bpo-37851: faulthandler allocates its stack on demand (GH-15358)
The faulthandler module no longer allocates its alternative stack at Python startup. Now the stack is only allocated at the first faulthandler usage. faulthandler no longer ignores memory allocation failure when allocating the stack. sigaltstack() failure now raises an OSError exception, rather than being ignored. The alternative stack is no longer used if sigaction() is not available. In practice, sigaltstack() should only be available when sigaction() is avaialble, so this change should have no effect in practice. faulthandler.dump_traceback_later() internal locks are now only allocated at the first dump_traceback_later() call, rather than always being allocated at Python startup.
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@ -0,0 +1,3 @@
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The :mod:`faulthandler` module no longer allocates its alternative stack at
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Python startup. Now the stack is only allocated at the first faulthandler
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usage.
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@ -125,7 +125,13 @@ static fault_handler_t faulthandler_handlers[] = {
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static const size_t faulthandler_nsignals = \
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Py_ARRAY_LENGTH(faulthandler_handlers);
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#ifdef HAVE_SIGALTSTACK
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/* Using an alternative stack requires sigaltstack()
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and sigaction() SA_ONSTACK */
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#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
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# define FAULTHANDLER_USE_ALT_STACK
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#endif
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#ifdef FAULTHANDLER_USE_ALT_STACK
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static stack_t stack;
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static stack_t old_stack;
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#endif
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@ -427,6 +433,36 @@ faulthandler_exc_handler(struct _EXCEPTION_POINTERS *exc_info)
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}
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#endif
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#ifdef FAULTHANDLER_USE_ALT_STACK
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static int
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faulthandler_allocate_stack(void)
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{
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if (stack.ss_sp != NULL) {
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return 0;
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}
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/* Allocate an alternate stack for faulthandler() signal handler
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to be able to execute a signal handler on a stack overflow error */
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stack.ss_sp = PyMem_Malloc(stack.ss_size);
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if (stack.ss_sp == NULL) {
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PyErr_NoMemory();
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return -1;
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}
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int err = sigaltstack(&stack, &old_stack);
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if (err) {
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/* Release the stack to retry sigaltstack() next time */
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PyMem_Free(stack.ss_sp);
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stack.ss_sp = NULL;
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PyErr_SetFromErrno(PyExc_OSError);
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return -1;
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}
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return 0;
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}
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#endif
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/* Install the handler for fatal signals, faulthandler_fatal_error(). */
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static int
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@ -437,32 +473,35 @@ faulthandler_enable(void)
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}
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fatal_error.enabled = 1;
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#ifdef FAULTHANDLER_USE_ALT_STACK
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if (faulthandler_allocate_stack() < 0) {
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return -1;
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}
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#endif
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for (size_t i=0; i < faulthandler_nsignals; i++) {
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fault_handler_t *handler;
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#ifdef HAVE_SIGACTION
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struct sigaction action;
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#endif
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int err;
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handler = &faulthandler_handlers[i];
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assert(!handler->enabled);
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#ifdef HAVE_SIGACTION
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struct sigaction action;
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action.sa_handler = faulthandler_fatal_error;
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sigemptyset(&action.sa_mask);
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/* Do not prevent the signal from being received from within
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its own signal handler */
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action.sa_flags = SA_NODEFER;
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#ifdef HAVE_SIGALTSTACK
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if (stack.ss_sp != NULL) {
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/* Call the signal handler on an alternate signal stack
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provided by sigaltstack() */
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action.sa_flags |= SA_ONSTACK;
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}
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#ifdef FAULTHANDLER_USE_ALT_STACK
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assert(stack.ss_sp != NULL);
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/* Call the signal handler on an alternate signal stack
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provided by sigaltstack() */
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action.sa_flags |= SA_ONSTACK;
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#endif
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err = sigaction(handler->signum, &action, &handler->previous);
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#else
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handler->previous = signal(handler->signum,
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faulthandler_fatal_error);
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faulthandler_fatal_error);
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err = (handler->previous == SIG_ERR);
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#endif
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if (err) {
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@ -676,17 +715,37 @@ faulthandler_dump_traceback_later(PyObject *self,
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}
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tstate = get_thread_state();
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if (tstate == NULL)
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if (tstate == NULL) {
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return NULL;
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}
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fd = faulthandler_get_fileno(&file);
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if (fd < 0)
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if (fd < 0) {
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return NULL;
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}
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if (!thread.running) {
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thread.running = PyThread_allocate_lock();
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if (!thread.running) {
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return PyErr_NoMemory();
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}
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}
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if (!thread.cancel_event) {
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thread.cancel_event = PyThread_allocate_lock();
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if (!thread.cancel_event || !thread.running) {
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return PyErr_NoMemory();
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}
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/* cancel_event starts to be acquired: it's only released to cancel
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the thread. */
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PyThread_acquire_lock(thread.cancel_event, 1);
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}
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/* format the timeout */
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header = format_timeout(timeout_us);
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if (header == NULL)
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if (header == NULL) {
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return PyErr_NoMemory();
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}
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header_len = strlen(header);
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/* Cancel previous thread, if running */
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@ -728,9 +787,10 @@ faulthandler_cancel_dump_traceback_later_py(PyObject *self,
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}
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#endif /* FAULTHANDLER_LATER */
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#ifdef FAULTHANDLER_USER
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static int
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faulthandler_register(int signum, int chain, _Py_sighandler_t *p_previous)
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faulthandler_register(int signum, int chain, _Py_sighandler_t *previous_p)
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{
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#ifdef HAVE_SIGACTION
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struct sigaction action;
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@ -745,19 +805,19 @@ faulthandler_register(int signum, int chain, _Py_sighandler_t *p_previous)
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own signal handler */
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action.sa_flags = SA_NODEFER;
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}
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#ifdef HAVE_SIGALTSTACK
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if (stack.ss_sp != NULL) {
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/* Call the signal handler on an alternate signal stack
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provided by sigaltstack() */
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action.sa_flags |= SA_ONSTACK;
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}
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#ifdef FAULTHANDLER_USE_ALT_STACK
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assert(stack.ss_sp != NULL);
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/* Call the signal handler on an alternate signal stack
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provided by sigaltstack() */
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action.sa_flags |= SA_ONSTACK;
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#endif
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return sigaction(signum, &action, p_previous);
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return sigaction(signum, &action, previous_p);
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#else
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_Py_sighandler_t previous;
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previous = signal(signum, faulthandler_user);
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if (p_previous != NULL)
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*p_previous = previous;
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if (previous_p != NULL) {
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*previous_p = previous;
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}
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return (previous == SIG_ERR);
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#endif
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}
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@ -861,6 +921,12 @@ faulthandler_register_py(PyObject *self,
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user = &user_signals[signum];
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if (!user->enabled) {
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#ifdef FAULTHANDLER_USE_ALT_STACK
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if (faulthandler_allocate_stack() < 0) {
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return NULL;
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}
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#endif
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err = faulthandler_register(signum, chain, &previous);
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if (err) {
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PyErr_SetFromErrno(PyExc_OSError);
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@ -1094,7 +1160,7 @@ faulthandler_fatal_error_py(PyObject *self, PyObject *args)
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Py_RETURN_NONE;
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}
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#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
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#if defined(FAULTHANDLER_USE_ALT_STACK)
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#define FAULTHANDLER_STACK_OVERFLOW
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#ifdef __INTEL_COMPILER
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@ -1153,7 +1219,7 @@ faulthandler_stack_overflow(PyObject *self, PyObject *Py_UNUSED(ignored))
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size, depth);
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return NULL;
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}
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#endif /* defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION) */
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#endif /* defined(FAULTHANDLER_USE_ALT_STACK) && defined(HAVE_SIGACTION) */
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static int
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@ -1318,35 +1384,18 @@ faulthandler_init_enable(void)
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PyStatus
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_PyFaulthandler_Init(int enable)
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{
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#ifdef HAVE_SIGALTSTACK
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int err;
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/* Try to allocate an alternate stack for faulthandler() signal handler to
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* be able to allocate memory on the stack, even on a stack overflow. If it
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* fails, ignore the error. */
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#ifdef FAULTHANDLER_USE_ALT_STACK
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memset(&stack, 0, sizeof(stack));
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stack.ss_flags = 0;
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/* bpo-21131: allocate dedicated stack of SIGSTKSZ*2 bytes, instead of just
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SIGSTKSZ bytes. Calling the previous signal handler in faulthandler
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signal handler uses more than SIGSTKSZ bytes of stack memory on some
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platforms. */
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stack.ss_size = SIGSTKSZ * 2;
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stack.ss_sp = PyMem_Malloc(stack.ss_size);
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if (stack.ss_sp != NULL) {
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err = sigaltstack(&stack, &old_stack);
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if (err) {
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PyMem_Free(stack.ss_sp);
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stack.ss_sp = NULL;
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}
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}
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#endif
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#ifdef FAULTHANDLER_LATER
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thread.file = NULL;
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thread.cancel_event = PyThread_allocate_lock();
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thread.running = PyThread_allocate_lock();
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if (!thread.cancel_event || !thread.running) {
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return _PyStatus_ERR("failed to allocate locks for faulthandler");
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}
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PyThread_acquire_lock(thread.cancel_event, 1);
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memset(&thread, 0, sizeof(thread));
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#endif
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if (enable) {
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@ -1386,7 +1435,8 @@ void _PyFaulthandler_Fini(void)
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/* fatal */
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faulthandler_disable();
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#ifdef HAVE_SIGALTSTACK
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#ifdef FAULTHANDLER_USE_ALT_STACK
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if (stack.ss_sp != NULL) {
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/* Fetch the current alt stack */
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stack_t current_stack;
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