xds: use defaults for unspecified ring_hash_lb_config values

[voidzcy]

Envoy uses default parameters for ring hash LB config (min_ring_size = 1024, max_ring_size = 8M). grpc/proposal#239 is updated to use the same default as Envoy.

The change follows the way that C-core does this:

• gRPC's ring_hash LB config parser sets default values if the raw config does not specify min_ring_size or max_ring_size.
  • Note, for us we need to do this in both ring_hash LB config parser and the CDS LB policy that instantiates the RingHashConfig directly.

/cc @apolcyn

[dapengzhang0]

Your change in CdsLoadBalancer2.java seems to allow both minRingSize and maxRingSize to be zero, but here it is not allowed?

[voidzcy]

Updated how default values are applied.

We want the XdsClient to do as much validation as possible and NACK invalid configurations earlier better than later. We need to know the default values in order to implement the full set of validations. For example, {min_ring_size = 100} should be considered as a valid config, as max_ring_size will be default to 8M. The validation has to take default values into consideration.

So we implement setting default values in XdsClient. However, there is another codepath we need to consider, which is the LB config parser. Currently, this part is dead code (and most LB policies except ring_hash do not implement it). It exists because ideally we would support pluggable LB policies with LB configs given by any name resolution mechanism.

Therefore, we inevitably introduce some duplicated logic, just because the architecture is intended to be pluggable for each component. Same thing is happening in C-core: XdsClient parsing ring_hash LB config and the ring_hash LB policy's own config parser parses JSON config.

I think we should do the same for circuit breaker's default (will send a PR for that after this one is resolved).

[dapengzhang0]

LGTM