My server is no longer hanging on stop after setting enable-rcon=false

I have a repro. The dangling NPE is caused by setting enable-rcon=true (in server.properties) while not having an rcon password configured - this causes "No rcon password set in server.properties, rcon disabled!" at startup and NPE at shutdown.

This NPE does not appear to be what is causing my server hang on shutdown - or at least not on my test server- so my search continues for that.

Well, the hang forever happened again after the initial surge died down and I don't think people were spamming in - but there was no NPE this time. Maybe a non-daemon thread somewhere.

I don't know, the point is I need to be able to experience the same issue on my PC to be able to rule it out.

I'm not convinced the last NPE is from players spam connecting which is why I would like you to test the same configuration but without spam connecting

How about this - tell me the command line(s) you'd like me to run against the process when it happens and i'll get the output to you.

Even if that is the case me and other developers don't have the luxury of people 'spam connecting' to our servers, so we need a controlled way to reproduce it.

I understand that, but my point is the repro of it happening has been 100% when people are spam connecting.

The cut off NPE is not the same as the others - note the different threads

The problem only occurs when people are spam connecting during shutdown. I don't recall running into it through the many other restarts I've done setting up the server and pregenerating for 2 days restarting every 30 min. I'll try to get a heap/thread dump one of these times... i'll have to lookup how to trigger one of those

edit:
Actually - that's not entirely true, it happened at least twice come to think of it. One time it was hung for 6 hours before I noticed - i'll see if that log has anything more useful in it.

I suggest you try and replicate the last line only, for example by running with your worlds on a non-public server

java.lang.IllegalStateException: AsyncPlayerPreLoginEvent cannot be 
triggered asynchronously from primary server 
thread.java.lang.IllegalStateException: AsyncPlayerPreLoginEvent cannot 
be triggered asynchronously from primary server thread. at 
org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:568)
 ~[spigot-1.16.1.jar:git-Spigot-6f4ff1b-983305b] at net.minecraft.server.v1_16_R1.LoginListener$LoginHandler.fireEvents(LoginListener.java:270) ~[spigot-1.16.1.jar:git-Spigot-6f4ff1b-983305b] at net.minecraft.server.v1_16_R1.LoginListener$2.run(LoginListener.java:220) [spigot-1.16.1.jar:git-Spigot-6f4ff1b-983305b] 

This error is not preventing your server from stopping. It is from a separate thread.

It is the last cut off line we need information from.