core/dbus: do not flush a user manager's bus that is not RUNNING

destroy_bus() flushes unwritten data for unprivileged managers so that
queued messages are not lost when a connection is torn down. However,
sd_bus_flush() first drives the connection to completion via
bus_ensure_running(): for a connection still in OPENING or
AUTHENTICATING this blocks the manager synchronously - a single-fd
ppoll, the event loop is not running - until the peer answers or
BUS_AUTH_TIMEOUT (= DEFAULT_TIMEOUT_USEC, 90 s by default) expires; a
connection in HELLO blocks the same way on the Hello call's own
method timeout. destroy_bus() is reached from four places: the
disconnect handler, manager_recheck_dbus(), the failed-setup path in
api_bus_instance_id_reply(), and bus_done() during normal shutdown or
reexec (via manager_free()).

Such a peer legitimately never answers: during session teardown,
dbus.socket can be (or re-enter) listening while the D-Bus service
behind it is hung or already gone, so connect() succeeds against the
socket backlog and the authentication request is never read. The user
manager then freezes mid-shutdown - or mid-reexec - for 90 s, with
every remaining unit stop (or the reexec itself) gated behind it.

This is the block traced in #16471 (2020, v245): the reporter's
strace shows the manager hanging in a single-fd ppoll with an ~89 s
timeout right after SIGTERM - bus_ensure_running() driving an
AUTHENTICATING reconnection - and their summary attributes it to the
flush. Their tested sd-bus-level patch (breaking the wait via a
SIGTERM-set flag) was met with "a work-around once things are already
bad, but we shouldn't even get in that state"; the same reply stated
the expectation this commit implements - the manager "should normally
protect itself ... and not issue dbus messages when the dbus service
isn't fully up". bus_foreach_bus() already applies that principle in
the other direction, skipping enqueue for connections that "haven't
started yet" via the same sd_bus_is_ready() check. 1166f4472d
("core/dbus: do not block the manager on GetId during bus
(re-)connection") removed the connect-time instance of the same class
of block, in code added in 2025. The flush here is twelve years
older: it dates back to the libsystemd-bus conversion (718db96199,
2013).

Only flush when the connection is currently RUNNING. This does not
change what gets delivered in the failure case this fixes: whatever a
non-RUNNING connection's write queue holds (auth/Hello traffic, and
any subscriber signal a per-unit or per-job bus_track attached without
a readiness check) was never actually sent by the old code either -
sd_bus_flush() calls bus_ensure_running() before it ever looks at the
write queue, so a connection that times out without reaching RUNNING
had its queued data silently discarded on close exactly as before,
just after blocking for up to 90 s first rather than immediately. The
one narrowing is a connection that would have completed authentication
within the timeout window: previously such traffic could still reach
the peer after the block; now it will not. Every destroy_bus() call
site is reached only once the manager has already decided the
connection is being torn down (disconnected, recognized as down, or
the process itself exiting/reexecuting), so this narrowing does not
trade a working delivery for a broken one.

Fixes #16471

src/core/dbus.c

@@ -1095,9 +1095,14 @@ static void destroy_bus(Manager *m, sd_bus **bus) {
         if (m->pending_reload_message_dbus && sd_bus_message_get_bus(m->pending_reload_message_dbus) == *bus)
                 m->pending_reload_message_dbus = sd_bus_message_unref(m->pending_reload_message_dbus);
 
-        /* Possibly flush unwritten data, but only if we are
-         * unprivileged, since we don't want to sync here */
-        if (!MANAGER_IS_SYSTEM(m))
+        /* Possibly flush unwritten data, but only if we are unprivileged, since we don't want to sync
+         * here, and only if the connection is currently RUNNING: sd_bus_flush() first drives the
+         * connection to completion via bus_ensure_running(), which blocks us synchronously for up to
+         * BUS_AUTH_TIMEOUT if the peer accepted the connection but never answers authentication (e.g. a
+         * socket-activated D-Bus service that is hung or already gone during session teardown).
+         * sd_bus_flush() only touches the write queue once that step succeeds, so anything queued on a
+         * connection that does not reach RUNNING is discarded either way - not worth blocking for. */
+        if (!MANAGER_IS_SYSTEM(m) && sd_bus_is_ready(*bus) > 0)
                 sd_bus_flush(*bus);
 
         /* And destroy the object */