Potential Bug

[xingyuuchen]

Hello, first thanks for your amazing work!

I found here may be a potential bug:

First you tried to lock the bucket:
if (bucket.state.compare_exchange_strong(bucketState, Bucket::State::BUSY, std::memory_order_relaxed))
Then you write key (and value) in:
bucket.entry.first = std::move(key);
Finally your change the state of the bucket to mark its state as ready:
bucket.state.store(Bucket::State::VALID, std::memory_order_release); // mark the bucket as valid

The problem, I think, lies in the situation when two threads try to insert the same key into the hashtable:
The two threads can probably insert the same key into two different buckets - when one thread already applied a free bucket but hasn't saved the key inside. At this time, the second thread cannot see the key in the table so it would apply a new bucket for that key.