2.0.14 won't start with just a WebSocket listener enabled

[hardillb]

Built 2.0.14 from source, enabled WebSocket support on Ubuntu 20.04 (fully updated)

Following config file fails at startup:

allow_anonymous true
listener 1884
protocol websockets

mosquitto -c ./test.conf 
1656419034: mosquitto version 2.0.14 starting
1656419034: Config loaded from ./test.conf.
1656419034: Opening websockets listen socket on port 1884.
1656419034: Error: Unable to start any listening sockets, exiting.

Adding a native MQTT listener works:

allow_anonymous true
listener 1883 127.0.0.1
listener 1884
protocol websockets

mosquitto -c ./test.conf 
1656419113: mosquitto version 2.0.14 starting
1656419113: Config loaded from ./test.conf.
1656419113: Opening ipv4 listen socket on port 1883.
1656419113: Opening websockets listen socket on port 1884.
1656419113: mosquitto version 2.0.14 running

I should be able to start with mosquitto only listening for WebSocket Connections

[ralight]

I believe this is down to that version of libwebsockets not being compiled with LWS_WITH_EXTERNAL_POLL support. There's not much to be done - without that the websockets support doesn't really work properly. If you've got a normal listener as well, it looks like it works but you'll get occasional latency problems.