shmounts: terminate the forked off mount namespace capture helper

Make sure to terminate the forked off mount namespace capture helper, otherwise
the child will continue execution of the remaining setup code and thus cause a
race with the parent which occasionally manifests itself with the
/var/snap/lxd/common/shmounts being missing from the parennt's mount namespace
after it has switched back to the moutn ns of pid 1.

Signed-off-by: Maciej Borzecki <maciej.borzecki@canonical.com>

shmounts/setup-shmounts.c

@@ -66,6 +66,12 @@ int mkdir_p(const char *dir, mode_t mode)
 	return 0;
 }
 
+__attribute__((noreturn))
+static void die(const char *s)  {
+	perror(s);
+	exit(1);
+}
+
 int setup_ns() {
 	int wstatus;
 	ssize_t ret;
@@ -94,40 +100,41 @@ int setup_ns() {
 		ret = lxc_read_nointr(pipe_fds[0], nspath, 1);
 		close(pipe_fds[0]);
 		if (ret < 0) {
-			return -1;
+			die("cannot read from pipe");
 		}
 
 		// Create the mountpoint
 		if (mkdir("/var/snap/lxd/common/ns", 0700) < 0 && errno != EEXIST) {
-			return -1;
+			die("cannot mkdir /var/snap/lxd/common/ns");
 		}
 
 		// Mount a tmpfs
 		if (mount("tmpfs", "/var/snap/lxd/common/ns", "tmpfs", 0, "size=1M,mode=0700") < 0) {
-			return -1;
+			die("cannot mount tmpfs on /var/snap/lxd/common/ns");
 		}
 
 		// Mark the tmpfs mount as MS_PRIVATE
 		if (mount("none", "/var/snap/lxd/common/ns", NULL, MS_REC|MS_PRIVATE, NULL) < 0) {
-			return -1;
+			die("cannot change propagation on /var/snap/lxd/common/ns");
 		}
 
 		// Store reference to the mntns
 		if (snprintf(nspath, PATH_MAX, "/proc/%u/ns/mnt", (unsigned)getppid()) < 0) {
-			return -1;
+			die("cannot capture reference to parent's mount ns");
 		}
 
 		fd = open("/var/snap/lxd/common/ns/shmounts", O_CREAT | O_RDWR, 0600);
 		if (fd < 0) {
-			return -1;
+			die("cannot open /var/snap/lxd/common/ns/shmounts");
 		}
 		close(fd);
 
 		if (mount(nspath, "/var/snap/lxd/common/ns/shmounts", NULL, MS_BIND, NULL) < 0) {
-			return -1;
+			die("cannot bind mount ns to /var/snap/lxd/common/ns/shmounts");
 		}
 
-		return 0;
+		/* the child is done */
+		exit(0);
 	}
 
 	close(pipe_fds[0]);