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"
@@ -47,11 +48,27 @@ 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);
+
+        /* 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.
+         */
+
+        if (streq(key, "systemd.unit") && !proc_cmdline_value_missing(key, value))
+                log_warning("Offline system update overriden by kernel command line systemd.unit= setting");
+        else if (!value && runlevel_to_target(key))
+                log_warning("Offline system update overriden by runlevel \"%s\" on the kernel command line", key);
+
         return 0;
 }
 
 int main(int argc, char *argv[]) {
-        int r;
+        int r, k;
 
         if (argc > 1 && argc != 4) {
                 log_error("This program takes three or no arguments.");
@@ -69,5 +86,11 @@ int main(int argc, char *argv[]) {
 
         r = generate_symlink();
 
+        if (r > 0) {
+                k = proc_cmdline_parse(parse_proc_cmdline_item, NULL, 0);
+                if (k < 0)
+                        log_warning_errno(k, "Failed to parse kernel command line, ignoring: %m");
+        }
+
         return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
 }