Clean up JoinGame packet encoding/decoding

Specifically, the JoinGame packet handling is now split between pre-1.16 encodings of the packet and post-1.16 handlings of the packet. This packet is one of the most amorphous packets in the entire Minecraft protocol, from Velocity's perspective.

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftVarintFrameDecoder.java

@@ -44,6 +44,10 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
     if (varintEnd == -1) {
       // We tried to go beyond the end of the buffer. This is probably a good sign that the
       // buffer was too short to hold a proper varint.
+      if (reader.getResult() == DecodeResult.RUN_OF_ZEROES) {
+        // Special case where the entire packet is just a run of zeroes. We ignore them all.
+        in.clear();
+      }
       return;
     }
 
@@ -54,7 +58,7 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
         in.clear();
         throw BAD_LENGTH_CACHED;
       } else if (readVarint == 0) {
-        // skip over the empty packet and ignore it
+        // skip over the empty packet(s) and ignore it
         in.readerIndex(varintEnd + 1);
       } else {
         int minimumRead = bytesRead + readVarint;

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/VarintByteDecoder.java

@@ -27,6 +27,15 @@ class VarintByteDecoder implements ByteProcessor {
 
   @Override
   public boolean process(byte k) {
+    if (k == 0 && bytesRead == 0) {
+      // tentatively say it's invalid, but there's a possibility of redemption
+      result = DecodeResult.RUN_OF_ZEROES;
+      return true;
+    }
+    if (result == DecodeResult.RUN_OF_ZEROES) {
+      result = DecodeResult.SUCCESS;
+      return false;
+    }
     readVarint |= (k & 0x7F) << bytesRead++ * 7;
     if (bytesRead > 3) {
       result = DecodeResult.TOO_BIG;
@@ -54,6 +63,7 @@ class VarintByteDecoder implements ByteProcessor {
   public enum DecodeResult {
     SUCCESS,
     TOO_SHORT,
-    TOO_BIG
+    TOO_BIG,
+    RUN_OF_ZEROES
   }
 }

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/JoinGame.java

@@ -99,7 +99,7 @@ public class JoinGame implements MinecraftPacket {
     return levelType;
   }
 
-  public void setLevelType(String levelType) {
+  public void setLevelType(@Nullable String levelType) {
     this.levelType = levelType;
   }
 
@@ -183,6 +183,46 @@ public class JoinGame implements MinecraftPacket {
 
   @Override
   public void decode(ByteBuf buf, ProtocolUtils.Direction direction, ProtocolVersion version) {
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_16) >= 0) {
+      // Minecraft 1.16 and above have significantly more complicated logic for reading this packet,
+      // so separate it out.
+      this.decode116Up(buf, version);
+    } else {
+      this.decodeLegacy(buf, version);
+    }
+  }
+
+  private void decodeLegacy(ByteBuf buf, ProtocolVersion version) {
+    this.entityId = buf.readInt();
+    this.gamemode = buf.readByte();
+    this.isHardcore = (this.gamemode & 0x08) != 0;
+    this.gamemode &= ~0x08;
+
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_9_1) >= 0) {
+      this.dimension = buf.readInt();
+    } else {
+      this.dimension = buf.readByte();
+    }
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_13_2) <= 0) {
+      this.difficulty = buf.readUnsignedByte();
+    }
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
+      this.partialHashedSeed = buf.readLong();
+    }
+    this.maxPlayers = buf.readUnsignedByte();
+    this.levelType = ProtocolUtils.readString(buf, 16);
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_14) >= 0) {
+      this.viewDistance = ProtocolUtils.readVarInt(buf);
+    }
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_8) >= 0) {
+      this.reducedDebugInfo = buf.readBoolean();
+    }
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
+      this.showRespawnScreen = buf.readBoolean();
+    }
+  }
+
+  private void decode116Up(ByteBuf buf, ProtocolVersion version) {
     this.entityId = buf.readInt();
     if (version.compareTo(ProtocolVersion.MINECRAFT_1_16_2) >= 0) {
       this.isHardcore = buf.readBoolean();
@@ -192,13 +232,13 @@ public class JoinGame implements MinecraftPacket {
       this.isHardcore = (this.gamemode & 0x08) != 0;
       this.gamemode &= ~0x08;
     }
-    String dimensionIdentifier = null;
+
-    String levelName = null;
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_16) >= 0) {
     this.previousGamemode = buf.readByte();
+
     ImmutableSet<String> levelNames = ImmutableSet.copyOf(ProtocolUtils.readStringArray(buf));
+
     CompoundBinaryTag registryContainer = ProtocolUtils.readCompoundTag(buf, JOINGAME_READER);
-      ListBinaryTag dimensionRegistryContainer = null;
+    ListBinaryTag dimensionRegistryContainer;
     if (version.compareTo(ProtocolVersion.MINECRAFT_1_16_2) >= 0) {
       dimensionRegistryContainer = registryContainer.getCompound("minecraft:dimension_type")
           .getList("value", BinaryTagTypes.COMPOUND);
@@ -210,6 +250,9 @@ public class JoinGame implements MinecraftPacket {
     ImmutableSet<DimensionData> readData =
         DimensionRegistry.fromGameData(dimensionRegistryContainer, version);
     this.dimensionRegistry = new DimensionRegistry(readData, levelNames);
+
+    String dimensionIdentifier;
+    String levelName = null;
     if (version.compareTo(ProtocolVersion.MINECRAFT_1_16_2) >= 0) {
       CompoundBinaryTag currentDimDataTag = ProtocolUtils.readCompoundTag(buf, JOINGAME_READER);
       dimensionIdentifier = ProtocolUtils.readString(buf);
@@ -219,43 +262,69 @@ public class JoinGame implements MinecraftPacket {
       dimensionIdentifier = ProtocolUtils.readString(buf);
       levelName = ProtocolUtils.readString(buf);
     }
-    } else if (version.compareTo(ProtocolVersion.MINECRAFT_1_9_1) >= 0) {
+
-      this.dimension = buf.readInt();
-    } else {
-      this.dimension = buf.readByte();
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_13_2) <= 0) {
-      this.difficulty = buf.readUnsignedByte();
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
     this.partialHashedSeed = buf.readLong();
-    }
     if (version.compareTo(ProtocolVersion.MINECRAFT_1_16_2) >= 0) {
       this.maxPlayers = ProtocolUtils.readVarInt(buf);
     } else {
       this.maxPlayers = buf.readUnsignedByte();
     }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_16) < 0) {
+
-      this.levelType = ProtocolUtils.readString(buf, 16);
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_14) >= 0) {
     this.viewDistance = ProtocolUtils.readVarInt(buf);
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_8) >= 0) {
     this.reducedDebugInfo = buf.readBoolean();
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
     this.showRespawnScreen = buf.readBoolean();
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_16) >= 0) {
     boolean isDebug = buf.readBoolean();
     boolean isFlat = buf.readBoolean();
     this.dimensionInfo = new DimensionInfo(dimensionIdentifier, levelName, isFlat, isDebug);
   }
-  }
 
   @Override
   public void encode(ByteBuf buf, ProtocolUtils.Direction direction, ProtocolVersion version) {
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_16) >= 0) {
+      // Minecraft 1.16 and above have significantly more complicated logic for reading this packet,
+      // so separate it out.
+      this.encode116Up(buf, version);
+    } else {
+      this.encodeLegacy(buf, version);
+    }
+  }
+
+  private void encodeLegacy(ByteBuf buf, ProtocolVersion version) {
+    buf.writeInt(entityId);
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_16_2) >= 0) {
+      buf.writeBoolean(isHardcore);
+      buf.writeByte(gamemode);
+    } else {
+      buf.writeByte(isHardcore ? gamemode | 0x8 : gamemode);
+    }
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_9_1) >= 0) {
+      buf.writeInt(dimension);
+    } else {
+      buf.writeByte(dimension);
+    }
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_13_2) <= 0) {
+      buf.writeByte(difficulty);
+    }
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
+      buf.writeLong(partialHashedSeed);
+    }
+    buf.writeByte(maxPlayers);
+    if (levelType == null) {
+      throw new IllegalStateException("No level type specified.");
+    }
+    ProtocolUtils.writeString(buf, levelType);
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_14) >= 0) {
+      ProtocolUtils.writeVarInt(buf, viewDistance);
+    }
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_8) >= 0) {
+      buf.writeBoolean(reducedDebugInfo);
+    }
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
+      buf.writeBoolean(showRespawnScreen);
+    }
+  }
+
+  private void encode116Up(ByteBuf buf, ProtocolVersion version) {
     buf.writeInt(entityId);
     if (version.compareTo(ProtocolVersion.MINECRAFT_1_16_2) >= 0) {
       buf.writeBoolean(isHardcore);
@@ -263,7 +332,6 @@ public class JoinGame implements MinecraftPacket {
     } else {
       buf.writeByte(isHardcore ? gamemode | 0x8 : gamemode);
     }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_16) >= 0) {
     buf.writeByte(previousGamemode);
     ProtocolUtils.writeStringArray(buf, dimensionRegistry.getLevelNames().toArray(new String[0]));
     CompoundBinaryTag.Builder registryContainer = CompoundBinaryTag.builder();
@@ -285,42 +353,18 @@ public class JoinGame implements MinecraftPacket {
       ProtocolUtils.writeString(buf, dimensionInfo.getRegistryIdentifier());
       ProtocolUtils.writeString(buf, dimensionInfo.getLevelName());
     }
-    } else if (version.compareTo(ProtocolVersion.MINECRAFT_1_9_1) >= 0) {
-      buf.writeInt(dimension);
-    } else {
-      buf.writeByte(dimension);
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_13_2) <= 0) {
-      buf.writeByte(difficulty);
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
     buf.writeLong(partialHashedSeed);
-    }
     if (version.compareTo(ProtocolVersion.MINECRAFT_1_16_2) >= 0) {
       ProtocolUtils.writeVarInt(buf, maxPlayers);
     } else {
       buf.writeByte(maxPlayers);
     }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_16) < 0) {
-      if (levelType == null) {
-        throw new IllegalStateException("No level type specified.");
-      }
-      ProtocolUtils.writeString(buf, levelType);
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_14) >= 0) {
     ProtocolUtils.writeVarInt(buf, viewDistance);
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_8) >= 0) {
     buf.writeBoolean(reducedDebugInfo);
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
     buf.writeBoolean(showRespawnScreen);
-    }
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_16) >= 0) {
     buf.writeBoolean(dimensionInfo.isDebugType());
     buf.writeBoolean(dimensionInfo.isFlat());
   }
-  }
 
   @Override
   public boolean handle(MinecraftSessionHandler handler) {