pid1: make clear that $WATCHDOG_USEC is set for the shutdown binary, noone else

We use the $WATCHDOG_USEC variable for two very closely uses: as part of
the sd_watchdog_enabled() protocol for implementing service watchdogs.
And as part of the protocol between the service manager and
systemd-shutdown across the PID 1 execve() transition during shutdown.

Apparently some exitrds tools got confused by the latter use. Let's
address that by setting $WATCHDOG_PID to 1, in accordance to the
sd_watchdog_enabled() protocol to make clear this is only intended for
PID 1 and nothing else.

Replaces: #35135

1 files changed, 5 insertions, 0 deletions

diff --git a/src/core/main.c b/src/core/main.c
index 38216fa7a6..172742c769 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -1689,6 +1689,11 @@ static int become_shutdown(int objective, int retval) {
         /* Tell the binary how often to ping, ignore failure */
         (void) strv_extendf(&env_block, "WATCHDOG_USEC="USEC_FMT, watchdog_timer);
 
+        /* Make sure that tools that look for $WATCHDOG_USEC (and might get started by the exitrd) don't get
+         * confused by the variable, because the sd_watchdog_enabled() protocol uses the same variable for
+         * the same purposes. */
+        (void) strv_extendf(&env_block, "WATCHDOG_PID=" PID_FMT, getpid_cached());
+
         if (arg_watchdog_device)
                 (void) strv_extendf(&env_block, "WATCHDOG_DEVICE=%s", arg_watchdog_device);