core/path: do not enqueue new job in .trigger_notify callback

Otherwise,
1. X.path triggered X.service, and the service has waiting start job,
2. systemctl stop X.service
3. the waiting start job is cancelled to install new stop job,
4. path_trigger_notify() is called, and may reinstall new start job,
5. the stop job cannot be installed, and triggeres assertion.

So, instead, let's add a defer event source, then enqueue the new start
job after the stop (or any other type) job finished.

Fixes https://github.com/systemd/systemd/issues/24577#issuecomment-1522628906.

src/core/path.c

@@ -10,6 +10,7 @@
 #include "dbus-path.h"
 #include "dbus-unit.h"
 #include "escape.h"
+#include "event-util.h"
 #include "fd-util.h"
 #include "glob-util.h"
 #include "inotify-util.h"
@@ -300,6 +301,7 @@ static void path_done(Unit *u) {
 
         assert(p);
 
+        p->trigger_notify_event_source = sd_event_source_disable_unref(p->trigger_notify_event_source);
         path_free_specs(p);
 }
 
@@ -575,6 +577,9 @@ static void path_enter_waiting(Path *p, bool initial, bool from_trigger_notify)
         Unit *trigger;
         int r;
 
+        if (p->trigger_notify_event_source)
+                (void) event_source_disable(p->trigger_notify_event_source);
+
         /* If the triggered unit is already running, so are we */
         trigger = UNIT_TRIGGER(UNIT(p));
         if (trigger && !UNIT_IS_INACTIVE_OR_FAILED(unit_active_state(trigger))) {
@@ -799,8 +804,28 @@ fail:
         return 0;
 }
 
-static void path_trigger_notify(Unit *u, Unit *other) {
+static void path_trigger_notify_impl(Unit *u, Unit *other, bool on_defer);
+
+static int path_trigger_notify_on_defer(sd_event_source *s, void *userdata) {
+        Path *p = ASSERT_PTR(userdata);
+        Unit *trigger;
+
+        assert(s);
+
+        trigger = UNIT_TRIGGER(UNIT(p));
+        if (!trigger) {
+                log_unit_error(UNIT(p), "Unit to trigger vanished.");
+                path_enter_dead(p, PATH_FAILURE_RESOURCES);
+                return 0;
+        }
+
+        path_trigger_notify_impl(UNIT(p), trigger, /* on_defer = */ true);
+        return 0;
+}
+
+static void path_trigger_notify_impl(Unit *u, Unit *other, bool on_defer) {
         Path *p = PATH(u);
+        int r;
 
         assert(u);
         assert(other);
@@ -826,13 +851,46 @@ static void path_trigger_notify(Unit *u, Unit *other) {
 
         if (p->state == PATH_RUNNING &&
             UNIT_IS_INACTIVE_OR_FAILED(unit_active_state(other))) {
-                log_unit_debug(UNIT(p), "Got notified about unit deactivation.");
-                path_enter_waiting(p, false, true);
+                if (!on_defer)
+                        log_unit_debug(u, "Got notified about unit deactivation.");
         } else if (p->state == PATH_WAITING &&
                    !UNIT_IS_INACTIVE_OR_FAILED(unit_active_state(other))) {
-                log_unit_debug(UNIT(p), "Got notified about unit activation.");
-                path_enter_waiting(p, false, true);
+                if (!on_defer)
+                        log_unit_debug(u, "Got notified about unit activation.");
+        } else
+                return;
+
+        if (on_defer) {
+                path_enter_waiting(p, /* initial = */ false, /* from_trigger_notify = */ true);
+                return;
         }
+
+        /* Do not call path_enter_waiting() directly from path_trigger_notify(), as this may be called by
+         * job_install() -> job_finish_and_invalidate() -> unit_trigger_notify(), and path_enter_waiting()
+         * may install another job and will trigger assertion in job_install().
+         * https://github.com/systemd/systemd/issues/24577#issuecomment-1522628906
+         * Hence, first setup defer event source here, and call path_enter_waiting() slightly later. */
+        if (p->trigger_notify_event_source) {
+                r = sd_event_source_set_enabled(p->trigger_notify_event_source, SD_EVENT_ONESHOT);
+                if (r < 0) {
+                        log_unit_warning_errno(u, r, "Failed to enable event source for triggering notify: %m");
+                        path_enter_dead(p, PATH_FAILURE_RESOURCES);
+                        return;
+                }
+        } else {
+                r = sd_event_add_defer(u->manager->event, &p->trigger_notify_event_source, path_trigger_notify_on_defer, p);
+                if (r < 0) {
+                        log_unit_warning_errno(u, r, "Failed to allocate event source for triggering notify: %m");
+                        path_enter_dead(p, PATH_FAILURE_RESOURCES);
+                        return;
+                }
+
+                (void) sd_event_source_set_description(p->trigger_notify_event_source, "path-trigger-notify");
+        }
+}
+
+static void path_trigger_notify(Unit *u, Unit *other) {
+        path_trigger_notify_impl(u, other, /* on_defer = */ false);
 }
 
 static void path_reset_failed(Unit *u) {

src/core/path.h

@@ -65,6 +65,8 @@ struct Path {
         PathResult result;
 
         RateLimit trigger_limit;
+
+        sd_event_source *trigger_notify_event_source;
 };
 
 struct ActivationDetailsPath {