PaperMC/paper-server/patches/sources/net/minecraft/server/network/LegacyQueryHandler.java.patch

--- a/net/minecraft/server/network/LegacyQueryHandler.java
+++ b/net/minecraft/server/network/LegacyQueryHandler.java
@@ -15,6 +15,7 @@
 
     private static final Logger LOGGER = LogUtils.getLogger();
     private final ServerInfo server;
+    private ByteBuf buf; // Paper
 
     public LegacyQueryHandler(ServerInfo server) {
         this.server = server;
@@ -23,6 +24,16 @@
     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;
 
@@ -34,11 +45,12 @@
 
                 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) {
                     LegacyQueryHandler.LOGGER.debug("Ping: (<1.3.x) from {}", socketaddress);
-                    s = LegacyQueryHandler.createVersion0Response(this.server);
+                    s = LegacyQueryHandler.createVersion0Response(this.server, event); // CraftBukkit
                     LegacyQueryHandler.sendFlushAndClose(channelhandlercontext, LegacyQueryHandler.createLegacyDisconnectPacket(channelhandlercontext.alloc(), s));
                 } else {
                     if (bytebuf.readUnsignedByte() != 1) {
@@ -46,16 +58,24 @@
                     }
 
                     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);
+                    if (s == null) s = LegacyQueryHandler.createVersion1Response(this.server, event); // CraftBukkit // Paper
                     LegacyQueryHandler.sendFlushAndClose(channelhandlercontext, LegacyQueryHandler.createLegacyDisconnectPacket(channelhandlercontext.alloc(), s));
                 }
 
@@ -106,14 +126,102 @@
         }
     }
 
-    private static String createVersion0Response(ServerInfo server) {
-        return String.format(Locale.ROOT, "%s\u00a7%d\u00a7%d", server.getMotd(), server.getPlayerCount(), server.getMaxPlayers());
+    // 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 static String createVersion1Response(ServerInfo server) {
-        return String.format(Locale.ROOT, "\u00a71\u0000%d\u0000%s\u0000%s\u0000%d\u0000%d", 127, server.getServerVersion(), server.getMotd(), server.getPlayerCount(), server.getMaxPlayers());
+    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());
+        // CraftBukkit end
+    }
+
+    // CraftBukkit start
+    private static String createVersion1Response(ServerInfo serverinfo, org.bukkit.event.server.ServerListPingEvent event) {
+        return String.format(Locale.ROOT, "\u00a71\u0000%d\u0000%s\u0000%s\u0000%d\u0000%d", 127, serverinfo.getServerVersion(), event.getMotd(), event.getNumPlayers(), event.getMaxPlayers());
+        // CraftBukkit end
+    }
+
     private static void sendFlushAndClose(ChannelHandlerContext context, ByteBuf buf) {
         context.pipeline().firstContext().writeAndFlush(buf).addListener(ChannelFutureListener.CLOSE);
     }