From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Minecrell <minecrell@minecrell.net>
Date: Wed, 11 Oct 2017 18:22:50 +0200
Subject: [PATCH] Make legacy ping handler more reliable

The Minecraft server often fails to respond to old ("legacy") pings
from old Minecraft versions using the protocol used before the switch
to Netty in Minecraft 1.7.

Due to packet fragmentation[1], we might not have all needed bytes
available when the LegacyPingHandler is called. In this case, it will
run into an error, remove the handler and continue using the modern
protocol.

This is unlikely to happen for the first two revisions of the legacy
ping protocol (used in Minecraft 1.5.x and older) since the request
consists of only one or two bytes, but happens frequently for the
last/third revision introduced in Minecraft 1.6.

It has much larger, variable packet sizes due to the inclusion of
the virtual host (the hostname/port used to connect to the server).

The solution[2] is simple: If we find more than two matching bytes,
we buffer the remaining bytes until we have enough to fully read and
respond to the request.

[1]: https://netty.io/wiki/user-guide-for-4.x.html#wiki-h3-11
[2]: https://netty.io/wiki/user-guide-for-4.x.html#wiki-h4-13

diff --git a/src/main/java/net/minecraft/server/network/LegacyQueryHandler.java b/src/main/java/net/minecraft/server/network/LegacyQueryHandler.java
index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
--- a/src/main/java/net/minecraft/server/network/LegacyQueryHandler.java
+++ b/src/main/java/net/minecraft/server/network/LegacyQueryHandler.java
@@ -0,0 +0,0 @@ public class LegacyQueryHandler extends ChannelInboundHandlerAdapter {
 
     private static final Logger LOGGER = LogUtils.getLogger();
     private final ServerInfo server;
+    private ByteBuf buf; // Paper
 
     public LegacyQueryHandler(ServerInfo server) {
         this.server = server;
@@ -0,0 +0,0 @@ public class LegacyQueryHandler extends ChannelInboundHandlerAdapter {
     public void channelRead(ChannelHandlerContext channelhandlercontext, Object object) {
         ByteBuf bytebuf = (ByteBuf) object;
 
+        // Paper start - Make legacy ping handler more reliable
+        if (this.buf != null) {
+            try {
+                readLegacy1_6(channelhandlercontext, bytebuf);
+            } finally {
+                bytebuf.release();
+            }
+            return;
+        }
+        // Paper end
         bytebuf.markReaderIndex();
         boolean flag = true;
 
@@ -0,0 +0,0 @@ public class LegacyQueryHandler extends ChannelInboundHandlerAdapter {
 
                 SocketAddress socketaddress = channelhandlercontext.channel().remoteAddress();
                 int i = bytebuf.readableBytes();
-                String s;
+                String s = null; // Paper
                 org.bukkit.event.server.ServerListPingEvent event = org.bukkit.craftbukkit.event.CraftEventFactory.callServerListPingEvent(socketaddress, this.server.getMotd(), this.server.getPlayerCount(), this.server.getMaxPlayers()); // CraftBukkit
 
                 if (i == 0) {
@@ -0,0 +0,0 @@ public class LegacyQueryHandler extends ChannelInboundHandlerAdapter {
                     }
 
                     if (bytebuf.isReadable()) {
-                        if (!LegacyQueryHandler.readCustomPayloadPacket(bytebuf)) {
-                            return;
+                        // Paper start - Replace with improved version below
+                        if (bytebuf.readUnsignedByte() != 250) {
+                            s = this.readLegacy1_6(channelhandlercontext, bytebuf);
+                            if (s == null) {
+                                return;
+                            }
                         }
-
-                        LegacyQueryHandler.LOGGER.debug("Ping: (1.6) from {}", socketaddress);
+                        // if (!LegacyQueryHandler.readCustomPayloadPacket(bytebuf)) {
+                        //     return;
+                        // }
+                        //
+                        // LegacyQueryHandler.LOGGER.debug("Ping: (1.6) from {}", socketaddress);
+                        // Paper end
                     } else {
                         LegacyQueryHandler.LOGGER.debug("Ping: (1.4-1.5.x) from {}", socketaddress);
                     }
 
-                    s = LegacyQueryHandler.createVersion1Response(this.server, event); // CraftBukkit
+                    if (s == null) s = LegacyQueryHandler.createVersion1Response(this.server, event); // CraftBukkit // Paper
                     LegacyQueryHandler.sendFlushAndClose(channelhandlercontext, LegacyQueryHandler.createLegacyDisconnectPacket(channelhandlercontext.alloc(), s));
                 }
 
@@ -0,0 +0,0 @@ public class LegacyQueryHandler extends ChannelInboundHandlerAdapter {
         }
     }
 
+    // Paper start
+    private static String readLegacyString(ByteBuf buf) {
+        int size = buf.readShort() * Character.BYTES;
+        if (!buf.isReadable(size)) {
+            return null;
+        }
+
+        String result = buf.toString(buf.readerIndex(), size, java.nio.charset.StandardCharsets.UTF_16BE);
+        buf.skipBytes(size); // toString doesn't increase readerIndex automatically
+        return result;
+    }
+
+    private String readLegacy1_6(ChannelHandlerContext ctx, ByteBuf part) {
+        ByteBuf buf = this.buf;
+
+        if (buf == null) {
+            this.buf = buf = ctx.alloc().buffer();
+            buf.markReaderIndex();
+        } else {
+            buf.resetReaderIndex();
+        }
+
+        buf.writeBytes(part);
+
+        if (!buf.isReadable(Short.BYTES + Short.BYTES + Byte.BYTES + Short.BYTES + Integer.BYTES)) {
+            return null;
+        }
+
+        String s = readLegacyString(buf);
+        if (s == null) {
+            return null;
+        }
+
+        if (!s.equals("MC|PingHost")) {
+            removeHandler(ctx);
+            return null;
+        }
+
+        if (!buf.isReadable(Short.BYTES) || !buf.isReadable(buf.readShort())) {
+            return null;
+        }
+
+        net.minecraft.server.MinecraftServer server = net.minecraft.server.MinecraftServer.getServer();
+        int protocolVersion = buf.readByte();
+        String host = readLegacyString(buf);
+        if (host == null) {
+            removeHandler(ctx);
+            return null;
+        }
+        int port = buf.readInt();
+
+        if (buf.isReadable()) {
+            removeHandler(ctx);
+            return null;
+        }
+
+        buf.release();
+        this.buf = null;
+
+        LOGGER.debug("Ping: (1.6) from {}", ctx.channel().remoteAddress());
+
+        String response = String.format("\u00a71\u0000%d\u0000%s\u0000%s\u0000%d\u0000%d",
+            Byte.MAX_VALUE, server.getServerVersion(), server.getMotd(), server.getPlayerCount(), server.getMaxPlayers());
+        return response;
+    }
+
+    private void removeHandler(ChannelHandlerContext ctx) {
+        ByteBuf buf = this.buf;
+        this.buf = null;
+
+        buf.resetReaderIndex();
+        ctx.pipeline().remove(this);
+        ctx.fireChannelRead(buf);
+    }
+
+    @Override
+    public void handlerRemoved(ChannelHandlerContext ctx) {
+        if (this.buf != null) {
+            this.buf.release();
+            this.buf = null;
+        }
+    }
+    // Paper end
+
     // CraftBukkit start
     private static String createVersion0Response(ServerInfo serverinfo, org.bukkit.event.server.ServerListPingEvent event) {
         return String.format(Locale.ROOT, "%s\u00a7%d\u00a7%d", event.getMotd(), event.getNumPlayers(), event.getMaxPlayers());
