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
(cherry picked from commit 4b20ae9a0e914e61d6bac095e5fc9664510ac03e)

diff --git a/src/core/main.c b/src/core/main.c
index 5b11c79e7911bc38abcf0d3af664baf3684d521d..6b9ca742f058e802aae63d2348ec45cac8ab2c9a 100644 (file)
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -1658,6 +1658,11 @@ 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);