X != false <==> X

Note that we'll have a similar problem when transitioning from a master that did not keep the routing table in sync to a master that now does. We will probably have to schedule a clean-up task that brings these two in sync again. There's no guarantee that that task will run before any other tasks, so the assertion might not only be violated while event.previousState().nodes().isLocalNodeElectedMaster() == false, but also on some follow-up cluster state changes. I'm wondering if we need to introduce some state to SnapshotsService to say whether we've properly cleaned up as a master and then make that assertion dependent on that boolean variable. Something we can look into in a follow-up.

I wonder if we can expect there to always exist the corresponding shard routing table?

I don't know but can't a ShardId that we got from the callback org.elasticsearch.cluster.routing.RoutingChangesObserver.AbstractChangedShardObserver#shardFailed lead to a null entry here? (I could be way off here ... not sure about the order of these things yet)

we are never adding or removing keys in the shard routing table during the AllocationService.reroute() phase. Can you add an assertion that the routing table is not null, but still keep the extra safety logic around?

can you add a comment that we are in the relocating state here

Added the assertion for that below.

perhaps move the assertion one level up, i.e.,

} else {
  assert priamryShardRouting.relocating();
  if (currentState == State.INIT || currentStatus.state() == State.ABORTED) { 
    ..
  } else {
    ..
  }
}