This adds the module I've been using for running my minecraft servers.
It is inspired by the declarative minecraft server module in nixpkgs,
but
* does not support a non-declarative mode.
* supports more than one server on the same machine.
* patches the fabric mod loader into the server.jar on startup.
* its stopping mechanism is more robust: It issues a `save-all` and
`stop` command over RCON and uses flock(1) for waiting on the
server's shutdown instead of relying on checking for the PID
via kill(1) in a loop.
It has some gaps in terms of features that I personally don't need, but
can be filled in over time.
Change-Id: I31b9139cab41a6398e5a08ecc72be33cd021ed2e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7291
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
