nbd not cleaning up socket on exit

[ari-s]

when configuring to serve on a unix socket, the socket is not cleaned up upon exit. This prevents restarting.

MWE

/etc/nbd-server/config:

[generic]
 	unixsock = /run/nbd-test.sock

[test]
	exportname = /tmp/nbd-test
	filesize = 10000000

# systemctl start nbd-server
# systemctl stop nbd-server
# [ -e /run/nbd-test.sock ] && echo "socket lingering"
socket lingering

I'm using nbd-server 3.21-1 on debian bullseye. Oddly enough, nbd-server itself reports:
This is nbd-server version 3.18