update-done: add minimal tool to manage system updates for /etc and /var, if /usr has changed

In order to support offline updates to /usr, we need to be able to run
certain tasks on next boot-up to bring /etc and /var in line with the
updated /usr. Hence, let's devise a mechanism how we can detect whether
/etc or /var are not up-to-date with /usr anymore: we keep "touch
files" in /etc/.updated and /var/.updated that are mtime-compared with
/usr. This means:

Whenever the vendor OS tree in /usr is updated, and any services that
shall be executed at next boot shall be triggered, it is sufficient to
update the mtime of /usr itself. At next boot, if /etc/.updated and/or
/var/.updated is older than than /usr (or missing), we know we have to
run the update tools once. After that is completed we need to update the
mtime of these files to the one of /usr, to keep track that we made the
necessary updates, and won't repeat them on next reboot.

A subsequent commit adds a new ConditionNeedsUpdate= condition that
allows checking on boot whether /etc or /var are outdated and need
updating.

This is an early step to allow booting up with an empty /etc, with
automatic rebuilding of the necessary cache files or user databases
therein, as well as supporting later updates of /usr that then propagate
to /etc and /var again.

src/update-done/Makefile

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

src/update-done/update-done.c

@@ -0,0 +1,104 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 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 "util.h"
+
+static int apply_timestamp(const char *path, struct timespec *ts) {
+        struct timespec twice[2];
+        struct stat st;
+
+        assert(path);
+        assert(ts);
+
+        if (stat(path, &st) >= 0) {
+                /* Is the timestamp file already newer than the OS? If so, there's nothing to do. */
+                if (st.st_mtim.tv_sec > ts->tv_sec ||
+                    (st.st_mtim.tv_sec == ts->tv_sec && st.st_mtim.tv_nsec >= ts->tv_nsec))
+                        return 0;
+
+                /* It is older? Then let's update it */
+                twice[0] = *ts;
+                twice[1] = *ts;
+
+                if (utimensat(AT_FDCWD, path, twice, AT_SYMLINK_NOFOLLOW) < 0) {
+
+                        if (errno == EROFS) {
+                                log_debug("Can't update timestamp file %s, file system is read-only.", path);
+                                return 0;
+                        }
+
+                        log_error("Failed to update timestamp on %s: %m", path);
+                        return -errno;
+                }
+
+        } else if (errno == ENOENT) {
+                _cleanup_close_ int fd = -1;
+
+                /* The timestamp file doesn't exist yet? Then let's create it. */
+
+                fd = open(path, O_CREAT|O_EXCL|O_WRONLY|O_TRUNC|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW, 0644);
+                if (fd < 0) {
+
+                        if (errno == EROFS) {
+                                log_debug("Can't create timestamp file %s, file system is read-only.", path);
+                                return 0;
+                        }
+
+                        log_error("Failed to create timestamp file %s: %m", path);
+                        return -errno;
+                }
+
+                twice[0] = *ts;
+                twice[1] = *ts;
+
+                if (futimens(fd, twice) < 0) {
+                        log_error("Failed to update timestamp on %s: %m", path);
+                        return -errno;
+                }
+        } else {
+                log_error("Failed to stat() timestamp file %s: %m", path);
+                return -errno;
+        }
+
+        return 0;
+}
+
+int main(int argc, char *argv[]) {
+        struct stat st;
+        int r, q;
+
+        log_set_target(LOG_TARGET_AUTO);
+        log_parse_environment();
+        log_open();
+
+        if (stat("/usr", &st) < 0) {
+                log_error("Failed to stat /usr: %m");
+                return EXIT_FAILURE;
+        }
+
+        r = apply_timestamp("/etc/.updated", &st.st_mtim);
+
+        q = apply_timestamp("/var/.updated", &st.st_mtim);
+        if (q < 0 && r == 0)
+                r = q;
+
+        return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
+}