Add swaylockd – a dumb launcher to spawn swaylock and ensure it’s running no matter what

meson.build

@@ -182,6 +182,11 @@ executable('swaylock',
 	install: true
 )
 
+executable('swaylockd',
+	'swaylockd.c',
+	install: true
+)
+
 install_data(
 	'pam/swaylock',
 	install_dir: sysconfdir + '/pam.d/'
@@ -192,6 +197,7 @@ if scdoc.found()
 	mandir = get_option('mandir')
 	man_files = [
 		'swaylock.1.scd',
+		'swaylockd.1.scd',
 	]
 	foreach filename : man_files
 		topic = filename.split('.')[-3].split('/')[-1]

swaylockd.1.scd

@@ -0,0 +1,60 @@
+swaylockd(1)
+
+# NAME
+
+swaylockd - launch swaylock(1) and re-launch if it dies
+
+
+# SYNOPSIS
+
+*swaylockd* [options...]
+
+All options are directly passed to *swaylock(1)*.
+
+
+# DESCRIPTION
+
+*swaylockd* is a dumb launcher to spawn *swaylock(1)* and ensure it's running no matter what - it immediately restarts *swaylock(1)* if terminated by a signal (e.g. the process crashed) and also blocks all signals (except `SIGKILL` and `SIGSTOP`).
+It also ensures that only one instance per user is running (using *flock(2)*).
+
+*IMPORTANT*: *swaylockd* is not compatible with *swaylock(1)* option *--daemonize*!
+
+
+# ENVIRONMENT
+
+*XDG_RUNTIME_DIR*:
+	This variable must be set and contain the path to the existing directory where the lock file will be created.
+	Refer to "XDG Base Directory Specification" for more information.
+
+
+# FILES
+
+*${XDG_RUNTIME_DIR}/swaylockd.lock*
+	A lock file created when *swaylockd* is executed and removed right before it quits.
+
+
+# LOGGING
+
+Error messages generated by *swaylockd* are printed to STDERR and logged to syslog with ident string "swaylockd" and facility code 1 (user).
+
+
+# HISTORY
+
+*swaylockd* has been developed mainly as a workaround for the *swaylock(1)* stability issues (see e.g. https://github.com/swaywm/swaylock/issues/181).
+
+*swaylock* is a critical piece of security software - as a screen locker, any bug in the program that causes it to crash will cause the screen to unlock.
+As soon as *swaylock* is no longer running, the screen is no longer locked.
+Therefore, great care must be taken to ensure that it never crash.
+
+*swaylockd* started (and is still available) as a standalone project at https://github.com/jirutka/swaylockd.
+The code has been merged into sway-effects.
+
+
+# AUTHORS
+
+Jakub Jirutka
+
+
+# SEE ALSO
+
+*swaylock(1)*

swaylockd.c

@@ -0,0 +1,101 @@
+// vim: set ts=4:
+// SPDX-FileCopyrightText: 2021 Jakub Jirutka <[email protected]>
+// SPDX-License-Identifier: MIT
+
+#define _POSIX_C_SOURCE 200809L
+
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <spawn.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <syslog.h>
+#include <unistd.h>
+#include <sys/file.h>
+#include <sys/wait.h>
+
+#define PROGNAME "swaylockd"
+
+#ifndef SWAYLOCK_PATH
+  #define SWAYLOCK_PATH "/usr/bin/swaylock"
+#endif
+
+#define logerr(format, ...) \
+	do { \
+		fprintf(stderr, PROGNAME ": " format "\n", __VA_ARGS__); \
+		if (syslog_enabled) syslog(LOG_ERR, format, __VA_ARGS__); \
+	} while (0)
+
+
+extern char **environ;
+
+static bool syslog_enabled = true;
+
+int main (int argc, char **argv) {
+	char lock_path[PATH_MAX] = "\0";
+	int lock_fd = -1;
+	int rc = 1;
+
+	// Open connection to syslog.
+	openlog(PROGNAME, LOG_PID, LOG_USER);
+
+	{
+		const char *xdg_rd = getenv("XDG_RUNTIME_DIR");
+		if (xdg_rd == NULL) {
+			logerr("%s", "XDG_RUNTIME_DIR not set in the environment");
+			return 100;
+		}
+		(void) snprintf(lock_path, sizeof(lock_path), "%s/%s.lock", xdg_rd, PROGNAME);
+	}
+
+	// Obtain exclusive lock.
+	if ((lock_fd = open(lock_path, O_CREAT | O_RDWR | O_CLOEXEC, 0600)) < 0) {
+		logerr("failed to write %s: %s", lock_path, strerror(errno));
+		return 101;
+	}
+	if (flock(lock_fd, LOCK_EX | LOCK_NB) < 0) {
+		logerr("another instance of %s is running", PROGNAME);
+		return 102;
+	}
+
+	// Block (ignore) all signals we can (i.e. except SIGKILL and SIGSTOP).
+	{
+		sigset_t mask;
+		sigfillset(&mask);
+		sigprocmask(SIG_SETMASK, &mask, NULL);
+	}
+
+	argv[0] = SWAYLOCK_PATH;
+
+	for (int i = 1;; i++) {
+		pid_t pid;
+		if (posix_spawn(&pid, SWAYLOCK_PATH, NULL, 0, argv, environ) != 0) {
+			logerr("failed to spawn %s: %s", SWAYLOCK_PATH, strerror(errno));
+			rc = 101;
+			goto done;
+		}
+
+		int status = -1;
+		(void) waitpid(pid, &status, 0);
+
+		// Exit only if the child terminated normally, not via signal.
+		if (WIFEXITED(status)) {
+			rc = WEXITSTATUS(status);
+			goto done;
+		}
+		logerr("%s terminated with signal %d, restarting (%d)",
+		       SWAYLOCK_PATH, WIFSIGNALED(status) ? WTERMSIG(status) : -1, i);
+
+		// Just to make sure we don't flood syslog in case of some bug...
+		if (i > 100) syslog_enabled = false;
+	}
+
+done:
+	if (unlink(lock_path) < 0) {
+		logerr("failed to remove lock file %s: %s", lock_path, strerror(errno));
+	}
+	return rc;
+}