system-update: add system update generator

http://freedesktop.org/wiki/Software/systemd/SystemUpdates

src/system-update-generator/Makefile

@@ -0,0 +1 @@
+../Makefile

src/system-update-generator/system-update-generator.c

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+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2012 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <errno.h>
+#include <unistd.h>
+
+#include "log.h"
+#include "util.h"
+#include "unit-name.h"
+#include "path-util.h"
+
+static const char *arg_dest = "/tmp";
+
+static int generate_symlink(void) {
+        struct stat st;
+        char *p;
+
+        if (lstat("/system-update", &st) < 0) {
+                if (errno == ENOENT)
+                        return 0;
+
+                log_error("Failed to check for system update: %m");
+                return -EINVAL;
+        }
+
+        p = strappend(arg_dest, "/default.target");
+        if (!p) {
+                log_error("Out of memory.");
+                return -ENOMEM;
+        }
+
+        if (symlink(SYSTEM_DATA_UNIT_PATH "/system-update.target", p) < 0) {
+                free(p);
+                log_error("Failed to create symlink: %m");
+                return -errno;
+        }
+
+        free(p);
+
+        return 0;
+}
+
+int main(int argc, char *argv[]) {
+        int r;
+
+        if (argc > 2) {
+                log_error("This program takes one or no arguments.");
+                return EXIT_FAILURE;
+        }
+
+        if (argc > 1)
+                arg_dest = argv[1];
+
+        log_set_target(LOG_TARGET_AUTO);
+        log_parse_environment();
+        log_open();
+
+        umask(0022);
+
+        r = generate_symlink();
+
+        return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
+}