unwrap ForwardingSubchannel during Picks

[shivaspeaks]

Unwap ForwardingSubchannels during picks (prerequisite to deleting Subchannel Attributes)

Some LB policies wrap the Subchannel (look for ForwardingSubchannel) to get events like shutdown(), but they don't un-wrap the subchannel when propagating the subchannel up the stack during picks. That means the identity of a subchannel is awkward, as you can't use object identity and instead need to store data inside the subchannel.

If the LB policies properly unwrap when doing picks, then if ManagedChannelImpl has Helper.createSubchannel() always return an instance of FooSubchannel, then during pickSubchannel() the subchannel should always be a FooSubchannel and the channel could blindly cast to FooSubchannel. Today we are using getInternalSubchannel() to unwrap.

So this task has two parts:

1. Fixing any LB policy that wraps subchannels (search for ForwardingSubchannel)
2. Adding an API to LB.PickResult to make it easier to unwrap without dropping data on the floor (like any tracer; this makes (1) less annoying)

[gemini-code-assist]

Summary of Changes

Hello @shivaspeaks, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a long-standing issue where load balancer policies would wrap Subchannel objects with ForwardingSubchannel but fail to unwrap them during the picking process. This behavior made it difficult to correctly identify the true subchannel and perform operations like casting. The changes introduce new API methods to PickResult for seamless subchannel replacement and stream tracer factory updates, and subsequently integrate these unwrapping mechanisms into various load balancer implementations. The overall goal is to streamline subchannel identity management and simplify future modifications related to subchannel attributes.

Highlights

• New PickResult Methods: Two new methods, withSubchannelReplacement and withStreamTracerFactory, were added to LoadBalancer.PickResult to facilitate easier unwrapping of subchannels and updating stream tracer factories while preserving other pick result attributes like authority override.
• Load Balancer Policy Updates: Several load balancer implementations, including HealthCheckingLoadBalancerFactory, HealthProducerHelper, OutlierDetectionLoadBalancer, ClusterImplLoadBalancer, WeightedRoundRobinLoadBalancer, and OrcaOobUtil, were updated to explicitly unwrap ForwardingSubchannel instances during the pickSubchannel process. This ensures that the underlying subchannel is exposed, addressing identity issues.
• Refactored ClusterImplLoadBalancer: The anonymous ForwardingSubchannel implementation within ClusterImplLoadBalancer was refactored into a dedicated inner class, ClusterImplSubchannel, improving code readability and maintainability.
• Test Coverage Expanded: New tests were added for the PickResult methods, and existing tests for OutlierDetectionLoadBalancer and WeightedRoundRobinLoadBalancer were updated to reflect the new unwrapping behavior and helper methods.

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Changelog

• api/src/main/java/io/grpc/LoadBalancer.java
  • Introduced withSubchannelReplacement(Subchannel subchannel) to PickResult for replacing the subchannel while retaining stream tracer factory and authority override.
  • Added withStreamTracerFactory(ClientStreamTracer.Factory streamTracerFactory) to PickResult for updating the stream tracer factory while retaining the subchannel and authority override.
• api/src/test/java/io/grpc/LoadBalancerTest.java
  • Added unit tests for the new PickResult.withSubchannelReplacement method.
  • Added unit tests for the new PickResult.withStreamTracerFactory method.
• services/src/main/java/io/grpc/protobuf/services/HealthCheckingLoadBalancerFactory.java
  • Implemented HealthCheckPicker to unwrap SubchannelImpl instances during pickSubchannel using withSubchannelReplacement.
• util/src/main/java/io/grpc/util/HealthProducerHelper.java
  • Imported ConnectivityState.
  • Implemented HealthProducerPicker to unwrap HealthProducerSubchannel instances during pickSubchannel using withSubchannelReplacement.
• util/src/main/java/io/grpc/util/OutlierDetectionLoadBalancer.java
  • Modified pickSubchannel to unwrap OutlierDetectionSubchannel instances using withSubchannelReplacement and withStreamTracerFactory.
• util/src/test/java/io/grpc/util/OutlierDetectionLoadBalancerTest.java
  • Updated delegatePick test to directly use the unwrapped subchannel from pickResult.getSubchannel().
• xds/src/main/java/io/grpc/xds/ClusterImplLoadBalancer.java
  • Refactored the anonymous ForwardingSubchannel into a named inner class ClusterImplSubchannel.
  • Modified pickSubchannel to unwrap ClusterImplSubchannel instances using withSubchannelReplacement.
  • Updated pickSubchannel to use withStreamTracerFactory when applying ORCA tracer factories.
• xds/src/main/java/io/grpc/xds/WeightedRoundRobinLoadBalancer.java
  • Modified pickSubchannel to unwrap WrrSubchannel instances using withSubchannelReplacement and withStreamTracerFactory.
• xds/src/main/java/io/grpc/xds/orca/OrcaOobUtil.java
  • Imported ConnectivityState, PickResult, PickSubchannelArgs, and SubchannelPicker.
  • Implemented OrcaOobPicker to unwrap SubchannelImpl instances during pickSubchannel using withSubchannelReplacement.
• xds/src/test/java/io/grpc/xds/WeightedRoundRobinLoadBalancerTest.java
  • Added a helper method getWrrPicker to correctly retrieve the WeightedRoundRobinPicker by unwrapping OrcaOobPicker if present.
  • Updated various test methods (pickChildLbTF, wrrLifeCycle, enableOobLoadReportConfig, pickByWeight, blackoutPeriod, updateWeightTimer, weightExpired, rrFallback, unknownWeightIsAvgWeight, pickFromOtherThread) to use the new getWrrPicker helper.

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[shivaspeaks]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces helper methods to PickResult for easier creation of new results from existing ones, and then refactors several load balancer implementations to use these new methods for unwrapping ForwardingSubchannel instances. This is a good improvement that simplifies the code and makes the logic clearer. The changes are consistent across different load balancers. I've found a few minor areas for improvement, mainly around Javadoc clarity and small code simplifications, which are detailed in the comments. Overall, this is a solid refactoring.

[gemini-code-assist]

Code Review

This pull request is a great step towards simplifying the load balancing logic by introducing helper methods on PickResult and using them to consistently unwrap ForwardingSubchannel instances within the load balancer policies. The changes make the code cleaner and more maintainable. I have a couple of minor suggestions to further improve consistency across the codebase.