core: Add trigger limit for path units

When conditions fail on a service unit, a path unit can cause
PID 1 to busy loop as it keeps trying to activate the service unit.
To avoid this from happening, add a trigger limit to the path unit,
identical to the trigger limit we have for socket units.

Initially, let's start with a high limit and not make it configurable.
If needed, we can add properties to configure the rate limit similar
to the ones we have for socket units.

src/core/path.c

@@ -265,6 +265,9 @@ static void path_init(Unit *u) {
         assert(u->load_state == UNIT_STUB);
 
         p->directory_mode = 0755;
+
+        p->trigger_limit.interval = 2 * USEC_PER_SEC;
+        p->trigger_limit.burst = 200;
 }
 
 void path_free_specs(Path *p) {
@@ -494,6 +497,12 @@ static void path_enter_running(Path *p) {
         if (unit_stop_pending(UNIT(p)))
                 return;
 
+        if (!ratelimit_below(&p->trigger_limit)) {
+                log_unit_warning(UNIT(p), "Trigger limit hit, refusing further activation.");
+                path_enter_dead(p, PATH_FAILURE_TRIGGER_LIMIT_HIT);
+                return;
+        }
+
         trigger = UNIT_TRIGGER(UNIT(p));
         if (!trigger) {
                 log_unit_error(UNIT(p), "Unit to trigger vanished.");
@@ -836,6 +845,7 @@ static const char* const path_result_table[_PATH_RESULT_MAX] = {
         [PATH_FAILURE_RESOURCES]            = "resources",
         [PATH_FAILURE_START_LIMIT_HIT]      = "start-limit-hit",
         [PATH_FAILURE_UNIT_START_LIMIT_HIT] = "unit-start-limit-hit",
+        [PATH_FAILURE_TRIGGER_LIMIT_HIT]    = "trigger-limit-hit",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(path_result, PathResult);