LBs should avoid calling LBs after lb.shutdown() (1.74.x backport) #12231
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LoadBalancers shouldn't be called after shutdown(), but RingHashLb could have enqueued work to the SynchronizationContext that executed after shutdown(). This commit fixes problems discovered when auditing all LBs usage of the syncContext for that type of problem. Similarly, PickFirstLb could have requested a new connection after shutdown(). We want to avoid that sort of thing too. RingHashLb's test changed from CONNECTING to TRANSIENT_FAILURE to get the latest picker. Because two subchannels have failed it will be in TRANSIENT_FAILURE. Previously the test was using an older picker with out-of-date subchannelView, and the verifyConnection() was too imprecise to notice it was creating the wrong subchannel. As discovered in b/430347751, where ClusterImplLb was seeing a new subchannel being called after the child LB was shutdown (the shutdown itself had been caused by RingHashConfig not implementing equals() and was fixed by a8de9f0, which caused ClusterResolverLb to replace its state): ``` java.lang.NullPointerException at io.grpc.xds.ClusterImplLoadBalancer$ClusterImplLbHelper.createClusterLocalityFromAttributes(ClusterImplLoadBalancer.java:322) at io.grpc.xds.ClusterImplLoadBalancer$ClusterImplLbHelper.createSubchannel(ClusterImplLoadBalancer.java:236) at io.grpc.util.ForwardingLoadBalancerHelper.createSubchannel(ForwardingLoadBalancerHelper.java:47) at io.grpc.util.ForwardingLoadBalancerHelper.createSubchannel(ForwardingLoadBalancerHelper.java:47) at io.grpc.internal.PickFirstLeafLoadBalancer.createNewSubchannel(PickFirstLeafLoadBalancer.java:527) at io.grpc.internal.PickFirstLeafLoadBalancer.requestConnection(PickFirstLeafLoadBalancer.java:459) at io.grpc.internal.PickFirstLeafLoadBalancer.acceptResolvedAddresses(PickFirstLeafLoadBalancer.java:174) at io.grpc.xds.LazyLoadBalancer$LazyDelegate.activate(LazyLoadBalancer.java:64) at io.grpc.xds.LazyLoadBalancer$LazyDelegate.requestConnection(LazyLoadBalancer.java:97) at io.grpc.util.ForwardingLoadBalancer.requestConnection(ForwardingLoadBalancer.java:61) at io.grpc.xds.RingHashLoadBalancer$RingHashPicker.lambda$pickSubchannel$0(RingHashLoadBalancer.java:440) at io.grpc.SynchronizationContext.drain(SynchronizationContext.java:96) at io.grpc.SynchronizationContext.execute(SynchronizationContext.java:128) at io.grpc.xds.client.XdsClientImpl$ResourceSubscriber.onData(XdsClientImpl.java:817) ```
kannanjgithub
approved these changes
Jul 18, 2025
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LoadBalancers shouldn't be called after shutdown(), but RingHashLb could have enqueued work to the SynchronizationContext that executed after shutdown(). This commit fixes problems discovered when auditing all LBs usage of the syncContext for that type of problem.
Similarly, PickFirstLb could have requested a new connection after shutdown(). We want to avoid that sort of thing too.
RingHashLb's test changed from CONNECTING to TRANSIENT_FAILURE to get the latest picker. Because two subchannels have failed it will be in TRANSIENT_FAILURE. Previously the test was using an older picker with out-of-date subchannelView, and the verifyConnection() was too imprecise to notice it was creating the wrong subchannel.
As discovered in b/430347751, where ClusterImplLb was seeing a new subchannel being called after the child LB was shutdown (the shutdown itself had been caused by RingHashConfig not implementing equals() and was fixed by a8de9f0, which caused ClusterResolverLb to replace its state):
Backport of #12226