close threads at server shutdown

To exit normally all non-daemon threads of the java process
are supposed to have stopped. There are several cases of
EventLoopGroup-created thread pools that are not shut down
as a part of the server shutdown.

These threads are spawned on the first Bedrock player login. The
identified cases are:
- The listener threads in GeyserServerInitializer
- The player threads in GeyserServerInitializer
- The EXECUTOR_SERVICE in the SkinProvider

This patch depends on the PR
GeyserMC/MCProtocolLib#748
submitted to MCProtocolLib to allow the TcpCLientSession to
shut down on process exit.

This is a very quickly thrown together proof of concept to verify that
the execution thread pools left running were what blocked a clean exit
with Geyser active.

Nullify EventLoopGroup variables after shutdown to allow them to be
restarted.

Author: Peter Svensson

core/src/main/java/org/geysermc/geyser/network/GeyserServerInitializer.java

@@ -47,6 +47,10 @@ public GeyserServerInitializer(GeyserImpl geyser) {
         this.geyser = geyser;
     }
 
+    public DefaultEventLoopGroup getEventLoopGroup() {
+        return eventLoopGroup;
+    }
+
     @Override
     public void initSession(@Nonnull BedrockServerSession bedrockServerSession) {
         try {
@@ -72,4 +76,4 @@ public void initSession(@Nonnull BedrockServerSession bedrockServerSession) {
     protected BedrockPeer createPeer(Channel channel) {
         return new GeyserBedrockPeer(channel, this::createSession);
     }
-}
+}

core/src/main/java/org/geysermc/geyser/network/netty/GeyserServer.java

@@ -26,6 +26,7 @@
 package org.geysermc.geyser.network.netty;
 
 import com.github.steveice10.packetlib.helper.TransportHelper;
+import com.github.steveice10.packetlib.tcp.TcpClientSession;
 import io.netty.bootstrap.ServerBootstrap;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelFuture;
@@ -57,6 +58,7 @@
 import org.geysermc.geyser.network.netty.proxy.ProxyServerHandler;
 import org.geysermc.geyser.ping.GeyserPingInfo;
 import org.geysermc.geyser.ping.IGeyserPingPassthrough;
+import org.geysermc.geyser.skin.SkinProvider;
 import org.geysermc.geyser.text.GeyserLocale;
 import org.geysermc.geyser.translator.text.MessageTranslator;
 
@@ -83,8 +85,9 @@ public final class GeyserServer {
     private static final Transport TRANSPORT = compatibleTransport();
 
     private final GeyserImpl geyser;
-    private final EventLoopGroup group;
+    private EventLoopGroup group;
     private final ServerBootstrap bootstrap;
+    private EventLoopGroup playerGroup;
 
     @Getter
     private final ExpiringMap<InetSocketAddress, InetSocketAddress> proxiedAddresses;
@@ -131,7 +134,15 @@ public CompletableFuture<Void> bind(InetSocketAddress address) {
     }
 
     public void shutdown() {
-        this.group.shutdownGracefully();
+        try {
+            this.group.shutdownGracefully().sync();
+            this.group = null;
+            this.playerGroup.shutdownGracefully().sync();
+            this.playerGroup = null;
+            SkinProvider.shutdown();
+        } catch (InterruptedException e) {
+           e.printStackTrace();
+        }
         this.future.channel().closeFuture().syncUninterruptibly();
     }
 
@@ -148,11 +159,13 @@ private ServerBootstrap createBootstrap(EventLoopGroup group) {
             }
         }
 
+        GeyserServerInitializer serverInitializer = new GeyserServerInitializer(this.geyser);
+        playerGroup = serverInitializer.getEventLoopGroup();
         return new ServerBootstrap()
                 .channelFactory(RakChannelFactory.server(TRANSPORT.datagramChannel()))
                 .group(group)
                 .option(RakChannelOption.RAK_HANDLE_PING, true)
-                .childHandler(new GeyserServerInitializer(this.geyser));
+                .childHandler(serverInitializer);
     }
 
     public boolean onConnectionRequest(InetSocketAddress inetSocketAddress) {

core/src/main/java/org/geysermc/geyser/skin/SkinProvider.java

@@ -58,7 +58,7 @@
 
 public class SkinProvider {
     private static final boolean ALLOW_THIRD_PARTY_CAPES = GeyserImpl.getInstance().getConfig().isAllowThirdPartyCapes();
-    static final ExecutorService EXECUTOR_SERVICE = Executors.newFixedThreadPool(ALLOW_THIRD_PARTY_CAPES ? 21 : 14);
+    static ExecutorService EXECUTOR_SERVICE;
 
     static final Skin EMPTY_SKIN;
     static final Cape EMPTY_CAPE = new Cape("", "no-cape", ByteArrays.EMPTY_ARRAY, -1, true);
@@ -133,6 +133,19 @@ public class SkinProvider {
         WEARING_CUSTOM_SKULL_SLIM = new SkinGeometry("{\"geometry\" :{\"default\" :\"geometry.humanoid.wearingCustomSkullSlim\"}}", wearingCustomSkullSlim, false);
     }
 
+    private static ExecutorService getExecutorService() {
+        if (EXECUTOR_SERVICE == null) {
+            EXECUTOR_SERVICE = Executors.newFixedThreadPool(ALLOW_THIRD_PARTY_CAPES ? 21 : 14);
+        }
+        return EXECUTOR_SERVICE;
+    }
+    public static void shutdown() {
+        if (EXECUTOR_SERVICE != null) {
+            EXECUTOR_SERVICE.shutdown();
+            EXECUTOR_SERVICE = null;
+        }
+    }
+
     public static void registerCacheImageTask(GeyserImpl geyser) {
         // Schedule Daily Image Expiry if we are caching them
         if (geyser.getConfig().getCacheImages() > 0) {
@@ -302,7 +315,7 @@ private static CompletableFuture<SkinAndCape> requestSkinAndCape(UUID playerId,
 
             GeyserImpl.getInstance().getLogger().debug("Took " + (System.currentTimeMillis() - time) + "ms for " + playerId);
             return skinAndCape;
-        }, EXECUTOR_SERVICE);
+        }, getExecutorService());
     }
 
     static CompletableFuture<Skin> requestSkin(UUID playerId, String textureUrl, boolean newThread) {
@@ -320,7 +333,7 @@ static CompletableFuture<Skin> requestSkin(UUID playerId, String textureUrl, boo
 
         CompletableFuture<Skin> future;
         if (newThread) {
-            future = CompletableFuture.supplyAsync(() -> supplySkin(playerId, textureUrl), EXECUTOR_SERVICE)
+            future = CompletableFuture.supplyAsync(() -> supplySkin(playerId, textureUrl), getExecutorService())
                     .whenCompleteAsync((skin, throwable) -> {
                         skin.updated = true;
                         CACHED_JAVA_SKINS.put(textureUrl, skin);
@@ -349,7 +362,7 @@ private static CompletableFuture<Cape> requestCape(String capeUrl, CapeProvider
 
         CompletableFuture<Cape> future;
         if (newThread) {
-            future = CompletableFuture.supplyAsync(() -> supplyCape(capeUrl, provider), EXECUTOR_SERVICE)
+            future = CompletableFuture.supplyAsync(() -> supplyCape(capeUrl, provider), getExecutorService())
                     .whenCompleteAsync((cape, throwable) -> {
                         CACHED_JAVA_CAPES.put(capeUrl, cape);
                         requestedCapes.remove(capeUrl);
@@ -388,7 +401,7 @@ private static CompletableFuture<Skin> requestEars(String earsUrl, boolean newTh
 
         CompletableFuture<Skin> future;
         if (newThread) {
-            future = CompletableFuture.supplyAsync(() -> supplyEars(skin, earsUrl), EXECUTOR_SERVICE)
+            future = CompletableFuture.supplyAsync(() -> supplyEars(skin, earsUrl), getExecutorService())
                     .whenCompleteAsync((outSkin, throwable) -> { });
         } else {
             Skin ears = supplyEars(skin, earsUrl); // blocking
@@ -620,7 +633,7 @@ public static CompletableFuture<String> requestTexturesFromUUID(String uuid) {
                 }
                 return null;
             }
-        }, EXECUTOR_SERVICE);
+        }, getExecutorService());
     }
 
     /**
@@ -646,7 +659,7 @@ public static CompletableFuture<String> requestTexturesFromUsername(String usern
                 }
                 return null;
             }
-        }, EXECUTOR_SERVICE).thenCompose(uuid -> {
+        }, getExecutorService()).thenCompose(uuid -> {
             if (uuid == null) {
                 return CompletableFuture.completedFuture(null);
             }