machined: watch leader PID's lifetime via pidfd

If we have a pidfd, we might as well track the machine's leader PID's
lifetime, and enqueue the machine for a GC run.

(This is similar to what we are already doing for logind's session
leaders)

src/machine/machine.c

@@ -118,6 +118,7 @@ Machine* machine_free(Machine *m) {
                         m->manager->host_machine = NULL;
         }
 
+        m->leader_pidfd_event_source = sd_event_source_disable_unref(m->leader_pidfd_event_source);
         if (pidref_is_set(&m->leader)) {
                 if (m->manager)
                         (void) hashmap_remove_value(m->manager->machine_leaders, PID_TO_PTR(m->leader.pid), m);
@@ -475,6 +476,38 @@ static int machine_ensure_scope(Machine *m, sd_bus_message *properties, sd_bus_e
         return 0;
 }
 
+static int machine_dispatch_leader_pidfd(sd_event_source *s, int fd, unsigned revents, void *userdata) {
+        Machine *m = ASSERT_PTR(userdata);
+
+        m->leader_pidfd_event_source = sd_event_source_disable_unref(m->leader_pidfd_event_source);
+        machine_add_to_gc_queue(m);
+
+        return 0;
+}
+
+static int machine_watch_pidfd(Machine *m) {
+        int r;
+
+        assert(m);
+        assert(m->manager);
+        assert(pidref_is_set(&m->leader));
+        assert(!m->leader_pidfd_event_source);
+
+        if (m->leader.fd < 0)
+                return 0;
+
+        /* If we have a pidfd for the leader, let's also track it for POLLIN, and GC the machine
+         * automatically if it dies */
+
+        r = sd_event_add_io(m->manager->event, &m->leader_pidfd_event_source, m->leader.fd, EPOLLIN, machine_dispatch_leader_pidfd, m);
+        if (r < 0)
+                return r;
+
+        (void) sd_event_source_set_description(m->leader_pidfd_event_source, "machine-pidfd");
+
+        return 0;
+}
+
 int machine_start(Machine *m, sd_bus_message *properties, sd_bus_error *error) {
         int r;
 
@@ -490,6 +523,10 @@ int machine_start(Machine *m, sd_bus_message *properties, sd_bus_error *error) {
         if (r < 0)
                 return r;
 
+        r = machine_watch_pidfd(m);
+        if (r < 0)
+                return r;
+
         /* Create cgroup */
         r = machine_ensure_scope(m, properties, error);
         if (r < 0)

src/machine/machine.h

@@ -49,6 +49,7 @@ struct Machine {
         char *scope_job;
 
         PidRef leader;
+        sd_event_source *leader_pidfd_event_source;
 
         dual_timestamp timestamp;