Probe from index less than match

[wego1236]

In probe mechanism there is chance for replication starting from an index less than match index in probe state, unlike that in replicate state. For example, a rejected probe response got delayed and is received when the leader falls into another probe state. This is a rare case of cause.

To my understanding, allowing such a smaller index implies we want to tolerate loss of already replicated log entries.
raft-rs/src/tracker/progress.rs

        if request_snapshot == INVALID_INDEX {
            self.next_idx = cmp::min(rejected, match_hint + 1);
            if self.next_idx < 1 {
                self.next_idx = 1;
            }

In this code, there is chance to make Next < Match. But the invariant in raft is that Next > Match.

[wego1236]

i also find in etcd it fix this bug in https://github.com/etcd-io/raft/pull/149

[BusyJay]

Good catch! Do you want to port the patch?

[wego1236]

Yes，i'll do it right now!