Deadlock distnifs

https://ampcode.com/threads/T-019beca5-7177-747e-9114-1022dc7a930a

Signed-off-by: Peter M <petermm@gmail.com>

Author: petermm

src/libAtomVM/dist_nifs.c

@@ -111,28 +111,58 @@ struct DistConnection
 static void dist_connection_dtor(ErlNifEnv *caller_env, void *obj)
 {
     struct DistConnection *conn_obj = (struct DistConnection *) obj;
+
+    // IMPORTANT: Remove from global list FIRST to maintain lock ordering.
+    // Lock order must always be: dist_connections -> remote_monitors -> pending_packets
+    // This prevents ABBA deadlock with code paths that hold dist_connections
+    // while acquiring pending_packets (e.g., dist_send_message, dist_send_link).
+    synclist_remove(&caller_env->global->dist_connections, &conn_obj->head);
+
     if (conn_obj->connection_process_id != INVALID_PROCESS_ID) {
         enif_demonitor_process(caller_env, conn_obj, &conn_obj->connection_process_monitor);
     }
+
+    // Collect monitors to demonitor outside the lock to avoid holding lock
+    // across potentially blocking enif_demonitor_process calls
+    struct ListHead monitors_to_free;
+    list_init(&monitors_to_free);
+
     struct ListHead *remote_monitors = synclist_wrlock(&conn_obj->remote_monitors);
     struct ListHead *item;
     struct ListHead *tmp;
     MUTABLE_LIST_FOR_EACH (item, tmp, remote_monitors) {
-        struct RemoteMonitor *remote_monitor = GET_LIST_ENTRY(item, struct RemoteMonitor, head);
-        enif_demonitor_process(caller_env, conn_obj, &remote_monitor->process_monitor);
         list_remove(item);
-        free(item);
+        list_append(&monitors_to_free, item);
     }
     synclist_unlock(&conn_obj->remote_monitors);
     synclist_destroy(&conn_obj->remote_monitors);
+
+    // Now demonitor and free outside the lock
+    MUTABLE_LIST_FOR_EACH (item, tmp, &monitors_to_free) {
+        struct RemoteMonitor *remote_monitor = GET_LIST_ENTRY(item, struct RemoteMonitor, head);
+        enif_demonitor_process(caller_env, conn_obj, &remote_monitor->process_monitor);
+        list_remove(item);
+        free(remote_monitor);
+    }
+
+    // Collect packets to free outside the lock
+    struct ListHead packets_to_free;
+    list_init(&packets_to_free);
+
     struct ListHead *pending_packets = synclist_wrlock(&conn_obj->pending_packets);
     MUTABLE_LIST_FOR_EACH (item, tmp, pending_packets) {
         list_remove(item);
-        free(item);
+        list_append(&packets_to_free, item);
     }
     synclist_unlock(&conn_obj->pending_packets);
     synclist_destroy(&conn_obj->pending_packets);
-    synclist_remove(&caller_env->global->dist_connections, &conn_obj->head);
+
+    // Free packets outside the lock
+    MUTABLE_LIST_FOR_EACH (item, tmp, &packets_to_free) {
+        struct DistributionPacket *packet = GET_LIST_ENTRY(item, struct DistributionPacket, head);
+        list_remove(item);
+        free(packet);
+    }
 }
 
 static void dist_enqueue_message(term control_message, term payload, struct DistConnection *connection, GlobalContext *global)