main: call gjs_dumpstack on aborts and traps and optionally on segfaults

In order to debug issues triggered by JS code we might need to
see the stack of it, this is not normally visible in static stack traces,
thus we need to call gjs_dumpstack () before dying the process.

Intercepting signals SIGABRT, SIGTRAP (needed for catching fatal glib
errors) SIGFPE and SIGIOT by default, while introducing a new
'backtrace-segfaults' flag for the SHELL_DEBUG environment variable to
do the same on SIGSEGV and SIGBUS (this is a precaution to avoid that we
corrupt the stack for automatic errors trackers).

In any case after dumping the stack we raise the signal again, in order
to make the system aware of it.

https://bugzilla.gnome.org/show_bug.cgi?id=789237

src/main.c

@@ -38,12 +38,14 @@ extern GType gnome_shell_plugin_get_type (void);
 
 static gboolean is_gdm_mode = FALSE;
 static char *session_mode = NULL;
+static int caught_signal = 0;
 
 #define DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER 1
 #define DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER 4
 
 enum {
   SHELL_DEBUG_BACKTRACE_WARNINGS = 1,
+  SHELL_DEBUG_BACKTRACE_SEGFAULTS = 2,
 };
 static int _shell_debug;
 
@@ -290,7 +292,8 @@ static void
 shell_init_debug (const char *debug_env)
 {
   static const GDebugKey keys[] = {
-    { "backtrace-warnings", SHELL_DEBUG_BACKTRACE_WARNINGS }
+    { "backtrace-warnings", SHELL_DEBUG_BACKTRACE_WARNINGS },
+    { "backtrace-segfaults", SHELL_DEBUG_BACKTRACE_SEGFAULTS },
   };
 
   _shell_debug = g_parse_debug_string (debug_env, keys,
@@ -328,6 +331,52 @@ shut_up (const char     *domain,
 {
 }
 
+static void
+dump_gjs_stack_alarm_sigaction (int        signo,
+                                siginfo_t *info,
+                                void      *data)
+{
+  g_warning ("Failed to dump Javascript stack, got stuck");
+
+  raise (caught_signal);
+}
+
+static void
+dump_gjs_stack_on_signal_handler (int signo)
+{
+  /* Waiting at least 5 seconds for the dumpstack, if it fails, we raise the error */
+  struct sigaction sa;
+
+  caught_signal = signo;
+  memset (&sa, 0, sizeof (sigaction));
+  sigemptyset (&sa.sa_mask);
+
+  sa.sa_flags     = SA_SIGINFO;
+  sa.sa_sigaction = dump_gjs_stack_alarm_sigaction;
+
+  sigaction (SIGALRM, &sa, NULL);
+
+  alarm (5);
+  gjs_dumpstack ();
+  alarm (0);
+
+  raise (signo);
+}
+
+static void
+dump_gjs_stack_on_signal (int signo)
+{
+  struct sigaction sa;
+
+  memset (&sa, 0, sizeof (sigaction));
+  sigemptyset (&sa.sa_mask);
+
+  sa.sa_flags   = SA_RESETHAND;
+  sa.sa_handler = dump_gjs_stack_on_signal_handler;
+
+  sigaction (signo, &sa, NULL);
+}
+
 static gboolean
 list_modes (const char  *option_name,
             const char  *value,
@@ -459,6 +508,17 @@ main (int argc, char **argv)
 
   shell_prefs_init ();
 
+  dump_gjs_stack_on_signal (SIGABRT);
+  dump_gjs_stack_on_signal (SIGFPE);
+  dump_gjs_stack_on_signal (SIGIOT);
+  dump_gjs_stack_on_signal (SIGTRAP);
+
+  if ((_shell_debug & SHELL_DEBUG_BACKTRACE_SEGFAULTS))
+    {
+      dump_gjs_stack_on_signal (SIGBUS);
+      dump_gjs_stack_on_signal (SIGSEGV);
+    }
+
   ecode = meta_run ();
 
   g_debug ("Doing final cleanup");