Fix handling of multiple SIGUSR1 signals

[mstoeckl]

The first commit fixes an issue in which, on receiving a second SIGUSR1, swaylock would exit without unlocking, printing User defined signal 1. The second fixes a deadlock which can occur if too many signals are sent in too short of a time; depending on how quickly signals are sent and on the pipe buffer size, the deadlock may prevent swaylock from exiting, possibly before it can send the unlock message.

To test this, I used a short program that sends signals in a tight loop:

signal_flood.c

// gcc signal_flood.c -o signal_flood
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>

int main(int argc, char **argv) {
	if (argc != 4) {
		printf("Usage: signal_flood PID SIGNUM NREPEAT\n");
		printf("SIGINT = 2\n");
		printf("SIGKILL = 9\n");
		printf("SIGUSR1 = 10\n");
		printf("SIGUSR2 = 12\n");
		printf("SIGTERM = 15\n");
		printf("SIGCONT = 18\n");
		printf("SIGSTOP = 19\n");
		return EXIT_FAILURE;
	}
	char *end;
	int sig = strtol(argv[2], &end, 10);
	if (*end) {
		printf("SIGNUM (%s) was not a number\n", argv[2]);
		return EXIT_FAILURE;
	}
	int nrepeat = strtol(argv[3], &end, 10);;
	if (*end) {
		printf("NREPEAT (%s) was not a number\n", argv[3]);
		return EXIT_FAILURE;
	}
	pid_t pid = strtol(argv[1], &end, 10);;
	if (*end) {
		printf("PID (%s) was not a number\n", argv[1]);
		return EXIT_FAILURE;
	}
	if (pid == 0) {
		printf("PID should be nonzero\n");
		return EXIT_FAILURE;
	}
	for (int i = 0; i < nrepeat; i++) {
		if (kill(pid, sig) == -1) {
			printf("Iteration %d, sending signal %d to %d failed\n",i,sig,pid);
			break;
		}
	}

	return EXIT_SUCCESS;
}