Thread-safe Buffer.__items__; proper KeysView, ItemsView, ValuesView

[crusaderky]

• Closes Dictionary views #61
• Partially closes Asynchronous Disk Access in Workers distributed#4424
• Fix RuntimeError when a thread is iterating on Buffer, e.g. distributed.worker_state_machine.WorkerStateMachine.validate_state(), while another is evicting/restoring a key

[hendrikmakait]

I have two questions regarding the implementation and a few nits.

[hendrikmakait]

Should we also override __contains__ in BufferItemsView to avoid changing the LRU?

[crusaderky]

The reason why __iter__ avoids changing the LRU is because it would raise RuntimeError halfway through. If you're testing if your buffer contains a specific key/value pair, and that causes the value to be unevicted from slow, then I don't see why it should bypass the principle of temporal locality.

[hendrikmakait]

Thanks for the explanation; now it makes sense to me. I had assumed we wanted this to be a general property of the views.

If you're testing if your buffer contains a specific key/value pair, and that causes the value to be unevicted from slow, then I don't see why it should bypass the principle of temporal locality.

Sounds good to me!

[crusaderky]

@hendrikmakait all review comments have been addressed

[hendrikmakait]

Thanks, @crusaderky!