logind: use SIGRTMIN to not conflict with xwayland

xwayland uses SIGUSR1 as startup notification. Make sure to use SIGRTMIN
and SIGRTMIN+1 for VT handling.

A bonus is SIGRT* signals can be queued multiple times, so we will be able
to correctly track them and will no longer lose signals (which wouldn't
really matter, but is confusing in logs).

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Tested-by: nerdopolis <bluescreen_avenger@verizon.net>

src/logind-util.c

@@ -692,14 +692,10 @@ signal_event(int fd, uint32_t mask, void *data)
 		return 0;
 	}
 
-	switch (sig.ssi_signo) {
-	case SIGUSR1:
+	if (sig.ssi_signo == (unsigned int)SIGRTMIN)
 		ioctl(wl->vt, VT_RELDISP, 1);
-		break;
-	case SIGUSR2:
+	else if (sig.ssi_signo == (unsigned int)SIGRTMIN + 1)
 		ioctl(wl->vt, VT_RELDISP, VT_ACKACQ);
-		break;
-	}
 
 	return 0;
 }
@@ -767,9 +763,21 @@ weston_logind_setup_vt(struct weston_logind *wl)
 		goto err_kbmode;
 	}
 
+	/*
+	 * SIGRTMIN is used as global VT-release signal, SIGRTMIN + 1 is used
+	 * as VT-acquire signal. Note that SIGRT* must be tested on runtime, as
+	 * their exact values are not known at compile-time. POSIX requires 32
+	 * of them to be available, though.
+	 */
+	if (SIGRTMIN + 1 > SIGRTMAX) {
+		weston_log("logind: not enough RT signals available: %u-%u\n",
+			   SIGRTMIN, SIGRTMAX);
+		return -EINVAL;
+	}
+
 	sigemptyset(&mask);
-	sigaddset(&mask, SIGUSR1);
-	sigaddset(&mask, SIGUSR2);
+	sigaddset(&mask, SIGRTMIN);
+	sigaddset(&mask, SIGRTMIN + 1);
 	sigprocmask(SIG_BLOCK, &mask, NULL);
 
 	wl->sfd = signalfd(-1, &mask, SFD_NONBLOCK | SFD_CLOEXEC);
@@ -790,8 +798,8 @@ weston_logind_setup_vt(struct weston_logind *wl)
 	}
 
 	mode.mode = VT_PROCESS;
-	mode.relsig = SIGUSR1;
-	mode.acqsig = SIGUSR2;
+	mode.relsig = SIGRTMIN;
+	mode.acqsig = SIGRTMIN + 1;
 	if (ioctl(wl->vt, VT_SETMODE, &mode) < 0) {
 		r = -errno;
 		weston_log("logind: cannot take over VT: %m\n");