Assertion for SpillBuffer breaks usage of custom Worker(data=...) types

[pentschev]

#5814 introduced https://github.com/dask/distributed/blob/main/distributed/worker.py#L3884 which I believe unintentionally breaks the flexibility of passing a custom type via Worker(data=...). In Dask-CUDA we use the data= kwarg to pass a specialized handler for spilling GPU memory, and it currently breaks one of our tests, see here.

Would it be possible to remove that assertion or make it somehow flexible so that a custom type wouldn't fail?

cc @crusaderky who's the author.

cc @quasiben @madsbk

[crusaderky]

Fixed in #5904. I would be grateful if you could review it and see if it is fit for purpose for you.

[pentschev]

Thanks @crusaderky for the reply, I responded in #5904 (comment) .