Make thread-safe api for generation management

[Alexander Knorre]

Describe the problem you are trying to solve.
When using pre-allocated manually managed id entities (e.g. allocating 100000 entities in advance, and never destruct-ing them, only clear-ing them) we want a way to lock-lessly recycle them by incrementing their generation, but currently the api for that is not thread safe:

ecs_set_entity_generation(world, id_with_new_generation); // not thread-safe (chunk alloc)
ecs_ensure(world, id_with_new_generation); // not thread-safe (chunk alloc + dense grow)

Describe the solution you'd like
Have a thread-safe api that never grows dense, never creates chunks, and only asserts that those already exist, and then we could "touch" our 100000 pre-allocated entities by giving them a dummy component like a name (to enforce chunk alloc & dense index), and then lock-lessly update their generation at arbitrary times from multi-threaded systems without sync-points, by never destruct-ing them - only clear-ing them (to keep chunk and dense intact).

[Sander Mertens]

This is possible with the current API:

if (ecs_exists(world, id_without_generation)) {
  ecs_set_entity_generation(world, id_with_new_generation);
}

Note that this is not thread safe, as multiple threads may try to overwrite the generation of the same entity.

[Alexander Knorre]

Note that this is not thread safe, as multiple threads may try to overwrite the generation of the same entity.

I have a guarantee that only one thread tries to write to generation of each entity, so I seem to be safe with current api, cheers!