fix(pick_first): ignore state updates from shut down or replaced subchannels - #12968
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…hannels In legacy PickFirstLoadBalancer, when handleNameResolutionError() or shutdown() is called, subchannel.shutdown() is invoked. Because ManagedChannelImpl delays subchannel shutdown by 5 seconds (SUBCHANNEL_SHUTDOWN_DELAY_SECONDS), the old subchannel may complete a connection attempt during this window and fire READY state updates. Without a check verifying if the calling subchannel is still the current active subchannel, PickFirstLoadBalancer publishes a READY picker for the obsolete subchannel right before its 5-second delayed shutdown task fires and kills the transport. This leaves the channel permanently stuck in READY with a dead subchannel picker. Fixes grpc#12958
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Aug 27, 2026
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In legacy PickFirstLoadBalancer, when handleNameResolutionError() or shutdown() is called, subchannel.shutdown() is invoked. Because ManagedChannelImpl delays subchannel shutdown by 5 seconds (SUBCHANNEL_SHUTDOWN_DELAY_SECONDS), the old subchannel may complete a connection attempt during this window and fire READY state updates. Without a check verifying if the calling subchannel is still the current active subchannel, PickFirstLoadBalancer publishes a READY picker for the obsolete subchannel right before its 5-second delayed shutdown task fires and kills the transport. This leaves the channel permanently stuck in READY with a dead subchannel picker.
Fixes #12958