3.17.0: Assertion `task && "When an ares socket is closed we should have a handle for it"' failed.

Description:

Our staging server (13 users, ~2 active) crashed for no apparent reason shortly after a user sync script ran (the script runs every 5 minutes, ran fine before and after manual restart of RC, and changed/POSTed nothing in this run.) After manual restart it works fine.

(Crash behaviour is similar to #22062, also reported by myself, but the error message is different.)

Steps to reproduce:

1. (Unclear)
2. Server dies with the log entries posted below
3. We had to manually restart RC.

Expected behavior:

Run, RC, keep on running! 😇 (Or at least attempt to restart, which could be achieved by systemd's Restart option. Since the systemd unit is created by rocketchatctl, I guess I'll file an issue there, too.)

Actual behavior:

systemd service exited, see logs below.

Server Setup Information:

• Version of Rocket.Chat Server: 3.17.0 (Commit Hash: d4ce52e098, Commit Branch: HEAD)
• Operating System: Debian GNU/Linux 9.13 (stretch)
• Deployment Method: rocketchatctl
• Number of Running Instances: 2 (one on this staging VM, one on a separate production VM)
• DB Replicaset Oplog: the startup log says "Enabled"
• NodeJS Version: v12.18.4 - x64
• MongoDB Version: 4.0.19 (mmapv1)

Client Setup Information

• Desktop App or Browser Version: n/a (no GUI client involved)
• Operating System: n/a

Additional context

What's (maybe) special about our setup is that:

• it's behind a corporate proxy (or more precisely, two different proxies, for HTTP and HTTPS) and not reachable from the outside world, which caused and causes a couple of problems (f.e. traefik can't request a LetsEncrypt certificate and creates an invalid "[0-9a-f.].traefik.default" cert)
• the machine has both IPv6 and IPv4 addresses, not all of them connected to the Internet, which caused (and might cause) a couple of problems (see f.e. https://github.com/RocketChat/install.sh/issues/41)

Relevant logs:

(last log entry, 8 seconds before the error:)
rocketchat[1678]: API ➔ debug POST: /api/v1/logout

rocketchat[1678]: /usr/local/bin/node[1678]: ../src/cares_wrap.cc:361:void node::cares_wrap::{anonymous}::ares_sockstate_cb(void*, ares_socket_t, int, int): Assertion `task && "When an ares socket is closed we should have a handle for it"' failed.
rocketchat[1678]:  1: 0xa093f0 node::Abort() [/usr/local/bin/node]
rocketchat[1678]:  2: 0xa0946e  [/usr/local/bin/node]
rocketchat[1678]:  3: 0x98e81a  [/usr/local/bin/node]
rocketchat[1678]:  4: 0x185d901 ares__close_sockets [/usr/local/bin/node]
rocketchat[1678]:  5: 0x186649e  [/usr/local/bin/node]
rocketchat[1678]:  6: 0x18668df  [/usr/local/bin/node]
rocketchat[1678]:  7: 0x1866ad5  [/usr/local/bin/node]
rocketchat[1678]:  8: 0x1348228  [/usr/local/bin/node]
rocketchat[1678]:  9: 0x1335e4f uv_run [/usr/local/bin/node]
rocketchat[1678]: 10: 0xa4b665 node::NodeMainInstance::Run() [/usr/local/bin/node]
rocketchat[1678]: 11: 0x9da5a8 node::Start(int, char**) [/usr/local/bin/node]
rocketchat[1678]: 12: 0x7f6b0d8d92e1 __libc_start_main [/lib/x86_64-linux-gnu/libc.so.6]
rocketchat[1678]: 13: 0x979215  [/usr/local/bin/node]
traefik[869]: time="2021-08-03T09:18:11Z" level=error msg="vulcand/oxy/forward/websocket: Error when copying from backend to client: websocket: close 1006 (abnormal closure): unexpected EOF"
traefik[869]: time="2021-08-03T09:18:11Z" level=error msg="vulcand/oxy/forward/websocket: Error when copying from backend to client: websocket: close 1006 (abnormal closure): unexpected EOF"
systemd[1]: rocketchat.service: Main process exited, code=killed, status=6/ABRT
systemd[1]: rocketchat.service: Unit entered failed state.
systemd[1]: rocketchat.service: Failed with result 'signal'.

Start with the reported Node.js v12.18.4 assertion in ../src/cares_wrap.cc and the accompanying systemd, Traefik, proxy, and IPv4/IPv6 setup details. Investigate whether the logout or user-sync activity can reproduce the ares socket failure under this deployment. Done means identifying a reproducible cause and confirming that the Rocket.Chat service no longer aborts unexpectedly.