Fix group node scan stall and UAF on server link teardown

- node_conn_direct_close: immediate conn cleanup when links are down
  (graceful fin_wait on a down conn left a lingering entry, so re-open
  in Phase3 stuck in 'pending' with no timer and scan never continued)
- tcp_link_close_cb: skip etcp_link_close if conn was already closed
  synchronously during etcp_on_link_down cascade (fixes use-after-free)

src/transport_layer/etcp_connections.c

@@ -1119,6 +1119,7 @@ static void tcp_link_close_cb(struct stcp_link *sl, int err, void *arg) {
     if (!link || !link->etcp) return;
     if (link->etcp->state == 2) return;
     int old_state = link->link_state;
+    struct ETCP_CONN *etcp = link->etcp;   /* сохранить до каскада: etcp_on_link_down может закрыть conn и освободить link */
     stcp_link_close(sl);
     link->tcp_link = NULL;
 
@@ -1126,8 +1127,13 @@ static void tcp_link_close_cb(struct stcp_link *sl, int err, void *arg) {
 
     if (link->is_server) {
         DEBUG_INFO(DEBUG_CATEGORY_ETCP, "[%s] TCP server link %d down err=%d, closing", link->etcp->log_name, link->local_link_id, err);
-        etcp_on_link_down(link->etcp, link);
+        etcp_on_link_down(etcp, link);
-        etcp_link_close(link);
+        if (etcp->state != 2) {
+            etcp_link_close(link);
+        } else {
+            DEBUG_INFO(DEBUG_CATEGORY_ETCP, "[%s] TCP server link %d already closed by conn teardown, skip",
+                       etcp->log_name, link->local_link_id);
+        }
     } else {
         DEBUG_INFO(DEBUG_CATEGORY_ETCP, "[%s] TCP link %d down err=%d, scheduling reconnect", link->etcp->log_name, link->local_link_id, err);
         etcp_on_link_down(link->etcp, link);

src/transport_layer/node_conn_direct.c

@@ -874,7 +874,7 @@ void node_conn_direct_close(struct NODE_CONN_DIRECT* h) {
         }
         h->cb = NULL; h->cb_arg = NULL;
         if (entry->connect_timer) { uasync_cancel_timeout(entry->ua, entry->connect_timer); entry->connect_timer = NULL; }
-        if (conn && conn->state != 2 && !entry->timed_out) {
+        if (conn && conn->state != 2 && !entry->timed_out && conn->links_up > 0) {
             /* устанавливаем fin_wait, отправляем CLOSE, ставим таймер */
             conn->fin_wait = 1;
             conn->fin_wait_clear_cb = ncd_fin_wait_cancelled;
@@ -903,8 +903,9 @@ void node_conn_direct_close(struct NODE_CONN_DIRECT* h) {
             /* закрываем conn синхронно чтобы линки сразу ушли из socket->links_queue:
              * иначе в том же тике новый open того же узла создаст дубль-conn и коллизию адресов.
              * u_free(conn) всё равно отложен (phase 2 в etcp_connection_close), UAF нет. */
-            DEBUG_INFO(DEBUG_CATEGORY_NCD, "[ncd] close: node=0x%016llx state=%d timed_out=%d cb_running=%d -> %s",
+            DEBUG_INFO(DEBUG_CATEGORY_NCD, "[ncd] close: node=0x%016llx state=%d timed_out=%d cb_running=%d links_up=%d -> %s",
                        (unsigned long long)node_id, conn->state, entry->timed_out, conn->callbacks_running,
+                       conn->links_up,
                        conn->callbacks_running ? "deferred" : "sync");
             if (conn->callbacks_running) uasync_call_soon(entry->ua, conn, ncd_deferred_close_conn);
             else etcp_connection_close(conn);