Edge case in generation system when external systems (e.g. in plugins) lock chunks

[dktapps]

Problem description

World->lockChunk() is used for advisory locking for async worldgen. Async generation won't take place if a chunk is locked.

However, we don't account for the possibility that the source of the chunk locking wasn't the generation subsystem. The assumption here that a generation request has locked the chunk is problematic:

PocketMine-MP/src/world/World.php

Line 3447 in 51cf681

//chunk is already in use by another generation request; queue the request for later

A deferred generation request is pushed onto the queue, but because the queue drainage is only triggered by other generation tasks completing, if there are no additional tasks, the request will never get processed.

This bug requires a conjunction of two things:

1. no subsequent generation requests to be submitted
2. the chunk to be locked by something outside the generation system (e.g. in a plugin)

Proposed solution

Some possible options come to mind:

1. a ticking function called by World to make sure the queue doesn't stagnate even if no new generation requests arrive
2. support callbacks (or Promises?) to get notified when a chunk is unlocked, which could be used to trigger queue processing
3. global callbacks for World->unlockChunk() to trigger stuff like population queue flushes - although this might get complicated during generation callbacks

Alternative solutions or workarounds

No response