xds: Allow child of cluster_impl LB to change (1.54.x backport) #10103
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Under normal conditions the child LB of `ClusterImplLoadBalancer` does not fluctuate, based on the field used to configure load balancing in the xDS `Cluster` proto it is either: 1. `WrrLocalityLoadBalancer` if the newer `load_balancing_policy` field is used 2. `WeightedTargetLoadBalancer` if the legacy `lb_policy` field is used `ClusterImplLoadBalancer` currently assumes that this child does not change and so does not change the child LB when the name resolver sends an update. If the control plane does switch to using a different field for LB config, that update will have an LB config meant for the other child LB type. This will result in a ClassCastException and a channel panic. To address this, `ClusterImplLoadBalancer` will now use `GracefulSwitchLoadBalancer` and makes sure if the child policy changes the correct LB implementation is switched to.
ejona86
approved these changes
Apr 25, 2023
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Backport of #10091
Under normal conditions the child LB ofClusterImplLoadBalancerdoes not fluctuate, based on the field used to configure load balancing in the xDSClusterproto it is either:1.WrrLocalityLoadBalancerif the newerload_balancing_policyfield is used2.
WeightedTargetLoadBalancerif the legacylb_policyfield is used(The premise set here is off, see comment)
ClusterImplLoadBalancercurrently assumes that this child does not change and so does not change the child LB when the name resolver sends an update. If the control plane does switch to using a different field for LB config, that update will have an LB config meant for the other child LB type. This will result in a ClassCastException and a channel panic.To address this,
ClusterImplLoadBalancerwill now useGracefulSwitchLoadBalancerand makes sure if the child policy changes the correct LB implementation is switched to.Closes #10006