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.

Misc/NEWS.d/next/Library/2019-08-21-13-43-04.bpo-37851.mIIfD_.rst

@@ -0,0 +1,3 @@
+The :mod:`faulthandler` module no longer allocates its alternative stack at
+Python startup. Now the stack is only allocated at the first faulthandler
+usage.

Modules/faulthandler.c

@@ -125,7 +125,13 @@ static fault_handler_t faulthandler_handlers[] = {
 static const size_t faulthandler_nsignals = \
     Py_ARRAY_LENGTH(faulthandler_handlers);
 
-#ifdef HAVE_SIGALTSTACK
+/* Using an alternative stack requires sigaltstack()
+   and sigaction() SA_ONSTACK */
+#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
+#  define FAULTHANDLER_USE_ALT_STACK
+#endif
+
+#ifdef FAULTHANDLER_USE_ALT_STACK
 static stack_t stack;
 static stack_t old_stack;
 #endif
@@ -427,6 +433,36 @@ faulthandler_exc_handler(struct _EXCEPTION_POINTERS *exc_info)
 }
 #endif
 
+
+#ifdef FAULTHANDLER_USE_ALT_STACK
+static int
+faulthandler_allocate_stack(void)
+{
+    if (stack.ss_sp != NULL) {
+        return 0;
+    }
+    /* Allocate an alternate stack for faulthandler() signal handler
+       to be able to execute a signal handler on a stack overflow error */
+    stack.ss_sp = PyMem_Malloc(stack.ss_size);
+    if (stack.ss_sp == NULL) {
+        PyErr_NoMemory();
+        return -1;
+    }
+
+    int err = sigaltstack(&stack, &old_stack);
+    if (err) {
+        /* Release the stack to retry sigaltstack() next time */
+        PyMem_Free(stack.ss_sp);
+        stack.ss_sp = NULL;
+
+        PyErr_SetFromErrno(PyExc_OSError);
+        return -1;
+    }
+    return 0;
+}
+#endif
+
+
 /* Install the handler for fatal signals, faulthandler_fatal_error(). */
 
 static int
@@ -437,27 +473,30 @@ faulthandler_enable(void)
     }
     fatal_error.enabled = 1;
 
+#ifdef FAULTHANDLER_USE_ALT_STACK
+    if (faulthandler_allocate_stack() < 0) {
+        return -1;
+    }
+#endif
+
     for (size_t i=0; i < faulthandler_nsignals; i++) {
         fault_handler_t *handler;
-#ifdef HAVE_SIGACTION
-        struct sigaction action;
-#endif
         int err;
 
         handler = &faulthandler_handlers[i];
         assert(!handler->enabled);
 #ifdef HAVE_SIGACTION
+        struct sigaction action;
         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
+#ifdef FAULTHANDLER_USE_ALT_STACK
-        if (stack.ss_sp != NULL) {
+        assert(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
@@ -676,17 +715,37 @@ faulthandler_dump_traceback_later(PyObject *self,
     }
 
     tstate = get_thread_state();
-    if (tstate == NULL)
+    if (tstate == NULL) {
         return NULL;
+    }
 
     fd = faulthandler_get_fileno(&file);
-    if (fd < 0)
+    if (fd < 0) {
         return NULL;
+    }
+
+    if (!thread.running) {
+        thread.running = PyThread_allocate_lock();
+        if (!thread.running) {
+            return PyErr_NoMemory();
+        }
+    }
+    if (!thread.cancel_event) {
+        thread.cancel_event = PyThread_allocate_lock();
+        if (!thread.cancel_event || !thread.running) {
+            return PyErr_NoMemory();
+        }
+
+        /* cancel_event starts to be acquired: it's only released to cancel
+           the thread. */
+        PyThread_acquire_lock(thread.cancel_event, 1);
+    }
 
     /* format the timeout */
     header = format_timeout(timeout_us);
-    if (header == NULL)
+    if (header == NULL) {
         return PyErr_NoMemory();
+    }
     header_len = strlen(header);
 
     /* Cancel previous thread, if running */
@@ -728,9 +787,10 @@ faulthandler_cancel_dump_traceback_later_py(PyObject *self,
 }
 #endif  /* FAULTHANDLER_LATER */
 
+
 #ifdef FAULTHANDLER_USER
 static int
-faulthandler_register(int signum, int chain, _Py_sighandler_t *p_previous)
+faulthandler_register(int signum, int chain, _Py_sighandler_t *previous_p)
 {
 #ifdef HAVE_SIGACTION
     struct sigaction action;
@@ -745,19 +805,19 @@ faulthandler_register(int signum, int chain, _Py_sighandler_t *p_previous)
            own signal handler */
         action.sa_flags = SA_NODEFER;
     }
-#ifdef HAVE_SIGALTSTACK
+#ifdef FAULTHANDLER_USE_ALT_STACK
-    if (stack.ss_sp != NULL) {
+    assert(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);
+    return sigaction(signum, &action, previous_p);
 #else
     _Py_sighandler_t previous;
     previous = signal(signum, faulthandler_user);
-    if (p_previous != NULL)
+    if (previous_p != NULL) {
-        *p_previous = previous;
+        *previous_p = previous;
+    }
     return (previous == SIG_ERR);
 #endif
 }
@@ -861,6 +921,12 @@ faulthandler_register_py(PyObject *self,
     user = &user_signals[signum];
 
     if (!user->enabled) {
+#ifdef FAULTHANDLER_USE_ALT_STACK
+        if (faulthandler_allocate_stack() < 0) {
+            return NULL;
+        }
+#endif
+
         err = faulthandler_register(signum, chain, &previous);
         if (err) {
             PyErr_SetFromErrno(PyExc_OSError);
@@ -1094,7 +1160,7 @@ faulthandler_fatal_error_py(PyObject *self, PyObject *args)
     Py_RETURN_NONE;
 }
 
-#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
+#if defined(FAULTHANDLER_USE_ALT_STACK)
 #define FAULTHANDLER_STACK_OVERFLOW
 
 #ifdef __INTEL_COMPILER
@@ -1153,7 +1219,7 @@ faulthandler_stack_overflow(PyObject *self, PyObject *Py_UNUSED(ignored))
         size, depth);
     return NULL;
 }
-#endif   /* defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION) */
+#endif   /* defined(FAULTHANDLER_USE_ALT_STACK) && defined(HAVE_SIGACTION) */
 
 
 static int
@@ -1318,35 +1384,18 @@ faulthandler_init_enable(void)
 PyStatus
 _PyFaulthandler_Init(int enable)
 {
-#ifdef HAVE_SIGALTSTACK
+#ifdef FAULTHANDLER_USE_ALT_STACK
-    int err;
+    memset(&stack, 0, sizeof(stack));
-
-    /* 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;
     /* bpo-21131: allocate dedicated stack of SIGSTKSZ*2 bytes, instead of just
        SIGSTKSZ bytes. Calling the previous signal handler in faulthandler
        signal handler uses more than SIGSTKSZ bytes of stack memory on some
        platforms. */
     stack.ss_size = SIGSTKSZ * 2;
-    stack.ss_sp = PyMem_Malloc(stack.ss_size);
-    if (stack.ss_sp != NULL) {
-        err = sigaltstack(&stack, &old_stack);
-        if (err) {
-            PyMem_Free(stack.ss_sp);
-            stack.ss_sp = NULL;
-        }
-    }
 #endif
+
 #ifdef FAULTHANDLER_LATER
-    thread.file = NULL;
+    memset(&thread, 0, sizeof(thread));
-    thread.cancel_event = PyThread_allocate_lock();
-    thread.running = PyThread_allocate_lock();
-    if (!thread.cancel_event || !thread.running) {
-        return _PyStatus_ERR("failed to allocate locks for faulthandler");
-    }
-    PyThread_acquire_lock(thread.cancel_event, 1);
 #endif
 
     if (enable) {
@@ -1386,7 +1435,8 @@ void _PyFaulthandler_Fini(void)
 
     /* fatal */
     faulthandler_disable();
-#ifdef HAVE_SIGALTSTACK
+
+#ifdef FAULTHANDLER_USE_ALT_STACK
     if (stack.ss_sp != NULL) {
         /* Fetch the current alt stack */
         stack_t current_stack;