmodule.c */
#if defined(PYOS_OS2) && !defined(PYCC_GCC)
#define NSIG 12
#endif
#ifndef NSIG
# if defined(_NSIG)
#  define NSIG _NSIG            /* For BSD/SysV */
# elif defined(_SIGMAX)
#  define NSIG (_SIGMAX + 1)    /* For QNX */
# elif defined(SIGMAX)
#  define NSIG (SIGMAX + 1)     /* For djgpp */
# else
#  define NSIG 64               /* Use a reasonable default value */
# endif
#endif

static void faulthandler_user(int signum);
#endif /* FAULTHANDLER_USER */


static fault_handler_t faulthandler_handlers[] = {
#ifdef SIGBUS
    {SIGBUS, 0, "Bus error", },
#endif
#ifdef SIGILL
    {SIGILL, 0, "Illegal instruction", },
#endif
    {SIGFPE, 0, "Floating point exception", },
    {SIGABRT, 0, "Aborted", },
    /* define SIGSEGV at the end to make it the default choice if searching the
       handler fails in faulthandler_fatal_error() */
    {SIGSEGV, 0, "Segmentation fault", }
};
static const unsigned char faulthandler_nsignals = \
    Py_ARRAY_LENGTH(faulthandler_handlers);

#ifdef HAVE_SIGALTSTACK
static stack_t stack;
#endif


/* Get the file descriptor of a file by calling its fileno() method and then
   call its flush() method.

   If file is NULL or Py_None, use sys.stderr as the new file.

   On success, return the new file and write the file descriptor into *p_fd.
   On error, return NULL. */

static PyObject*
faulthandler_get_fileno(PyObject *file, int *p_fd)
{
    PyObject *result;
    long fd_long;
    int fd;

    if (file == NULL || file == Py_None) {
        file = PySys_GetObject("stderr");
        if (file == NULL) {
            PyErr_SetString(PyExc_RuntimeError, "unable to get sys.stderr");
            return NULL;
        }
    }

    result = PyObject_CallMethod(file, "fileno", "");
    if (result == NULL)
        return NULL;

    fd = -1;
    if (PyLong_Check(result)) {
        fd_long = PyLong_AsLong(result);
        if (0 <= fd_long && fd_long < INT_MAX)
            fd = (int)fd_long;
    }
    Py_DECREF(result);

    if (fd == -1) {
        PyErr_SetString(PyExc_RuntimeError,
                        "file.fileno() is not a valid file descriptor");
        return NULL;
    }

    result = PyObject_CallMethod(file, "flush", "");
    if (result != NULL)
        Py_DECREF(result);
    else {
        /* ignore flush() error */
        PyErr_Clear();
    }
    *p_fd = fd;
    return file;
}

/* Get the state of the current thread: only call this function if the current
   thread holds the GIL. Raise an exception on error. */
static PyThreadState*
get_thread_state(void)
{
    PyThreadState *tstate = PyThreadState_Get();
    if (tstate == NULL) {
        PyErr_SetString(PyExc_RuntimeError,
                        "unable to get the current thread state");
        return NULL;
    }
    return tstate;
}

static PyObject*
faulthandler_dump_traceback_py(PyObject *self,
                               PyObject *args, PyObject *kwargs)
{
    static char *kwlist[] = {"file", "all_threads", NULL};
    PyObject *file = NULL;
    int all_threads = 1;
    PyThreadState *tstate;
    const char *errmsg;
    int fd;

    if (!PyArg_ParseTupleAndKeywords(args, kwargs,
        "|Oi:dump_traceback", kwlist,
        &file, &all_threads))
        return NULL;

    file = faulthandler_get_fileno(file, &fd);
    if (file == NULL)
        return NULL;

    tstate = get_thread_state();
    if (tstate == NULL)
        return NULL;

    if (all_threads) {
        errmsg = _Py_DumpTracebackThreads(fd, tstate->interp, tstate);
        if (errmsg != NULL) {
            PyErr_SetString(PyExc_RuntimeError, errmsg);
            return NULL;
        }
    }
    else {
        _Py_DumpTraceback(fd, tstate);
    }
    Py_RETURN_NONE;
}


/* Handler for SIGSEGV, SIGFPE, SIGABRT, SIGBUS and SIGILL signals.

   Display the current Python traceback, restore the previous handler and call
   the previous handler.

   On Windows, don't explicitly call the previous handler, because the Windows
   signal handler would not be called (for an unknown reason). The execution of
   the program continues at faulthandler_fatal_error() exit, but the same
   instruction will raise the same fault (signal), and so the previous handler
   will be called.

   This function is signal-safe and should only call signal-safe functions. */

static void
faulthandler_fatal_error(int signum)
{
    const int fd = fatal_error.fd;
    unsigned int i;
    fault_handler_t *handler = NULL;
    PyThreadState *tstate;
    int save_errno = errno;

    if (!fatal_error.enabled)
        return;

    for (i=0; i < faulthandler_nsignals; i++) {
        handler = &faulthandler_handlers[i];
        if (handler->signum == signum)
            break;
    }
    if (handler == NULL) {
        /* faulthandler_nsignals == 0 (unlikely) */
        return;
    }

    /* restore the previous handler */
#ifdef HAVE_SIGACTION
    (void)sigaction(signum, &handler->previous, NULL);
#else
    (void)signal(signum, handler->previous);
#endif
    handler->enabled = 0;

    PUTS(fd, "Fatal Python error: ");
    PUTS(fd, handler->name);
    PUTS(fd, "\n\n");

#ifdef WITH_THREAD
    /* SIGSEGV, SIGFPE, SIGABRT, SIGBUS and SIGILL are synchronous signals and
       are thus delivered to the thread that caused the fault. Get the Python
       thread state of the current thread.

       PyThreadState_Get() doesn't give the state of the thread that caused the
       fault if the thread released the GIL, and so this function cannot be
       used. Read the thread local storage (TLS) instead: call
       PyGILState_GetThisThreadState(). */
    tstate = PyGILState_GetThisThreadState();
#else
    tstate = PyThreadState_Get();
#endif

    if (fatal_error.all_threads)
        _Py_DumpTracebackThreads(fd, fatal_error.interp, tstate);
    else {
        if (tstate != NULL)
            _Py_DumpTraceback(fd, tstate);
    }

    errno = save_errno;
#ifdef MS_WINDOWS
    if (signum == SIGSEGV) {
        /* don't explicitly call the previous handler for SIGSEGV in this signal
           handler, because the Windows signal handler would not be called */
        return;
    }
#endif
    /* call the previous signal handler: it is called immediatly if we use
       sigaction() thanks to SA_NODEFER flag, otherwise it is deferred */
    raise(signum);
}

/* Install the handler for fatal signals, faulthandler_fatal_error(). */

static PyObject*
faulthandler_enable(PyObject *self, PyObject *args, PyObject *kwargs)
{
    static char *kwlist[] = {"file", "all_threads", NULL};
    PyObject *file = NULL;
    int all_threads = 1;
    unsigned int i;
    fault_handler_t *handler;
#ifdef HAVE_SIGACTION
    struct sigaction action;
#endif
    int err;
    int fd;
    PyThreadState *tstate;

    if (!PyArg_ParseTupleAndKeywords(args, kwargs,
        "|Oi:enable", kwlist, &file, &all_threads))
        return NULL;

    file = faulthandler_get_fileno(file, &fd);
    if (file == NULL)
        return NULL;

    tstate = get_thread_state();
    if (tstate == NULL)
        return NULL;

    Py_XDECREF(fatal_error.file);
    Py_INCREF(file);
    fatal_error.file = file;
    fatal_error.fd = fd;
    fatal_error.all_threads = all_threads;
    fatal_error.interp = tstate->interp;

    if (!fatal_error.enabled) {
        fatal_error.enabled = 1;

        for (i=0; i < faulthandler_nsignals; i++) {
            handler = &faulthandler_handlers[i];
#ifdef HAVE_SIGACTION
            action.sa_handler = faulthandler_fatal_error;
            sigemptyset(&action.sa_mask);
            /* Do not prevent the signal from being received from within
               its own signal handler */
            action.sa_flags = SA_NODEFER;
#ifdef HAVE_SIGALTSTACK
            if (stack.ss_sp != NULL) {
                /* Call the signal handler on an alternate signal stack
                   provided by sigaltstack() */
                action.sa_flags |= SA_ONSTACK;
            }
#endif
            err = sigaction(handler->signum, &action, &handler->previous);
#else
            handler->previous = signal(handler->signum,
                                       faulthandler_fatal_error);
            err = (handler->previous == SIG_ERR);
#endif
            if (err) {
                PyErr_SetFromErrno(PyExc_RuntimeError);
                return NULL;
            }
            handler->enabled = 1;
        }
    }
    Py_RETURN_NONE;
}

static void
faulthandler_disable(void)
{
    unsigned int i;
    fault_handler_t *handler;

    if (fatal_error.enabled) {
        fatal_error.enabled = 0;
        for (i=0; i < faulthandler_nsignals; i++) {
            handler = &faulthandler_handlers[i];
            if (!handler->enabled)
                continue;
#ifdef HAVE_SIGACTION
            (void)sigaction(handler->signum, &handler->previous, NULL);
#else
            (void)signal(handler->signum, handler->previous);
#endif
            handler->enabled = 0;
        }
    }

    Py_CLEAR(fatal_error.file);
}

static PyObject*
faulthandler_disable_py(PyObject *self)
{
    if (!fatal_error.enabled) {
        Py_INCREF(Py_False);
        return Py_False;
    }
    faulthandler_disable();
    Py_INCREF(Py_True);
    return Py_True;
}

static PyObject*
faulthandler_is_enabled(PyObject *self)
{
    return PyBool_FromLong(fatal_error.enabled);
}

#ifdef FAULTHANDLER_LATER

static void
faulthandler_thread(void *unused)
{
    PyLockStatus st;
    const char* errmsg;
    PyThreadState *current;
    int ok;
#if defined(HAVE_PTHREAD_SIGMASK) && !defined(HAVE_BROKEN_PTHREAD_SIGMASK)
    sigset_t set;

    /* we don't want to receive any signal */
    sigfillset(&set);
    pthread_sigmask(SIG_SETMASK, &set, NULL);
#endif

    do {
        st = PyThread_acquire_lock_timed(thread.cancel_event,
                                         thread.timeout_us, 0);
        if (st == PY_LOCK_ACQUIRED) {
            PyThread_release_lock(thread.cancel_event);
            break;
        }
        /* Timeout => dump traceback */
        assert(st == PY_LOCK_FAILURE);

        /* get the thread holding the GIL, NULL if no thread hold the GIL */
        current = _Py_atomic_load_relaxed(&_PyThreadState_Current);

        write(thread.fd, thread.header, thread.header_len);

        errmsg = _Py_DumpTracebackThreads(thread.fd, thread.interp, current);
        ok = (errmsg == NULL);

        if (thread.exit)
            _exit(1);
    } while (ok && thread.repeat);

    /* The only way out */
    PyThread_release_lock(thread.running);
}

static void
cancel_dump_tracebacks_later(void)
{
    /* Notify cancellation */
    PyThread_release_lock(thread.cancel_event);

    /* Wait for thread to join */
    PyThread_acquire_lock(thread.running, 1);
    PyThread_release_lock(thread.running);

    /* The main thread should always hold the cancel_event lock */
    PyThread_acquire_lock(thread.cancel_event, 1);

    Py_CLEAR(thread.file);
    if (thread.header) {
        free(thread.header);
        thread.header = NULL;
    }
}

static char*
format_timeout(double timeout)
{
    unsigned long us, sec, min, hour;
    double intpart, fracpart;
    char buffer[100];

    fracpart = modf(timeout, &intpart);
    sec = (unsigned long)intpart;
    us = (unsigned long)(fracpart * 1e6);
    min = sec / 60;
    sec %= 60;
    hour = min / 60;
    min %= 60;

    if (us != 0)
        PyOS_snprintf(buffer, sizeof(buffer),
                      "Timeout (%lu:%02lu:%02lu.%06lu)!\n",
                      hour, min, sec, us);
    else
        PyOS_snprintf(buffer, sizeof(buffer),
                      "Timeout (%lu:%02lu:%02lu)!\n",
                      hour, min, sec);

    return strdup(buffer);
}

static PyObject*
faulthandler_dump_tracebacks_later(PyObject *self,
                                   PyObject *args, PyObject *kwargs)
{
    static char *kwlist[] = {"timeout", "repeat", "file", "exit", NULL};
    double timeout;
    PY_TIMEOUT_T timeout_us;
    int repeat = 0;
    PyObject *file = NULL;
    int fd;
    int exit = 0;
    PyThreadState *tstate;
    char *header;
    size_t header_len;

    if (!PyArg_ParseTupleAndKeywords(args, kwargs,
        "d|iOi:dump_tracebacks_later", kwlist,
        &timeout, &repeat, &file, &exit))
        return NULL;
    if ((timeout * 1e6) >= (double) PY_TIMEOUT_MAX) {
        PyErr_SetString(PyExc_OverflowError,  "timeout value is too large");
        return NULL;
    }
    timeout_us = (PY_TIMEOUT_T)(timeout * 1e6);
    if (timeout_us <= 0) {
        PyErr_SetString(PyExc_ValueError, "timeout must be greater than 0");
        return NULL;
    }

    tstate = get_thread_state();
    if (tstate == NULL)
        return NULL;

    file = faulthandler_get_fileno(file, &fd);
    if (file == NULL)
        return NULL;

    /* format the timeout */
    header = format_timeout(timeout);
    if (header == NULL)
        return PyErr_NoMemory();
    header_len = strlen(header);

    /* Cancel previous thread, if running */
    cancel_dump_tracebacks_later();

    Py_XDECREF(thread.file);
    Py_INCREF(file);
    thread.file = file;
    thread.fd = fd;
    thread.timeout_us = timeout_us;
    thread.repeat = repeat;
    thread.interp = tstate->interp;
    thread.exit = exit;
    thread.header = header;
    thread.header_len = header_len;

    /* Arm these locks to serve as events when released */
    PyThread_acquire_lock(thread.running, 1);

    if (PyThread_start_new_thread(faulthandler_thread, NULL) == -1) {
        PyThread_release_lock(thread.running);
        Py_CLEAR(thread.file);
        free(header);
        thread.header = NULL;
        PyErr_SetString(PyExc_RuntimeError,
                        "unable to start watchdog thread");
        return NULL;
    }

    Py_RETURN_NONE;
}

static PyObject*
faulthandler_cancel_dump_tracebacks_later_py(PyObject *self)
{
    cancel_dump_tracebacks_later();
    Py_RETURN_NONE;
}
#endif  /* FAULTHANDLER_LATER */

#ifdef FAULTHANDLER_WATCHDOG

static void
file_watchdog(void *unused)
{
    PyLockStatus st;
    PY_TIMEOUT_T timeout;

    const int MAXDATA = 1024;
    char buf1[MAXDATA], buf2[MAXDATA];
    char *data = buf1, *old_data = buf2;
    Py_ssize_t data_len, old_data_len = -1;

#if defined(HAVE_PTHREAD_SIGMASK) && !defined(HAVE_BROKEN_PTHREAD_SIGMASK)
    sigset_t set;

    /* we don't want to receive any signal */
    sigfillset(&set);
    pthread_sigmask(SIG_SETMASK, &set, NULL);
#endif

    /* On first pass, feed file contents immediately */
    timeout = 0;
    do {
        st = PyThread_acquire_lock_timed(watchdog.cancel_event,
                                         timeout, 0);
        timeout = watchdog.period_us;
        if (st == PY_LOCK_ACQUIRED) {
            PyThread_release_lock(watchdog.cancel_event);
            break;
        }
        /* Timeout => read and write data */
        assert(st == PY_LOCK_FAILURE);

        if (lseek(watchdog.rfd, 0, SEEK_SET) < 0) {
            break;
        }
        data_len = read(watchdog.rfd, data, MAXDATA);
        if (data_len < 0) {
            break;
        }
        if (data_len != old_data_len || memcmp(data, old_data, data_len)) {
            char *tdata;
            Py_ssize_t tlen;
            /* Contents changed, feed them to wfd */
            long x = (long) data_len;
            /* We can't do anything if the consumer is too slow, just bail out */
            if (write(watchdog.wfd, (void *) &x, sizeof(x)) < sizeof(x))
                break;
            if (write(watchdog.wfd, data, data_len) < data_len)
                break;
            tdata = data;
            data = old_data;
            old_data = tdata;
            tlen = data_len;
            data_len = old_data_len;
            old_data_len = tlen;
        }
    } while (1);

    close(watchdog.rfd);
    close(watchdog.wfd);

    /* The only way out */
    PyThread_release_lock(watchdog.running);
}

static void
cancel_file_watchdog(void)
{
    /* Notify cancellation */
    PyThread_release_lock(watchdog.cancel_event);

    /* Wait for thread to join */
    PyThread_acquire_lock(watchdog.running, 1);
    PyThread_release_lock(watchdog.running);

    /* The main thread should always hold the cancel_event lock */
    PyThread_acquire_lock(watchdog.cancel_event, 1);
}

static PyObject*
faulthandler_file_watchdog(PyObject *self,
                           PyObject *args, PyObject *kwargs)
{
    static char *kwlist[] = {"rfd", "wfd", "period", NULL};
    double period;
    PY_TIMEOUT_T period_us;
    int rfd, wfd;

    if (!PyArg_ParseTupleAndKeywords(args, kwargs,
        "iid:_file_watchdog", kwlist,
        &rfd, &wfd, &period))
        return NULL;
    if ((period * 1e6) >= (double) PY_TIMEOUT_MAX) {
        PyErr_SetString(PyExc_OverflowError,  "period value is too large");
        return NULL;
    }
    period_us = (PY_TIMEOUT_T)(period * 1e6);
    if (period_us <= 0) {
        PyErr_SetString(PyExc_ValueError, "period must be greater than 0");
        return NULL;
    }

    /* Cancel previous thread, if running */
    cancel_file_watchdog();

    watchdog.rfd = rfd;
    watchdog.wfd = wfd;
    watchdog.period_us = period_us;

    /* Arm these locks to serve as events when released */
    PyThread_acquire_lock(watchdog.running, 1);

    if (PyThread_start_new_thread(file_watchdog, NULL) == -1) {
        PyThread_release_lock(watchdog.running);
        PyErr_SetString(PyExc_RuntimeError,
                        "unable to start file watchdog thread");
        return NULL;
    }

    Py_RETURN_NONE;
}

static PyObject*
faulthandler_cancel_file_watchdog(PyObject *self)
{
    cancel_file_watchdog();
    Py_RETURN_NONE;
}
#endif  /* FAULTHANDLER_WATCHDOG */

#ifdef FAULTHANDLER_USER
static int
faulthandler_register(int signum, int chain, _Py_sighandler_t *p_previous)
{
#ifdef HAVE_SIGACTION
    struct sigaction action;
    action.sa_handler = faulthandler_user;
    sigemptyset(&action.sa_mask);
    /* if the signal is received while the kernel is executing a system
       call, try to restart the system call instead of interrupting it and
       return EINTR. */
    action.sa_flags = SA_RESTART;
    if (chain) {
        /* do not prevent the signal from being received from within its
           own signal handler */
        action.sa_flags = SA_NODEFER;
    }
#ifdef HAVE_SIGALTSTACK
    if (stack.ss_sp != NULL) {
        /* Call the signal handler on an alternate signal stack
           provided by sigaltstack() */
        action.sa_flags |= SA_ONSTACK;
    }
#endif
    return sigaction(signum, &action, p_previous);
#else
    _Py_sighandler_t previous;
    previous = signal(signum, faulthandler_user);
    if (p_previous != NULL)
        *p_previous = previous;
    return (previous == SIG_ERR);
#endif
}

/* Handler of user signals (e.g. SIGUSR1).

   Dump the traceback of the current thread, or of all threads if
   thread.all_threads is true.

   This function is signal safe and should only call signal safe functions. */

static void
faulthandler_user(int signum)
{
    user_signal_t *user;
    PyThreadState *tstate;
    int save_errno = errno;

    user = &user_signals[signum];
    if (!user->enabled)
        return;

#ifdef WITH_THREAD
    /* PyThreadState_Get() doesn't give the state of the current thread if
       the thread doesn't hold the GIL. Read the thread local storage (TLS)
       instead: call PyGILState_GetThisThreadState(). */
    tstate = PyGILState_GetThisThreadState();
#else
    tstate = PyThreadState_Get();
#endif

    if (user->all_threads)
        _Py_DumpTracebackThreads(user->fd, user->interp, tstate);
    else {
        if (tstate == NULL)
            return;
        _Py_DumpTraceback(user->fd, tstate);
    }
#ifdef HAVE_SIGACTION
    if (user->chain) {
        (void)sigaction(signum, &user->previous, NULL);
        /* call the previous signal handler */
        raise(signum);
        (void)faulthandler_register(signum, user->chain, NULL);
    }
#else
    if (user->chain) {
        /* call the previous signal handler */
        user->previous(signum);
    }
#endif
    errno = save_errno;
}

static int
check_signum(int signum)
{
    unsigned int i;

    for (i=0; i < faulthandler_nsignals; i++) {
        if (faulthandler_handlers[i].signum == signum) {
            PyErr_Format(PyExc_RuntimeError,
                         "signal %i cannot be registered, "
                         "use enable() instead",
                         signum);
            return 0;
        }
    }
    if (signum < 1 || NSIG <= signum) {
        PyErr_SetString(PyExc_ValueError, "signal number out of range");
        return 0;
    }
    return 1;
}

static PyObject*
faulthandler_register_py(PyObject *self,
                         PyObject *args, PyObject *kwargs)
{
    static char *kwlist[] = {"signum", "file", "all_threads", "chain", NULL};
    int signum;
    PyObject *file = NULL;
    int all_threads = 1;
    int chain = 0;
    int fd;
    user_signal_t *user;
    _Py_sighandler_t previous;
    PyThreadState *tstate;
    int err;

    if (!PyArg_ParseTupleAndKeywords(args, kwargs,
        "i|Oii:register", kwlist,
        &signum, &file, &all_threads, &chain))
        return NULL;

    if (!check_signum(signum))
        return NULL;

    tstate = get_thread_state();
    if (tstate == NULL)
        return NULL;

    file = faulthandler_get_fileno(file, &fd);
    if (file == NULL)
        return NULL;

    if (user_signals == NULL) {
        user_signals = calloc(NSIG, sizeof(user_signal_t));
        if (user_signals == NULL)
            return PyErr_NoMemory();
    }
    user = &user_signals[signum];

    if (!user->enabled) {
        err = faulthandler_register(signum, chain, &previous);
        if (err) {
            PyErr_SetFromErrno(PyExc_OSError);
            return NULL;
        }
    }

    Py_XDECREF(user->file);
    Py_INCREF(file);
    user->file = file;
    user->fd = fd;
    user->all_threads = all_threads;
    user->chain = chain;
    user->previous = previous;
    user->interp = tstate->interp;
    user->enabled = 1;

    Py_RETURN_NONE;
}

static int
faulthandler_unregister(user_signal_t *user, int signum)
{
    if (!user->enabled)
        return 0;
    user->enabled = 0;
#ifdef HAVE_SIGACTION
    (void)sigaction(signum, &user->previous, NULL);
#else
    (void)signal(signum, user->previous);
#endif
    Py_CLEAR(user->file);
    user->fd = -1;
    return 1;
}

static PyObject*
faulthandler_unregister_py(PyObject *self, PyObject *args)
{
    int signum;
    user_signal_t *user;
    int change;

    if (!PyArg_ParseTuple(args, "i:unregister", &signum))
        return NULL;

    if (!check_signum(signum))
        return NULL;

    if (user_signals == NULL)
        Py_RETURN_FALSE;

    user = &user_signals[signum];
    change = faulthandler_unregister(user, signum);
    return PyBool_FromLong(change);
}
#endif   /* FAULTHANDLER_USER */


static PyObject *
faulthandler_read_null(PyObject *self, PyObject *args)
{
    int *x = NULL, y;
    int release_gil = 0;
    if (!PyArg_ParseTuple(args, "|i:_read_null", &release_gil))
        return NULL;
    if (release_gil) {
        Py_BEGIN_ALLOW_THREADS
        y = *x;
        Py_END_ALLOW_THREADS
    } else
        y = *x;
    return PyLong_FromLong(y);

}

static PyObject *
faulthandler_sigsegv(PyObject *self, PyObject *args)
{
#if defined(MS_WINDOWS)
    /* For SIGSEGV, faulthandler_fatal_error() restores the previous signal
       handler and then gives back the execution flow to the program (without
       explicitly calling the previous error handler). In a normal case, the
       SIGSEGV was raised by the kernel because of a fault, and so if the
       program retries to execute the same instruction, the fault will be
       raised again.

       Here the fault is simulated by a fake SIGSEGV signal raised by the
       application. We have to raise SIGSEGV at lease twice: once for
       faulthandler_fatal_error(), and one more time for the previous signal
       handler. */
    while(1)
        raise(SIGSEGV);
#else
    raise(SIGSEGV);
#endif
    Py_RETURN_NONE;
}

static PyObject *
faulthandler_sigfpe(PyObject *self, PyObject *args)
{
    /* Do an integer division by zero: raise a SIGFPE on Intel CPU, but not on
       PowerPC. Use volatile to disable compile-time optimizations. */
    volatile int x = 1, y = 0, z;
    z = x / y;
    /* If the division by zero didn't raise a SIGFPE (e.g. on PowerPC),
       raise it manually. */
    raise(SIGFPE);
    /* This line is never reached, but we pretend to make something with z
       to silence a compiler warning. */
    return PyLong_FromLong(z);
}

static PyObject *
faulthandler_sigabrt(PyObject *self, PyObject *args)
{
#ifdef _MSC_VER
    /* Visual Studio: configure abort() to not display an error message nor
       open a popup asking to report the fault. */
    _set_abort_behavior(0, _WRITE_ABORT_MSG | _CALL_REPORTFAULT);
#endif
    abort();
    Py_RETURN_NONE;
}

#ifdef SIGBUS
static PyObject *
faulthandler_sigbus(PyObject *self, PyObject *args)
{
    raise(SIGBUS);
    Py_RETURN_NONE;
}
#endif

#ifdef SIGILL
static PyObject *
faulthandler_sigill(PyObject *self, PyObject *args)
{
    raise(SIGILL);
    Py_RETURN_NONE;
}
#endif

static PyObject *
faulthandler_fatal_error_py(PyObject *self, PyObject *args)
{
    char *message;
    if (!PyArg_ParseTuple(args, "y:fatal_error", &message))
        return NULL;
    Py_FatalError(message);
    Py_RETURN_NONE;
}

#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
static void*
stack_overflow(void *min_sp, void *max_sp, size_t *depth)
{
    /* allocate 4096 bytes on the stack at each call */
    unsigned char buffer[4096];
    void *sp = &buffer;
    *depth += 1;
    if (sp < min_sp || max_sp < sp)
        return sp;
    buffer[0] = 1;
    buffer[4095] = 0;
    return stack_overflow(min_sp, max_sp, depth);
}

static PyObject *
faulthandler_stack_overflow(PyObject *self)
{
    size_t depth, size;
    char *sp = (char *)&depth, *stop;

    depth = 0;
    stop = stack_overflow(sp - STACK_OVERFLOW_MAX_SIZE,
                          sp + STACK_OVERFLOW_MAX_SIZE,
                          &depth);
    if (sp < stop)
        size = stop - sp;
    else
        size = sp - stop;
    PyErr_Format(PyExc_RuntimeError,
        "unable to raise a stack overflow (allocated %zu bytes "
        "on the stack, %zu recursive calls)",
        size, depth);
    return NULL;
}
#endif


static int
faulthandler_traverse(PyObject *module, visitproc visit, void *arg)
{
#ifdef FAULTHANDLER_USER
    unsigned int signum;
#endif

#ifdef FAULTHANDLER_LATER
    Py_VISIT(thread.file);
#endif
#ifdef FAULTHANDLER_USER
    if (user_signals != NULL) {
        for (signum=0; signum < NSIG; signum++)
            Py_VISIT(user_signals[signum].file);
    }
#endif
    Py_VISIT(fatal_error.file);
    return 0;
}

PyDoc_STRVAR(module_doc,
"faulthandler module.");

static PyMethodDef module_methods[] = {
    {"enable",
     (PyCFunction)faulthandler_enable, METH_VARARGS|METH_KEYWORDS,
     PyDoc_STR("enable(file=sys.stderr, all_threads=True): "
               "enable the fault handler")},
    {"disable", (PyCFunction)faulthandler_disable_py, METH_NOARGS,
     PyDoc_STR("disable(): disable the fault handler")},
    {"is_enabled", (PyCFunction)faulthandler_is_enabled, METH_NOARGS,
     PyDoc_STR("is_enabled()->bool: check if the handler is enabled")},
    {"dump_traceback",
     (PyCFunction)faulthandler_dump_traceback_py, METH_VARARGS|METH_KEYWORDS,
     PyDoc_STR("dump_traceback(file=sys.stderr, all_threads=True): "
               "dump the traceback of the current thread, or of all threads "
               "if all_threads is True, into file")},
#ifdef FAULTHANDLER_LATER
    {"dump_tracebacks_later",
     (PyCFunction)faulthandler_dump_tracebacks_later, METH_VARARGS|METH_KEYWORDS,
     PyDoc_STR("dump_tracebacks_later(timeout, repeat=False, file=sys.stderrn, exit=False):\n"
               "dump the traceback of all threads in timeout seconds,\n"
               "or each timeout seconds if repeat is True. If exit is True, "
               "call _exit(1) which is not safe.")},
    {"cancel_dump_tracebacks_later",
     (PyCFunction)faulthandler_cancel_dump_tracebacks_later_py, METH_NOARGS,
     PyDoc_STR("cancel_dump_tracebacks_later():\ncancel the previous call "
               "to dump_tracebacks_later().")},
#endif

#ifdef FAULTHANDLER_WATCHDOG
    {"_file_watchdog",
     (PyCFunction)faulthandler_file_watchdog, METH_VARARGS|METH_KEYWORDS,
     PyDoc_STR("_file_watchdog(rfd, wfd, period):\n"
               "feed the contents of 'rfd' to 'wfd', if changed,\n"
               "every 'period seconds'.")},
    {"_cancel_file_watchdog",
     (PyCFunction)faulthandler_cancel_file_watchdog, METH_NOARGS,
     PyDoc_STR("_cancel_file_watchdog():\ncancel the previous call "
               "to _file_watchdog().")},
#endif

#ifdef FAULTHANDLER_USER
    {"register",
     (PyCFunction)faulthandler_register_py, METH_VARARGS|METH_KEYWORDS,
     PyDoc_STR("register(signum, file=sys.stderr, all_threads=True, chain=False): "
               "register an handler for the signal 'signum': dump the "
               "traceback of the current thread, or of all threads if "
               "all_threads is True, into file")},
    {"unregister",
     faulthandler_unregister_py, METH_VARARGS|METH_KEYWORDS,
     PyDoc_STR("unregister(signum): unregister the handler of the signal "
                "'signum' registered by register()")},
#endif

    {"_read_null", faulthandler_read_null, METH_VARARGS,
     PyDoc_STR("_read_null(release_gil=False): read from NULL, raise "
               "a SIGSEGV or SIGBUS signal depending on the platform")},
    {"_sigsegv", faulthandler_sigsegv, METH_VARARGS,
     PyDoc_STR("_sigsegv(): raise a SIGSEGV signal")},
    {"_sigabrt", faulthandler_sigabrt, METH_VARARGS,
     PyDoc_STR("_sigabrt(): raise a SIGABRT signal")},
    {"_sigfpe", (PyCFunction)faulthandler_sigfpe, METH_NOARGS,
     PyDoc_STR("_sigfpe(): raise a SIGFPE signal")},
#ifdef SIGBUS
    {"_sigbus", (PyCFunction)faulthandler_sigbus, METH_NOARGS,
     PyDoc_STR("_sigbus(): raise a SIGBUS signal")},
#endif
#ifdef SIGILL
    {"_sigill", (PyCFunction)faulthandler_sigill, METH_NOARGS,
     PyDoc_STR("_sigill(): raise a SIGILL signal")},
#endif
    {"_fatal_error", faulthandler_fatal_error_py, METH_VARARGS,
     PyDoc_STR("_fatal_error(message): call Py_FatalError(message)")},
#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
    {"_stack_overflow", (PyCFunction)faulthandler_stack_overflow, METH_NOARGS,
     PyDoc_STR("_stack_overflow(): recursive call to raise a stack overflow")},
#endif
    {NULL, NULL}  /* sentinel */
};

static struct PyModuleDef module_def = {
    PyModuleDef_HEAD_INIT,
    "faulthandler",
    module_doc,
    0, /* non-negative size to be able to unload the module */
    module_methods,
    NULL,
    faulthandler_traverse,
    NULL,
    NULL
};

PyMODINIT_FUNC
PyInit_faulthandler(void)
{
    return PyModule_Create(&module_def);
}

/* Call faulthandler.enable() if the PYTHONFAULTHANDLER environment variable
   is defined, or if sys._xoptions has a 'faulthandler' key. */

static int
faulthandler_env_options(void)
{
    PyObject *xoptions, *key, *module, *res;

    if (!Py_GETENV("PYTHONFAULTHANDLER")) {
        int has_key;

        xoptions = PySys_GetXOptions();
        if (xoptions == NULL)
            return -1;

        key = PyUnicode_FromString("faulthandler");
        if (key == NULL)
            return -1;

        has_key = PyDict_Contains(xoptions, key);
        Py_DECREF(key);
        if (!has_key)
            return 0;
    }

    module = PyImport_ImportModule("faulthandler");
    if (module == NULL) {
        return -1;
    }
    res = PyObject_CallMethod(module, "enable", "");
    Py_DECREF(module);
    if (res == NULL)
        return -1;
    Py_DECREF(res);
    return 0;
}

int _PyFaulthandler_Init(void)
{
#ifdef HAVE_SIGALTSTACK
    int err;

    /* Try to allocate an alternate stack for faulthandler() signal handler to
     * be able to allocate memory on the stack, even on a stack overflow. If it
     * fails, ignore the error. */
    stack.ss_flags = 0;
    stack.ss_size = SIGSTKSZ;
    stack.ss_sp = PyMem_Malloc(stack.ss_size);
    if (stack.ss_sp != NULL) {
        err = sigaltstack(&stack, NULL);
        if (err) {
            PyMem_Free(stack.ss_sp);
            stack.ss_sp = NULL;
        }
    }
#endif
#ifdef FAULTHANDLER_LATER
    thread.file = NULL;
    thread.cancel_event = PyThread_allocate_lock();
    thread.running = PyThread_allocate_lock();
    if (!thread.cancel_event || !thread.running) {
        PyErr_SetString(PyExc_RuntimeError,
                        "could not allocate locks for faulthandler");
        return -1;
    }
    PyThread_acquire_lock(thread.cancel_event, 1);
#endif
#ifdef FAULTHANDLER_WATCHDOG
    watchdog.cancel_event = PyThread_allocate_lock();
    watchdog.running = PyThread_allocate_lock();
    if (!watchdog.cancel_event || !watchdog.running) {
        PyErr_SetString(PyExc_RuntimeError,
                        "could not allocate locks for faulthandler");
        return -1;
    }
    PyThread_acquire_lock(watchdog.cancel_event, 1);
#endif

    return faulthandler_env_options();
}

void _PyFaulthandler_Fini(void)
{
#ifdef FAULTHANDLER_USER
    unsigned int signum;
#endif

#ifdef FAULTHANDLER_LATER
    /* later */
    cancel_dump_tracebacks_later();
    if (thread.cancel_event) {
        PyThread_release_lock(thread.cancel_event);
        PyThread_free_lock(thread.cancel_event);
        thread.cancel_event = NULL;
    }
    if (thread.running) {
        PyThread_free_lock(thread.running);
        thread.running = NULL;
    }
#endif

#ifdef FAULTHANDLER_WATCHDOG
    /* file watchdog */
    cancel_file_watchdog();
    if (watchdog.cancel_event) {
        PyThread_release_lock(watchdog.cancel_event);
        PyThread_free_lock(watchdog.cancel_event);
        watchdog.cancel_event = NULL;
    }
    if (watchdog.running) {
        PyThread_free_lock(watchdog.running);
        watchdog.running = NULL;
    }
#endif

#ifdef FAULTHANDLER_USER
    /* user */
    if (user_signals != NULL) {
        for (signum=0; signum < NSIG; signum++)
            faulthandler_unregister(&user_signals[signum], signum);
        free(user_signals);
        user_signals = NULL;
    }
#endif

    /* fatal */
    faulthandler_disable();
#ifdef HAVE_SIGALTSTACK
    if (stack.ss_sp != NULL) {
        PyMem_Free(stack.ss_sp);
        stack.ss_sp = NULL;
    }
#endif
}