core: only refuse Type=dbus service enqueuing if dbus has stop job

Follow-up for systemd#27579

In systemd#27579 we refused all StartUnit requests for Type=dbus units
if dbus is not running, which means if dbus is manually stopped,
user can't use systemctl to start Type=dbus units again, which
is incorrect.

The only culprit that leads to the cancellation of the whole
transaction mentioned in systemd#26799 is job type conflict on dbus.
So let's relax the restriction and only refuse job enqueuing
if dbus has a stop job.

To summarize, the case we want to avoid is:

1. dbus has a stop job installed
2. StartUnit/ActivationRequest is received
3. Type=dbus service gets started, which has Requires=dbus.socket
4. dbus is pulled in again, resulting in job type conflict

What we can support is:

1. dbus is already stopped
2. StartUnit is received (possibly through systemctl, i.e. on private bus)
3. Type=dbus service gets started, which will wait for dbus to start
4. dbus is started again, thus the job for Type=dbus service

Replaces systemd#27590
Fixes systemd#27588

src/core/dbus-unit.c

@@ -1875,13 +1875,30 @@ int bus_unit_queue_job(
             (type == JOB_STOP && u->refuse_manual_stop) ||
             (IN_SET(type, JOB_RESTART, JOB_TRY_RESTART) && (u->refuse_manual_start || u->refuse_manual_stop)) ||
             (type == JOB_RELOAD_OR_START && job_type_collapse(type, u) == JOB_START && u->refuse_manual_start))
-                return sd_bus_error_setf(error, BUS_ERROR_ONLY_BY_DEPENDENCY, "Operation refused, unit %s may be requested by dependency only (it is configured to refuse manual start/stop).", u->id);
-
-        /* dbus-broker issues StartUnit for activation requests, so let's apply the same check
-         * used in signal_activation_request(). */
-        if (type == JOB_START && u->type == UNIT_SERVICE &&
-            SERVICE(u)->type == SERVICE_DBUS && !manager_dbus_is_running(u->manager))
-                return sd_bus_error_set(error, BUS_ERROR_SHUTTING_DOWN, "Refusing activation, D-Bus is not running.");
+                return sd_bus_error_setf(error,
+                                         BUS_ERROR_ONLY_BY_DEPENDENCY,
+                                         "Operation refused, unit %s may be requested by dependency only (it is configured to refuse manual start/stop).",
+                                         u->id);
+
+        /* dbus-broker issues StartUnit for activation requests, and Type=dbus services automatically
+         * gain dependency on dbus.socket. Therefore, if dbus has a pending stop job, the new start
+         * job that pulls in dbus again would cause job type conflict. Let's avoid that by rejecting
+         * job enqueuing early.
+         *
+         * Note that unlike signal_activation_request(), we can't use unit_inactive_or_pending()
+         * here. StartUnit is a more generic interface, and thus users are allowed to use e.g. systemctl
+         * to start Type=dbus services even when dbus is inactive. */
+        if (type == JOB_START && u->type == UNIT_SERVICE && SERVICE(u)->type == SERVICE_DBUS)
+                FOREACH_STRING(dbus_unit, SPECIAL_DBUS_SOCKET, SPECIAL_DBUS_SERVICE) {
+                        Unit *dbus;
+
+                        dbus = manager_get_unit(u->manager, dbus_unit);
+                        if (dbus && unit_stop_pending(dbus))
+                                return sd_bus_error_setf(error,
+                                                         BUS_ERROR_SHUTTING_DOWN,
+                                                         "Operation for unit %s refused, D-Bus is shutting down.",
+                                                         u->id);
+                }
 
         r = sd_bus_message_new_method_return(message, &reply);
         if (r < 0)