core: add Manager::honor_device_enumeration flag

When system manager is started first time or after switching root,
then the udev's device tag data do not exist yet.
So, let's not honor the enumeration results.

Fixes #11997.

src/core/device.c

@@ -666,9 +666,13 @@ static void device_found_changed(Device *d, DeviceFound previous, DeviceFound no
 }
 
 static void device_update_found_one(Device *d, DeviceFound found, DeviceFound mask) {
+        Manager *m;
+
         assert(d);
 
-        if (MANAGER_IS_RUNNING(UNIT(d)->manager)) {
+        m = UNIT(d)->manager;
+
+        if (MANAGER_IS_RUNNING(m) && (m->honor_device_enumeration || MANAGER_IS_USER(m))) {
                 DeviceFound n, previous;
 
                 /* When we are already running, then apply the new mask right-away, and trigger state changes

src/core/manager.c

@@ -1619,6 +1619,8 @@ static void manager_ready(Manager *m) {
 
         /* Let's finally catch up with any changes that took place while we were reloading/reexecing */
         manager_catchup(m);
+
+        m->honor_device_enumeration = true;
 }
 
 static Manager* manager_reloading_start(Manager *m) {
@@ -3131,6 +3133,9 @@ int manager_serialize(
         (void) serialize_bool(f, "taint-logged", m->taint_logged);
         (void) serialize_bool(f, "service-watchdogs", m->service_watchdogs);
 
+        /* After switching root, udevd has not been started yet. So, enumeration results should not be emitted. */
+        (void) serialize_bool(f, "honor-device-enumeration", !switching_root);
+
         t = show_status_to_string(m->show_status);
         if (t)
                 (void) serialize_item(f, "show-status", t);
@@ -3359,6 +3364,15 @@ int manager_deserialize(Manager *m, FILE *f, FDSet *fds) {
                         else
                                 m->service_watchdogs = b;
 
+                } else if ((val = startswith(l, "honor-device-enumeration="))) {
+                        int b;
+
+                        b = parse_boolean(val);
+                        if (b < 0)
+                                log_notice("Failed to parse honor-device-enumeration flag '%s', ignoring.", val);
+                        else
+                                m->honor_device_enumeration = b;
+
                 } else if ((val = startswith(l, "show-status="))) {
                         ShowStatus s;
 
@@ -3549,6 +3563,11 @@ int manager_reload(Manager *m) {
         assert(m->n_reloading > 0);
         m->n_reloading--;
 
+        /* On manager reloading, device tag data should exists, thus, we should honor the results of device
+         * enumeration. The flag should be always set correctly by the serialized data, but it may fail. So,
+         * let's always set the flag here for safety. */
+        m->honor_device_enumeration = true;
+
         manager_ready(m);
 
         m->send_reloading_done = true;

src/core/manager.h

@@ -395,6 +395,8 @@ struct Manager {
          * multiple times on the same unit. */
         unsigned sigchldgen;
         unsigned notifygen;
+
+        bool honor_device_enumeration;
 };
 
 #define MANAGER_IS_SYSTEM(m) ((m)->unit_file_scope == UNIT_FILE_SYSTEM)