[gnome-shell] main: use SA_NODEFER to track signals to being able to raise from alarm

[Marco Trevisan <marcotrevi src gnome org>]

commit 891bc45e360b70bffbd081d7c992d73647ac5259
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Tue Oct 24 00:28:33 2017 -0500

    main: use SA_NODEFER to track signals to being able to raise from alarm
    
    After we receive one of the tracked signals, in case we get stuck inside
    the gjs_dumpstack () call, we use an alarm to raise the previously emitted
    signal, however without using SA_NODEFER, the raise inside the alarm handler
    will be ignored.
    
    To avoid to handle new signals caused by the handler calls, once we get the
    first signal, we just ignore them all as they could only lead to dirty traces.
    
    Also, cleaning up a bit the code, and disabling the shell log handler
    in dump_gjs_stack_alarm_sigaction since this might lead to a new
    gjs_dumpstack () request.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=789237

 src/main.c |   38 +++++++++++++++++++++++---------------
 1 files changed, 23 insertions(+), 15 deletions(-)
---
diff --git a/src/main.c b/src/main.c
index 214d2a5..934fbd5 100644
--- a/src/main.c
+++ b/src/main.c
@@ -48,6 +48,7 @@ enum {
   SHELL_DEBUG_BACKTRACE_SEGFAULTS = 2,
 };
 static int _shell_debug;
+static gboolean _tracked_signals[NSIG] = { 0 };
 
 static void
 shell_dbus_acquire_name (GDBusProxy  *bus,
@@ -332,11 +333,11 @@ shut_up (const char     *domain,
 }
 
 static void
-dump_gjs_stack_alarm_sigaction (int        signo,
-                                siginfo_t *info,
-                                void      *data)
+dump_gjs_stack_alarm_sigaction (int signo)
 {
+  g_log_set_default_handler (g_log_default_handler, NULL);
   g_warning ("Failed to dump Javascript stack, got stuck");
+  g_log_set_default_handler (default_log_handler, NULL);
 
   raise (caught_signal);
 }
@@ -344,16 +345,24 @@ dump_gjs_stack_alarm_sigaction (int        signo,
 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;
+  struct sigaction sa = { 0 };
+  gsize i;
+
+  /* Ignore all the signals starting this point, a part the one we'll raise
+   * (which is implicitly ignored here through SA_RESETHAND), this is needed
+   * not to get this handler being called by other signals that we were
+   * tracking and that might be emitted by code called starting from now.
+   */
+  for (i = 0; i < G_N_ELEMENTS (_tracked_signals); ++i)
+    {
+      if (_tracked_signals[i] && i != signo)
+        signal (i, SIG_IGN);
+    }
 
+  /* Waiting at least 5 seconds for the dumpstack, if it fails, we raise the error */
   caught_signal = signo;
-  memset (&sa, 0, sizeof (sigaction));
+  sa.sa_handler = dump_gjs_stack_alarm_sigaction;
   sigemptyset (&sa.sa_mask);
-
-  sa.sa_flags     = SA_SIGINFO;
-  sa.sa_sigaction = dump_gjs_stack_alarm_sigaction;
-
   sigaction (SIGALRM, &sa, NULL);
 
   alarm (5);
@@ -366,15 +375,14 @@ dump_gjs_stack_on_signal_handler (int signo)
 static void
 dump_gjs_stack_on_signal (int signo)
 {
-  struct sigaction sa;
+  struct sigaction sa = { 0 };
 
-  memset (&sa, 0, sizeof (sigaction));
-  sigemptyset (&sa.sa_mask);
-
-  sa.sa_flags   = SA_RESETHAND;
+  sa.sa_flags   = SA_RESETHAND | SA_NODEFER;
   sa.sa_handler = dump_gjs_stack_on_signal_handler;
+  sigemptyset (&sa.sa_mask);
 
   sigaction (signo, &sa, NULL);
+  _tracked_signals[signo] = TRUE;
 }
 
 static gboolean