libink/finit: properties, and org.finit on the system bus

Runlevel and version are state, not actions, so they belong behind
org.freedesktop.DBus.Properties rather than another method each.

Finit also claims org.finit on the system bus when it finds one, so
ordinary D-Bus clients can reach it without knowing about
/run/finit/bus.  Opportunistic on purpose: no dbus-daemon is a normal
state for the systems Finit runs on, not an error to report.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

libink/server.c

@@ -5,6 +5,7 @@
  */
 
 #include <errno.h>
+#include <fcntl.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/socket.h>
@@ -160,3 +161,44 @@ int link_server_accept(link_server_t *srv, link_connection_t **out)
 	*out = conn;
 	return 0;
 }
+
+link_connection_t *link_server_attach(link_server_t *srv, int fd, uid_t peer_uid)
+{
+	link_connection_t *conn;
+	int flags;
+
+	/* On entry we always own `fd` -- close it on every failure path
+	 * so callers don't have to track whether we touched fcntl state. */
+	if (!srv || fd < 0) {
+		if (fd >= 0)
+			close_save_errno(fd);
+		errno = EINVAL;
+		return NULL;
+	}
+
+	/* Match server_accept's fd setup: CLOEXEC first (so a fork-and-
+	 * exec between the two calls cannot leak the fd), then NONBLOCK
+	 * so process_binary's read loop can drain without hanging. */
+	flags = fcntl(fd, F_GETFD, 0);
+	if (flags < 0 || fcntl(fd, F_SETFD, flags | FD_CLOEXEC) < 0)
+		goto err_close;
+	flags = fcntl(fd, F_GETFL, 0);
+	if (flags < 0 || fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0)
+		goto err_close;
+
+	conn = calloc(1, sizeof(*conn));
+	if (!conn)
+		goto err_close;
+
+	conn->fd       = fd;
+	conn->auth     = LINK_AUTH_DONE;	/* caller already handshook */
+	conn->server   = srv;
+	conn->peer_uid = peer_uid;
+	__auth_generate_guid(conn->guid);
+
+	return conn;
+
+err_close:
+	close_save_errno(fd);
+	return NULL;
+}