xds: add RlsClusterSpecifierPlugin for RLS-in-xDS

[dapengzhang0]

Add RlsClusterSpecifierPlugin as per the design doc

The structure of ClusterSpecifierPlugin is very similar to io.grpc.xds.Filter.

The following changes to the existing code are made:

• move ConfigOrError class out of Filter class to be shared with ClusterSpecifierPlugin
• make io.grpc.rls.RlsProtoData public to be accessible by io.grpc.xds
• treat empty defaultTarget in io.grpc.rls.RlsProtoData.RouteLookupConfig as null to support both json and proto config without defaultTarget field specified.

[sergiitk]

LGTM, some notes.

[sergiitk]

Thank you for addressing my nits :)

[ejona86]

Why is this exposing RlsProtoData to xds? That is not what the design called for. The design seems to imply that the proto would be converted to json, but certainly it is json being passed to the existing LB policy parsing logic. Is that not feasible?

[dapengzhang0]

ejona86: Why is this exposing RlsProtoData to xds? That is not what the design called for. The design seems to imply that the proto would be converted to json, but certainly it is json being passed to the existing LB policy parsing logic. Is that not feasible?

It is feasible to use json if the plugin TypedExtension is encoded with TypedStruct (typeUrl = "type.googleapis.com/envoy.config.route.v3.FilterConfig") instead of RlsProtoData, and the plugin implementation should avoid using RlsProtoData proto. But this should be called out in the design. cc @dfawley

[ejona86]

TypedStruct (typeUrl = "type.googleapis.com/envoy.config.route.v3.FilterConfig")

That would not be the correct type. It would be grpc.lookup.v1.RlsClusterSpecifier.

It is never encoded with RlsProtoData. That's our internal representation. Even if it is encoded as a proto, we can convert RLSClusterSpecifier to JSON using the protobuf tooling and use the existing JSON-based parsing API. No need to re-create the parsing logic.

[dapengzhang0]

That would not be the correct type. It would be grpc.lookup.v1.RlsClusterSpecifier.

Sorry, I copy & paste in mistake. It should be (typUrl="type.googleapis.com/udpa.type.v1.TypedStruct") in the top level of TypedExtension. Then inside the TypedStruct, the inner level typeUrl is "grpc.lookup.v1.RlsClusterSpecifier"

[dfawley]

this should be called out in the design

The design does note that the RLS cluster specifier plugin should emit, as part of the config, "routeLookupConfig": <JSON form of RLSClusterSpecifier.config>.