Commits on Jul 8, 2019

1. raft: allow voter to become learner through snapshot …

   At the time of writing, we don't allow configuration changes to change
   voters to learners directly, but note that a snapshot may compress
   multiple changes to the configuration into one: the voter could have
   been removed, then readded as a learner and the snapshot reflects both
   changes. In that case, a voter receives a snapshot telling it that it is
   now a learner. In fact, the node has to accept that snapshot, or it is
   permanently cut off from the Raft log.
   
   I think this just wasn't realized in the original work, but this is just
   my guess since there generally is very little rationale on the various
   decisions made. I also generally haven't been able to figure out whether
   the decision to prevent voters from becoming learners without first
   having been removed was motivated by some particular concern, or if it
   just wasn't deemed necessary. I suspect it is the latter because
   demoting a voter seems perfectly safe.
   
   See etcd-io#8751 (comment).

   

   tbg committed Jul 8, 2019
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