Background & Motivation

This assertion is hit in mongoc_client_pool_destroy under these conditions:

• The topology type is LoadBalanced
• The topology description fails to add the LoadBalancer server.
• A TopologyClosed callback is set.

CDRIVER-4145 fixed a similar assertion on the TopologyOpening event. The solution was to not emit a TopologyOpening event if the topology is marked invalid (i.e. a failure to resolve any hosts, or invalid options applied through TXT lookup).

Reproducing

This can be reproduced by modifying the _test_dns_maybe_pooled function to set a topology closed callback with mongoc_apm_set_topology_closed_cb and running the test loadBalanced-replicaSet-errors.

Scope
Do not emit a TopologyClosed event if the topology is marked invalid. A TopologyOpening event is already not emitted in this case.