I had some free time so I fixed a bug that prevented players from logging in.
It was strange, and only had been there after a week on a seemingly stable build. A dormant bug came to light after nothing at all changed; here is an informal summary for game developers to read:
The client was receiving CONNECT event, but the server was not
But for some reason this only broke after months of successful server uptime
So the server never did its processing for the full login handshake
If you just send any data it will work
But just enet_host_flush alone wasn't enough, you have to actually respond with a packet, even if the packet has nothing in it.
But I never had this issue before. It just randomly happened on a build of the game that was working fine
Anyway, if I didn't use a library, I wouldn't have this problem
Things being as they were, I had to investigate it at every level to find this, like looking at the hex data of the packets from tcpdump