Mongos hangs without any log messages

Symptoms: mongos listen port, but don't serve client requests for few hours (until restart). Nothing suspicious in the logs found.

I was make core dump and backtrace for all threads (backtrace file is attached to this issue).

How often: around twice a month per production environment.

Version:

mongos version v4.0.4
{{ git version: f288a3bdf201007f3693c58e140056adf8b04839}}
{{ OpenSSL version: OpenSSL 1.0.2g 1 Mar 2016}}
{{ allocator: tcmalloc}}
{{ modules: none}}
{{ build environment:}}
{{ distmod: ubuntu1604}}
{{ distarch: x86_64}}
{{ target_arch: x86_64}}

Topology:

• single shard cluster
• 3-nodes configuration replicaset;
• 4-nodes shard replicaset;
• mongos instance on each client host.