Implement default signal handlers (#20257)

This implements best-effort attempt to implement default signal handlers as per https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html:
 - Terminate action is mapped to `_Exit` aka `wasi_proc_exit` - variant of exit that doesn't run `atexit` handlers.
 - Abort action (terminate with core dump) is mapped to `__builtin_trap` aka Wasm `unreachable` instruction.
 - Process pause/continue actions are ignored as it's unclear what that would do in Wasm (maybe use Asyncify in the future?).

Fixes #20254

Author: RReverser

system/lib/libc/raise.c

@@ -17,6 +17,39 @@ extern sigset_t __sig_pending;
 
 bool __sig_is_blocked(int sig);
 
+// Default handler actions ~auto-generated from https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html.
+// Note that stop and continue actions are not supported and treated as ignored.
+
+typedef enum {
+  ACTION_IGNORE = 0,
+  ACTION_TERMINATE,
+  ACTION_ABORT,
+} default_action_t;
+
+static default_action_t default_actions[_NSIG] = {
+  [SIGABRT] = ACTION_ABORT,
+  [SIGALRM] = ACTION_TERMINATE,
+  [SIGBUS] = ACTION_ABORT,
+  [SIGFPE] = ACTION_ABORT,
+  [SIGHUP] = ACTION_TERMINATE,
+  [SIGILL] = ACTION_ABORT,
+  [SIGINT] = ACTION_TERMINATE,
+  [SIGKILL] = ACTION_TERMINATE,
+  [SIGPIPE] = ACTION_TERMINATE,
+  [SIGQUIT] = ACTION_ABORT,
+  [SIGSEGV] = ACTION_ABORT,
+  [SIGTERM] = ACTION_TERMINATE,
+  [SIGUSR1] = ACTION_TERMINATE,
+  [SIGUSR2] = ACTION_TERMINATE,
+  [SIGPOLL] = ACTION_TERMINATE,
+  [SIGPROF] = ACTION_TERMINATE,
+  [SIGSYS] = ACTION_ABORT,
+  [SIGTRAP] = ACTION_ABORT,
+  [SIGVTALRM] = ACTION_TERMINATE,
+  [SIGXCPU] = ACTION_ABORT,
+  [SIGXFSZ] = ACTION_ABORT,
+};
+
 int raise(int sig) {
   if (__sig_is_blocked(sig)) {
     sigaddset(&__sig_pending, sig);
@@ -26,14 +59,25 @@ int raise(int sig) {
     siginfo_t t = {0};
     __sig_actions[sig].sa_sigaction(sig, &t, NULL);
   } else {
-    if (__sig_actions[sig].sa_handler == SIG_DFL || __sig_actions[sig].sa_handler == SIG_IGN) {
-
-      return 0;
+    void (*handler)(int) = __sig_actions[sig].sa_handler;
+    if (handler == SIG_DFL) {
+      switch (default_actions[sig]) {
+        case ACTION_IGNORE:
+          break;
+        case ACTION_TERMINATE:
+          // Intentionally exiting via a function that doesn't call atexit handlers.
+          _Exit(128 + sig);
+        case ACTION_ABORT:
+          // TODO: should this integrate with `Module['onAbort'] once `abort` is migrated
+          // to raise SIGABRT as per spec?
+          __builtin_trap();
+      }
+    } else if (handler != SIG_IGN) {
+        // Avoid a direct call to the handler, and instead call via JS so we can
+        // avoid strict signature checking.
+        // https://github.com/emscripten-core/posixtestsuite/issues/6
+        __call_sighandler(__sig_actions[sig].sa_handler, sig);
     }
-    // Avoid a direct call to the handler, and instead call via JS so we can
-    // avoid strict signature checking.
-    // https://github.com/emscripten-core/posixtestsuite/issues/6
-    __call_sighandler(__sig_actions[sig].sa_handler, sig);
   }
   return 0;
 }

test/other/test_standalone_syscalls.cpp

@@ -37,9 +37,9 @@ int main() {
   // check we can call this, but the test doesn't check the output
   emscripten_get_now();
 
-  // This doesn't do anything because we have not handler registered, but it
+  // This doesn't do anything because we have no handler registered, but it
   // verifies that `raise` can be included in the build without any non-standard
   // imports being generated.
-  raise(SIGUSR1);
+  raise(SIGCHLD);
   return 0;
 }

test/test_core.py

@@ -10,6 +10,7 @@
 import random
 import re
 import shutil
+import signal
 import sys
 import time
 import unittest
@@ -6179,6 +6180,19 @@ def test_sigalrm(self):
   def test_signals(self):
     self.do_core_test(test_file('test_signals.c'))
 
+  @parameterized({
+    'sigint': (signal.SIGINT, 128 + signal.SIGINT, True),
+    'sigabrt': (signal.SIGABRT, 7, False)
+  })
+  def test_sigaction_default(self, signal, exit_code, assert_identical):
+    self.set_setting('EXIT_RUNTIME')
+    self.do_core_test(
+      test_file('test_sigaction_default.c'),
+      args=[str(int(signal))],
+      assert_identical=assert_identical,
+      assert_returncode=exit_code
+    )
+
   @no_windows('https://github.com/emscripten-core/emscripten/issues/8882')
   @requires_node
   def test_unistd_access(self):

test/test_sigaction_default.c

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2023 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <assert.h>
+
+static void cleanExit() {
+  puts("3");
+}
+
+int main(int argc, char **argv) {
+  atexit(cleanExit);
+  puts("1");
+  raise(atoi(argv[1]));
+  puts("2");
+  return 0;
+}

test/test_sigaction_default.out

@@ -0,0 +1 @@
+1