I'm getting unix netty errors with 1.12,
Server1.log and server2.log
This is server grade hardware and this error did not show
until this morning after 1.12 patch.
Server3.log says allot for being so short, I would read it
attached.
Linked issues
is duplicated by 2
Attachments
Comments 11

(https://bugs.mojang.com/browse/MC-118733)
The same issue here, it keeps happening to me for days and several times! It is totally unacceptable for any servers like us which are supposed to provide stable and meaningful service to the public. Just take a simple look at https://bstats.org/global/bukkit you will find that over 80% of server are running on an Unix-like system. It means that those server, including some big networks, will never have a chance to upgrade to 1.12 if it is not fixed!
I believe it is very simple to fix this issue: just upgrade netty to 4.1.10 version (see https://github.com/netty/netty/issues/6607 ).
If this issue could not be fix, or at least provide some temporary solutions for people, I have no choice but to downgrade my whole network back to 1.11.2.

OK, let's see whether that will work. Download netty-all 4.1.11-Final, put it in the same directory as your server, and launch the server with
java -cp netty-all-4.1.11.Final.jar:minecraft_server.1.12.jar net.minecraft.server.MinecraftServer --nogui
which should force a newer version of netty (no guarantees). Does that work?
It takes several days on testing since the issue occurs randomly (depending on how and how many player join). So I will report our results on Monday or if it breaks.
For those who need to run cb/spigot, here is the command you need:
java -server -Xmx2G -cp netty-all-4.1.11.Final.jar:spigot-1.12.jar org.bukkit.craftbukkit.Main
It has NO guarantee. I'm not working for Mojang or Spigotmc, so good luck to you! (and me lol)
After 3 days of testing, although my server still throw this warning to me occasionally:
[21:19:09] [Netty Server IO #3/WARN]: Selector.select() returned prematurely 512 times in a row; rebuilding Selector io.netty.channel.nio.SelectedSelectionKeySetSelector@b338db7.
[21:19:09] [Netty Server IO #3/INFO]: Migrated 3 channel(s) to the new Selector.
Good news is no more player is kicked or prevented. The newer version of netty seems working again.
For the warning it self, I did some dig in about this. Founds out that it looks like that there might be something to do with incompatibility with certain version of kernel and netty. However my kernel is 4.4.0 running Ubuntu 16.04LTS which should be pretty main-stream at this moment, so more people must be affected by this (even though 4.1.11 don't kick player anymore). But what really bugs me is that this issue was first reported way before 4.0.22. Remember, we didn't have such a problem before 1.12 when Minecraft was using netty 4.0.23 by then. My guess is there should be more bugs in netty which should be fixed before it breaks things again.
I highly suggest that people who working at Mojang should not only upgrade netty to a newer version, but report the warning to the netty project team as well. Only you guys can do it because we don't have access to the minecraft code and we can not debug with it.
Thank all of you for the consideration and effort!

Try updating netty as per this comment. The latest build of netty is 4.1.16-Final.
I'm running a forge server, and I'm confused as to to how to modify my startup script to use the new netty version. Help?
I posted a support topic at the forge forums. I'll leave this here for any other forge users having the same issue.

Forge is complicated (it's got a couple of levels of launching) and I really am not qualified to provide forge support; you'd be better off asking in one of the forge help areas. Sorry I can't be helpful here.
When I run the command as per this comment, I get the following:
Error: Could not find or load main class net.minecraft.server.MinecraftServer
Caused by: java.lang.ClassNotFoundException: net.minecraft.server.MinecraftServer

That means that the Minecraft server jar wasn't on the classpath. The most likely cause is platform distinctions between :
and ;
as a classpath separator; on windows you use ;
. (n.b. the latest release is also now 4.1.25.Final)
Netty was updated from 4.0.23 to 4.1.9 in 1.12, so it's possible that this is a new bug from that.
I can find a few references to that message: #3539, #6266. Both were supposedly fixed, but it might be that it's still an issue that's exposed in a different way.