Commits on Sep 4, 2018

1. raft: fix correctness bug in CommittedEntries pagination …

   In etcd-io#9982, a mechanism to limit the size of `CommittedEntries` was
   introduced. The way this mechanism worked was that it would load
   applicable entries (passing the max size hint) and would emit a
   `HardState` whose commit index was truncated to match the limitation
   applied to the entries. Unfortunately, this was subtly incorrect
   when the user-provided `Entries` implementation didn't exactly
   match what Raft uses internally. Depending on whether a `Node` or
   a `RawNode` was used, this would either lead to regressing the
   HardState's commit index or outright forgetting to apply entries,
   respectively.
   
   Asking implementers to precisely match the Raft size limitation
   semantics was considered but looks like a bad idea as it puts
   correctness squarely in the hands of downstream users. Instead, this
   PR removes the truncation of `HardState` when limiting is active
   and tracks the applied index separately. This removes the old
   paradigm (that the previous code tried to work around) that the
   client will always apply all the way to the commit index, which
   isn't true when commit entries are paginated.
   
   See [1] for more on the discovery of this bug (CockroachDB's
   implementation of `Entries` returns one more entry than Raft's when the
   size limit hits).
   
   [1]: cockroachdb/cockroach#28918 (comment)

   

   tbg committed Sep 4, 2018
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