system-update-generator: warn if the command line blocks updates

If "3", "5", "systemd.unit=", or similar are present on the kernel command line,
the system will not enter into offline update. This behaviour is in line with the
general logic that configuration on the kernel command line has higher priority
than the configuration on disk, but is rather surprising. Emit a warning to help
users diagnose the situation.

https://bugzilla.redhat.com/show_bug.cgi?id=1405439#c4

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

@@ -22,6 +22,7 @@
 
 #include "fs-util.h"
 #include "log.h"
+#include "proc-cmdline.h"
 #include "special.h"
 #include "string-util.h"
 #include "util.h"
@@ -47,9 +48,41 @@ static int generate_symlink(void) {
         if (symlink(SYSTEM_DATA_UNIT_PATH "/system-update.target", p) < 0)
                 return log_error_errno(errno, "Failed to create symlink %s: %m", p);
 
+        return 1;
+}
+
+static int parse_proc_cmdline_item(const char *key, const char *value, void *data) {
+        assert(key);
+
+        if (streq(key, "systemd.unit")) {
+                if (proc_cmdline_value_missing(key, value))
+                        return 0;
+
+                log_warning("Offline system update overriden by kernel command line systemd.unit= setting");
+
+        } else if (!value) {
+                const char *target;
+
+                target = runlevel_to_target(key);
+                if (target)
+                        log_warning("Offline system update overriden by runlevel \"%s\" on the kernel command line", key);
+        }
+
         return 0;
 }
 
+static void maybe_warn(void) {
+        /* Check if a run level is specified on the kernel command line. The
+         * command line has higher priority than any on-disk configuration, so
+         * it'll make any symlink we create moot.
+         */
+        int r;
+
+        r = proc_cmdline_parse(parse_proc_cmdline_item, NULL, 0);
+        if (r < 0)
+                log_warning_errno(r, "Failed to parse kernel command line, ignoring: %m");
+}
+
 int main(int argc, char *argv[]) {
         int r;
 
@@ -69,5 +102,8 @@ int main(int argc, char *argv[]) {
 
         r = generate_symlink();
 
+        if (r > 0)
+                maybe_warn();
+
         return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
 }