Unselectable flag race fix #1773
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In short, we could get in a state where `IsReplica` could be true, but we couldn't use it because it was unselectable via this path, especially on a reconnect race. This caused failures in other Sentinel testing, specifically the `NoConnectionAvailable` variety on `DemandMaster` sanity check gets, because we had a replica, it was connected, but the flags said we couldn't use it. That should _never_ be the case and this additional check at the flag set site ensures it.
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In short, we could get in a state where
IsReplicacould be true, but we couldn't use it because it was unselectable via this path, especially on a reconnect race. This caused failures in other Sentinel testing, specifically theNoConnectionAvailablevariety onDemandMastersanity check gets, because we had a replica, it was connected, but the flags said we couldn't use it. That should never be the case and this additional check at the flag set site ensures it.In testing, it looked like this (local changes here are logging this condition):