Chunk generation priority (PM4)

[dktapps]

Description

Chunk generation should be able to have an associated priority, to speed up the generation of more useful chunks in the generation queue.

Since the introduction of chunk generation promises, the combination of high chunks-per-tick sending and low generation queue size causes some problematic side effects on chunk loading when flying around in a new world, because chunk generation requests are FIFO. This means that if you move to a different position, and are still using some of the same chunks that you were using at the old position, those chunks will be generated first, even though they are now (usually) further away, and you have closer ungenerated chunks which need to be generated.

This results in the player being able to fly into ungenerated terrain even on very high performance servers if misconfigured.

To address this, generation requests should have an associated priority so that they can be processed in the most efficient order possible.

[dktapps]

Implementing this would have a very high complexity cost, because the generation queue might end up getting continuously reordered due to players moving around.

The older system which just bounces the request over rate limit dealt with this much more effectively, because the player would just re-request chunks with bounced requests. Since the whole chunk order is rebuilt starting from the player's new position when it moves, this ensures that generation is always producing chunks near the player. It wasn't a perfect solution, but it worked better than the current system, which unconditionally accepts generation requests even when the queue is full, and just defers them until the generator is not busy.

[dktapps]

This problem is the result of an over-engineered solution for #3947 that in the end turned out not to be very useful anyway.