[mce] Use sigaction() for registering signal handlers. Fixes JB#38515

Under hard to reproduce conditions mce can crash due to SIGPIPE when
attempting to writev() to libdsme related unix domain socket.

Assumption is that it might have something to do with mce setting up
SIGPIPE handler via signal() and attempting to pass the signals to
mainloop just to be ignored there.

Simply ignore SIGPIPE altogether instead of attempting to move it from
signal handler context to mainloop.

Also use sigaction() for setting up signal handlers, separate handling of
terminal failure signals from expected control signals, and make sure that
signal() function is used only for SIG_IGN and SIG_DFL purposes.

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>

mce.c

@@ -206,56 +206,100 @@ static void signal_handler(const gint signr)
 		mce_quit_mainloop();
 		break;
 
-	case SIGPIPE:
-		break;
-
 	default:
 		/* Should never happen */
+		mce_log(LL_WARN, "stray signal %d received in mainloop", signr);
 		break;
 	}
 }
 
-/** Array of signals that should be trapped */
-static const int mce_signals_to_trap[] =
+/** Array of signals that should be ignored */
+static const int mce_signals_to_ignore[] =
 {
-	SIGUSR1,
-	SIGUSR2,
-	SIGHUP,
+	/* We want error return from write() & co, not a signal */
+	SIGPIPE,
 
-	SIGINT,
-	SIGQUIT,
-	SIGTERM,
+	/* Ignore tty signals even if mce is run from terminal */
+	SIGTSTP,
+	SIGTTOU,
+	SIGTTIN,
+	-1
+};
 
+/** Array of signals that should terminate mce */
+static const int mce_signals_to_exit_on[] =
+{
 #ifdef ENABLE_WAKELOCKS
 	SIGABRT,
 	SIGILL,
 	SIGFPE,
 	SIGSEGV,
-	SIGPIPE,
 	SIGALRM,
 	SIGBUS,
 	SIGTSTP,
 #endif
+	-1
+};
 
+/** Array of signals that should be trapped */
+static const int mce_signals_to_trap[] =
+{
+	SIGUSR1,
+	SIGUSR2,
+	SIGHUP,
+	SIGINT,
+	SIGQUIT,
+	SIGTERM,
 	-1
 };
 
 /** Install handlers for signals we need to trap
  */
 static void mce_signal_handlers_install(void)
 {
-	for( size_t i = 0; mce_signals_to_trap[i] != -1; ++i ) {
-		signal(mce_signals_to_trap[i], mce_tx_signal_cb);
-	}
+	struct sigaction sa;
+
+	/* Signals that are completely ignored */
+	for( size_t i = 0; mce_signals_to_ignore[i] != -1; ++i )
+		signal(mce_signals_to_ignore[i], SIG_IGN);
+
+	/* Unrecoverable situations that require immediate exit,
+	 * but we should still attempt to disable autosuspend
+	 * and clean up wakelocks: Reset default behavior when
+	 * triggered and do not block while attempting to handle.
+	 */
+
+	memset(&sa, 0, sizeof sa);
+	sigemptyset(&sa.sa_mask);
+	sa.sa_handler = mce_tx_signal_cb;
+	sa.sa_flags   = SA_RESETHAND | SA_NODEFER;
+
+	for( size_t i = 0; mce_signals_to_exit_on[i] != -1; ++i )
+		sigaction(mce_signals_to_exit_on[i], &sa, 0);
+
+	/* Signals that should be ok to handle via mainloop */
+
+	memset(&sa, 0, sizeof sa);
+	sigemptyset(&sa.sa_mask);
+	sa.sa_handler = mce_tx_signal_cb;
+	sa.sa_flags   = SA_RESTART;
+
+	for( size_t i = 0; mce_signals_to_trap[i] != -1; ++i )
+		sigaction(mce_signals_to_trap[i], &sa, 0);
 }
 
 /** Restore default handlers for trapped signals
  */
 void mce_signal_handlers_remove(void)
 {
-	for( size_t i = 0; mce_signals_to_trap[i] != -1; ++i ) {
+	for( size_t i = 0; mce_signals_to_ignore[i] != -1; ++i )
+		signal(mce_signals_to_ignore[i], SIG_DFL);
+
+	for( size_t i = 0; mce_signals_to_exit_on[i] != -1; ++i )
+		signal(mce_signals_to_exit_on[i], SIG_DFL);
+
+	for( size_t i = 0; mce_signals_to_trap[i] != -1; ++i )
 		signal(mce_signals_to_trap[i], SIG_DFL);
-	}
 }
 
 /** Pipe used for transferring signals out of signal handler context */
@@ -303,17 +347,16 @@ static void mce_tx_signal_cb(int sig)
 	static volatile int exit_tries = 0;
 
 	static const char msg[] = "\n*** BREAK ***\n";
-#ifdef ENABLE_WAKELOCKS
 	static const char die[] = "\n*** UNRECOVERABLE FAILURE ***\n";
-#endif
-
-	/* FIXME: Should really use sigaction() to avoid having
-	 * the default handler active until we manage to restore
-	 * our handler here ... */
-	signal(sig, mce_tx_signal_cb);
 
 	switch( sig )
 	{
+	case SIGUSR1:
+	case SIGUSR2:
+	case SIGHUP:
+		/* Just pass to mainloop */
+		break;
+
 	case SIGINT:
 	case SIGQUIT:
 	case SIGTERM:
@@ -333,25 +376,13 @@ static void mce_tx_signal_cb(int sig)
 		}
 		break;
 
-#ifdef ENABLE_WAKELOCKS
-	case SIGABRT:
-	case SIGILL:
-	case SIGFPE:
-	case SIGSEGV:
-	case SIGALRM:
-	case SIGBUS:
-		/* Unrecoverable failures can't be handled in the mainloop
-		 * Terminate but disable suspend first */
+	default:
+		/* Assume unrecoverable failure that can't be handled in
+		 * the mainloop - disable autosuspend and then terminate
+		 * via default signal handler. */
 		no_error_check_write(STDERR_FILENO, die, sizeof die - 1);
 		mce_exit_via_signal(sig);
 		break;
-
-	case SIGTSTP:
-		/* Stopping mce could also lead to unrecoverable suspend */
-		break;
-#endif
-	default:
-		break;
 	}
 
 	/* transfer the signal to mainloop via pipe */