MergingHnswGraphBuilder is unstable

[msokolov]

Description

If you create an index and merge it, even under very controlled conditions, MergingHnswGraphBuilder will produce different graphs on subsequent runs. I think it's because of the iteration order of the IntHashSet keys which it relies on. This is a problem for writing tests where you want to assert that the correct graph is being produced. I'm not sure what can be done -- perhaps we can fetch all the node ids and sort them rather than relying on the hashset iteration order

Version and environment details

No response

[dweiss]

IntHashSet will have a random iteration order - it is by design to prevent problems with collisions. You could probably hack your way into the internals and iterate over values in any way you like... but it's still a hash set - the ordering isn't guaranteed and may change, so tests shouldn't rely on it.

[msokolov]

also backported to 10x