seatd: s6-style readiness notification support

This adds the ability to specify the number of an fd that is inherited
by the process as open. Once seatd is read to serve requests, it will
write a single newline and close the fd.

2 files changed, 22 insertions, 1 deletions

diff --git a/man/seatd.1.scd b/man/seatd.1.scd
index ab79ee1..5945782 100644
--- a/man/seatd.1.scd
+++ b/man/seatd.1.scd
@@ -13,6 +13,11 @@ seatd - A seat management daemon
 *-h*
 	Show help message and quit.
 
+*-n*
+	FD to notify readiness on. A single newline will be written and the fd
+	closed when seatd is ready to serve requests. This is compatible with
+	s6's notification protocol.
+
 *-u <user>*
 	User to own the seatd socket.
 
diff --git a/seatd/seatd.c b/seatd/seatd.c
index 278f857..420847d 100644
--- a/seatd/seatd.c
+++ b/seatd/seatd.c
@@ -70,6 +70,7 @@ int main(int argc, char *argv[]) {
 	const char *usage = "Usage: seatd [options]\n"
 			    "\n"
 			    "  -h		Show this help message\n"
+			    "  -n <fd>          FD to notify readiness on\n"
 			    "  -u <user>	User to own the seatd socket\n"
 			    "  -g <group>	Group to own the seatd socket\n"
 			    "  -s <path>	Where to create the seatd socket\n"
@@ -78,9 +79,17 @@ int main(int argc, char *argv[]) {
 
 	int c;
 	int uid = 0, gid = 0;
+	int readiness = -1;
 	const char *socket_path = getenv("SEATD_SOCK");
-	while ((c = getopt(argc, argv, "vhs:g:u:")) != -1) {
+	while ((c = getopt(argc, argv, "vhn:s:g:u:")) != -1) {
 		switch (c) {
+		case 'n':
+			readiness = atoi(optarg);
+			if (readiness < 0) {
+				fprintf(stderr, "Invalid readiness fd: %s\n", optarg);
+				return 1;
+			}
+			break;
 		case 's':
 			socket_path = optarg;
 			break;
@@ -149,6 +158,13 @@ int main(int argc, char *argv[]) {
 
 	log_info("seatd started");
 
+	if (readiness != -1) {
+		if (write(readiness, "\n", 1) == -1) {
+			log_errorf("Could not write readiness signal: %s\n", strerror(errno));
+		}
+		close(readiness);
+	}
+
 	while (server.running) {
 		if (poller_poll(&server.poller) == -1) {
 			log_errorf("Poller failed: %s", strerror(errno));