Committing entries from previous terms

[as58545652]

raft/src/replication.c

Lines 1539 to 1574 in 4c71b94

	void replicationQuorum(struct raft *r, const raft_index index)
	{
	size_t votes = 0;
	size_t i;
	assert(r->state == RAFT_LEADER);
	if (index <= r->commit_index) {
	return;
	}
	/* TODO: fuzzy-test --seed 0x8db5fccc replication/entries/partitioned
	* fails the assertion below. */
	if (logTermOf(&r->log, index) == 0) {
	return;
	}
	// assert(logTermOf(&r->log, index) > 0);
	assert(logTermOf(&r->log, index) <= r->current_term);
	for (i = 0; i < r->configuration.n; i++) {
	struct raft_server *server = &r->configuration.servers[i];
	if (server->role != RAFT_VOTER) {
	continue;
	}
	if (r->leader_state.progress[i].match_index >= index) {
	votes++;
	}
	}
	if (votes > configurationVoterCount(&r->configuration) / 2) {
	r->commit_index = index;
	tracef("new commit index %llu", r->commit_index);
	}
	return;
	}

From section 3.6.2:
Raft never commits log entries from previous terms by counting replicas. Only log entries from the leader’s current term are committed by counting replicas.

In the code, I have not found the implementation of the logic described above.

[MathieuBordere]

Thanks for logging this issue, good catch!

The quorum logic should implement the part in Rules for Servers - Leaders at the bottom of figure 2 in raft extended

If there exists an N such that N > commitIndex, 
a majority of matchIndex[i] ≥ N, 
and log[N].term == currentTerm 
: set commitIndex = N.

This does not seem to be the case in our implementation.

[calvin2021y]

Any plan to fix this？

[MathieuBordere]

Yes, it is still the plan to fix this.

[calvin2021y]

This bugs find almost a year, Is there a schedule plan for it.

[MathieuBordere]

I have a branch where I'm implementing a fix as it has some unintended consequences in dqlite, however, unfortunately, things keep coming up in between to finish it.

[calvin2021y]

Thanks for the quick response, look forward to it.

If you can share your branch here, we can try it and maybe give some advice.

[MathieuBordere]

https://github.com/MathieuBordere/raft/commits/quorum-bugfix it's behind the current master somewhat though.

[cole-miller]

I'm in the process of picking up Mathieu's previous work on this, will have a draft PR up soon.