core: make sure initrd-switch-root command survives PID1's killing spree (#4730)

This is a different way to implement the fix proposed by commit
a4021390fef27f4136497328f suggested by Lennart Poettering.

In this patch we instruct PID1 to not kill "systemctl switch-root" command
started by initrd-switch-root service using the "argv[0][0]='@'" trick.

See: https://www.freedesktop.org/wiki/Software/systemd/RootStorageDaemons/ for
more details.

We had to backup argv[0] because argv is modified by dispatch_verb().

src/systemctl/systemctl.c

@@ -142,6 +142,7 @@ static const char *arg_kill_who = NULL;
 static int arg_signal = SIGTERM;
 static char *arg_root = NULL;
 static usec_t arg_when = 0;
+static char *argv_cmdline = NULL;
 static enum action {
         _ACTION_INVALID,
         ACTION_SYSTEMCTL,
@@ -5638,6 +5639,13 @@ static int switch_root(int argc, char *argv[], void *userdata) {
                         init = NULL;
         }
 
+        /* Instruct PID1 to exclude us from its killing spree applied during
+         * the transition from the initrd to the main system otherwise we would
+         * exit with a failure status even though the switch to the new root
+         * has succeed. */
+        if (in_initrd())
+                argv_cmdline[0] = '@';
+
         r = acquire_bus(BUS_MANAGER, &bus);
         if (r < 0)
                 return r;
@@ -8373,6 +8381,8 @@ static int logind_cancel_shutdown(void) {
 int main(int argc, char*argv[]) {
         int r;
 
+        argv_cmdline = argv[0];
+
         setlocale(LC_ALL, "");
         log_parse_environment();
         log_open();

units/initrd-switch-root.service.in

@@ -17,10 +17,4 @@ AllowIsolate=yes
 Type=oneshot
 # we have to use "--force" here, otherwise systemd would umount /run
 ExecStart=@rootbindir@/systemctl --no-block --force switch-root /sysroot
-
-# Just before switching to the new rootfs, systemd might send us a TERM signal
-# depending on how fast we are to execute the main command and exit. If we get
-# the SIGTERM signal that simply means that we succeed but haven't got enough
-# time to exit properly. Since systemd considers SIGTERM as a failure for
-# short-running process (aka Type=oneshot), instruct it to ignore this case.
-SuccessExitStatus=SIGTERM
+KillMode=none