libink: a brokerless D-Bus implementation for Finit

Finit had no way to answer the question every service manager gets
asked: what is running, and change it.  D-Bus is how the rest of
userspace asks, but linking libdbus, sd-bus or GIO into PID 1 buys a
dependency, an allocator and a main loop we do not control.

So libink: the wire format, an object tree, and a bus of Finit's own
at /run/finit/bus, gated like INIT_SOCKET.  It speaks the standard
org.freedesktop.DBus, .Peer, .Introspectable interfaces, and Finit's
own Manager1, Service1 and Cond1 on top.  Methods that change
something are marked privileged and answered only for a caller the
kernel vouched for, via SO_PEERCRED.

Server and client both, since initctl is the first thing that needs
to talk to it, and its Start/Stop/Restart/Reload now go over the bus
rather than the legacy socket.

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

src/initctl.c

@@ -268,19 +268,105 @@ static int do_startstop(int cmd, char *arg)
 	return do_svc(cmd, arg);
 }
 
-static int do_start  (char *arg) { return do_startstop(INIT_CMD_START_SVC,   arg); }
-static int do_stop   (char *arg) { return do_startstop(INIT_CMD_STOP_SVC,    arg); }
+#ifdef HAVE_DBUS
+#include "link.h"
+
+/* Try the D-Bus path for a Manager1 method.  Returns:
+ *    0  succeeded via D-Bus
+ *    1  D-Bus replied with an error -- callers should error out
+ *   -1  D-Bus not reachable -- callers should fall back to the
+ *       legacy INIT_SOCKET transport
+ *
+ * On D-Bus error replies the function maps the org.* error name to
+ * the same exit code initctl historically printed for that case
+ * (e.g. NoSuchService -> 69 with the legacy message). */
+static int try_dbus_manager(const char *method, const char *arg_sig,
+			    const char *arg)
+{
+	link_client_t *c;
+	const char    *err;
+	int            rc;
+
+	c = link_client_open(FINIT_BUS_SOCKET);
+	if (!c)
+		return -1;
+
+	/* "s" methods take the service identity (arg may be NULL ->
+	 * empty string); void methods pass no body. */
+	if (arg_sig && !strcmp(arg_sig, "s"))
+		rc = link_client_call_v(c, "/org/finit/manager",
+					"org.finit.Manager1", method,
+					"s", arg ? arg : "");
+	else if (!arg_sig || !*arg_sig)
+		rc = link_client_call_v(c, "/org/finit/manager",
+					"org.finit.Manager1", method, NULL);
+	else
+		rc = LINK_CALL_FAIL;
+
+	if (rc == LINK_CALL_ERROR) {
+		const link_reply_t *r = link_client_reply(c);
+
+		err = (r && r->error_name) ? r->error_name : "";
+		/* Exact match on the fully-qualified error name; substring
+		 * matching would misfire on a future name that contains
+		 * one of these as a substring. */
+		if (!strcmp(err, "org.finit.Error.NoSuchService")) {
+			link_client_close(c);
+			ERRX(noerr ? 0 : 69, "no such task or service(s): %s",
+			     arg ? arg : "");
+		}
+		if (!strcmp(err, "org.freedesktop.DBus.Error.AccessDenied")) {
+			link_client_close(c);
+			ERRX(1, "permission denied: %s requires root", method);
+		}
+		link_client_close(c);
+		ERRX(1, "%s: %s", method, *err ? err : "D-Bus error");
+	}
+	link_client_close(c);
+	if (rc == LINK_CALL_OK)
+		return 0;
+	return -1;	/* LINK_CALL_FAIL or anything else: fall back */
+}
+#endif /* HAVE_DBUS */
+
+static int do_start  (char *arg)
+{
+#ifdef HAVE_DBUS
+	int rc = try_dbus_manager("Start", "s", arg);
+	if (rc >= 0) return rc;
+#endif
+	return do_startstop(INIT_CMD_START_SVC, arg);
+}
+
+static int do_stop   (char *arg)
+{
+#ifdef HAVE_DBUS
+	int rc = try_dbus_manager("Stop", "s", arg);
+	if (rc >= 0) return rc;
+#endif
+	return do_startstop(INIT_CMD_STOP_SVC, arg);
+}
 
 static int do_reload (char *arg)
 {
-	if (!arg || !arg[0])
+	if (!arg || !arg[0]) {
+#ifdef HAVE_DBUS
+		int rc = try_dbus_manager("Reload", "", NULL);
+		if (rc >= 0) return rc;
+#endif
 		return do_svc(INIT_CMD_RELOAD, NULL);
+	}
 
 	return do_startstop(INIT_CMD_RELOAD_SVC, arg);
 }
 
 static int do_restart(char *arg)
 {
+#ifdef HAVE_DBUS
+	int rc = try_dbus_manager("Restart", "s", arg);
+	if (rc == 0) return 0;
+	if (rc == 1) ERRX(noerr ? 0 : 7, "failed restarting %s", arg);
+#endif
 	if (do_startstop(INIT_CMD_RESTART_SVC, arg))
 		ERRX(noerr ? 0 : 7, "failed restarting %s", arg);
 
@@ -643,9 +729,47 @@ static int do_cmd(int cmd)
 	return 0;
 }
 
-int do_reboot  (char *arg) { return do_cmd(INIT_CMD_REBOOT);   }
-int do_halt    (char *arg) { return do_cmd(INIT_CMD_HALT);     }
-int do_poweroff(char *arg) { return do_cmd(INIT_CMD_POWEROFF); }
+#ifdef HAVE_DBUS
+static int do_reboot_dbus(const char *method)
+{
+	int rc = try_dbus_manager(method, "", NULL);
+
+	if (rc == 0) {
+		sleep(5);	/* match legacy: wait for finit to shut down */
+		return 0;
+	}
+	return rc;	/* 1 = error, -1 = fall back */
+}
+#endif
+
+int do_reboot  (char *arg)
+{
+#ifdef HAVE_DBUS
+	int rc = do_reboot_dbus("Reboot");
+	if (rc >= 0) return rc;
+#endif
+	return do_cmd(INIT_CMD_REBOOT);
+}
+
+int do_halt    (char *arg)
+{
+#ifdef HAVE_DBUS
+	int rc = do_reboot_dbus("Halt");
+	if (rc >= 0) return rc;
+#endif
+	return do_cmd(INIT_CMD_HALT);
+}
+
+int do_poweroff(char *arg)
+{
+#ifdef HAVE_DBUS
+	int rc = do_reboot_dbus("Poweroff");
+	if (rc >= 0) return rc;
+#endif
+	return do_cmd(INIT_CMD_POWEROFF);
+}
+
+/* Suspend has no Manager1 equivalent yet; uses the legacy IPC. */
 int do_suspend (char *arg) { return do_cmd(INIT_CMD_SUSPEND);  }
 
 /**