CP-52880: Avoid O(N^2) behaviour in Xapi_vdi.update_allowed_operations

[edwintorok]

Activate old xapi_vdi.update_allowed_operations optimization: get_internal_records_where does a linear scan currently, so operating on N VDIs is O(N^2).

Look at the VBD records directly, like before this 2013 commit which regressed it: 5097475

(We are going to optimize get_record separately so it doesn't go through serialization)

This still requires validation and measurements (e.g. using @contificate 's benchmark)

[psafont]

So the change here is avoiding the None value? Why can't the function's signature be change to remove the optional codepath to explicitly remove the n^2 behaviour?

[robhoes]

I agree with Pau: the get_internal_records_where branch is still there, but effectively made dead code?

Also, the trade-off here, which triggered the original change 11 years ago, is that if you are not no the coordinator, the get_internal_records_where call is a single round-trip over the network, where now you have a round-trip for each VBD. The network overhead may outweigh the DB handling speedup in that case. It depends whether this function may run on any host or just on the coordinator.