ChunkMap#getPlayers(ChunkPos,boolean) can return players that are not actually tracking the chunk

ChunkMap#getPlayers(ChunkPos,boolean) when passed in false should return all players that are tracking the chunk, meaning the chunk’s data should already have been sent to the client. However, this is not currently the case. Specifically when a chunk has just loaded in on the server, this method can return players that do not have the chunk’s data yet.

This affects the Fabric API (seen here) and the Neoforge API (seen here) as they use this method to check if chunk packets should be sent to a client.

I understand that Mojang typically does not accept bug reports for issues relating to modloaders, but as will be detailed below, this is a vanilla bug, just that the bug is manifesting itself via the Fabric/Neoforge API.

Code Analysis

ChunkMap#getPlayers(ChunkPos,boolean) relies on ChunkMap#isChunkTracked to determine if a player is tracking a chunk.

boolean isChunkTracked(ServerPlayer player, int chunkX, int chunkZ) {
	return player.getChunkTrackingView().contains(chunkX, chunkZ) && !player.connection.playerChunkSender.isPending(ChunkPos.toLong(chunkX, chunkZ));
}

ChunkMap#isChunkTracked checks if the chunk’s data has already been sent to the client via the PlayerChunkSender . If the PlayerChunkSender still has the chunk in its pending chunks, then the chunk is considered to not be tracked by the player yet.

Looking at where pending chunks are added to PlayerChunkSender , we see that ChunkMap#markChunkPendingToSend adds new chunks to a player’s PlayerChunkSender . There are two similar methods, one taking in a ChunkPos and one taking in a WorldChunk. The one we are interested in is the ChunkPos one.

private void markChunkPendingToSend(ServerPlayer player, ChunkPos pos) {
	LevelChunk levelChunk = this.getChunkToSend(pos.toLong());
	if (levelChunk != null) {
		markChunkPendingToSend(player, levelChunk); // this then adds it to the PlayerChunkSender
	}
}

If a chunk is “not ready” to be sent to the client yet, then it won’t get added to the player’sPlayerChunkSender . While this may seem correct, it means that the assumption made by ChunkMap#isChunkTracked as seen above is incorrect, as it will return true for chunks that are not ready to be sent but already within the client’s view distance. This usually happens when a chunk first loads in on the server.

Possible Solution

Change ChunkMap#markChunkPendingToSend to always add the chunk to the player’s PlayerChunkSender regardless of whether it is ready or not. PlayerChunkSender actually already accounts for whether a chunk is ready to be sent. PlayerChunkSender#collectChunksToSend will exclude chunks that are not ready yet (via the filter Objects::nonNull), so unready chunks will just remain in the pending chunks queue until they are ready.

PlayerChunkSender#markChunkPendingToSend currently takes in a LevelChunk, but if you look at its code it really only needs a ChunkPos or a long.