Poll future for server shutdown instead of blocking on it

Should work around race condition where server exits before
processing future

src/main/java/org/embeddedt/vintagefix/mixin/bugfix/exit_freeze/IntegratedServerMixin.java

@@ -1,19 +1,43 @@
 package org.embeddedt.vintagefix.mixin.bugfix.exit_freeze;
 
+import com.mojang.authlib.GameProfileRepository;
+import com.mojang.authlib.minecraft.MinecraftSessionService;
+import com.mojang.authlib.yggdrasil.YggdrasilAuthenticationService;
 import net.minecraft.server.MinecraftServer;
 import net.minecraft.server.integrated.IntegratedServer;
+import net.minecraft.server.management.PlayerProfileCache;
+import net.minecraft.util.datafix.DataFixer;
 import net.minecraftforge.fml.common.ObfuscationReflectionHelper;
+import org.embeddedt.vintagefix.VintageFix;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Redirect;
 
+import java.io.File;
+import java.net.Proxy;
+import java.util.concurrent.*;
+
 @Mixin(IntegratedServer.class)
-public class IntegratedServerMixin {
+public abstract class IntegratedServerMixin extends MinecraftServer {
+    public IntegratedServerMixin(File anvilFileIn, Proxy proxyIn, DataFixer dataFixerIn, YggdrasilAuthenticationService authServiceIn, MinecraftSessionService sessionServiceIn, GameProfileRepository profileRepoIn, PlayerProfileCache profileCacheIn) {
+        super(anvilFileIn, proxyIn, dataFixerIn, authServiceIn, sessionServiceIn, profileRepoIn, profileCacheIn);
+    }
+
     /**
      * Sometimes the server thread is already gone at this point, try to prevent a freeze.
      */
-    @Redirect(method = "initiateShutdown", at = @At(value = "INVOKE", target = "Lnet/minecraft/server/integrated/IntegratedServer;isServerRunning()Z"), require = 0)
-    private boolean checkThreadStatus(IntegratedServer instance) {
-        return instance.isServerRunning() && ((Thread)ObfuscationReflectionHelper.getPrivateValue(MinecraftServer.class, instance, "field_175590_aa")).isAlive();
+    @Redirect(method = "initiateShutdown", at = @At(value = "INVOKE", target = "Lcom/google/common/util/concurrent/Futures;getUnchecked(Ljava/util/concurrent/Future;)Ljava/lang/Object;"), require = 0)
+    private <V> V checkThreadStatus(Future<V> future) {
+        while(this.isServerRunning() && ((Thread)ObfuscationReflectionHelper.getPrivateValue(MinecraftServer.class, this, "field_175590_aa")).isAlive()) {
+            try {
+                return future.get(500, TimeUnit.MILLISECONDS);
+            } catch(InterruptedException e) {
+                break;
+            } catch(ExecutionException | CancellationException e) {
+                throw new RuntimeException(e);
+            } catch(TimeoutException ignored) {}
+        }
+        VintageFix.LOGGER.warn("Server thread has already exited, skipping logout process");
+        return null;
     }
 }