seatd-launch: Add seatd launch wrapper

This launch wrapper is used to conveniently start a new seatd instance,
wait for it to be ready, and launch a target application.

meson.build

@@ -204,6 +204,13 @@ if with_server
 		install: true,
 		dependencies: [realtime],
 	)
+	executable(
+		'seatd-launch',
+		[ 'seatd-launch/seatd-launch.c' ],
+		include_directories: [include_directories('.', 'include')],
+		install: true,
+		dependencies: [realtime],
+	)
 endif
 
 if get_option('examples') == 'enabled'

seatd-launch/seatd-launch.c

@@ -0,0 +1,123 @@
+#include <errno.h>
+#include <poll.h>
+#include <pwd.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+int main(int argc, char *argv[]) {
+	(void)argc;
+	char sockbuf[256];
+
+	sprintf(sockbuf, "/tmp/seatd.%d.sock", getpid());
+	unlink(sockbuf);
+
+	int fds[2];
+	if (pipe(fds) == -1) {
+		perror("Could not create pipe");
+		goto error;
+	}
+
+	pid_t seatd_child = fork();
+	if (seatd_child == -1) {
+		perror("Could not fork seatd process");
+		goto error;
+	} else if (seatd_child == 0) {
+		close(fds[0]);
+
+		char pipebuf[8];
+		sprintf(pipebuf, "%d", fds[1]);
+		struct passwd *user = getpwuid(getuid());
+
+		// TODO: Make seatd accept the numeric UID
+		execlp("seatd", "seatd", "-n", pipebuf, "-u", user->pw_name, "-s", sockbuf, NULL);
+		perror("Could not start seatd");
+		goto error;
+	}
+	close(fds[1]);
+
+	// Drop privileges
+	if (setgid(getgid()) == -1) {
+		perror("Could not set gid to drop privileges");
+		goto error_seatd;
+	}
+	if (setuid(getuid()) == -1) {
+		perror("Could not set uid to drop privileges");
+		goto error_seatd;
+	}
+
+	char buf[1] = {0};
+	while (true) {
+		pid_t p = waitpid(seatd_child, NULL, WNOHANG);
+		if (p == seatd_child) {
+			fprintf(stderr, "seatd exited prematurely\n");
+			goto error_seatd;
+		} else if (p == -1 && (errno != EINTR && errno != ECHILD)) {
+			perror("Could not wait for seatd process");
+			goto error_seatd;
+		}
+
+		struct pollfd fd = {
+			.fd = fds[0],
+			.events = POLLIN,
+		};
+
+		// We poll with timeout to avoid a racing on a blocking read
+		if (poll(&fd, 1, 1000) == -1) {
+			if (errno == EAGAIN || errno == EINTR) {
+				continue;
+			} else {
+				perror("Could not poll notification fd");
+				goto error_seatd;
+			}
+		}
+
+		if (fd.revents & POLLIN) {
+			ssize_t n = read(fds[0], buf, 1);
+			if (n == -1 && errno != EINTR) {
+				perror("Could not read from pipe");
+				goto error_seatd;
+			} else if (n > 0) {
+				break;
+			}
+		}
+	}
+	close(fds[0]);
+
+	pid_t child = fork();
+	if (child == -1) {
+		perror("Could not fork target process");
+		goto error_seatd;
+	} else if (child == 0) {
+		setenv("SEATD_SOCK", sockbuf, 1);
+		execv(argv[1], &argv[1]);
+		perror("Could not start target");
+		goto error_seatd;
+	}
+
+	while (true) {
+		pid_t p = waitpid(child, NULL, 0);
+		if (p == child) {
+			break;
+		} else if (p == -1 && errno != EINTR) {
+			perror("Could not wait for target process");
+			goto error_seatd;
+		}
+	}
+
+	unlink(sockbuf);
+	kill(seatd_child, SIGTERM);
+	return 0;
+
+error_seatd:
+	unlink(sockbuf);
+	kill(seatd_child, SIGTERM);
+error:
+	return 1;
+}